Category Archives: Lipids

The term lipids refers to a very heterogeneous group of organic compounds generally identified on the basis of their solubility in nonpolar solvents such as chloroform or ether.
From a chemical point of view, they consist mainly of carbon and hydrogen atoms joined by covalent bonds. In some lipids a part of the molecule has polar properties, generally due to the presence of oxygen and/or phosphorus atoms; examples are fatty acids and phospholipids. Compared to non-polar lipids, polar ones are amphipathic molecules, namely, they are soluble in both polar and nonpolar solvents, and some are soluble in water but not in nonpolar solvents.
From a physiological point of view they can be divided into three categories.

  • Storage lipids, essentially composed by triglycerides. They are a major energy reserve in biological systems.
  • Lipids with structural functions, such as cholesterol, phospholipids and glycolipids, which are a component, together with proteins, of cell membranes.
  • Lipids that are involved in cell signaling, such as those that play a role in the transduction of the signals that cells receive from the outside. For example, such molecules can be released from cell membrane as a result of the binding of messengers to receptors, like G proteins, of the target cell. In turn, these lipid signals can act in autocrine and or paracrine manner.

The group also includes several vitamins, called fat-soluble vitamins, namely, vitamins A, E, D and K.
In addition to fat-soluble vitamins, there are other lipids that humans and many other animals can’t synthesize and must be taken with food, namely, the essential fatty acids.
Although the term fat is often used as a synonym for lipids, the two terms are not synonymous, as fats are a subgroup of them.

Chemical composition of olive oil

From a chemical point of view, we can identify in the olive oil two fractions, depending on the behavior in the presence of heating and strong alkaline solutions (concentrated solutions of KOH or NaOH):

  • the saponifiable fraction, which represents 98-99% of the total weight, is composed of substances that form soaps in the above conditions;
  • the unsaponifiable fraction, which represents the remaining 1-2% of the total weight, is composed of substances that fail to form soaps in the above conditions.

CONTENTS

Saponifiable fraction

It is composed of saturated fatty acids and unsaturated fatty acids, esterified almost entirely to glycerol to form triglycerides (or triacylglycerols). To a much lesser extent, diglycerides (or diacylglycerols), monoglycerides (monoacylglycerols), and free fatty acids are also found.
Unsaturated fatty acids make up 75 to 85% of the total fatty acids. Oleic (O) and linoleic (L) acids are the most abundant ones; palmitoleic, eptadecenoic, gadoleic and alpha-linolenic (Ln) acids are present in lower/trace amounts.

International Olive Oil Council (IOOC) requirements for fatty acids in olive oil
IOOC Requirements for Olive Oil

Oleic acid is the major fatty acid in olive oils. According to the rules laid down by the International Olive Oil Council (IOOC), its concentration must range from 55% to 83% of total fatty acids.
Linoleic acid is the most abundant polyunsaturated fatty acid in olive oil; its concentration must vary between 2.5% and 21% (IOOC). Because of its high degree of unsaturation, it is subject to oxidation; this means that an oil high in linoleic acid becomes rancid easily, and thus it may be stored for a shorter time.
In a Mediterranean-type diet, olive oil is the main source of fat: therefore, oleic acid, among monounsaturated fatty acids, and linoleic acid, among polyunsaturated fatty acids, are the most abundant fatty acids.
alpha-Linolenic acid must be present in very low amount, according to the IOOC standards ≤1%. It is an omega-3 polyunsaturated fatty acid, which may have health benefits. However, because of to its high degree of unsaturation (higher than that of linoleic acid), it is very susceptible to oxidation, and therefore it promotes rancidity of the olive oil that contains it.
Saturated fatty acids make up 15 to 25% of the total fatty acids.
Palmitic (P) (7.5-20%) and stearic (S) acids (0.5-5%) are the most abundant saturated fatty acids; myristic, heptadecanoic, arachidic, behenic and lignoceric acids may be present in trace amounts.

The presence of fatty acids that should be absent or present in amounts different than those found is a marker of adulteration with other vegetable oils. On this regard, particular attention is paid to myristic, arachidic, behenic, lignoceric, gadoleic and alpha-linolenic acids, whose limits are set by IOOC.

Fatty acid composition is influenced by several factors.

  • The climate.
  • The latitude.
  • The zone of production.
    Italian, Spanish and Greek olive oils are high in oleic acid and low in palmitic and linoleic acids, while Tunisian olive oils are high in palmitic and linoleic acids but lower in oleic acid. Therefore, oils can be divided into two groups:

one rich in oleic acid and low in palmitic and linoleic acids;
the other high in palmitic and linoleic acids and low in oleic acid.

  • The cultivar.
  • The degree of olive ripeness at the time of oil extraction.
    It should be noted that oleic acid is formed first in the fruit, and data seem to indicate a competitive relationship between oleic acid and palmitic, palmitoleic, and linoleic acids.

Triglycerides

As previously said, fatty acids in olive oil are almost entirely present as triglycerides.
In small percentage, they are also present as diglycerides, monoglycerides, and in free form.

Sterospecific numbering of triglycerides

During triglyceride biosynthesis, thanks to the presence of specific enzymes, only about 2% of glycerol binds palmitic acid in the sn-2 position (also the percentage of stearic acid in the sn-2 position is very low); for the most part, the sn-2 position is occupied by oleic acid.
On the contrary, if we consider oils that have undergone a nonenzymatic esterification, the percentage of palmitic acid in the sn-2 position increases significantly.
Note: sn = Stereospecific numbering

Among triglycerides present in significant proportions in olive oil, there are:

  • OOO: 40-59%;
  • POO: 12-20%;
  • OOL: 12.5-20%;
  • POL:  5.5-7%;
  • SOO: 3- 7%.

POP, POS, OLnL, OLnO, PLL, PLnO are present in smaller amounts.
Trilinolein (LLL) is a triglyceride that contains three molecules of linoleic acid. Its low content is an indicator of an oil of good quality.
Triglycerides containing three saturated fatty acids or three molecules of alpha-linolenic acid have not been reported.

Diglycerides and monoglycerides

Their presence is due to an incomplete synthesis and/or a partial hydrolysis of triglycerides.
The content of diglycerides in virgin olive oil ranges from 1% to 2.8%. 1,2-Diglycerides prevail in fresh olive oil, representing over 80% of the diglycerides. During oil storage, isomerization occurs with a progressive increase of the more stable 1-3 isomers, which after about 10 months become the major isomers.
Therefore, the ratio 1,2/1,3-diglycerides may be used as an indicator of the age of the oil.
Monoglycerides are present in amounts lower than diglycerides, <0.25%, with 1-monoglycerides far more abundant than 2-monoglycerides.

Unsaponifiable fractions

It is composed of a large number of different molecules, very important from a nutritional point of view, as they contribute significantly to the health effects of olive oil.
Furthermore, they are responsible for the stability and the taste of olive oil, and are also used to detect adulteration with other vegetable oils.
This fraction includes tocopherols, sterols, polyphenols, pigments, hydrocarbons, aromatic and aliphatic alcohol, triterpene acids, waxes, and minor constituents.
Their content is influenced by factors similar to those seen for fatty acid composition, such as:

  • the cultivar;
  • the degree of ripeness of the olive;
  • the zone of production;
  • the crop year and olive harvesting practices;
  • the storage time of olives;
  • the oil extraction process;
  • the storage conditions of the oil.

It should be noted that many of these compounds are not present in refined olive oils, as they are removed during the refining processes.

Polyphenols

They make up 18 to 37% of the unsaponifiable fraction.
They are a very heterogeneous group of molecules with nutritional and organoleptic properties  (for example, oleuropein and hydroxytyrosol give oil its bitter and pungent taste).
For a more extensive discussion, see: ” Polyphenols in olive oil: variability and composition.”

Hydrocarbons

They make up 30 to 50% of the unsaponifiable fraction.
Squalene and beta-carotene are the main molecules.
Squalene, isolated for the first time from shark liver, is the major constituent of the unsaponifiable fraction, and constitutes more than 90% of the hydrocarbons. Its concentration ranges from 200 to 7500 mg/kg of olive oil.

Skeletal formula of squalene, an hydrocarbon of the unsaponifiable fraction of olive oil

It is an intermediate in the biosynthesis of the four-ring structure of steroids, and it seems to be responsible of several health effects of olive oil.
In the hydrocarbon fraction of virgin olive oil, n-paraffins, diterpene and triterpene hydrocarbons, isoprenoidal polyolefins are also found.
Beta-carotene acts both as antioxidant, protecting oil during storage, and as dye (see below).

Sterols

They are important lipids of olive oil, and are:

  • linked to many health benefits for consumers;
  • important to the quality of the oil;
  • widely used for checking its genuineness.
    On this regard, it is to underline that sterols are species-specific molecules; for example, the presence of high concentrations of brassicasterol, a sterol typically found in Brassicaceae (Cruciferae) family, such as rapeseed, indicates adulteration of olive oil with canola oil.

Four classes of sterols are present in olive oil: common sterols, 4-methylsterols, triterpene alcohols, and triterpene dialcohols. Their content ranges from 1000 mg/kg, the minimum value required by the IOOC standard, to 2000 mg/kg. The lowest values are found in refined oils because of the refining processes may cause losses up to 25%.

Common sterols or 4α-desmethylsterols

Common sterols are present mainly in the free and esterified form; however they have been also found as lipoproteins and sterylglucosides.
The main molecules are beta-sitosterol, which makes up 75 to 90% of the total sterol, Δ5-avenasterol, 5 to  20%, and campesterol, 4%. Other components found in lower amounts or traces are, for example, stigmasterol, 2%, cholesterol, brassicasterol, and ergosterol.Skeletal formula of beta-sitosterol, a sterol of the unsaponifiable fraction of olive oil

4-Methylsterols

They are intermediates in the biosynthesis of sterols, and are present both in the free and esterified form. They are present in small amounts, much lower than those of common sterols and triterpene alcohols, varying between 50 and 360 mg/kg. The main molecules are obtusifoliol, cycloeucalenol, citrostadienol, and gramisterol.

