Omega-3/6 conversion efficiency

FADS1 controls how efficiently your body converts plant oils into long-chain omega-3 and omega-6 fats like EPA, DHA and arachidonic acid. Lower efficiency means dietary fish or algae matter more.

What the FADS1 variant does

FADS1 and FADS2 make the desaturase enzymes that convert short-chain plant fats into the long-chain fats the body actually uses: alpha-linolenic acid into EPA and DHA, and linoleic acid into arachidonic acid. This variant marks the region that sets how efficiently that conversion runs.

Fast converters make more long-chain omega-3 and omega-6 from the same diet. Slow converters make less, which matters most for people whose only omega-3 source is plant-based.

How strong the evidence is

This is one of the strongest genetic determinants of blood fatty acid composition known. In a study of 239,268 people the association with the proportion of polyunsaturated fatty acids reached a p-value at the limit of what is reported, and it has been replicated across many populations and dietary contexts.

Diet is where this becomes practical

If you eat oily fish, conversion efficiency matters less, because you are consuming EPA and DHA directly rather than making them. If you are vegetarian or vegan, it matters more: flaxseed, chia and walnuts supply alpha-linolenic acid, and a slow converter turns less of it into the DHA that the brain and retina use.

The practical option in that case is an algal oil supplement, which supplies DHA directly from a plant source. That is a reasonable thing to consider on general grounds for people avoiding fish, and this genotype is a mild argument in the same direction rather than a new instruction.

An unusual evolutionary history

Frequencies differ sharply across populations, and diet appears to be why. In 1000 Genomes reference data, about 34% of East Asian-ancestry and 11% of European-ancestry individuals carry two copies of the allele read here, against roughly 1% in African-ancestry samples.

Populations with long agricultural histories tend toward the efficient-conversion haplotype, which suits a plant-heavy diet, while populations with marine diets, most strikingly Greenlandic Inuit, carry variants in the opposite direction. It is one of the better-documented cases of diet shaping the genome.

What this variant does not tell you

It cannot tell you your omega-3 status, which is measurable directly as an omega-3 index from a blood sample. It also says nothing about cardiovascular benefit: large trials of omega-3 supplements have produced mixed results, and the genotype does not resolve that debate.

What each rs174537 genotype means

rs174537 has three possible genotypes: GG, GT and TT.

Efficient omega-3 conversion (rs174537 GG). You have the efficient version of FADS1, so your body readily converts plant oils into the long-chain omega-3 and omega-6 fats (like EPA and DHA) your brain and heart use. You make these well from a normal diet.

Intermediate omega-3 conversion (rs174537 GT). You carry one copy of the less-efficient FADS1 variant, so you convert plant oils into long-chain omega-3s a bit less effectively than the most efficient type. Eating some fish or algae-based omega-3 helps cover the gap.

Lower omega-3 conversion (rs174537 TT). You have two copies of the less-efficient FADS1 variant, so your body is slower at turning plant oils (like those in flax or walnuts) into the long-chain omega-3s EPA and DHA. Getting omega-3 directly from fish or algae oil is a more reliable source for you.

Evidence & sources

Across global populations, about 11% of people carry two copies of this variant, and about 53% carry none.

This variant appears in 4 published genetic studies: PubMed 25646338 · PubMed 26584805 · PubMed 34503513 · PubMed 40545721.

Common questions

Should I take fish oil because of this result? If you eat oily fish regularly, the conversion question is largely bypassed. If you avoid fish, a DHA source such as algal oil is worth considering on general grounds, and a slow-converter genotype is a mild additional reason rather than a decisive one.

Does this affect my omega-6 to omega-3 balance? The same enzymes act on both pathways, so the variant shifts arachidonic acid production as well. What that means for health is not settled, and the ratio itself is a much weaker concept than popular nutrition writing suggests.

Related

Adult lactose digestion · Alcohol response (flushing & dependence risk) · Caffeine metabolism speed · LDL cholesterol · Sweet tooth / carbohydrate preference (FGF21) · Total cholesterol · Vitamin D status (low-D genetic burden)