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

Essential fatty acids or EFAs are fatty acids, a class of lipids, that cannot be synthesized by animals, and, like other essential nutrients, must be obtained from the diet. They are linoleic acid (LA) or 18:2n-6, and alpha-linolenic acid (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.

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, lipids with 20, 22, or 24 carbon atoms, and up to 6 double bonds, such as arachidonic acid (ARA) or 20:4n6 and docosahexaenoic acid (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
• Functions of EFAs and their PUFA derivatives
• Foods rich in essential fatty acids
• References

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.

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, and soybean oil, ~ 75 mg/g. Other foods rich in ALA include nuts, ~ 70 mg/g, and soybeans, ~ 10 mg/g.

References

1. Akoh C.C. and Min D.B. “Food lipids: chemistry, nutrition, and biotechnology” 3th ed. 2008
2. 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
3. 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
4. Chow Ching K. “Fatty acids in foods and their health implication” 3th ed. 2008
5. 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
6. 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
7. 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