Nutrition

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

Essential fatty acids or EFAs are unsaturated fatty acids that cannot be synthesized by animals, and, like other essential nutrients, must be obtained from the diet. They are linoleic acid and alpha-linolenic acid.

Animals cannot synthesize the 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).[1][9][15] These enzymes introduce cis double bonds beyond carbon 9, and are present in plants and some microorganisms such as some bacteria, fungi and molds.[10]

  • Delta-12 desaturase 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.[15]
  • Delta-15 desaturase or omega-3 desaturase or fatty acid desaturase 3 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. The enzyme is only present in the plastids and in the endoplasmic reticulum of phytoplankton and vascular terrestrial plants.[1][9]
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. In the absence of dietary EFAs, a rather rare condition, the other omega-3 and omega-6 fatty acids become essential, too. For this reason, these lipids are defined by some as conditionally essential fatty acids.[9]

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

Contents

The discovery

The first evidence of the existence of essential fatty acids 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.[2]

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.[11]

Eleven years after Aron work, in 1929, Burr and his wife Mildred Lawson hypothesized that warm-blooded animals were not able to synthesize appreciable amounts of certain fatty acids.[7] One year later, they discovered that linoleic acid was essential for animals, and it was they who coined the term essential fatty acid.[8][19]

However, EFA deficiency in humans was first described by Arild Hansen only in 1958, in infants fed a milk-based formula lacking them.[14]

And in 1964, thanks to the research of Van Dorp and Bergstroem, one of their biological functions was discovered: being the precursors for the synthesis of prostaglandins.[3][21]

Role

Essential fatty acids play important biological functions.

  • Linoleic acid and alpha-linolenic acid can be used as an energy source through beta-oxidation.[22]
  • Linoleic acid and alpha-linolenic acid are the precursors to omega-6 polyunsaturated fatty acids and omega-3 polyunsaturated fatty acids. Indeed, many animals, including humans, can synthesize, although with variable efficiency, the other omega-3 and omega-6 polyunsaturated fatty acids, for example arachidonic acid, eicosapentaenoic acid, and docosahexaenoic acid.[5][6][18]
  • Essential fatty acids are structural components of cellular membranes, modulating, for example, their fluidity.[20][22]
  • 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.[23]
  • They have a crucial role in the prevention of many diseases, particularly coronary heart disease.[12][16]

Food sources

Linoleic acid is the most abundant polyunsaturated fatty acid in the Western diet, and accounts for 85-90 percent of dietary omega-6 polyunsaturated fatty acids.[13] The richest dietary sources are vegetable oils and seeds of many plants.[4]

Food sources Linoleic acid (mg/g)
Safflower oil ∼ 740
Sunflower oil ∼ 600
Soybean oil ∼ 530
Corn oil ∼ 500
Cottonseed oil ∼ 480
Walnuts ∼ 340
Brazil nuts ∼ 250
Peanut oil ∼ 240
Rapeseed oil ∼ 190
Peanuts ∼ 140
Flaxseed oil ∼ 135

Linoleic acid is present in fair amounts also in animal products such as chicken eggs or lard, because it is present in their feed.[9]
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.[17]

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 alpha-linolenic acid include nuts, ~ 70 mg/g, and soybeans, ~ 10 mg/g.[1]

