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.
- Long-chain-fatty-acid–CoA ligases (ACSL1, ACSL3 to ACSL6)ATP, CoAReleases AMP and diphosphate
- Acyl-CoA 6-desaturase (FADS2)O2, 2 reduced cytochrome b5First, rate-limiting desaturation; shared with linoleoyl-CoA (n-6)
- several stepsMalonyl-CoA, 2 NADPHElongation cycle: ELOVL5, HSD17B12, HACD1–4, TECR; releases CO2
- Acyl-CoA (8-3)-desaturase, Δ5-desaturase (FADS1)O2, 2 reduced cytochrome b5
- several stepsMalonyl-CoA, 2 NADPHElongation cycle; ELOVL2 or ELOVL5 catalyses the condensation
- several stepsMalonyl-CoA, 2 NADPHElongation cycle; ELOVL2 catalyses the condensation
- Acyl-CoA 6-desaturase (FADS2)O2, 2 reduced cytochrome b5Second Δ6-desaturation; human activity inferred by similarity
- several stepsFAD, O2, H2O, NAD+, CoAOne peroxisomal β-oxidation cycle: ACOX1, HSD17B4, ACAA1; releases acetyl-CoA
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.