Last reviewed: July 2026

Omega-3s: ALA vs EPA and DHA, and what plant sources can and cannot do

Not all omega-3 is interchangeable, and the difference decides whether your sources are working. Alpha-linolenic acid (ALA), the form in flaxseed, chia and walnuts, is essential and useful, but the body converts it to eicosapentaenoic acid (EPA) at only about 5 to 10 percent and to docosahexaenoic acid (DHA) at under 1 percent in most adults. EPA and DHA are the forms tied to heart and brain outcomes in research. Two portions of oily fish a week, one of them oily, covers it comfortably: a 140 g serving of salmon supplies roughly 2 to 3 grams of EPA plus DHA, which is most of a week's target in one meal.

Three molecules, one label word

Packaging says omega-3 as though it were a single ingredient. It is a family, and three members matter. ALA has 18 carbons and comes from plants: flaxseed, chia, hemp, walnuts and rapeseed oil. EPA and DHA are longer, 20 and 22 carbons, and come almost entirely from marine sources, because the algae at the base of the ocean food chain make them and fish accumulate them. Only ALA is strictly essential in the sense that the body cannot build it from scratch. The complication is that being able to make EPA and DHA from ALA in principle is not the same as making useful quantities of them in practice.

The conversion problem, in numbers

Tracer studies put ALA to EPA conversion at roughly 5 to 10 percent and ALA to DHA at under 1 percent, sometimes near zero in men. Women of reproductive age convert somewhat better, plausibly an adaptation for pregnancy, but the ceiling is still low. High intakes of omega-6 linoleic acid compete for the same enzymes and push conversion lower still, which is one of the few genuinely load-bearing points in the wider omega-6 argument. The practical consequence is blunt: a tablespoon of flaxseed a day is a fine source of ALA and a poor way to raise DHA.

What the fish actually delivers

The gap in that table is the whole article. Canned fish deserves particular attention because it makes the cheap version of this obvious: a tin of sardines delivers omega-3 in the same league as fresh salmon for a fraction of the price, with calcium from the soft bones as a bonus. Fast and cheap beats aspirational and unbought.

What the evidence supports, honestly

Observational data linking fish consumption to lower cardiovascular mortality is consistent. Randomised trials of omega-3 supplements are more mixed: the large Cochrane review of supplementation found little or no effect on all-cause mortality or cardiovascular events for most people, while trials using high-dose prescription EPA in people with elevated triglycerides have shown benefit. Triglyceride lowering itself is well established at doses of 2 to 4 g a day. So the fair summary is that eating fish looks good, taking capsules to prevent heart disease in an otherwise healthy person looks unimpressive, and specific high-dose clinical uses are real but are prescribing decisions, not shopping decisions.

DHA's structural role in the brain and retina is not controversial, and it matters most during pregnancy, breastfeeding and infancy, where guidelines commonly add an explicit DHA recommendation. That is one situation where a supplement is a reasonable default rather than an optional extra, and it should be discussed with a midwife or doctor.

The vegan and vegetarian route

Plant-based diets can hit ALA effortlessly and still land low on DHA. The clean solution is algae oil, which supplies DHA and increasingly EPA directly from the organism that manufactures it, skipping the conversion bottleneck entirely. It is one of the short list of supplements with a clear rationale, alongside the case made in the B12 explainer. Alongside that, keeping omega-6 intake moderate rather than extreme helps the conversion pathway do slightly more of the work.

Practical targets

• General adults: two portions of fish a week, at least one oily, which averages roughly 250 to 500 mg of EPA plus DHA per day.
• Not a fish eater: an algae oil supplement supplying around 250 mg DHA daily is the straightforward substitute.
• Pregnancy and breastfeeding: follow local guidance, which usually adds DHA and also lists fish to limit for mercury.
• Mercury caution: the concern applies to large predatory fish, not to the salmon, sardines and mackerel that supply most of the omega-3.
• Everyone: ALA foods such as walnuts and flax are still worth eating for their own sake; just do not count them towards DHA.

Bottom line

ALA and DHA are not substitutes. If you eat oily fish twice a week you are done and no supplement is needed. If you do not, algae oil solves it cheaply. And if you were relying on a daily spoon of ground flax to cover your omega-3, it is doing a decent job of one form and almost none of the other. Compare fish options side by side in the comparison tool if price per portion is the deciding factor, because the cheapest tins are frequently the best value per gram of omega-3 in the shop.