Triterpene alcohols or 4,4-dimethylsterols

They are a complex class of sterols, present both in the free and esterified form. They are found in amounts ranging from 350 to 1500 mg/kg.
The main components are beta-amyrin, 24-methylenecycloartanol, cycloartenol, and butyrospermol; other molecules present in lower/trace amounts are, for example, cyclosadol, cyclobranol, germanicol, and dammaradienol.

Triterpene dialcohols

The main triterpene dialcohols found in olive oil are erythrodiol and uvaol.
Erythrodiol is present both in the free and esterified form; in virgin olive oil, its level varies between 19 and 69 mg/kg, and the free form is generally lower than 50 mg/kg.

Tocopherols

They make up 2 to 3% of the unsaponifiable fraction, and include vitamin E.
Of the eight E-vitamers, alpha-tocopherol represents about 90% of tocopherols in virgin olive oil. It is present in the free form and in very variable amount, but on average higher than 100 mg/kg of olive oil. Thanks to its in vivo antioxidant properties, its presence is a protective factor for health. Alpha-tocopherol concentration seems to be related to the high levels of chlorophylls and to the concomitant requirement for deactivation of singlet oxygen.
Beta-tocopherol, delta-tocopherol, and gamma-tocopherol are usually present in low amounts.

Pigments

In this group we find chlorophylls and carotenoids.
In olive oil, chlorophylls are present as phaeophytins, mainly  phaeophytin a (i.e. a chlorophyll from which magnesium has been removed and substituted with two hydrogen ions), and confer the characteristic green color to olive oil. They are photosensitizer molecules that contribute to the photooxidation of olive oil itself.
Beta-carotene and lutein are the main carotenoids in olive oil. Several xanthophylls are also present, such as antheraxanthin, beta-cryptoxanthin, luteoxanthin, mutatoxanthin, neoxanthin, and violaxanthin.
Olive oil’s color is the result of the presence of chlorophylls and carotenoids and of their green and yellow hues. Their presence is closely related.

Triterpene acids

They are important components of the olive, and are present in trace amounts in the oil.
Oleanolic and maslinic acids are the main triterpene acids in virgin olive oil: they are present in the olive husk, from which they are extracted in small amount during processing.

Aliphatic and aromatic alcohols

Fatty alcohols and diterpene alcohols are the most important ones.
Aliphatic alcohols have a number of carbon atoms between 20 and 30, and are located mostly inside the olive stones, from where they are partially extracted by milling.

Fatty alcohols

They are linear saturated alcohols with more than 16 carbon atoms.
They are found in the free and esterified form and are present, in virgin olive oil, in amount not generally higher than 250 mg/kg.
Docosanol (C22), tetracosanol (C24), hexacosanol (C26), and octacosanol (C28) are the main fatty alcohols in olive oil, with tetracosanol and hexacosanol present in larger amounts.
Waxes, which are minor constituents of olive oil, are esters of fatty alcohols with fatty acids, mainly of palmitic acid and oleic acid. They can be used as a criterion to discriminate between different types of oils; for example, they must be present in virgin and extra virgin olive oil at levels <150 mg/kg, according to the IOOC standards.

Diterpene alcohols

Geranylgeraniol and phytol are two acyclic diterpene alcohols, present in the free and esterified form. Among esters present in the wax fraction of extra virgin olive oil, oleate, eicosenoate , eicosanoate, docosanoate, and tetracosanoate have been found, mainly as phytyl derivatives.

Volatile compounds

More than 280 volatile compounds have been identified in olive oil, such as hydrocarbons, the most abundant fraction, alcohols, aldehydes, ketones, esters, acids, ethers and many others. However, only about 70 of them are present at levels higher than the perception threshold beyond which they may contribute to the aroma of virgin olive oil.

Minor components

Phospholipids are found among the minor components of olive oil; the main ones are phosphatidylserine, phosphatidylethanolamine, phosphatidylcholine, phosphatidylinositol.
In the unfiltered oils, trace amounts of proteins may be found.

References

Gunstone F.D. Vegetable oils in food technology: composition, properties and uses. 2th Edition. Wiley J. & Sons, Inc., Publication, 2011

Caponio F., Bilancia M.T., Pasqualone A., Sikorska E., Gomes T. Influence of the exposure to light on extra virgin olive oil quality during storage. Eur Food Res Technol 2005;221:92-98. doi:10.1007/s00217-004-1126-8

Servili M., Sordini B., Esposto S., Urbani S., Veneziani G., Di Maio I., Selvaggini R. and Taticchi A. Biological activities of phenolic compounds of extra virgin olive oil. Antioxidants 2014;3:1-23. doi:10.3390/antiox3010001

GLA and physiology and pathophysiology of the skin

gamma-Linolenic acid (GLA), an omega-6 PUFA, like its precursor linoleic acid (the most abundant polyunsaturated fatty acid in human skin epidermis, where it’s involved in the maintenance of the epidermal water barrier), plays important roles in the physiology and pathophysiology of the skin.
Studies conducted on humans revealed that gamma-linolenic acid:

  • improves skin moisture, firmness, roughness;
  • decreases transepidermal water loss (one of the abnormalities of the skin in essential fatty acid deficiency animals).
Skelatal formula of Prostaglandin E1, a derivative of gamma-linolenic acid, an omega-6 PUFA
Prostaglandin E1

Using guinea pig skin epidermis as a model of human epidermis (they are functionally similar), it was demonstrated that supplementation of animals with gamma-linolenic acid-rich foods results in a major production of PGE1 and 15-HETrE in the skin (as previously demonstrated in in vitro experiments).
Because these molecules have both anti-inflammatory/anti-proliferative properties supplementation of diet with gamma-linolenic acid acid-rich foods may be an adjuncts to standard therapy for inflammatory/proliferative skin disorders.

Supplemental sources of GLA

The main supplemental sources of gamma-linolenic acid are oils of the seeds of:

  • borage (20%-27% of the total fatty acids);
  • black currant (from 15% to 19% of the total fatty acids);
  • evening primrose (from 7% to 14% of the total fatty acids), and

Role of gamma-linolenic acid in lowering blood pressure

The relationship between dietary fatty acid intake and blood pressure mainly comes from studies conducted on genetically modified rats that spontaneously develops hypertension (a commonly used animal model for human hypertension).
In these studies many membrane abnormalities were seen so hypertension in rat model may be related to change in polyunsaturated fatty acid metabolism at cell membrane level.
About polyunsaturated fatty acids, several research teams have reported that gamma-linolenic acid reduces blood pressure in normal and genetically modified rats (greater effect) and it was purported by interfering with Renin-Angiotensin System (that promote vascular resistance and renal retention) altering the properties of the vascular smooth muscle cell membrane and so interfering with the action of angiotensin II.
Another possible mechanism of action of gamma-linolenic acid to lower blood pressure could be by its metabolite dihomo-gamma-linolenic acid: it may be incorporated in vascular smooth muscle cell membrane phospholipids, then released by the action of phospholipase A2 and transformed by COX-1 in PGE1 that induces vascular smooth muscle relaxation.

Role gamma-linolenic acid in treatment of rheumatoid arthritis

In a study conducted by Leventhal et al. on 1993 it was demonstrated the dietary intake of higher concentration of borage oil (about 1400 mg of gamma-linolenic acid/day) for 24 weeks resulted in clinically significant reductions in signs and symptoms of rheumatoid arthritis activity.
In a subsequent study by Zurier et al. on 1996 the dietary intake of an higher dose (about 2.8 g/day gamma-linolenic acid) for 6 months reduced, in a clinically relevant manner, signs and symptoms of the disease activity; patients who remained for 1 year on the 2.8 g/day dietary gamma-linolenic acid exhibited continued improvement in symptoms (the use of gamma-linolenic acid also at the above higher dose is well tolerated, with minimal deleterious effects). These data underscore that the daily amount and the duration of gamma-linolenic acid dietary intake do correlate with the clinical efficacy.

References

Akoh C.C. and Min D.B. “Food lipids: chemistry, nutrition, and biotechnology” 3th ed. 2008

Chow Ching K. “Fatty acids in foods and their health implication” 3th ed. 2008

Fan Y.Y. and Chapkin R.S. Importance of dietary γ-linolenic acid in human health and nutrition. J Nutr 1998;128:1411-1414. doi:10.1093/jn/128.9.1411

Leventhal L.J., Boyce E.G. and Zurier R.B. Treatment of rheumatoid arthritis with gammalinolenic acid. Ann Intern Med 1993 119:867-873. doi:10.7326/0003-4819-119-9-199311010-00001

Miller C.C. and Ziboh V.A. Gammalinolenic acid-enriched diet alters cutaneous eicosanoids. Biochem Biophys Res Commun 1988 154:967-974. doi:10.1016/0006-291X(88)90234-3

Zurier R.B., Rossetti R.G., Jacobson E.W., DeMarco D.M., Liu N.Y., Temming J.E., White B.M. and Laposata M. Gamma-linolenic acid treatment of rheumatoid arthritis. A randomized, placebocontrolled trial. Arthritis Rheum 1996 39:1808-1817. doi:10.1002/art.1780391106

Trans fatty acids (TFA): structure, sources, health effects, examples

Trans fatty acids (TFA) or trans-unsaturated fatty acids or trans fats are unsaturated fatty acids with at least one a double bond in the trans or E configuration.
Carbon-carbon double bonds show planar conformation, and so they can be considered as plains from whose opposite sides carbon chain attaches and continues. “The entry” and “the exit” of the carbon chain from the plain may occur on the same side of the plain, and in this case double bond is defined in cis or Z configuration, or on opposite side, and in that case it is defined in trans configuration.

Examples of C18:1 cis/trans isomers, and of a saturated fatty acid
Fig. 1 – C18:1 cis/trans Isomers

Unsaturated fatty acids most commonly have their double bonds in cis configuration; the other, less common configuration is trans.
Cis bond causes a bend in the fatty acid chain, whereas the geometry of trans bond straightens the fatty acid chain, imparting a structure more similar to that of saturated fatty acids.

CONTENTS

Properties of fats rich in trans fatty acids

Below, some distinctive characteristics of the fats rich in trans fats, that make them particularly suited for the production of margarines and vegetable shortening used in home and commercial cooking, and manufacturing processes.

