Omega-3 fatty acids. I could write about this until my fingers are on fire. But the place to start is that "omega-3" is not one thing — it is three, and they are not interchangeable.
The trio: ALA, EPA and DHA
ALA (alpha-linolenic acid) is the plant form. It is in walnuts, flaxseed, chia and canola oil. ALA is genuinely essential — the body cannot make it — and it is the only omega-3 for which US nutrition authorities have set an intake figure: the Institute of Medicine's Adequate Intake is 1.6 grams a day for men and 1.1 grams for women.
EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) are the long-chain marine forms, and they are the ones most of the research is about. EPA is involved in the pathways that resolve inflammation. DHA is a major structural fat in the brain, the retina and sperm.
The body can convert ALA into EPA and DHA, but it does so inefficiently — commonly cited conversion rates are in the low single digits to around 10%, and they vary with genetics, sex and overall diet. This is the crux of it: eating flaxseed is not the same as eating salmon. Seafood delivers EPA and DHA ready-made, so your body does not have to do the conversion at all.
Where the omega-3s actually come from
Here is the part most people find surprising: fish do not manufacture omega-3s. They inherit them.
EPA and DHA originate with marine microalgae — diatoms are major EPA producers, while dinoflagellates and other single-celled algae supply much of the ocean's DHA. Those fats move up the food web through zooplankton, krill and forage fish. A king salmon has never eaten algae directly, but every gram of DHA in its flesh traces back to it.
Which is why a wild fish feeding in a cold, productive marine ecosystem carries so much of it, and why what a fish eats shows up in what you eat.
How much is in what
Not all seafood is an omega-3 food, and it is worth being straight about that. These are EPA+DHA figures for a 3-ounce cooked serving, from USDA composition data:
- Sablefish (black cod) — about 1.5 g
- Wild king (Chinook) salmon — about 1.5 g
- Wild coho salmon — about 1.2 g
- Wild sockeye salmon — about 0.7–1.0 g
- Canned albacore tuna — about 0.7–1.1 g
- Snow crab — about 0.4 g
- Red king crab and Dungeness crab — about 0.2–0.35 g
- Shrimp and prawns — about 0.2–0.3 g
- Pacific halibut — about 0.2–0.37 g
- Scallops — about 0.15 g
- Pacific cod — about 0.14–0.29 g
Figures vary with season, run and how the fish is cooked, so treat them as typical rather than exact. But the shape of the list is the point: the oily fish are in a different league. Wild salmon and sablefish carry four to seven times what lean whitefish and shellfish do.
That does not make crab, shrimp or halibut and cod lesser foods — they are lean, high in protein, and rich in selenium, zinc and B12. Eat them because they are superb. Just don't count on them for your omega-3s.
How much do you need?
There is no official US requirement for EPA and DHA. The Institute of Medicine set an Adequate Intake for ALA only, and, as the NIH Office of Dietary Supplements puts it, did not establish specific intake recommendations for EPA, DHA or the other long-chain omega-3s.
What exists instead are advisory targets. The American Heart Association recommends two servings of fish a week, about 3 ounces cooked each, preferably fatty fish. EFSA and WHO/FAO point to roughly 250 mg a day of EPA+DHA. The FDA and EPA advise two to three servings a week from their "Best Choices" list.
In practice, two servings of oily fish a week clears all of those comfortably. One 3-ounce piece of king salmon is roughly six days' worth of the 250 mg figure on its own.
Low omega-3 status
Frank omega-3 deficiency is rare in the US, and its classic signs — rough, scaly skin and dermatitis — come from severe, prolonged deprivation rather than an ordinary low-fish diet. What is common is simply low intake: most Americans do not reach the recommended two servings a week.
If you want to know where you stand rather than guess, there is a blood test for it. The Omega-3 Index measures the proportion of EPA and DHA in your red blood cell membranes, which reflects the last few months of intake rather than what you ate yesterday. Your doctor can order one, or you can ask about it at your next physical.
None of this is complicated. Eat oily fish a couple of times a week, enjoy the lean stuff for its own sake, and you have covered it.
Ashley Besecker is the co-founder of Premier Catch and Director of Health and Nutrition. She is a registered dietitian nutritionist, certified dietitian, and nutritional genetics specialist. Ashley's writing covers the nutritional benefits of eating sustainable, wild seafood and more detail on Premier Catch's sourcing commitments.