Tag Archives: linoleic acid

Chemical composition of olive oil

Olive oil constituents

Olive Oil
Fig. 1 – EVOO

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.

Saponifiable fraction of olive oil

It is composed of saturated 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.

Oleic Acid
Fig. 2 – IOOC and Fatty Acids

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 of olive oil

Olive Oil
Fig. 3 – The sn Positions of 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.
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 of olive oil

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 of olive oil

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

Olive Oil
Fig. 4 – Squalene

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.
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
Olive Oil
Fig. 5 – beta-Sitosterol

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.

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

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;221:92-8 [Abstract]

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 [Abstract]

Skin, blood pressure, rheumatoid arthritis and gamma-linolenic acid

Healthy skin and gamma-linolenic acid

gamma-Linolenic acid (GLA), an omega-6 essential fatty acid, 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).
Skin and gamma-Linolenic Acid
Fig. 1 – GLA

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

Borago officinalis
Fig. 2 – Borage

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 gamma-linolenic acid in human health and nutrition. J Nutr 1998;128:1411-14 [Abstract]

Leventhal L.J., Boyce E.G. and Zurier R.B. Treatment of rheumatoid arthritis with gammalinolenic acid. Ann Intern Med 1993 119:867-73 [Abstract]

Miller C.C. and Ziboh V.A. Gammalinolenic acid-enriched diet alters cutaneous eicosanoids. Biochem Biophys Res Commun 1988 154:967-74 [Abstract]

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 39:1808-17 [Abstract]

Omega-6 polyunsaturated fatty acids

The synthesis of 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.

Omega-6 polyunsaturated fatty acids and linoleic acid

Metabolism of Omega-6 Polyunsaturated Fatty Acids
Fig. 1 – Metabolism of Omega-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.
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).

Omega-6 polyunsaturated fatty acids: 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 families 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 [Full Text]

Burr G. and Burr M. A new deficiency disease produced by the rigid exclusion of fat from the diet. J Biol Chem 1929;82:345-67 [Full Text]

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

Cozzani I. e Dainese E. “Biochimica degli alimenti e della nutrizione”. Piccin Editore, 2006

Mahan L.K., Escott-Stump S.: “Krause’s foods, nutrition, and diet therapy” 10th ed. 2000

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

Shils M.E., Olson J.A., Shike M., Ross A.C.: “Modern nutrition in health and disease” 9th ed. 1999

Stipanuk M.H.. Biochemical and physiological aspects of human nutrition. W.B. Saunders Company-An imprint of Elsevier Science, 2000

Van D., Beerthuis R.K., Nugteren D.H. and Vonkeman H. Enzymatic conversion of all-cis-polyunsaturated fatty acids into prostaglandins. Nature 1964;203:839-41

Omega-3 polyunsaturated fatty acids

The synthesis of omega-3 polyunsaturated fatty acids

Within omega-3 (ω-3) polyunsaturated fatty acid family:

are important fatty acids.

Omega-3 polyunsaturated fatty acids and α-linolenic acid

Omega-3 Polyunsaturated Fatty Acids: Omega-3 Fatty Acid Metabolism
Fig. 1 – Omega-3 Fatty Acid Metabolism

Like linoleic acid (omega-6 fatty acid), alpha-linolenic acid (ALA) is a primary product of plant polyunsaturated fatty acids (PUFA) synthesis and is the precursor of all the omega-3 polyunsaturated fatty acids.
It is produced de novo from linoleic acid only by plants (by the chloroplasts of marine phytoplankton and land plants) in a reaction catalyzed by Δ15-desaturase, i.e. the enzyme that forms the omega-3 polyunsaturated fatty acid family from omega-6 one catalyzing the insertion of the double bond between carbon atoms 3 and 4, numbered from methyl end of the molecule.
Note: while many land plants lack the ability to synthesize omega-3 polyunsaturated fatty acids, aquatic ones and planktons in colder water produce abundant amounts of them.
Animals, lacking Δ15-desaturase, can’t synthesize alpha-linolenic acid, and all the omega-3 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-3 polyunsaturated fatty acids are considered essential fatty acids, so called EFA.