  • Bent molecules can’t pack together easily, but linear ones can do it.
    This means that trans fatty acids contribute, together with the geometrically similar saturated fatty acids, to the hardness of the fats in which they are, giving them a higher melting point.
    Heightening the melting point of fats means that it is possible to convert them from liquid form to semi-solids and solids at room temperature.
    Note: trans fats tend to be less solid than saturated fatty acids.
  • They have:

a melting point, consistency and “mouth feel” similar to those of butter;
a long shelf life at room temperature;
a flavor stability.

  • They are stable during frying.

Sources of trans fatty acids

Dietary TFA come from different sources briefly reviewed below.

  • In industrialized countries, greater part of the consumed trans fatty acids, in USA about 80 percent of the total, are produced industrially, in varying amounts, during partial hydrogenation of edible oils containing unsaturated fatty acids (see below).
  • They are produced at home during frying with vegetable oils containing unsaturated fatty acids.
  • They come from bacterial transformation of unsaturated fatty acids ingested by ruminants in their rumen (see below).
  • Another natural source is represented by some plant species, such as leeks, peas, lettuce and spinach, that contain trans-3-hexadecenoic acid, and rapeseed oil, that contains brassidic acid (22:1∆13t) and gondoic acid (20:1∆11t). In these sources trans fatty acids are present in small amounts.
  • Very small amounts, less than 2 percent, are formed during deodorization of vegetable oils, a process necessary in the refining of edible oils. During this process trans fatty acids with more than one double bond are formed in small amounts. These isomers are also present in fried foods and in considerable amounts in some partially hydrogenated vegetable oils (see below).

Industrial trans fatty acids

Hydrogenation is a chemical reaction in which hydrogen atoms react, in the presence of a catalyst, with a molecule.
The hydrogenation of unsaturated fatty acids involves the addition of hydrogen atoms to double bonds on the carbon chains of fatty acids. The reaction occurs in presence of metal catalyst and hydrogen, and is favored by heating vegetable oils containing unsaturated fatty acids.

Partial hydrogenation of vegetable oils

The process of hydrogenation was first discovered in 1897 by French Nobel prize in Chemistry, jointly with fellow Frenchman Victor Grignard, Paul Sabatier using a nickel catalyst.
Partially hydrogenated vegetable oils were developed in 1903 by a German chemist, Wilhelm Normann, who files British patent on “Process for converting unsaturated fatty acids or their glycerides into saturated compounds”. The term trans fatty acids or trans fats appeared for the first time in the Remark column of the 5th edition of the “Standard Tables of Food Composition” in Japan.
During partial hydrogenation, an incomplete saturation of the unsaturated sites on the carbon chains of unsaturated fatty acids occurs. For example, with regard to fish oil, trans fatty acid content in non-hydrogenated oils and in highly hydrogenated oils is 0.5 and 3.6%, respectively, whereas in partially hydrogenated oils is 30%.

The cis to trans isomerization of oleic acid to vaccenic acid
Fig. 2 – From Oleic Acid to Vaccenic Acid

But, most importantly, some of the remaining cis double bonds may be moved in their positions on the carbon chain, producing geometrical and positional isomers, that is, double bonds can be modified in both conformation and position.
Below, other changes that occur during partial hydrogenation are listed.

Partially hydrogenated vegetable oils were developed for the production of vegetable fats, a cheaper alternative to animal fats. In fact, through hydrogenation, oils such as soybean, safflower and cottonseed oils, which are rich in unsaturated fatty acids, are converted into semi-solid fats (see above).
The first hydrogenated oil was cottonseed oil in USA in 1911 to produce vegetable shortening.
In the 1930’s, partial hydrogenation became popular with the development of margarine.
Currently, per year in USA, 6-8 billion tons of hydrogenated vegetable oil are produced.

Ruminant trans fatty acids

Ruminant trans fats are produced by bacteria in the rumen of the animals, for example cows, sheep and goats, using as a substrate a proportion of the relatively small amounts of unsaturated fatty acids present in their feedstuffs, that is, feed, plants and herbs. And, considering an animal that lives at least a year, and has the opportunity to graze and/or eat hay, there is a season variability in unsaturated fatty acids intake, and trans fats produced. In fact, in summer and spring, pasture plants and herbs may contain more unsaturated fatty acids than the winter feed supply.
Then, TFA are present at low levels in meat and full fat dairy products, typically <5% of total fatty acids, and are located in the sn-1 and sn-3 positions of the triacylglycerols, whereas in margarines and other industrially hydrogenated products they appear to be concentrated in the sn-2 position of the triacylglycerols.
Ruminant trans fatty acids are mainly monounsaturated fatty acids, with 16 to 18 carbon atoms, and constitute a small percentage of the trans fatty acids in the diet (see below).

Isomers of dietary trans fatty acids

The most important cluster of trans fatty acids is trans-C18:1 isomers, that is, fatty acids containing 18 carbon atoms plus one double bond, whose position varies between Δ6 and Δ16 carbon atoms. In both sources, the most common isomers are those with double bonds between positions Δ9 and Δ11.
However, even if these molecules are present both in industrial and ruminant TFA, there is a considerable quantitative difference. For example, vaccenic acid (C18:1 Δ11t) represents over 60 percent of the trans-C18:1 isomers in ruminant trans fatty acids, whereas in industrial ones elaidic acid (C18:1Δ9t) comprises 15-20 percent and C18:1 Δ10t and vaccenic acid over 20 percent each others.

trans-C18:1 fatty acid isomers, the most important cluster of trans fatty acids
Fig. 3 – trans-C18:1 Isomers

Trans fatty acids: effects on human health

Ruminant trans fatty acids, in amounts actually consumed in diets, are not harmful for human health (see below).
Conversely, consumption of industrial trans fats has neither apparent benefit nor intrinsic value, above their caloric contribution, and, from human health standpoint they are only harmful, having adverse effects on:

  • serum lipid levels;
  • endothelial cells;
  • systemic inflammation;
  • other risk factors for cardiovascular disease.

Moreover, they are positively associated with the risk of coronary heart disease (CHD), and sudden death from cardiac causes and diabetes.

Note: further in the text, we will refer to industrial trans fatty acids as trans fats or trans fatty acids.

Trans fatty acids: effects at plasmatic level

Low-density lipoprotein cholesterol (LDL-C) and high-density lipoprotein cholesterol (HDL-C) plasma levels are well-documented risk markers for the development of coronary heart disease (CHD).

  • High LDL-C levels are associated with an increased incidence of ischemic heart disease.
  • High HDL-C levels are associated with a reduced incidence of the risk.

For this reason, the ratio between total cholesterol level and HDL-C is often used as a combined risk marker for these two components in relation to the development of heart disease: the higher the ratio, the higher the risk.

TFA, as previously said, have adverse effects on serum lipids.
These effects have been evaluated in numerous controlled dietary trials by isocaloric replacement of saturated fatty acids or cis-unsaturated fatty acids with trans fats. It was demonstrated that such replacement:

  • raises LDL-C levels;
  • lowers HDL-C levels, in contrast to saturated fatty acids that increase HDL-C levels when used as replacement in similar study;
  • increases the ratio of total cholesterol to HDL-C, approximately twice that for saturated fatty acids, and, on the basis of this effect alone, trans fatty acids has been estimated to cause about 6% of coronary events in the USA.

Furthermore, trans fats:

  • produce a deleterious increase in small, dense LDL-C subfractions, that is associated with a marked increased in the risk of CHD, even in the presence of relatively normal LDL-C;
  • increase the blood levels of triglycerides, and this is an independent risk factor for CHD;
  • increase levels of Lp(a)lipoprotein, another important coronary risk factor.

But on 2004 prospective studies have shown that the relation between the intake of trans fatty acids and the incidence of CHD is greater than that predicted by changes in serum lipid levels alone. This suggests that trans fats influence other risk factors for CHD, such as inflammation and endothelial-cell dysfunction.

Trans fatty acids, inflammation and endothelial-cell dysfunction

The role of inflammation in atherosclerosis, and consequently in CHD, is burgeoned in the last decade.
Interleukin-6, C-reactive protein (CRP), and an increased activity of tumor necrosis factor (TNF) system are markers of inflammation.
In women greater intake of trans fatty acids is associated with increased activity of TNF system, and in those with a higher body mass index with increased levels of interleukin-6 and CRP. For example, the difference in CRP seen with an average intake of trans fats of 2.1% of the total daily energy intake, as compared with 0.9%, correspond to an increased risk of cardiovascular disease of 30%. Similar results have been reported in patients with established heart disease, in randomized, controlled trials, in in vitro studies, and in studies in which it has been analyzed membrane levels of trans fatty acids, a biomarker of their dietary intake.
So, trans fats promote inflammation, and their inflammatory effects may account at least in part for their effects on CHD that, as seen above, are greater than would be predicted by effects on serum lipoproteins alone.
Attention: the presence of inflammation is an independent risk factor not only for CHD but also for insulin resistance, diabetes, dyslipidemia, and heart failure.

Another site of action of TFA may be endothelial function.
Several studies have suggested the association between greater intake of trans fats and increased levels of circulating biomarkers of endothelial dysfunction, such as E-selectin, sICAM-1, and sVCAM-1.

Other effects of trans fatty acids

In vitro studies have demonstrate that trans fats affect lipid metabolism through several pathways.

  • They alter secretion, lipid composition, and size of apolipoprotein B-100 (apo B-100).
  • They increase cellular accumulation and secretion of free cholesterol and cholesterol esters by hepatocytes.
  • They alter expression in adipocytes of genes for peroxisome proliferator-activated receptor-γ (PPAR- γ), lipoprotein lipase, and resistin, proteins having a central roles in the metabolism of fatty acids and glucose.

Industrial trans fatty acids and CHD

Industrial trans fats are independent cardiovascular risk factor.
Since the early 1990s attention has been focused on the effect of trans fatty acids on plasma lipid and lipoprotein concentrations (see above).
Furthermore, four major prospective studies covering about 140,000 subjects, monitored for 6-14 years, have all found positive epidemiological evidence relating their levels in the diet, assessed with the aid of a detailed questionnaire on the composition of the diet, to the risk of CHD. These four studies are:

  • “The Health Professionals Follow-up study” (2005);
  • “The Alpha-Tocopherol Beta-Carotene Cancer Prevention Study” (1997);
  • “The Nurses’ Health Study” (2005);
  • “The Zutphen Elderly Study” (2001).

