Why two oily fish meals beat a daily flax sprinkle

In brief

Plant omega-3 from flax and chia is essential but converts poorly to the EPA and DHA forms that actually move cardiovascular and longevity outcomes. Two oily fish meals a week, or an algae-based capsule for vegetarians, hits the same target.

There are three meaningful omega-3 fats in the human diet: alpha-linolenic acid (ALA) from flax, chia, and walnut; and the two long-chain forms EPA and DHA, found preformed in oily fish and algae. The cardiovascular, neurological, and inflammatory benefits of omega-3 in the evidence base are driven almost entirely by EPA and DHA. ALA is essential, but it has to be converted, and that is where the issue starts.

The conversion problem

Brenna and colleagues, reviewing tracer studies in the American Journal of Clinical Nutrition (2009), found ALA-to-EPA conversion in adult men at 0.5 to 5 percent and ALA-to-DHA conversion at under 1 percent. Pre-menopausal women convert slightly more thanks to oestrogen, but the upper bound is around 9 percent. For a 1 tablespoon serving of ground flax (about 2.4 g of ALA), this means at most 100 mg of EPA reaches the bloodstream, against a 1 to 2 g daily target.

What hits the target

Two 100 g servings per week of oily fish (salmon, sardines, mackerel, anchovies) deliver around 3 g of EPA plus DHA across the week, the consensus dose for raising the Omega-3 Index toward 8 percent. The Index, a measure of EPA + DHA in red cell membranes, is one of the strongest single nutritional predictors of cardiovascular and all-cause mortality. Harris and colleagues, in Prog Cardiovasc Dis (2021), showed that an Index above 8 percent was associated with about five years of additional life expectancy versus an Index below 4 percent. For vegetarians and vegans, 1 to 2 g per day of an algae oil capsule (the original source of fish DHA) hits the same target.

Key Takeaways

• •Plant omega-3 from flax and chia converts to the long-chain forms at under 5 percent in most adults.
• •Two servings of oily fish per week, or 1 to 2 g daily of algae oil, hits the dose linked to longer life expectancy.
• •The Omega-3 Index above 8 percent is a stronger mortality predictor than total cholesterol.