Omega-3 polyunsaturated fatty acids: from α-linolenic acid to EPA and DHA

Omega-3 Polyunsaturated Fatty Acids: Foods Rich in Omega-3 Fatty Acids
Fig. 2 – Foods Rich in Omega-3 Fatty Acids

Animals are able to elongate and desaturase dietary alpha-linolenic acid in a cascade of reactions to form very long polyunsaturated omega-3 fatty acids but terrestrial animals have limited ability to do it. The efficiency of synthesis decreases down the cascade: conversion of alpha-linolenic acid to EPA is limited (the activity of Δ6-desaturase is the rate limiting in humans) and to DHA is even more restricted than that of EPA. This metabolic pathway occurs mainly in the liver and cerebral microvasculature of the blood brain barrier, but also in the cerebral endothelium and astrocytes.

Fish and shellfish, unlike terrestrial animals, are able to convert efficiently alpha-linolenic acid, obtained from chloroplast of marine phytoplankton, in EPA and DHA (the last one is present in high concentration in many fish oils but pay attention: many fish oils are also rich in saturated fatty acids).
It should be noted that polyunsaturated fatty acids of the ω-3 family, and of any other n-families, can be interconverted by enzymatic processed only within the same family, not among families.

EPA and DHA are primarily found in marine algae (in genetically engineered algae DHA represents approximately 50% of the total fatty acids), fish, shellfish, and marine products (particularly oil from cold-water marine fish).

Some functions of omega-3 polyunsaturated fatty acids

  • Omega-3 polyunsaturated fatty acid are capable of increasing high-density lipoprotein (HDL), “good cholesterol”, and of interleukin-2 levels. On the other hand, they decrease the levels of low-density lipoprotein (LDL), “bad cholesterol“, and very low density lipoprotein cholesterol (VLDL) and of interleukin-1 levels.
  • They are essential for the normal functioning of the brain and retina, especially in premature borns.
  • They are essential for growth and development throughout the life; for example if in children diet there is not enough omega-3 polyunsaturated fatty acids they may suffer dermatitis, growth retardation, neurological and visual disturbances.
  • C-20 polyunsaturated fatty acids, belonging to omega-3 and also omega-6 polyunsaturated fatty acid families, are the precursors eicosanoids (prostaglandins, prostacyclin, thromboxanes, and leukotrienes), potent, short-acting, local hormones.
  • While the omission in the diet of omega-6 polyunsaturated fatty acids results in a manifest systemic dysfunction, the deprivation of omega-3 polyunsaturated fatty acids causes dysfunction in a wide range of behavioral and physiological modalities.
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 [Full Text]

Burr G. and Burr M. A new deficiency disease produced by the rigid exclusion of fat from the diet. J Biol Chem 1929;82:345-67 [Full Text]

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

Cozzani I. e Dainese E. “Biochimica degli alimenti e della nutrizione”. Piccin Editore, 2006

Mahan L.K., Escott-Stump S.: “Krause’s foods, nutrition, and diet therapy” 10th ed. 2000

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

Shils M.E., Olson J.A., Shike M., Ross A.C.: “Modern nutrition in health and disease” 9th ed. 1999

Stipanuk M.H.. Biochemical and physiological aspects of human nutrition. W.B. Saunders Company-An imprint of Elsevier Science, 2000

Van D., Beerthuis R.K., Nugteren D.H. and Vonkeman H. Enzymatic conversion of all-cis-polyunsaturated fatty acids into prostaglandins. Nature 1964;203:839-41

Essential fatty acids: definition, functions, food sources, recommended intake

What are essential fatty acids?

Essential Fatty Acids
Fig. 1 – EFA

Essential fatty acids or EFA are fatty acids which cannot be synthesized de novo by animals, but by plants and microorganisms, such as bacteria, fungi and molds, and whose deficiency can be reversed by dietary addition.
There are two essential fatty acids: linoleic acid (LA, 18:2n-6) and α-linolenic acid (ALA, 18:3n-3), polyunsaturated fatty acids (PUFA) with 18 carbon atoms, belonging to omega-6 and omega-3 families, respectively.
Animals cannot synthesize these two fatty acids because they lack desaturases that introduce double bonds beyond the Δ9 position (carbon atoms numbered from the methyl end), namely:

  • Δ12-desaturase (E.C. 1.14.19.6), which catalyzes the synthesis of LA from oleic acid;
  • Δ15-desaturase (EC 1.14.19.25), present also in phytoplankton, which catalyzes the synthesis of ALA from linoleic acid.