These studies cover such different populations that the results very probably hold true for the populations as a whole.
A meta-analysis of these studies have shown that a 2% increase in energy intake from industrial TFA was associated with a 23% increase in the incidence of CHD. The relative risk of heart disease was 1.36 in “The Health Professionals Follow-up Study”, 1.14 in “The Alpha-Tocopherol Beta-Carotene Cancer Prevention Study”; 1.93 (1.43-2.61) in “The Nurses’ Health Study”, and 1.28 (1.01-1.61) in “The Zutphen Elderly Study”.
So,  there is a substantially increased risk even at low levels of intake: 2% of total energy intake, for a 2,000 Kcal diet is 40 Kcal or about 4-5 g of fat corresponding to a teaspoonful of fat!
Moreover, in three of the studies, the association between the intake of industrial trans fats and the risk of CHD was stronger than a corresponding association between the intake of saturated fatty acids and the risk of heart disease. In “The Zutphen Elderly Study”, this association was not investigated.
Because of the adverse effects of industrial trans fatty acids, for the same authors are unethical conducting randomized long-term trials to test their effects on the incidence of CHD.
So, avoidance of industrial trans fats, or a consumption of less 0.5% of total daily energy intake is necessary to avoid their adverse effects, far stronger on average than those of food contaminants or pesticide residues.

Further evidence
A study conducted in an Australia population with a first heart attack and no preceding history of CHD or hyperlipidemia has showed a positive association between levels of trans fatty acids in adipose tissue and the risk of nonfatal myocardial infarction.
It was shown that adipose tissue C18:1Δ7t, found in both animal and vegetable fats, was an independent predictor of a first myocardial infarction, that is, its adipose tissue level is still a predictor for heart disease after adjustment for total cholesterol. Again, it appears that only a minor part of the negative effects of trans fats occurs via plasma lipoproteins.
During the course of this study, mid-1996, TFA were eliminated from margarines sold in Australia (see below). This was a unique opportunity to investigate the temporal relationship between trans fat intake and their adipose tissue levels. It was demonstrated that trans fats disappear from adipose tissue of both case-patients and controls with a rate about 15% of total trans fats/y.
Another study conduct in Costa Rica have found a positive association between myocardial infarction and trans fatty acids.
Interestingly, in a larger, community-based case-control study, levels of trans fats in red blood cell membranes were associated, after adjustment for other risk factors, with an increase in the risk of sudden cardiac death. Moreover, the increased risk appeared to be related to trans-C18:2 levels, that were associated with a tripling of the risk, but not with cell membrane levels of trans-C18:1,  the major trans fatty acids in foods (see above).

Trans fatty acids and diabetes

In a prospective study covering 84,204 female nurses, from “The Nurses’ Health Study”, aged 34–59 y, analyzed from the 1980 to 1996, with no cancer, diabetes, or cardiovascular disease at base line, the intake of trans fatty acids was significantly related to the risk of developing type 2 diabetes. And, after adjustment for other risk factors trans fat intake was positively associated with the incidence of diabetes with a risk up to 39% greater.
Data from controlled intervention studies showed that TFA could impair insulin sensitivity in subjects with insulin resistance and type 2 diabetes (saturated fatty acids do the analogous response, with no significant difference between TFA and them) more than unsaturated fatty acids, in particular the isomer of conjugated linoleic acid (CLA) trans-10, cis-12-CLA. Be careful because some dietary supplements contain CLA isomers and may be diabetogenic and proatherogenic in insulin-resistant subjects.

No significant effect was seen in insulin sensitivity of lean, healthy subjects.

Ruminant trans fatty acids and the risk of CHD

Four prospective studies have evaluated the relation between the intake of ruminant trans fatty acids and the risk of CHD: no significant association was identified.
In another study published on 2008 was analyzed data from four Danish cohort studies that cover 3,686 adults enrolled between 1974 and 1993, and followed for a median of 18 years. In Denmark, consumption of dairy products is relatively high and the range of ruminant trans fat intake is relatively broad, up to 1.1% of energy. Conversely, in the other countries, ruminant trans fatty acid consumption for most people is substantially lower than 1% of energy, in USA about 0.5% of energy. After adjustment for other risk factors, no significant associations between ruminant TFA consumption and incidence of CHD were found, confirming, in a population with relatively high intake of ruminant trans fatty acids, conclusions of four previous prospective studies.
So ruminant trans fats, in amounts actually consumed in diets, do not raise CHD risk.
The absence of risk of CHD with trans fats from ruminants as compared with industrial trans fatty acids  may be due to a lower intake. In the USA, greater part of trans fats have industrial origin (see above); moreover trans fat levels in milk and meats are relatively low, 1 to 8% of total fats.
The absence of a higher risk of CHD may be due also to the presence of different isomers. Ruminant and industrial sources share many common isomers, but there are some quantitative difference (see fig. 4):

  • vaccenic acid level is higher in ruminant fats, 30-50% of trans isomers;
  • trans-C18:2 isomers, present in deodorized and fried vegetable oils, as well as in some partially hydrogenated vegetable oils, are not present in appreciable amounts in ruminants fats.

Finally other, still unknown, potentially protective factors could outweigh harmful effects of ruminant trans fats.

Trans fatty acids: legislation regulating their content

USA
Until 1985 no adverse effects of trans fatty acids on human health was demonstrated, and in 1975 a Procter & Gamble study showed no effect of trans fats on cholesterol.
Their use in fast food preparation grew up from 1980’s, when the role of dietary saturated fats in increasing cardiac risk began clear. Then, it was led a successful campaign to get McDonald’s to switch from beef tallow to vegetable oil for frying its French fries. Meanwhile, studies began to raise concerns about their effects on health. On 1985 Food and Drug Administration (FDA) concluded that TFA and oleic acid affected serum cholesterol level similarly, but from the second half of 1985 their harmful began clear, and the final proof came from both controlled feeding trials and prospective epidemiologic studies.
On 2003 FDA ruled that food labels, for conventional foods and supplements, show their content beginning January 1, 2006. Notably, this ruling was the first substantive change to food labeling since the requirement for per-serving food labels information was added in 1990.
On 2005 the US Department of Agriculture made a minimized intake of trans fatty acids a key recommendation of the new food-pyramid guidelines.
On 2006 American Heart Association recommended to limit their intake to 1% of daily calorie consumption, and suggested food manufacturers and restaurants switch to other fats.
On 2006 New York City Board of Health announced trans fat ban in its 40,000 restaurants within July 1, 2008, followed by the state of California in 2010-2011.

Australia
After June 1996 they were eliminated from margarine sold in Australia, that before contributed about 50% of their dietary intake.

Europe
On March 11, 2003 the Danish government, after a debate started in 1994 and two new reports in 2001 and 2003, decided to phase out the use of industrial trans fats in food before the end of 2003. Two years later, however, the European Commission (EC) asked Denmark to withdraw this law, which was not accepted on the European Community level, unfortunately. However, in 2007, EC decided to closes its infringement procedure against Denmark because of increasing scientific evidence of the danger of this type of fatty acids.
The Danish example was followed by Austria and Switzerland in 2009, Iceland, Norway, and Hungary in 2011, and most recently, Estonia and Georgia in 2014. So, about 10% of the European Union population, about 500 million people, lives in countries where it is illegal to sell food high in industrial trans fats.
Governments of other European Union countries instead rely on the willingness of food producers to reduce trans fatty acid content in their products. This strategy has proved effective only for Western European countries (see below).

Canada
Canada is considering legislation to eliminate them from food supplies, and, in 2005, ruled that pre-packaged food labels show their content.

Therefore, with the exception of the countries where the use of trans fats in the food industry was banned, the only way to reduce their intake in the other countries is consumer’s decision to choose foods free in such fatty acids, avoiding those known containing them, and always reading nutrition facts and ingredients because they may come from margarine, vegetable oil and frying. Indeed, for example in the USA, the producers of foods that contain less than 0.5 g of industrial trans fatty acids per serving can list their content as 0 on the packaging. This content is low but if a consumer eats multiple servings, he consumes substantial amount of them.

Be careful: food labels are not obligatory in restaurants, bakeries, and many other retail food outlets.

Trans fatty acids and food reformulation

Public health organizations, including the World Health Organization in September 2006, have recommended reducing the consumption of industrial trans fatty acids; only in USA the near elimination of these fatty acids might avoid between 72,000 and 280,000 of the 1.2 million of CHD events every year.
Food manufacturers and restaurants may reduce industrial TFA use choosing alternatives to partially hydrogenated oils.
In Denmark, their elimination (see above) from vegetable oils did not increase consumption of saturated fatty acids because they were mostly replaced with cis-unsaturated fatty acids. Moreover, there were no noticeable effects for the consumer: neither increase in the cost nor reduction in availability and quality of foods.
In 2009, Stender et al. have shown that industrial trans fatty acids in food such as French fries, cookies, cakes, and microwave-oven popcorn purchased in USA, South Africa, and many European Country can be replaced, at similar prices, with a mixture of saturated, monounsaturated, and polyunsaturated fatty acids. Such substitution has even greater nutritional benefit than one-to-one substitution of industrial trans fats with saturated fatty acids alone. However, be careful because only in French fries with low industrial trans fats the percentage of saturate fatty acids remains constant, whereas in cookies and cakes is in average +33 percentage points and microwave-oven popcorn +24 percentage points: saturated fatty acids are less dangerous than industrial trans fats but more than mono- and polyunsaturated fatty acids.
The same research group, analyzing some popular foods in Europe, purchased in supermarkets, even of the same supermarket chain, and fast food, namely, McDonald’s and Kentucky Fried Chicken (KFC), from 2005 to 2014, showed that their TFA content was reduced or even absent in several Western European countries while remaining high in Eastern and Southeastern Europe.
In 2010 Mozaffarian et al. evaluated  the levels of industrial trans fats and saturated fatty acids in major brand-name U.S. supermarket and restaurant foods after reformulation to reduce industrial trans fatty acid content, in two time: from 1993 through 2006 and from 2008 through 2009. They found a generally reduction in industrial trans fat content without any substantial or equivalent increase in saturated fatty acid content.

