Omega-3: Oily Fish, Plant Sources and What Supplement Evidence Shows

Omega-3 is one of the few supplement categories where large, long-term randomised trials exist — and where the results have quietly changed what can honestly be said. The nutrients are genuinely important, UK intakes of oily fish are genuinely low, and the case for capsules is more nuanced than the shelf suggests.

Key points

• UK advice is to eat at least two portions of fish a week, at least one of which should be oily.
• DHA contributes to maintenance of normal brain function and normal vision, at 250 mg a day. EPA and DHA together contribute to normal heart function at 250 mg a day, and contribute to the maintenance of normal blood pressure and normal blood triglyceride levels at higher intakes.
• ALAClinical, the plant form, contributes to the maintenance of normal blood cholesterol levels at 2 g a day, and converts only poorly into EPA and DHA.
• Large trials have not shown that routine omega-3 supplements prevent heart disease in the general population.
• Oily fish intake in the UK is well below recommendations, so food is the first place to look.

What omega-3 fats are

Omega-3s are a family of polyunsaturated fats. Three matter nutritionally:

• ALAClinical (alpha-linolenic acid) — the essential one, meaning the body cannot make it. Found in plants.
• EPA (eicosapentaenoic acid) — mainly from marine sources.
• DHA (docosahexaenoic acid) — mainly marine; structurally important in the brain and retina.

The body can convert ALA into EPA and then DHA, but conversion is inefficient — commonly cited as a few per cent for EPA and under 1% for DHA, with some evidence that conversion is somewhat higher in women. This is the central reason plant-based diets are advised to consider an algal source of DHA.

What they do in the body

DHA is a major structural fat in neuronal membranes and the retina. EPA and DHA are precursors to signalling molecules involved in inflammation resolution and vascular function. Under Great Britain claims rules, permitted statements are specific: normal brain function and normal vision for DHA, normal heart function for EPA and DHA, and effects on blood triglycerides and blood pressure at defined higher intakes. Broad language about "reducing inflammation" or "protecting the heart" is not permitted, and would overstate the evidence.

Food sources and UK examples

Oily fish (EPA and DHA):

• Salmon
• Mackerel
• Sardines and pilchards
• Herring and kippers
• Trout
• Sprats
• Fresh tuna (tinned tuna does not count as oily, as the processing reduces omega-3)

One 140 g portion of salmon or mackerel supplies well above 250 mg of EPA plus DHA — for many people, two portions a week is the whole job done.

Plant sources (ALA):

• Flaxseed (linseed) and flaxseed oil
• Chia seeds
• Walnuts
• Rapeseed oil
• Hemp seeds
• Soya beans and tofu

Algal oil supplies DHA, and often EPA, directly from the source fish get it from — the practical option for vegans and vegetarians.

Fortified foods such as some eggs and spreads contribute modest amounts.

UK fish advice and safety limits

The NHS sets maximums for some groups because oily fish can contain low levels of pollutants:

• Girls, women planning a pregnancy, pregnant or breastfeeding: no more than two portions of oily fish a week, and avoid shark, swordfish and marlin. Limit tuna as advised.
• Boys, men, and women not planning pregnancy: up to four portions of oily fish a week.
• Pregnant women should not take cod liver oil or supplements containing retinol (preformed vitamin A) because of the vitamin A content.

Who may have lower intakes

• People who do not eat fish at all, including vegans and vegetarians
• People who dislike oily fish specifically, which is common
• People on restricted diets or with limited food budgets, since fresh oily fish can be expensive (tinned sardines and mackerel are inexpensive and count)

What research says about supplements

Cardiovascular prevention. This is the best-studied question. Large randomised trials and Cochrane reviews of omega-3 supplementation in general populations have found little or no effect on cardiovascular mortality or events. The picture differs from older observational studies of fish-eating populations, and the current position is that a capsule is not a substitute for a dietary pattern.

