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food source · 2
Salmon stores its oil in its muscle, so the fillet you eat carries the long-chain omega-3 fats EPA and DHA itself: about 1.8–2.5 g per 100 g of farmed Atlantic salmon.
Mechanism · Why the oil is in the flesh
Salmon and its relatives store oil in the muscles they swim with. Cod stores its oil in the liver instead, which is why cod-liver oil carries omega-3 and vitamin D while a cod fillet has almost none. Salmon fat is woven between the muscle fibers and cannot be trimmed away; cutting off the layer under the skin throws out the most valuable part of the fish.
Evidence · Eating fish is not taking fish oil
Farmed Atlantic salmon has about 1.8–2.5 g of EPA and DHA per 100 g. The same double bonds that keep cell membranes supple in cold water also make this fat sensitive to long, high heat, so steaming it or searing it lightly until just pink in the center keeps more of it. Cochrane 2020 pooled 86 randomized trials with more than 160,000 people, mostly of fish-oil capsules, and found with high certainty that taking more omega-3 has little or no effect on deaths or cardiovascular events. But it judged capsules, not eating whole fish: the benefits of eating fish come mainly from observational studies, and a piece of fish also brings protein, vitamin D, selenium and B12, and often replaces a serving of red meat.
EPA and DHA are first made by marine microalgae, and sardines feed on plankton, taking them close to the source. Canned sardines have about 1.4–1.5 g of EPA and DHA per 100 g, among the highest of common foods.
Mechanism · At the source of the food chain
EPA and DHA are first made by marine microalgae. Sardines feed on plankton all their lives and take these fats straight from the source, instead of through several links of the food chain as large predatory fish do; they are also small and short-lived, so they build up little mercury.
Evidence · A can of fish and a few capsules
Gram for gram, canned sardines have about 1.4–1.5 g of EPA and DHA per 100 g, so one can roughly matches several standard fish-oil capsules and also brings protein, vitamin D, calcium and B12. That does not mean the evidence for eating fish is stronger than for capsules: people who eat more fatty fish have lower cardiovascular risk, but that is an association from observational studies, while randomized trials of fish-oil capsules in the general population have mostly been negative. The two kinds of study answer different questions.
synergy · 4
All fats are told apart by the same structure: how long the carbon chain is, how many double bonds it has, and where the first one sits. Omega-3 fats are the polyunsaturated family whose first double bond sits at the third carbon from the methyl end, so the rules for classifying fats are also where understanding omega-3 starts.
Highly unsaturated fatty acids such as EPA and DHA oxidize more easily, so cell membranes need more vitamin E to protect them. The more omega-3 you take in, the more vitamin E you need; fish-oil supplements themselves may already be oxidized in the bottle.
In the REDUCE-IT trial, a high-dose pure EPA drug (icosapent ethyl, 4 g a day) cut the relative risk of major cardiovascular events by 25% in high-risk patients already on statins with raised triglycerides; ordinary-dose fish oil showed small or no effects in the VITAL and ASCEND trials. Dose, form and how high the patient's risk was to begin with explain the difference.
The Mocking 2016 meta-analysis pooled randomized trials in major depression: omega-3 supplements gave a small-to-moderate overall improvement, larger at higher EPA doses and in people already taking an antidepressant. The Mediterranean-style diet used in the SMILES trial (2017) also included eating more fish.
cofactor · 3
DHA is not just an anti-inflammatory molecule but a building material for cell membranes: about half the membrane fatty acids in the retina's rod outer segments are DHA, and about 15–20% in the brain's gray matter. It keeps membranes flexible and affects how receptors and ion channels work. A fetus's developing brain and vision rely heavily on this supply.
Plant alpha-linolenic acid (ALA) has to pass through the enzyme delta-6 desaturase to be lengthened into EPA and DHA, and the omega-6 linoleic acid in the diet queues for the same enzyme. Adults convert only about 5–10% of ALA to EPA and often less than 1% to DHA, so the practical source of the long-chain EPA and DHA the body needs is fish or algae oil; flaxseed cannot stand in.
Mechanism · Two fatty acids, one enzyme
Once inside a liver cell, ALA and linoleic acid queue for the same enzyme, delta-6 desaturase, and whichever is more plentiful takes more of it. Estimates from US food-supply data put the ratio of linoleic acid to ALA at about 6.4 in 1909 and about 10 in 1999 (Blasbalg 2011); by the mechanism, this rate-limiting enzyme is mostly occupied by linoleic acid, and ALA conversion gets crowded out. The long-chain EPA and DHA stall at this step: not because the body cannot convert, but because the workstation is taken. Both pathways share the same chain of enzymes, delta-6 desaturase, elongase and delta-5 desaturase, so they are competing for one set of equipment rather than working in parallel.
blasbalg-2011-ajcn-la-trendsNumbers · How much ALA gets converted
Even when the enzyme is free, the pathway itself is narrow. Burdge and Calder 2005 pooled human tracer studies: dietary ALA converts to EPA at about 5–10% and to DHA often at less than 1%, and a considerable share of ALA is simply burned for energy. So eating more flaxseed does not raise long-chain omega-3 in the body in proportion. Burdge 2006 puts it plainly: the conversion of dietary ALA is limited.
In practice · Getting around the conversion
The way around the whole conversion pathway is to eat EPA and DHA directly: fatty fish twice a week, or algae oil if you do not eat fish. When EFSA assessed dietary fats in 2010, it explicitly declined to set an omega-6 to omega-3 ratio and instead gave separate adequate intakes: linoleic acid at 4% of energy, ALA at 0.5%, and 250 mg of EPA plus DHA a day for adults. Get enough long-chain omega-3 and the ratio falls on its own; pushing omega-6 down to zero is unnecessary, and no human evidence shows it helps. Flaxseed can still supply ALA; it just cannot stand in for fish.
Every phospholipid in your cell membranes is one head plus two tails: the tails come from the fat you eat, and the head is most often choline. Your omega-3 intake affects how fluid the membrane is, and choline decides whether the membrane can be built at all; short on either, it does not get made properly.
regulates · 2
←After birth and breastfeeding
The fatty acid make-up of breast milk follows what the mother eats. In an analysis of 65 studies, breast-milk DHA ranged from 0.06% to 1.4% of fatty acids, with the highest levels in coastal populations who eat a lot of seafood.
Period pain is driven mainly by prostaglandins released from the uterine lining, and EPA and DHA shift the prostaglandins the body makes toward the less inflammatory side, acting on the same pathway as painkillers such as ibuprofen, only much more slowly and weakly. A small crossover trial saw period pain ease, but a Cochrane review judges the evidence for fish oil to be very limited.
contrast · 2
In cell experiments, curcumin can suppress inflammatory signals such as NF-κB and COX-2; in the body, EPA and DHA can be turned into molecules that help inflammation resolve (resolvins and protectins). They act at different points and are often sold together as an anti-inflammatory combination, but good human trials of whether the pair works better in chronic inflammatory disease are lacking.
Do not give up fish because of microplastics: the omega-3 fats, protein and vitamin D in fish are clear benefits, while the health effects of microplastics are still unclear. To cut exposure, remove the guts of small fish you would eat whole, rather than crossing a whole food group off the list.