Essential Fatty AcidsInstead, animals have the enzymes needed to elongate and desaturate, though with low efficiency, the two EFA to form PUFA with 20, 22, or 24 carbon atoms and up to 6 double bonds, such as for example  dihomo-gamma-linolenic acid (DGLA, 20:3n6), arachidonic acid (AA, 20:4n6), eicosapentaenoic acid (EPA, 20:5n3), and docosahexaenoic acid (DHA, 22:6n3).
If diet is deficient in EFA, also fatty acids synthesized from them become essential. For this reason they may be termed conditionally essential fatty acids.
It should be noted 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.

Functions of essential fatty acids and their PUFA derivatives

Essential Fatty Acids
Fig. 2 – DHA

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, besides its caloric contribution, there was a inherent nutritive value deriving from 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 in the fat of an essential substance they called vitamin F.
Eleven years after Aron work, in 1929, George Burr and his wife Mildred developed the hypothesis 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 only in 1958, in infants fed a milk-based formula lacking them.
And in 1964, thanks to the research by Van Dorp et al. and Bergstroem et al., one of their biological functions, that is, being precursor for the synthesis of prostaglandins, was discovered.
Now, it is clear that EFA and derived PUFA play many important roles, some of which are listed below.

  • They are fundamental components of biological membranes, modulating, for example, their fluidity, particularly DHA.
  • They are essential for the proper development and functioning of the nervous system, particularly AA and DHA.
  • They are involved in membrane signal transduction, particularly omega-6 fatty acids, such as membrane phospholipid arachidonic acid.
  • They are involved in the regulation of genes encoding lipolytic and lipogenic enzymes. In fact they are strong inducers of  fatty acid oxidation, as well as inhibitors of their synthesis and that of triglycerides, at least in animal models, by acting, for example, as:

activators of the peroxisome proliferator-activated receptor α or PPAR-α, which stimulates, among other things, the transcription of genes encoding lipolytic enzymes as well as mitochondrial and peroxisomal β-oxidation enzymes, and inhibits the transcription of genes encoding for enzymes involved in lipogenesis;

inhibitors of sterol responsive element binding protein-1c (SREBP-1c) gene transcription, a hepatic transcription factor required for liver fatty acid and triglyceride synthesis induced by insulin.
Note: PUFA also increase SREBP 1c mRNA degradation as well as SREBP-1 protein degradation.

  • They are precursors of signaling molecules, with autocrine and paracrine action, which act as mediators in many cellular processes. Eicosanoids, a group of oxygenated, 20 carbon fatty acids, are probably the most studied. They derive from linoleic acid, dihomo-gamma-linolenic acid, arachidonic acid, and EPA, and include prostaglandins, thromboxanes, leukotrienes, lipoxins, and  epoxyeicosatrienoic acids.
  • They are essential, especially LA present in sphingolipids of the stratum corneum of the skin, for the formation of the barrier against water loss from the skin itself.
  • They have a crucial role in the prevention of many diseases, particularly coronary heart disease or CHD, acting as antihypertensive, antithrombotic, and triglyceridelowering agents (increasing in the latter case mitochondrial β-oxidation).
  • Finally, energy storage function is marginal.