High-dose prescription therapy. Prescription-strength omega-3 preparations at doses of several grams a day are used clinically for hypertriglyceridaemia under medical supervision. Trials of these agents have produced mixed results and this is a treatment decision for a clinician, not a self-care choice.

Triglycerides. Reduction of blood triglycerides with EPA and DHA at 2–4 g a day is well documented and reflected in authorised claim conditions. Intakes at this level should be discussed with a healthcare professional.

Brain and vision. DHA's role in maintaining normal brain function and vision is authorised at 250 mg a day. Trials of omega-3 supplements for preventing cognitive decline in older adults have not shown consistent benefit.

Pregnancy. DHA maternal intake contributes to normal brain and eye development of the foetus and breastfed infants, at defined intakes in addition to the adult requirement. Pregnancy supplementation should be discussed with a midwife or GP.

Joints, mood and skin. Evidence is mixed and generally does not meet the bar for authorised claims. Statements suggesting relief of joint pain or improvement of mood are not permitted.

Safety, interactions and upper limits

• The Food Standards Agency has advised that supplementing with up to about 3 g a day of EPA and DHA combined is unlikely to be harmful for most adults.
• Common side effects are fishy aftertaste, reflux and loose stools. Taking capsules with food or freezing them can help.
• High doses may increase bleeding tendency. If you take warfarin, a DOAC or antiplatelet medicines, or are due surgery, check with your GP or pharmacist.
• Cod liver oil is not the same as fish oil — it contains vitamin A and vitamin DClinical, and vitamin A has an upper limit. Do not double up with a multivitamin without checking, and avoid it in pregnancy.
• Fish allergy: algal oil is an alternative.

Choosing a supplement responsibly

• Check EPA and DHA content, not capsule size. A 1,000 mg fish oil capsule may contain only 300 mg of EPA plus DHA.
• Match the dose to your goal. 250 mg a day of DHA covers the general maintenance claims; higher intakes belong in a conversation with a clinician.
• Vegan or vegetarian? Choose algal oil for DHA rather than relying on flaxseed for conversion.
• Look for purity and freshness testing — oxidised oil tastes and smells rancid.
• Avoid cod liver oil if you already take vitamin A or are pregnant.
• Prefer food where possible. Two portions of fish a week, one oily, remains the primary UK recommendation.

Frequently asked questions

How much oily fish should I eat a week?

UK advice is at least two portions of fish a week, at least one of which is oily. Women who are pregnant, breastfeeding or may become pregnant should have no more than two portions of oily fish a week.

Do omega-3 supplements prevent heart disease?

Large trials and systematic reviews have not shown that routine supplementation prevents cardiovascular events in the general population. The authorised claim relates to contributing to normal heart function at 250 mg a day of EPA and DHA, which is a different statement.

Can I get enough omega-3 from flaxseed or chia?

They supply ALAClinical, which is essential and useful, but conversion to EPA and especially DHA is limited. If you avoid fish, an algal DHA supplement is the more reliable route.

Is fish oil the same as cod liver oil?

No. Cod liver oil also contains vitamins A and D. Because vitamin A has an upper limit and is a concern in pregnancy, cod liver oil should not be taken during pregnancy or alongside other retinol-containing supplements without advice.

How much omega-3 is too much?

The Food Standards Agency has indicated up to around 3 g a day of EPA and DHA combined is unlikely to cause harm in most adults. Higher intakes, and use alongside blood-thinning medicines, should be discussed with a healthcare professional.

Which is better, EPA or DHA?

They have different authorised roles. DHA alone carries the brain and vision claims; EPA and DHA together carry the heart function claim. Most marine oils supply both.

Conclusion

Omega-3 fats are essential, UK oily fish intake is low, and the practical advice has not changed: eat fish twice a week, once oily, or use an algal DHA supplement if you do not eat fish. Capsules are a reasonable way to cover a gap in intake; they are not a proven substitute for a healthy dietary pattern, and the trial evidence for routine cardiovascular prevention does not support that framing.