Foods high in trans fatty acids: examples and values

Many foods high in trans fats are popularly consumed worldwide.
In USA greater part of these fatty acids comes from partially hydrogenated vegetable oils, with an average consumption from this source that has been constant since the 1960′s.
It should be noted that the following trans fatty acid values must be interpreted with caution because, as previously said, many fast food establishments, restaurants and industries may have changed, or had to change the type of fat used for frying and cooking since the analysis were done.
The reported values, unless otherwise specified, refer to percentage in trans fatty acids/ 100 g of fatty acids.

Margarine

Among foods with trans fats, stick or hard margarine had the highest percentage of them, but levels of these fatty acids have declined as improved technology allowed the production of softer margarines which have become popular. But there are difference in trans fatty acid content of margarine from different countries. Below some examples.

  • The highest content, 13-16.5%, is found in soft margarine from Iceland, Norway, and the UK.
  • Less content is found in Italy, Germany, Finland, and Greece, 5.1%, 4.8%, 3.2%, and 2.9% respectively).
  • In Portugal, The Netherlands, Belgium, Denmark, France, Spain, and Sweden margarine trans fat content is less than 2%.

USA and Canada lag behind Europe, but in the USA, with the advent of trans fat labeling of foods and the greater knowledge of the risk associated with their consumption by the buyers, change is occurring. For this reason, at now, in the USA margarine is considered to be a minor contributor to the intake of TFA, whereas the major sources are commercially baked and fast food products like cake, cookies, wafer, snack crackers, chicken nuggets, French fries or microwave-oven popcorn (see below).

Vegetable shortenings

Trans fatty acid content of vegetable shortenings ranges from 6% to 50%, and varies in different country: in Germany, Austria and New Zealand it is less than France or USA.
However, like margarines, their trans fat content is decreasing. In Germany it decreased from 12% in 1994 to 6% in 1999, in Denmark is 7% (1996) while in New Zealand is about 6% (1997).

Vegetable oils

At now, non-hydrogenated vegetable oils for salad and cooking contain no or only small amounts of trans fats.
Processing of these oils can produce minimal level of them, ranged from 0.05g/100 food for extra virgin oil to 2.42 g/100 g food for canola oil. So, their contribution to trans fat content of the current food supply is very little.
One exception is represented by Pakistani hydrogenated vegetable oils whose TFA content ranges from 14% to 34%.

Prepared soups

Among foods with trans fats, prepared soups contain significant amount of them, ranging from 10% of beef bouillon to 35% of onion cream. So, they contribute great amount of such fatty acids to the diet if frequently consumed.

Processed foods

Thanks to their properties (see above), trans fatty acids are used in many processed foods as cookies, cakes, croissants, pastries and other baked goods. And, baked goods are the greatest source of these fats in the North American diet. Of course, their trans fat content depends on the type of fat used in processing.

Sauces

Mayonnaise, salad dressings and other sauces contain only small or no-amounts of trans fats.

Human milk and infant foods

Trans fat content of human milk reflects the trans fat content of maternal diet in the previous day, is comprised between 1 and 7%, and is decreasing from 7.1% in 1998 to 4.6% in 2005/2006.
Infant formulas have trans fat values on average 0.1%-4.5%, with a brand up to 15.7%.
Baby foods contain greater than 5% of trans fats.

Fast foods and restaurant’s foods

Vegetable shortenings high in trans fats are used as frying fats, so fast foods and many restaurant’s foods may contain relatively large amounts of them. Foods are fried pies, French fries, chicken nuggets, hamburgers, fried fish as well as fried chicken.
In articles published by Stender et al. from 2006 to 2009, it is showed that for French fries and chicken nuggets their content varies largely from nation to nation, but also within the same fast food chain in the same country, and even in the same city, because of the cooking oil used. For example, oil used in USA and Peru outlets of a famous fast food chain contained 23-24% of trans fats, whereas oil used in many European countries of the same fast food chain contained about 10%, with some countries, such as Denmark, as low as 5% and 1%.
And, considering a meal of French fries and chicken nuggets, in serving size of 171 and 160 g respectively, purchased at McDonald‘s in New York City, it contained over 10 g of TFA, while if purchased at KFC in Hungary they were almost 25 g.
Below, again from the work of Stender et al. it can see a cross-country comparison of trans fat contents of chicken nuggets and French fries purchased at McDonald ‘s or KFC.

Chicken nuggets and French fries from McDonald’s:

  • less than 1 g only if the meals were purchased in Denmark;
  • 1-5 g in Portugal, the Netherlands, Russia, Czech Republic, or Spain;
  • 5-10 g in the United States, Peru, UK, South Africa, Poland, Finland, France, Italy, Norway, Spain, Sweden, Germany, or Hungary.

Chicken and French fries from KFC:

  • less than 2 g if the meals were purchased UK (Aberdeen), Denmark, Russia, or Germany (Wiesbaden);
  • 2-5 in Germany (Hamburg), France, UK (London or Glasgow), Spain, or Portugal;
  • 5-10 in the Bahamas, South Africa, or USA;
  • 10-25 g in Hungary, Poland, Peru, or Czech Republic.

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Long chain fatty acid synthesis in plants and animals

When excess calories are consumed from carbohydrates or proteins, such surplus is used to synthesize fatty acids and then triacylglycerols, while it doesn’t occur if the excess come from fats.

Metabolic pathways for saturated and unsaturated long chain fatty acid synthesis
Long Chain Fatty Acids Biosynthesis

CONTENTS

De novo fatty acid synthesis in plants and animals

De novo fatty acid synthesis is largely similar among plants and animals.
It occurs in chloroplasts of photosynthetic cells of higher plants, and in cytosol of animal cells by the concerted action of two enzymes: acetyl-CoA carboxylase (EC 6.4.1.2) and fatty acid synthase (EC 2.3.1.85).
Fatty acid synthase catalyzes a repeating four-step sequence by which the fatty acyl chain is extended by two carbons, at the carboxyl end, every each passage through the cycle; this four-step process is the same in all organisms.
In animals, the primary site for lipid metabolism is liver, not the adipose tissue. However, adipose tissue is a major organ system in which fatty acid synthesis occurs, though in humans it is less active than in many other animal species.
Although myristic acid, lauric acid and a trace of stearic acid may also be produced, in animals and plants the main product of these reactions is palmitic acid.
It should be noted that in certain plants, such as palm and coconut, chain termination occurs earlier than palmitic acid release: up to 90% of the fatty acids produced and then present in the oils of these plants are between 8 (caprylic acid) and 14 (myristic acid) carbons long (palmitic acid: 16 carbon atoms).

Synthesis of long chain saturated and unsaturated fatty acids

Palmitic acid is the commonest saturated fatty acid in plant and animal lipids, but generally it is not present in very large proportions because it may be undergo into several metabolic pathways.
In fact:

  • it is the precursor of stearic acid;
  • it may be desaturated (insertion of a double bond into fatty acid chain) to palmitoleic acid, the precursor of all fatty acids of omega-7 or n-7 family, in a reaction catalyzed by Δ9-desaturase (EC 1.14.19.1), an ubiquitous enzyme in both plant and animal kingdoms and the most active lipid enzyme in mammalian tissues, the same enzyme that catalyzes the desaturation of stearic acid to oleic acid (see below).
    Note: Δ9- desaturase inserts double bounds in the 9-10 position of the fatty acid carbon chain, position numbered from the carboxyl end of the molecule, and:

if the substrate is palmitic acid, the double bond is inserted between n-7 and n-8 position of the chain (in this case numbered from the methyl end of the molecule), so producing palmitoleic acid, the founder of omega-7 series;Numbering of carbons of palmitic acid, delta-9 desaturase and insertion of a double bond at the omega-7 position

if the substrate is stearic acid, the double bond is inserted between n-9 and n-10 position of the chain and oleic acid will be produced.Numbering of carbons of stearic, delta-9 desaturase and insertion of a double bond at the omega-9 position

  • It may be esterified into complex lipids.

Of course, in plants and animals there are fatty acids longer and/or more unsaturated than these just seen thanks to modification systems (again desaturation and elongation) that catalyze reactions of fatty acid synthesis that are organism- tissue- and cell- specific.
For example, stearic acid may be:

  • elongated to arachidic acid, behenic acid and lignoceric acid, all saturated fatty acids, in reactions catalyzed by elongases. Again, chain elongation occurs, both in mitochondria and in the smooth endoplasmic reticulum, by the addition of two carbon atom units at a time at the carboxylic end of the fatty acid through the action of fatty acid elongation systems (particularly long and very long saturated fatty acids, from 18 to 24 carbon atoms, are synthesized only on cytosolic face of the smooth endoplasmic reticulum);
  • desaturated, as seen, to oleic acid, an omega-9 or n-9 fatty acid, in a reaction catalyzed by Δ9-desaturase.
    Several researchers have postulated that the reason for which stearic acid is not hypercholesterolemic is its rapid conversion to oleic acid.

Oleic acid is the start point for the synthesis of many other unsaturated fatty acids by reactions of elongation and/or desaturation.
In fact:

  • gadoleic acid, erucic acid and nervonic acid, all monounsaturated fatty acids.
    Saturated fatty acids, and unsaturated fatty acids of the omega-9 series, usually oleic acid (but also palmitoleic acid and other omega-7 fatty acids) are the only fatty acids produced de novo in mammal systems.
  • Thanks to the consecutive action of the enzymes Δ12-desaturase (1.14.19.6) and Δ15-desaturase (EC 1.14.19.25), that insert a double bond respectively in the 12-13 and 15-16 position of the carbon chain of the fatty acid, oleic acid is converted first to linoleic acid, founder of all the omega-6 polyunsaturated fatty acids, and then to alpha-linolenic acid, founder of all the omega-3 polyunsaturated fatty acids (omega-3 and omega-6 PUFA that will be produced from these precursors through repeating reactions of elongation and desaturation).Numbering of carbon atoms of oleic acid and specific targets of desaturase action
  • In the case of essential fatty acid (EFA) deficiency, oleic acid may be desaturated and elongated to omega-9 polyunsaturated fatty acids, with accumulation especially of Mead acid.

