Omega-3 and fish oil: what the evidence says

Omega-3 fatty acids are a type of fat that's important for health. The most talked-about are EPA and DHA, found mainly in oily fish, plus ALA from some plant foods. Eating oily fish is recommended. For omega-3 supplements such as fish oil, the picture is more mixed than the adverts suggest: large trials have been largely neutral for preventing heart disease in the general population. This guide explains what the evidence does and doesn't support.

This is general information, not personalised medical advice. If you have a health condition or take medication, speak to a doctor or pharmacist before starting a supplement.

What are omega-3 fatty acids?

There are three main omega-3s:

• EPA and DHA — long-chain omega-3s found mainly in oily fish. These are the forms most linked to heart health.
• ALA — a plant omega-3 found in foods such as walnuts, flaxseed and rapeseed oil. The body can convert only small amounts of ALA into EPA and DHA.

The body uses omega-3s in cell membranes and many processes; DHA is also important for a baby's developing nervous system.

What does the evidence support?

Eating oily fish is recommended, and people who regularly eat it tend to have a lower risk of heart disease. But for omega-3 supplements, the evidence for preventing heart disease is mixed, and large trials have largely been neutral for general prevention.

The British Heart Foundation's position is clear: a balanced diet including oily fish provides omega-3 and other nutrients, and you don't need fish oil supplements unless a doctor prescribes them. UK guidance does not recommend routine omega-3 supplementation to prevent cardiovascular disease.

Does fish oil lower cholesterol?

This is a common misconception. Omega-3s mainly affect triglycerides (a type of blood fat), not LDL ("bad") cholesterol — in fact high doses can slightly raise LDL.

High-dose prescription omega-3 (see below) can meaningfully lower triglycerides when these are high. But that's different from lowering cholesterol, and it's different from over-the-counter fish oil capsules. If you're trying to manage cholesterol, the proven steps are diet, physical activity and, where appropriate, medicines such as statins prescribed by your doctor.

Should I eat oily fish or take a supplement?

A food-first approach has the strongest support. The NHS advises a balanced diet including at least 2 portions of fish a week, one of them oily. Oily fish includes salmon, mackerel, sardines, herring and trout.

Whole oily fish provides omega-3 alongside protein and other nutrients, and is the option backed by the most evidence. Supplements may be useful for people who eat little or no fish, but they aren't a guaranteed substitute and shouldn't be seen as heart-disease insurance.

Krill oil vs fish oil — is one better?

Krill oil is often marketed as better absorbed than fish oil, but the evidence that it's actually superior is weak and limited. Both supply EPA and DHA; krill oil tends to be more expensive and provides them in a different chemical form.

There's no good basis for paying a premium for krill oil on absorption claims alone. If you choose a supplement, the amount of EPA and DHA it provides matters more than whether it comes from fish or krill.

What are "omega-3 acid ethyl esters"?

This is the prescription form of omega-3. Omega-3 acid ethyl esters are concentrated, purified EPA and/or DHA prescribed by a doctor — most often to help lower high triglyceride levels — at doses higher than typical supplements. They are medicines, used under medical supervision, and are not the same as buying fish oil capsules off the shelf.

Is omega-3 safe, and who should be cautious?

For most people, omega-3 from food and standard supplement doses is well tolerated. Some points to be aware of:

• Blood thinning — high doses can have a mild blood-thinning effect, so take care if you're on anticoagulant or antiplatelet medicines, or before surgery, and check with a doctor.
• Side effects — fishy aftertaste, burping and mild stomach upset are common.
• Pregnancy — oily fish is beneficial in pregnancy, but limit it to no more than 2 portions a week, and avoid fish liver oil supplements (such as cod liver oil), which are high in vitamin A and can harm a developing baby. Follow NHS guidance on fish to avoid in pregnancy.
• Allergies — avoid fish or krill oil if you have a fish or shellfish allergy.

Omega-3 supplements are sometimes taken alongside other supplements such as turmeric or vitamin D; none of these is a substitute for a balanced diet or medical care. If you have heart disease, high triglycerides or take regular medication, talk to a doctor before starting omega-3.

This page provides general, evidence-based information. It is not a substitute for advice from a qualified healthcare professional.