Foods rich in essential fatty acids and derived PUFA

Linoleic acid, produced mainly by terrestrial plants, is the most abundant polyunsaturated fatty acid in the Western diet, and accounts for 85-90% of dietary omega-6 fatty acids.
In the human diet, the richest 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
  • cotton seed oil, ~ 480 mg/g
  • walnuts, ~ 340 mg/g
  • brazil nuts, ~ 250 mg/g
  • peanut oil, ~ 240 mg/100 g
  • canola oil, ~ 190 mg/g
  • peanuts, ~140 mg/g
  • flaxseed oil, ~ 135 mg/g

Linoleic acid is present in fair amount also in animal products such as chicken eggs or lard, but only because it is present in their feed.
It should be noted that some of the major sources of LA such as walnuts, flax seed oil, soybean oil, and canola oil are also rich sources of α-linolenic acid (see below).
In seed oils, omega-6 fatty acids with a chain length longer than 18 carbon atoms, such as DGLA and arachidonic acid, are present only in traces. Instead, AA is found in all animal tissues and animal-based foods.

α-Linolenic acid is produced by plants, also cold water vegetation such as algae and phytoplankton.
In the human diet, some of the richest sources are:

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

Other foods rich in ALA are nuts, ~ 70 mg/g, and soybeans, ~ 10 mg/g.
EPA and DHA are mainly found in marine algae, and in engineered algae DHA can represent about 50% of the total fatty acids. In the human diet, EPA and DHA derive from fish, shellfish and fish oil, particularly that derived from cold-water fatty fish.

Essential fatty acids in Western diets

Over the past 50 years, Western diet has been enriched in saturated fatty acids and omega-6 fatty acids, whereas has become poor  in omega-3 fatty acids, with an omega-6/omega-3 ratio between 10/1 and 20/1, and hence, far from the recommended ratio of 5:1.
This high ratio is due to several factors, some of which are listed below.

  • While wild plant foods are typically high in omega-3 fatty acids, in industrial agriculture crops rich in omega-6 fatty acids have had greater success than those rich in omega-3 fatty acids.
  • The low consumption of seafood and fish oil.
  • The high consumption of animal products derived from animals, such as chickens, cattle and pigs, raised on corn-based feed. In addition to this, omega-3 fatty acid content, of some species of farmed fish is lower than their wild counterparts, again because of the feed used.
  • The high consumption of vegetable oils low in omega-3 fatty acids and high in omega-6 fatty acids, such as safflower oil, sunflower oil, soybean oil and corn oil.
  • The increased shelf life of those foods in which omega-6 fatty acids predominate over omega-3-fatty acids.

So, although it is desirable to increase consumption of omega-3 fatty acids, this will not occur easily.

Omega-6/omega-3 ratio

Many evidences, like lower rates of incidence of cancer, autoimmunity and coronary heart disease in populations whose diet has a high ratio of omega-3 to omega-6 fatty acids, such as Eskimos, Japanese and others who consume a large amount of seafood, suggest that the change of this ratio has affected human physiology adversely, promoting, together with other factors such as smoking and a sedentary lifestyle, the development of the main classes of diseases.
Note: Japanese are the only people with an omega-3/omega-6 ratio of 1/2-4.

Recommended dietary intake of essential fatty acids

Hereinafter, the recommended dietary intake for omega-3 and omega-6 fatty acids for healthy adults, according to the recommendation of some of the major scientific societies and international organizations, and, as you will see, there is no common position.

  • Omega-3 fatty acids
    WHO recommends a dietary intake of omega-3-fatty acids between 0.5 and 2% of energy/day, with 300-500 mg of EPA/DHA per day, and 0.8-1.1 g per day of α-linolenic acid.
    Academy of Nutrition and Dietetics recommends a dietary intake of 500 mg of EPA/DHA per day.
    European Food Safety recommends a dietary intake of 250 mg of EPA/DHA per day.
    American Heart Association and American Diabetes Association recommend to eat fish at least twice a week, particularly fatty fish.
    American Heart Association recommends to include oils and foods rich in α-linolenic acid.
  • Omega-6 fatty acids
    In the past, dietary recommendations for omega-6 fatty acid intakes, and so especially linoleic acid, were focused on the prevention of their deficiency, while currently they are focused on the determination of the optimal intake to reduce the risk of chronic diseases, with special attention to CHD.
    Currently, most scientific societies recommend a daily intake of linoleic acid between 5 and 10% of energy/day. This daily intake seems able to reduce the risk of CHD and coronary heart disease deaths compared to lower intakes.
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