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Essential fatty acids: history, biochemistry, functions and food sources

Dr. Nicola Tazzini

Essential fatty acids are lipids that cannot be synthesized by mammals and, like other essential nutrients, must be obtained through the diet. Specifically, they consist of two unsaturated fatty acids: linoleic acid and alpha-linolenic acid.[1]

The inability of mammals to synthesize these two fatty acids stems from the absence of two desaturases, which are present in other species, both animal and plant.[2][3][4][5]

Discovered in the early decades of the 20th century, they perform energetic and structural functions and are the precursors of omega-6 and omega-3 polyunsaturated fatty acids.[6][7]

The main dietary sources for humans are various vegetable oils and the seeds of many plants.[8]

Summary: Key Points

  • Definition: essential fatty acids include linoleic acid and alpha-linolenic acid, indispensable molecules in the diet as they cannot be synthesized by mammals.
  • Enzymatic block: the inability of endogenous synthesis is linked to the lack of Δ12 and Δ15 desaturases, which are instead present in plants, fungi, some invertebrates, and phytoplankton.
  • Energy yield: they can be used for energy purposes via β-oxidation, though showing a slightly lower ATP yield compared to stearic acid, the corresponding saturated fatty acid.
  • Functions: they act as precursors of bioactive lipid mediators and are fundamental structural components of cell membranes and the skin barrier.
  • Food sources: the main dietary sources are represented by various vegetable oils, seeds, and nuts, which sometimes contain significant amounts of both fatty acids.

Contents

Historical background

The first indication of the existence of essential fatty acids dates back to 1918, when Hans Aron suggested that dietary fat might be essential for normal growth in animals and that, in addition to its caloric contribution, there was a special intrinsic nutritional value derived from the presence of certain specific lipid molecules.[9]

In 1927, Herbert M. Evans and George Oswald Burr demonstrated that, despite the addition of the fat-soluble vitamins A, D, and E to the diet, a deficiency in fat severely affected both the growth and reproduction of experimental animals, suggesting the presence of a new essential substance in fat, which they named vitamin F. The term “vitamin F” was later abandoned because fatty acids did not fit the classical definition of a vitamin, namely, a micronutrient required in quantities on the order of milligrams or micrograms, since the daily requirements for essential fatty acids are on the order of grams and they also play energetic and structural roles, in addition to serving as precursors.[10]

In 1929, eleven years after Aron’s work, Burr and his wife Mildred Lawson hypothesized that warm-blooded animals were unable to synthesize appreciable amounts of certain fatty acids, and a year later, they discovered that linoleic acid was essential for animals.[6] These two researchers are credited with coining the term “essential fatty acid.”[7][11]

It was not until 1958, thanks to the work of Arild Hansen, that the first description of an essential fatty acid deficiency in humans was published, specifically in infants fed a milk-based diet lacking these fats.[12]

In 1964, thanks to the research of Van Dorp and Bergström, one of their biological functions was discovered: they serve as precursors for the synthesis of prostaglandins.[13][14]

Role of Δ12– and Δ15-desaturases

The inability of mammals to insert cis double bonds at 3 and 6 carbon atoms from the methyl end of fatty acids, referred to as the ω-end, or beyond carbon 9 from the carboxyl end, referred to as the Δ-end, stems from the absence of two specific desaturases: Δ12-desaturase (EC 1.14.19.6) and Δ15-desaturase.[3][8][15]

Δ12-desaturase is present in the chloroplasts of terrestrial plants and phytoplankton, as well as in bacteria, protozoa, fungi, and animals such as the house cricket, the American cockroach (Periplaneta americana), and other insects and invertebrates, including various nematodes, crustaceans, and mites.
It catalyzes the synthesis of linoleic acid (LA) from oleic acid by inserting a second cis double bond between carbons 6 and 7 from the methyl end of the molecule (thus between carbons 12 and 13 from the carboxyl end). LA is the first of the omega-6 polyunsaturated fatty acids to be synthesized.[5]

Δ15-desaturase is present in the chloroplasts of phytoplankton and some terrestrial vascular plants.
It catalyzes the synthesis of α-linolenic acid (ALA) from LA by inserting a third cis double bond between carbons 3 and 4 from the methyl end of the molecule (thus between carbons 15 and 16 from the carboxyl end).
ALA is the first member of the omega-3 polyunsaturated fatty acid family to be synthesized.[1][16][17]

Biosynthesis of Essential Fatty Acids

If the diet lacks these two essential fatty acids, the other derived omega-6 and omega-3 fatty acids also become essential; for this reason, some researchers refer to these lipids as conditionally essential fatty acids.[1][11]

Note: 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.[17]

Energy role

Unlike long-chain polyunsaturated fatty acids, essential fatty acids can be present in the diet in substantial amounts. Consequently, their accumulation in the triglycerides of adipose tissue, typically at the sn-1 and sn-3 positions, can also be significant, as can their role in energy production through mitochondrial and peroxisomal β-oxidation.[17]

The presence of cis double bonds results in a slightly lower ATP yield compared to the corresponding saturated fatty acid, stearic acid. While 122 ATP can be generated from stearic acid, 116.5 ATP can be generated from LA, and 115 ATP from ALA, with percentage differences in yield of 4.51% and 5.74% less, respectively.[18][19]

Functions

Essential fatty acids perform structural, metabolic, and regulatory functions and serve as precursors to bioactive lipid mediators.

Dietary sources

LA is the most abundant polyunsaturated fatty acid in the Western diet, accounting for 85–90% of all ingested omega-6 fatty acids.[33] The main sources for humans are various vegetable oils and the seeds of many plants.[8]

LA and ALA content (mg/g of total lipids) in major human dietary sources
Foods LA (omega-6) ALA (omega-3)
Safflower oil ≈ 740 ≈ 1
Sunflower oil ≈ 600 ≈ 1
Hemp seed oil ≈ 550 ≈ 200
Soybean oil ≈ 530 ≈ 75
Corn oil ≈ 500 ≈ 10
Cottonseed oil ≈ 480 ≈ 2
Walnuts ≈ 340 ≈ 70
Brazil nuts ≈ 250 ≈ 2
Peanut oil ≈ 240 —
Rapeseed oil (canola) ≈ 190 ≈ 85
Chia seed oil ≈ 180 ≈ 620
Peanuts ≈ 140 —
Flaxseed oil ≈ 135 ≈ 550

LA is also present in moderate amounts in animal products, such as chicken eggs and lard, but only because of the lipid content of the feed used in animal husbandry, since animals are unable to synthesize it.[1]
It should be noted that some of the main sources of LA, such as walnuts, flaxseed oil, soybean oil, and rapeseed oil (canola), also contain significant amounts of ALA.[32]

Among the richest sources of ALA in the human diet are chia seed oil, flaxseed oil, hemp oil, canola oil, soybean oil, and walnuts.[17]

References

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