Omega-3 and omega-6 PUFA synthesis

Animal tissues can desaturate fatty acids in the 9-10 position of the chain, thanks to the presence of Δ9 desaturase; as previously seen, if the substrate of the reaction is palmitic acid, the double bond will appear between n-7 and n-8 position, with stearic acid between n-9 and n-10 position, so leading to formation respectively of palmitoleic acid and oleic acid.
Animals lack Δ12- and Δ15-desaturases, enzymes able to desaturate carbon carbon bonds beyond the 9-10 position of the chain. For these reason, they can’t produce de novo omega-3 and omega-6 PUFA (which have double bonds also beyond the 9-10 position), that are so essential fatty acids.
Δ12- and Δ15-desaturases are present in plants; though many land plants lack Δ15-desaturase, also called omega-3 desaturase, planktons and aquatic plants in colder water possess it and produce abundant amounts of the omega-3 fatty acids.

References

Akoh C.C. and Min D.B. “Food lipids: chemistry, nutrition, and biotechnology”. CRC Press Taylor & Francis Group, 2008 3th ed. 2008

Bender D.A. “Benders’ dictionary of nutrition and food technology”. 2006, 8th Edition. Woodhead Publishing. Oxford

Burr G.O. and Burr M.M. A new deficiency disease produced by the rigid exclusion of fat from the diet. Nutr Rev 1973;31(8):148-149. doi:10.1111/j.1753-4887.1973.tb06008.x

Chow Ching K. “Fatty acids in foods and their health implication”. 3rd Edition. CRC Press Taylor & Francis Group, 2008

Rosenthal M.D., Glew R.H. Mediacal biochemistry. Human metabolism in health and disease. John Wiley & Sons, Inc. 2009

Stipanuk M.H., Caudill M.A. Biochemical, physiological, and molecular aspects of human nutrition. 3rd Edition. Elsevier health sciences, 2012

Omega-6 polyunsaturated fatty acids

Omega-6 polyunsaturated fatty acids are the major polyunsaturated fatty acids (PUFA) in the Western diet (about 90% of all of them in the diet), being components of most animal and vegetable fats.

CONTENTS:

The synthesis of ω-6 polyunsaturated fatty acids

Within the omega-6 (ω-6) family, linoleic acid is one of the most important and widespread fatty acids and the precursor of all omega-6 polyunsaturated fatty acids. It is produced de novo from oleic acid (an omega-9 fatty acid) only by plant in a reaction catalyzed by Δ12-desaturase, i.e. the enzyme that forms the omega-6 polyunsaturated fatty acid family from omega-9 one.
Δ12-desaturase catalyzes the insertion of the double bond between carbon atoms 6 and 7, numbered from the methyl end of the molecule.
Linoleic acid, together with alpha-linolenic acid, is a primary product of plant polyunsaturated fatty acids synthesis.

Biosynthesis and metabolism of omega-6 polyunsaturated fatty acids
Omega-6 Polyunsaturated Fatty Acid Metabolism

Animals, lacking Δ12-desaturase, can’t synthesize it, and all the omega-6 polyunsaturated fatty acid family de novo, and they are obliged to obtain it from plant foodstuff and/or from animals that eat them; for this reason omega-6 polyunsaturated fatty acid are considered essential fatty acids, so called EFA (the essentiality of omega-6 polyunsaturated fatty acids, in particular just the essentiality of linoleic acid, was first reported in 1929 by Burr and Burr).

ω-6 PUFA: from linoleic acid to arachidonic acid

Animals are able to elongate and desaturase dietary linoleic acid in a cascade of reactions to form very omega-6 polyunsaturated fatty acids.
Linoleic acid is first desaturated to gamma-linolenic acid, another important ω-6 fatty acid with significant physiologic effects, in the reaction catalyzed by Δ6-desaturase. It is thought that the rate of this reaction is limiting in certain conditions like in the elderly, under certain disease states and in premature infants; for this reason, and because it is found in relatively small amounts in the diet, few oils containing it (black currant, evening primrose, and borage oils) have attracted attention.
In turn gamma-linolenic acid may be elongated to dihomo-gamma-linolenic acid by an elongase (it catalyzes the addition of two carbon atoms from glucose metabolism to lengthen the fatty acid chain) that may be further desaturated in a very limited amount to arachidonic acid, in a reaction catalyzed by another rate limiting enzyme, Δ5-desaturase.
Arachidonic acid can be elongated and desaturated to adrenic acid.

It should be noted that polyunsaturated fatty acids in the omega-6 family, and in any other n-families, can be interconverted by enzymatic processes only within the same family, not among families.

C-20 polyunsaturated fatty acids belonging to omega-6 and omega-3 polyunsaturated fatty acids are the precursors of eicosanoids (prostaglandins, prostacyclin, thromboxanes, and leukotrienes), powerful, short-acting, local hormones.

While the deprivation of omega-3 polyunsaturated fatty acids causes dysfunction in a wide range of behavioral and physiological modalities, the omission in the diet of omega-6 polyunsaturated fatty acids results in manifest systemic dysfunction.

In plant seed oils omega-6 fatty acids with chain length longer than 18 carbons are present only in trace while arachidonic acid is found in all animal tissues and animal-based food products.

References

Akoh C.C. and Min D.B. “Food lipids: chemistry, nutrition, and biotechnology” 3th ed. 2008

Aron H. Uber den Nahvert (On the nutritional value). Biochem Z. 1918;92:211–233 (German)

Bender D.A. “Benders’ dictionary of nutrition and food technology”. 2006, 8th Edition. Woodhead Publishing. Oxford

Bergstroem S., Danielsson H., Klenberg D. and Samuelsson B. The enzymatic conversion of essential fatty acids into prostaglandins. J Biol Chem 1964;239:PC4006-PC4008.

Burr G.O. and Burr M.M. A new deficiency disease produced by the rigid exclusion of fat from the diet. Nutr Rev 1973;31(8):148-149. doi:10.1111/j.1753-4887.1973.tb06008.x

Chow Ching K. “Fatty acids in foods and their health implication” 3th ed. 2008

Rosenthal M.D., Glew R.H. Mediacal biochemistry. Human metabolism in health and disease. John Wiley & Sons, Inc. 2009

Stipanuk M.H., Caudill M.A. Biochemical, physiological, and molecular aspects of human nutrition. 3rd Edition. Elsevier health sciences, 2012

Van D., Beerthuis R.K., Nugteren D.H. and Vonkeman H. Enzymatic conversion of all-cis-polyunsaturated fatty acids into prostaglandins. Nature 1964;203:839-841

Omega-3 fatty acids: synthesis, mechanism of action, health benefits, and foods

Omega-3 polyunsaturated fatty acids or omega-3 PUFAs or omega-3 fatty acids are unsaturated fatty acids that have a double bond three carbons from the methyl end of the carbon chain. For humans, the most important omega-3 PUFAs are:

  • alpha-linolenic acid or ALA or 18:3n-3, with 18 carbon atoms and 3 double bonds;
  • eicosapentaenoic acid or EPA or 20:5n-3, with 20 carbon atoms and 5 double bonds;
  • docosahexaenoic acid or DHA or 22:6n-3, that, with 22 carbon atoms and 6 double bonds, is the most complex.

EPA and DHA are termed long-chain polyunsaturated fatty acids or LC-PUFAs.
Animals cannot synthesize linoleic acid or LA and alpha-linolenic acid, the precursors to omega-6 polyunsaturated fatty acids and omega-3 PUFAs, respectively, due to the lack of two desaturases: delta-12 desaturase (EC 1.14.19.6) and delta-15 desaturase (EC 1-14.19.13). Such desaturases insert double bonds at positions 6 and 3 from the methyl end of the molecule, respectively. Linoleic acid and alpha-linolenic acid are therefore essential fatty acids. Humans and many other animals can produce, from dietary ALA, all the other omega-3 polyunsaturated fatty acids. Then, such omega-3 PUFAs become essential in the absence of dietary ALA, and for this reason they are termed conditionally essential fatty acids.
EPA and DHA are important structural components of cell membranes, where they are mainly found, especially in muscle and nerve tissues. Conversely, many other fatty acids are stored mainly in adipose tissue triglycerides.
DHA is the main component of cell membrane phospholipids of neural tissues of vertebrates, including photoreceptor of the retina, where it performs important functions. In addition to their structural functions, omega-3 PUFAs are substrates for the production of bioactive lipid mediators with anti-inflammatory action, such as eicosanoids, maresins, resolvins, and protectins.
Omega-3 polyunsaturated fatty acids are essential in neurological development of the fetus, and their intake during pregnancy is especially important in the third trimester of pregnancy, when significant brain growth occurs. In the course of life their intake has been associated with a reduction in the risk of developing many chronic diseases, particularly cardiovascular diseases.
The major dietary sources for humans are fishery products, especially those obtained from cold waters.

CONTENTS

Synthesis of omega-3 polyunsaturated fatty acids

Alpha-linolenic acid, the precursors to omega-3 polyunsaturated fatty acids, is produced from linoleic acid, an omega-6 PUFAs, only in the plastids of phytoplankton and vascular terrestrial plants, where delta-15 desaturase inserts a double bond between carbon 3 and 4 from the methyl end of LA. In turn, ALA undergoes desaturation reactions, catalyzed by delta-5 desaturase (EC 1.14.19.44) and delta-6 desaturase (EC 1.14.19.3), elongation reactions, catalyzed by elongase 5 (EC 2.3.1.199) and elongase 5 and/or by elongase 2 (EC 2.3.1.199), and a limited beta-oxidation in peroxisomes, to produce DHA. For more details see the article on DHA.

Synthesis and metabolism of omega-3 polyunsaturated fatty acids
Omega-3 Fatty Acid Metabolism

The enzymes that catalyze the conversion of ALA to DHA are shared with the synthetic pathways leading to the synthesis of omega-6, omega-7 and omega-9 PUFAs. Omega-3 PUFAs appear to be the preferred substrates for delta-5 desaturase and delta-6 desaturase. However, because in many Western diets there is a high intake of linoleic acid relative to alpha-linolenic acid intake, the omega-6 pathway would be preferred over the other pathways. This could be one of the explanations for the low conversion rate of alpha-linolenic acid into the other omega-3 PUFAs, although the synthesis of arachidonic acid or ARA from linoleic acid seems to be very low, too. Note that both the omega-3 and omega-6 families inhibit the synthesis of omega-9 polyunsaturated fatty acids.

