Flaxseed oil or fish: why plant omega-3 isn’t the same as marine omega-3

Flaxseed oil bottles often say “source of omega-3” — and that’s true. But the omega-3 in fish and the omega-3 in plant oil are chemically different, and the body converts one into the other reluctantly at best. That’s why the calculator tracks them as two separate nutrients, not one.

One name, two different substances

ALA (alpha-linolenic acid) is the plant omega-3 in flaxseed oil, walnuts, chia and canola oil. EPA and DHA are the forms actually used by the brain, retina and blood vessels, and are present in meaningful amounts only in fish and seafood. The human body can elongate ALA into EPA and then DHA, but how efficiently it does that is the whole question.

How much ALA actually becomes EPA and DHA

In a study by Burdge and Wootton (2002), women were given labelled ALA and tracked for 21 days to see where it went. 21% of the dose became EPA, 6% became DPA, and 9% became DHA. Roughly another 22% was simply burned as fuel within the first day.

In men, the picture is different: in the same kind of study, labelled DHA didn’t show up in any of the blood measures over 21 days — conversion of ALA to DHA was “either very low or absent” (Burdge et al., 2002). The authors suggest women’s greater conversion capacity relates to the fetus’s and newborn’s need for DHA during pregnancy and lactation.

Why ALA supplements don’t raise blood DHA

A review by the International Society for the Study of Fatty Acids and Lipids sums it up: ALA supplements raise blood and breast-milk EPA and DPA, but have little effect on DHA — while preformed DHA from food raises it effectively (Brenna et al., 2009). So “eat more flaxseed oil to get DHA” doesn’t work the way it sounds like it should.

Where plant ALA actually is

A spoonful of flaxseed oil or a handful of walnuts easily covers the daily ALA target (1.1 g). But that’s not an equivalent to fish oil — it’s a separate nutrient with its own benefits.

Where fish-derived EPA and DHA actually are

100 g of mackerel or salmon comfortably covers the EPA+DHA target (250 mg). This is the form of omega-3 the body can’t efficiently make on its own from plant oils.

If fish rarely makes it to your plate

There’s a way around fish: in a study of vegetarians, 0.94 g of DHA a day from microalgae oil for 8 weeks raised the blood “omega-3 index” from 4.8% to 8.4%, and 69% of participants reached the desirable level — versus none in the placebo group (Geppert et al., 2005). Supplements like this, made from algae rather than fish, are sold as vegan DHA.

In the CookCount diary, ALA (plant) and EPA+DHA (marine) are tracked separately, each against its own target. Log your meals in the diary to see which of the two omega-3s you’re actually short on — they come from different foods and call for different fixes.