What it is

Oily fish supply two long omega-3 fats, EPA and DHA; some plant oils supply a shorter one, α-linolenic acid (ALA) 12.

Per 100 g, raw farmed Atlantic salmon contains 0.862 g eicosapentaenoic acid (EPA, 20:5 n-3) and 1.1 g docosahexaenoic acid (DHA, 22:6 n-3), and cold-pressed linseed oil 53.4 g ALA (18:3 n-3) 1. Fish are the main dietary source of methylmercury; predatory species contain the most mercury 3.

In the body

The body converts only small amounts of ALA to EPA and DHA 4.

Humans lack Δ12- and Δ15-desaturases, so ALA is essential 4. FADS2 catalyses the first, rate-limiting Δ6-desaturation; ELOVL5 and FADS1 complete EPA, and ELOVL2, FADS2 and one peroxisomal β-oxidation cycle convert EPA to DHA 5.

α-Linolenic acid to docosahexaenoic acidEndoplasmic reticulum membrane, then peroxisome
  1. α-Linolenic acid (18:3 n-3)
  2. Long-chain-fatty-acid–CoA ligases (ACSL1, ACSL3 to ACSL6)ATP, CoAReleases AMP and diphosphate
  3. α-Linolenoyl-CoA
  4. Acyl-CoA 6-desaturase (FADS2)O2, 2 reduced cytochrome b5First, rate-limiting desaturation; shared with linoleoyl-CoA (n-6)
  5. Stearidonoyl-CoA (18:4 n-3)
  6. several stepsMalonyl-CoA, 2 NADPHElongation cycle: ELOVL5, HSD17B12, HACD1–4, TECR; releases CO2
  7. Eicosatetraenoyl-CoA (20:4 n-3)
  8. Acyl-CoA (8-3)-desaturase, Δ5-desaturase (FADS1)O2, 2 reduced cytochrome b5
  9. Eicosapentaenoyl-CoA (EPA, 20:5 n-3)
  10. several stepsMalonyl-CoA, 2 NADPHElongation cycle; ELOVL2 or ELOVL5 catalyses the condensation
  11. Docosapentaenoyl-CoA (DPA, 22:5 n-3)
  12. several stepsMalonyl-CoA, 2 NADPHElongation cycle; ELOVL2 catalyses the condensation
  13. Tetracosapentaenoyl-CoA (24:5 n-3)
  14. Acyl-CoA 6-desaturase (FADS2)O2, 2 reduced cytochrome b5Second Δ6-desaturation; human activity inferred by similarity
  15. Tetracosahexaenoyl-CoA (24:6 n-3)
  16. several stepsFAD, O2, H2O, NAD+, CoAOne peroxisomal β-oxidation cycle: ACOX1, HSD17B4, ACAA1; releases acetyl-CoA
  17. Docosahexaenoyl-CoA (DHA, 22:6 n-3)

Estimated conversion is 8–12% to EPA and possibly under 1% to DHA; dietary linoleic acid, converted by the same enzymes, lowers it 4. Homozygotes for the derived FADS haplotype D had 24% more plasma DHA than those for haplotype A 6.

Methylmercury crosses the placenta and blood–brain barrier; EFSA derived its tolerable weekly intake, 1.3 µg/kg body weight, from prenatal neurodevelopment, and high fish consumers may exceed it up to about sixfold 3.

What studies show

In independently funded research, fish-oil capsules did not reduce heart attacks and strokes taken together 78.

VITAL gave 25,871 US adults without cardiovascular disease 1 g fish oil daily (460 mg EPA, 380 mg DHA) or placebo; over a median 5.3 years, major cardiovascular events occurred in 3.0% against 3.2% (hazard ratio 0.92, 95% CI 0.80 to 1.06) 7.

In a meta-analysis of 10 trials in 77,917 high-risk people (mean 4.4 years), the rate ratio for major vascular events was 0.97 (0.93 to 1.01) 8.