Omega-3 PUFA synthesis in humans

Humans, like many other animals, can convert alpha-linolenic acid to docosahexaenoic acid, a metabolic pathway found mainly in the liver and cerebral microcirculation of the hematoencephalic barrier, but also in the cerebral endothelium and astrocytes. It is common opinion that humans, like other terrestrial animals, have a limited capacity to synthesize LC-PUFAs, and therefore need an adequate intake of EPA and DHA from food.
It has been shown that the yield of the synthesis decreases along the pathway: the rate of conversion of alpha-linolenic acid to eicosapentaenoic acid is low, and the limiting factor seems to be the activity of delta-6 desaturase, and the rate of conversion to docosahexaenoic acid is extremely low. However recent studies have demonstrated the existence of a marked polymorphism in the fatty acid desaturase (FADS) gene cluster, especially for the contiguous genes FADS1 and FADS2 coding for delta-5 desaturase and delta-6 desaturase, respectively, which are present on chromosome 11q12.2. By analyzing genome-wide sequencing data from Bronze Age individuals and present-day Europeans, a  comprehensive overview was obtained of the changes in allele frequency of FADS genes. In European populations, the transition from a hunter-gatherer society to an agricultural society would have resulted in an increase in the intake of linoleic acid and alpha-linolenic acid, and a reduction in the intake of EPA and ARA. Natural selection would then have favored the haplotype associated with the increase in the expression of FADS1 and the decrease in the expression of FADS2. This pattern is opposite to that found in the Greenlander Inuit, where it is hypothesized that natural selection would have favored alleles associated with a decrease in the rate of conversion of linoleic acid and alpha-linolenic acid into LC-PUFAs, in order to compensate for their relatively high dietary intake in such population.

Do other animals need EPA and DHA?

Organisms lacking delta-15 desaturase cannot synthesize alpha-linolenic acid and hence the other omega-3 PUFAs, and, if needed, must obtain it from dietary sources. However, many animals do not need to get EPA and DHA from diet.
Terrestrial herbivorous vertebrates satisfy their need for long chain omega-3 polyunsaturated fatty acids by synthesizing them from alpha-linolenic acid obtained from the green parts of plants.
And there are animals that do not need EPA and DHA, and practically do not have them. These include terrestrial insects, that have very low levels of EPA. In such animals, EPA is synthesized from dietary alpha-linolenic acid and used for eicosanoid production.
Conversely, aquatic insects have high levels of EPA, whereas DHA is practically absent.
Some classes of phytoplankton, such as Cryptophyceae and Dinophyceae, are very rich in EPA and DHA, whereas Bacillariophyceae or diatoms are very rich in EPA. In general, microalgae are the primary producers of EPA and DHA, and then, aquatic ecosystems are the main source of omega-3 LC-PUFAs in the biosphere. EPA and DHA are then transferred from these microalgae along the food chain, from invertebrates to fish, and from fish to terrestrial animals, including humans. So, from microalgae to humans.

Benefits of omega-3 polyunsaturated fatty acids for humans

Omega-3 polyunsaturated fatty acids are essential components of a healthy and balanced diet. They are needed throughout development, starting from fetal life, and are associated with health improvements and reduced risk of disease. Indeed, many epidemiological studies have associated high intake of EPA and DHA with a lower cardiovascular mortality, especially for cardiac diseases, than predicted, probably due to the improvements in many risk factors such as plasma levels of triglycerides, HDL-cholesterol, C-reactive protein, blood pressure, both systolic and diastolic, and heart rate.
EPA and DHA have also been shown to be useful in the treatment of diseases such as rheumatoid arthritis, and could be useful in the treatment of other inflammatory conditions such as asthma, psoriasis or inflammatory bowel disease, due to their ability to modulate many aspects of the inflammatory processes.
Conversely, LC-PUFAs seem to have little or no effects on measures of glucose metabolism, such as insulin, insulin resistance, fasting glucose, and  glycated haemoglobin, or on type 2 diabetes.

Omega-3/omega-6 ratio

Epidemiological studies suggest that the consumption of a diet with a low omega-3/omega-6 ratio has had a negative impact on human health, contributing to the development, together with other risk factors such as sedentary life and smoking, of the main classes of diseases. Indeed, a lower incidence of cancer, autoimmunity and coronary heart disease has been observed in populations whose diet has a high omega-3/omega-6 ratio, such as Eskimos and Japanese, populations with a high fish consumption.
Despite these evidences, Western diet has become rich in saturated fatty acids and omega-6 polyunsaturated fatty acids, and poor in omega-3 polyunsaturated fatty acids, with an omega-3/omega-6 ratio between 1:10 and 1:20, then, far from the recommended ratio of 1:5.
The low value of the omega-3/omega-6 ratio is due to several factors, some of which are listed below.

  • Although wild plant foods are generally high in omega-3 PUFAs, crops high in omega-6 PUFAs have been much more successful in industrial agriculture than those high in omega-3 PUFAs.
  • Low consumption of fishery products and fish oils.
  • The high consumption of animals raised on corn-based feed, such as chickens, cattle, and pigs. Added to this is the fact that the omega-3 PUFA content of some farmed fish species is lower than that of  their wild counterparts.
  • The high consumption of oils rich in omega-6 PUFAs and poor in omega-3 PUFAs, such as safflower, sunflower, soybeans and corn oils.

Note: there is no evidence that the omega-3/omega-6 ratio is important for prevention and treatment of type 2
diabetes mellitus.

Effects at the molecular level of EPA and DHA

In recent years, the molecular mechanisms underlying the functional effects attributed to omega-3 polyunsaturated fatty acids, especially to EPA and DHA, are being clarified, and most of these require their incorporation into membrane phospholipids.
Omega-3 PUFAs are structural components of cell membranes where they play an essential role in regulating fluidity. Due to this effect, omega-3, especially EPA and DHA, can modulate cellular responses that depend upon membrane protein functions. This is particularly important in the eye, where DHA allows for optimal activity of rhodopsin, a photoreceptor protein. The effect on membrane fluidity is essential for animals living in cold water, as EPA and DHA also have an antifreeze function.
EPA and DHA can modify the formation of lipid raft, microdomains with a specific lipid composition that act as platforms for receptor activities and the initiation of intracellular signaling pathways. By modifying lipid raft formation, they affect intracellular signaling pathways in different cell types, such as neurons, immune system cells, and cancer cells. In this way, EPA and DHA can modulate the activation of transcription factors, such as NF-κB, PPARs and SREBPs, and so the corresponding gene expression patterns. This is  central to their role in controlling adipocyte differentiation, the metabolism of fatty acids and triacylglycerols, and inflammation.
EPA, DHA, and ARA are substrates for the synthesis of bioactive lipid mediators, such as eicosanoids, that are involved in the regulation of inflammation, immunity, platelet aggregation, renal function, and smooth muscle contraction. Eicosanoids produced from arachidonic acid, that is the major substrate for their synthesis, have important physiological roles, but an excessive production has been associated with numerous disease processes. The increase in EPA and DHA content in membrane phospholipids is paralleled by a reduction in ARA content and associated with a decreased production of lipid mediators form ARA and an increased production of lipid mediators from the two omega-3. Moreover, among the molecules derived from EPA and DHA, there are eicosanoids analogous to those produced from ARA, but with lower activity, resolvins, and, from DHA, protectins and maresins. These molecules appear to be responsible for many of the immune-modulating and anti-inflammatory actions attributed to the omega-3 polyunsaturated fatty acids EPA and DHA.
EPA and DHA can also play a role in the non-esterified form, acting directly through receptors coupled to G proteins, modulating their activity.
Finally, they can reduce the intestinal absorption of omega-6 PUFAs, and, at the enzymatic level, competitively inhibit cyclooxygenase-1 or COX-1 (EC 1.14.99.1) and lipoxygenases, and compete with omega-6 PUFAs for acyltransferases.

Major sources of EPA and DHA for humans

In general, fish and aquatic invertebrates, such as molluscs and crustaceans, are the major sources of EPA and DHA for humans. These animals can get EPA and DHA from food, namely, from phytoplankton, or synthesize them from alpha-linolenic acid. Moreover, DHA is present in high concentrations in many fish oils, too, especially those from coldwater fish. However, it should be underscored that such oils are also high in saturated fatty acids. For those who do not eat fishery products, good sources of omega-3 LC-PUFAs are the liver of terrestrial animals and several birds of the order Passeriformes.
Regarding the recommended intake of omega-3 polyunsaturated fatty acids, it is not yet clear what it is. The following table shows the values suggested by Food and Agriculture Organization (FAO) of the United Nations and the European Food Safety Authority (EFSA).

Omega-3 polyunsaturated fatty acids and culinary treatments

As omega-3 polyunsaturated fatty acids are particularly susceptible to oxidation due to heating, cooking and other culinary treatments could reduce their content. However, this is only partially true. In food, EPA and DHA are not in free form but mainly esterified into membrane phospholipids and, in such form, are much less susceptible to oxidation.
Considering the content of EPA and DHA, to express it as a percentage of the total fatty acids instead of as absolute content, namely, mg/g wet weight, leads to erroneous conclusions. For example, a fatty fish like salmon has a high EPA + DHA content, ~8 mg/g wet weight, and expressed as a percentage of the total fatty acids ~20%; conversely Atlantic code has a low EPA + DHA content, ~3 mg/g of wet weight, but, if expressed as a percentage of the total fatty acids ~40%. Atlantic code has a high percentage of EPA + DHA because is a lean fish, whereas in fatty fish EPA + DHA content is diluted by the high fatty acid content of the adipose tissue of the animal.
And when EPA + DHA content is expressed in mg/g of product, no decrease in LC-PUFAs content is observed following most common culinary treatments.

References

AbuMweis S., Jew S., Tayyem R. Agraib L. Eicosapentaenoic acid and docosahexaenoic acid containing supplements modulate risk factors for cardiovascular disease: a meta-analysis of randomised placebo-control human clinical. J Hum Nutr Diet 2018 31(1):67-84.  doi:10.1111/jhn.12493

Akoh C.C. and Min D.B. “Food lipids: chemistry, nutrition, and biotechnology” 3th ed. 2008

Aron H. Uber den Nahvert (On the nutritional value). Biochem Z. 1918;92:211-233 (German)

Bergstroem S., Danielsson H., Klenberg D. and Samuelsson B. The enzymatic conversion of essential fatty acids into prostaglandins. J Biol Chem 1964;239:PC4006-PC4008.