References

  1. ^ a b c Akoh C.C. and Min D.B. Food lipids: chemistry, nutrition, and biotechnology. 3th Edition. CRC Press, Taylor & Francis Group, 2008
  2. ^ Aron H. Uber den Nahrwert. Biochem Z 1918; 92: 211-33
  3. ^ Bergstroem S., Danielsson H., Klenberg D., Samuelsson B. The enzymatic conversion of essential fatty acids into prostaglandins. J Biol Chem 1964;239:PC4006-8. doi:10.1016/S0021-9258(18)91234-2
  4. ^ Blasbalg T. L., Hibbeln J. R., Ramsden C. E., Majchrzak S. F. & Rawlings R. R. Changes in consumption of omega-3 and omega-6 fatty acids in the United States during the 20th century. Am J Clin Nutr 2011;93(5):950-962. doi:10.3945/ajcn.110.006643
  5. ^ Burdge G.C, Jones A.E, Wootton S.A. Eicosapentaenoic and docosapentaenoic acids are the principal products of α-linolenic acid metabolism in young men. Br J Nutr 2002;88(4):355-364. doi:10.1079/BJN2002662
  6. ^ Burdge G.C., Wootton S.A. Conversion of α-linolenic acid to eicosapentaenoic, docosapentaenoic and docosahexaenoic acids in young women. Br J Nutr 2002;88(4):411-420. doi:10.1079/BJN2002689
  7. ^ 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. doi:10.1016/S0021-9258(20)78281-5
  8. ^ Burr G.O., Burr M.M., Miller E.S. On the fatty acids essential in nutrition. III. J Biol Chem 1932;97(1):1-9. doi:10.1016/S0021-9258(18)76213-3
  9. ^ a b c d Chow Ching K. Fatty acids in foods and their health implication. 3th Edition. CRC Press, Taylor & Francis Group, 2008
  10. ^ Das U.N. Essential fatty acids: biochemistry, physiology and pathology. Biotechnol J 2006;1;420-439. doi:10.1002/biot.200600012
  11. ^ 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
  12. ^ Farvid M.S., Ding M., Pan A., Sun Q., Chiuve S.E., Steffen L.M., Willett W.C., Hu F.B. Dietary linoleic acid and risk of coronary heart disease: a systematic review and meta-analysis of prospective cohort studies. Circulation 2014;130:1568-1578. doi:10.1161/CIRCULATIONAHA.114.010236
  13. ^ Guyenet S.J., Carlson S.E. Increase in adipose tissue linoleic acid of US adults in the last half century. Adv Nutr 2015;6(6):660-664. doi:https://DOI.org/10.3945/an.115.009944
  14. ^ Hansen A.E., Haggard M.E., Boelsche A.N., Adam D.J., Wiese H.F. Essential fatty acids in infant nutrition. III. Clinical manifestations of linoleic acid deficiency. J Nutr 1958;66(4):565-76. doi:10.1093/jn/66.4.565
  15. ^ a b Malcicka M., Visser B., Ellers J. An evolutionary perspective on linoleic acid synthesis in animals. Evol Biol 2018;45:15-26. doi:10.1007/s11692-017-9436-5
  16. ^ Mensink R.P., Zock P.L., Kester A.D., Katan M.B. Effects of dietary fatty acids and carbohydrates on the ratio of serum total to HDL cholesterol and on serum lipids and apolipoproteins: a meta-analysis of 60 controlled trials. Am J Clin Nutr 2003;77(5):1146-55. doi:10.1093/ajcn/77.5.1146
  17. ^ Rajaram S. Health benefits of plant-derived α-linolenic acid. Am J Clin Nutr 2014;100 Suppl 1:443S-8S. doi:10.3945/ajcn.113.071514
  18. ^ Rett B.S., Whelan J. Increasing dietary linoleic acid does not increase tissue arachidonic acid content in adults consuming Western-type diets: a systematic review. Nutr Metab (Lond) 2011;8:36. doi:10.1186/1743-7075-8-36
  19. ^ 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
  20. ^ Stubbs C.D., Smith A.D. The modification of mammalian membrane polyunsaturated fatty acid composition in relation to membrane fluidity and function. Biochim Biophys Acta 1984;779(1):89-137. doi:10.1016/0304-4157(84)90005-4
  21. ^ 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
  22. ^ a b Whelan J. and Fritsche K. Linoleic acid. Adv Nutr 2013;4(3): 311-312. doi:10.3945/an.113.003772
  23. ^ Ziboh V.A. Prostaglandins, leukotrienes, and hydroxy fatty acids in epidermis. Semin Dermatol 1992;11(2):114-20