Brown T.J., Brainard J., Song F., Wang X., Abdelhamid A., Hooper L. Omega-3, omega-6, and total dietary polyunsaturated fat for prevention and treatment of type 2 diabetes mellitus: systematic review and meta-analysis of randomised controlled trials. BMJ 2019;366:l4697. doi:10.1136/bmj.l4697

Buckley M.T., Racimo F., Allentoft M.E., et al. Selection in Europeans on fatty acid desaturases associated with dietary changes. Mol Biol Evol 2017;34(6):1307-1318. doi:10.1093/molbev/msx103

Calder P.C. Very long-chain n-3 fatty acids and human health: fact, fiction and the future. Proc Nutr Soc 2018 77(1):52-72. doi:10.1017/S0029665117003950

Chow Ching K. “Fatty acids in foods and their health implication” 3th ed. 2008

De Meester F., Watson R.R.,Zibadi S. Omega-6/3 fatty acids: functions, sustainability strategies and perspectives. Springer Science & Business Media, 2012

EFSA Panel on Dietetic Products, Nutrition, and Allergies (NDA). Scientific opinion on dietary reference values for fats, including saturated fatty acids, polyunsaturated fatty acids, monounsaturated fatty acids, trans fatty acids, and cholesterol. 2010. doi:10.2903/j.efsa.2010.1461

FAO. Global Recommendations for EPA and DHA Intake (As of 30 June 2014)

Gladyshev M.I.  and Sushchik N.N. Long-chain omega-3 polyunsaturated fatty acids in natural ecosystems and the human diet: assumptions and challenges. Biomolecules 2019;9(9):485. doi:10.3390/biom9090485

Oh D.Y., Talukdar S., Bae E.J., Imamura T., Morinaga H., Fan WQ, Li P., Lu W.J., Watkins S.M., Olefsky J.M. GPR120 is an omega-3 fatty acid receptor mediating potent anti-inflammatory and insulin-sensitizing effects. Cell 2010 142(5):687-698. doi:10.1016/j.cell.2010.07.041

Simopoulos A.P. The importance of the omega-6/omega-3 fatty acid ratio in cardiovascular disease and other chronic diseases. Exp Biol Med 2008;233(6):6746-88. doi:10.1016/S0753-3322(02)00253-6

Van D., Beerthuis R.K., Nugteren D.H. and Vonkeman H. Enzymatic conversion of all-cis-polyunsaturated fatty acids into prostaglandins. Nature 1964;203:839-841

Essential fatty acids: definition, synthesis, functions, and foods

Essential fatty acids or EFAs are fatty acids that cannot be synthesized by animals, and, like other essential nutrients, must be obtained from the diet. They are linoleic acid or LA or 18:2n-6, and alpha-linolenic acid or ALA or 18:3n-3.
Animals cannot synthesize these two fatty acids due to the lack of delta-12 desaturase (E.C. 1.14.19.6) and delta-15 desaturase (EC 1.14.19.25). These enzymes introduce cis double bonds beyond carbon 9, and are present in plants and microorganisms, such as some bacteria, fungi and molds. In particular, in plants:

  • delta-12 desaturase, present in the plastids, catalyzes the synthesis of linoleic acid from oleic acid, by introducing a double bond at delta-12 position, namely, between carbons 6 and 7 from the methyl end of the fatty acid;
  • delta-15 desaturase, present in the plastids and in the endoplasmic reticulum of phytoplankton and vascular terrestrial plants, catalyzes the synthesis of alpha-linolenic acid from linoleic acid by introducing a double bond at delta-15 position, namely, between carbons 3 and 4 from the methyl end of the fatty acid.
Synthesis of the essential fatty acids linoleic acid and alpha-linolenic acid
Synthesis of EFAs

Linoleic acid and alpha-linolenic acid are the precursors to omega-6 polyunsaturated fatty acids and omega-3 polyunsaturated fatty acids. Indeed, animals can synthesize, although with variable efficiency, the other omega-3 and omega-6 polyunsaturated fatty acids, molecules with 20, 22, or 24 carbon atoms, and up to 6 double bonds, such as arachidonic acid or ARA or 20:4n6 and docosahexaenoic acid or DHA or 22:6n3, due to the presence of desaturases that introduce double bonds at delta-5 and delta-6 positions and elongases that catalyze  the elongation of the carbon chain.
In the absence of dietary essential fatty acids, a rather rare condition, the other omega-3 and omega-6 fatty acids become essential, too. For this reason, they are defined by some as conditionally essential fatty acids.

It should be pointed out that all essential fatty acids are polyunsaturated molecules, but not all polyunsaturated fatty acids are essential, such as those belonging to the omega-7 and omega-9 families.

CONTENTS

Discovery of essential fatty acids

The first evidence of their existence dates back to 1918, when Hans Aron suggested that dietary fat could be essential for the healthy growth of animals and that, in addition to their caloric contribution, there was a inherent nutritive value due to the presence of certain lipid molecules.
In 1927, Herbert M. Evans and George Oswald Burr demonstrated that, despite the addition of vitamins A, D, and E to the diet, a deficiency of fat severely affected both growth and reproduction of experimental animals. Therefore, they suggested the presence of an essential substance in the fat that they called vitamin F.
Eleven years after Aron work, in 1929, George Burr and his wife Mildred Lawson hypothesized that warm-blooded animals were not able to synthesize appreciable amounts of certain fatty acids. One year later, they discovered that linoleic acid was essential for animals, and it was they who coined the term essential fatty acid.
However, EFA deficiency in humans was first described by Arild Hansen et al. only in 1958, in infants fed a milk-based formula lacking them.
And in 1964, thanks to the research of Van Dorp et al. and Bergstroem et al., one of their biological functions was discovered: being the precursors for the synthesis of prostaglandins.

Functions of EFAs and their PUFA derivatives

EFAs and their polyunsaturated fatty acid derivatives play important biological functions.

  • They are structural components of cellular membranes, modulating, for example, their fluidity, particularly DHA.
  • They are essential for the development and functioning of the nervous system, particularly ARA and DHA.
  • They are involved in signal transduction, particularly omega-6 polyunsaturated fatty acids, such as ARA.
  • They are involved in the regulation of genes encoding lipolytic and lipogenic enzymes, being strong inducers of fatty acid oxidation, as well as inhibitors of their synthesis and that of triglycerides, at least in animal models.
    They act, for example, as:

    • activators of the peroxisome proliferator-activated receptor α (PPAR-α) that stimulates the transcription of genes encoding lipolytic enzymes as well as mitochondrial and peroxisomal beta-oxidation enzymes, and inhibitors of transcription of genes encoding enzymes involved in lipogenesis;
    • inhibitors of sterol responsive element binding protein-1c (SREBP-1c) gene transcription, a transcription factor required for liver fatty acid and triglyceride synthesis induced by insulin.
      Note: PUFAs also increase SREBP-1c mRNA degradation as well as SREBP-1 degradation.
  • They are precursors for signaling molecules, with autocrine and paracrine action, that act as mediators in many cellular processes, such as eicosanoids
  • They are essential in the skin, especially linoleic acid in sphingolipids of the stratum corneum, where they contribute to the formation of the barrier against water loss.
  • They have a crucial role in the prevention of many diseases, particularly coronary heart disease, acting as antihypertensive, antithrombotic, and triglyceride-lowering agents.
  • Noteworthy, their energy storage function is quantitatively unimportant.

Foods rich in essential fatty acids

Linoleic acid is the most abundant polyunsaturated fatty acid in the Western diet, and accounts for 85-90% of dietary omega-6 polyunsaturated fatty acids.
The richest dietary sources are vegetable oils and seeds of many plants, such as:

  • safflower oil, ~ 740 mg/g;
  • sunflower oil, ~ 600 mg/g;
  • soybean oil, ~ 530 mg/g;
  • corn oil, ~ 500 mg/g;
  • cottonseed oil, ~ 480 mg/g;
  • walnuts, ~ 340 mg/g;
  • brazil nuts, ~ 250 mg/g;
  • peanut oil, ~ 240 mg/100 g;
  • rapeseed oil, ~ 190 mg/g;
  • peanuts, ~140 mg/g;
  • flaxseed oil, ~ 135 mg/g.

Linoleic acid is present in fair amounts also in animal products such as chicken eggs or lard, because it is present in their feed.
It should be noted that some of the major sources of linoleic acid, such as walnuts, flaxseed oil, soybean oil, and rapeseed oil are also high in alpha-linolenic acid.

Some of the richest dietary sources of alpha-linolenic acid are:

  • flaxseed oil, ~ 550 mg/g
  • rapeseed oil, ~ 85 mg/g
  • soybean oil, ~ 75 mg/g

Other foods rich in ALA include nuts, ~ 70 mg/g, and soybeans, ~ 10 mg/g.

References

Akoh C.C. and Min D.B. “Food lipids: chemistry, nutrition, and biotechnology” 3th ed. 2008

Bergstroem S., Danielsson H., Klenberg D. and Samuelsson B. The enzymatic conversion of essential fatty acids into prostaglandins. J Biol Chem 1964;239:PC4006-PC4008

Burr G. and Burr M. A new deficiency disease produced by the rigid exclusion of fat from the diet. J Biol Chem 1929;82:345-367

Chow Ching K. “Fatty acids in foods and their health implication” 3th ed. 2008

Evans H. M. and G. O. Burr. A new dietary deficiency with highly purified diets. III. The beneficial effect of fat in the diet. Proc Soc Exp Biol Med 1928;25:390-397. doi:10.3181/00379727-25-3867

Smith W., Mukhopadhyay R. Essential fatty acids: the work of George and Mildred Burr. J Biol Chem 2012;287(42):35439-35441. doi:10.1074/jbc.O112.000005

Van Dorp. D.A., Beerthuis R.K., Nugteren D.H. and Vonkeman H. Enzymatic conversion of all-cis-polyunsaturated fatty acids into prostaglandins. Nature 1964;203:839-841. doi:10.1038/203839a0