Quick answer

Omega-3s are fats your body builds into its cell membranes. Most research is on EPA and DHA, found in fish and fish oil. US adults get about 90 mg a day from food. Supplements reliably raise blood levels; large trials in healthy adults at about 1 g a day found no clear effect on their main heart outcomes.^1,2,3

Key facts

The body turns less than 15% of the plant omega-3 ALA into EPA and DHA, so fish or fish oil is the practical way to raise them.¹
A typical 1,000 mg fish oil softgel holds 180 mg of EPA and 120 mg of DHA, so read the EPA and DHA lines on a label, not the total oil.¹
The US has set no daily amount for EPA or DHA. The Dietary Guidelines advise adults to eat at least 8 ounces of seafood a week.^4,5
FDA considers supplements with up to 5 g a day of EPA and DHA safe when used as recommended.^1,4
High doses can affect clotting, so ask first if you take a blood thinner.^4,6

What omega-3s are

The three main omega-3s are ALA, found mainly in plant oils, and EPA and DHA, found in fish and other seafood.⁴

ALA is an essential fat: the body cannot make it, so it has to come from food. EPA and DHA are called long-chain omega-3s because their carbon chains are longer, with 20 and 22 carbons against the 18 in ALA.^1,4

The body converts ALA into EPA and then DHA only in very small amounts, with reported rates under 15%.^1,4

Omega-3s are part of the membrane around every cell in the body. DHA levels are especially high in the retina, the brain and sperm cells.^1,4

Beyond their place in membranes, omega-3s are a source of energy, and the body turns them into signaling molecules called eicosanoids. These act on the heart, blood vessels, lungs, immune system and hormone glands.^1,4

What the evidence shows

Two claims pass our evidence standard. The first, about blood levels, is graded strong. The second is a nutrient-function statement, graded moderate. Under each is the amount the studies used, and the grade reflects how much human research there is and how well it agrees, not the size of the effect.

Raises EPA and DHA levels in the blood*^1,2

Strong evidence

In a five-month randomized trial in 115 healthy adults, 600 mg a day of EPA plus DHA from standard fish oil raised the omega-3 index (the share of EPA and DHA in red blood cell membranes) by 59%, with no change on placebo. NIH notes that all the common forms of omega-3 supplement raise blood EPA and DHA. Typical US intake from food is only about 90 mg a day.

Amount studied: 300 to 1,800 mg of EPA plus DHA a day; the 59% rise was at 600 mg a day for about five months

Provides EPA and DHA, which contribute to normal heart function*^1,3,7

Moderate evidence

This is the nutrient-function statement authorized in the EU, on condition that the daily intake is at least 250 mg of EPA plus DHA. It rests mainly on studies linking fish and omega-3 intake with heart health. It is not a promise of fewer heart problems: in the 25,871-person VITAL trial, about 840 mg a day did not improve the trial's main heart outcome.

Amount studied: At least 250 mg of EPA plus DHA a day, the EU condition for this statement

What the largest trial did not find matters as much as what passed.

In the VITAL trial, 25,871 US adults (men 50 and older, women 55 and older) took 1 g of fish oil a day, with 460 mg of EPA and 380 mg of DHA, or a placebo for a median of 5.3 years. The supplement did not significantly lower the rate of the trial's main combined heart outcome.^1,3

The one large trial with a clear result, REDUCE-IT, gave 4 g a day of a prescription EPA product to people under medical care, and a similar trial at the same dose, STRENGTH, found no benefit. Those results come from a different dose and a different group, so they are not carried over to a supplement here.^1,5

For some health outcomes the evidence for eating seafood is stronger than for taking omega-3 supplements. Other nutrients in seafood may play a part, some of its value may come from eating it in place of less healthful foods, and people who eat seafood tend to have healthier lifestyles.⁵

How much people take

Men need 1.6 g of the plant omega-3 ALA a day and women 1.1 g. These adequate intakes are the only official US targets for omega-3s in adults.^1,4

The European Food Safety Authority uses 250 mg a day of EPA plus DHA as its reference amount for adults.¹

Food gives most US adults well under that, about 90 mg a day. One typical fish oil softgel adds about 300 mg of EPA plus DHA. Prescription omega-3 medicines are a separate case, taken at 4 g a day under a doctor's care.^1,6

Food and supplement forms

Cold-water fatty fish such as salmon, mackerel, tuna, herring and sardines are rich in EPA and DHA. Leaner fish such as bass, tilapia and cod, and shellfish, have less.^1,4

Flaxseed, chia seeds, walnuts and canola oil supply ALA rather than EPA or DHA, and some brands of eggs, yogurt, milk and juice have DHA added.^1,4

Supplements come as fish oil, krill oil, cod liver oil and algal oil, the vegetarian choice, and they differ in how much EPA and DHA each serving holds. Flaxseed oil supplements supply ALA.^4,5

What's in ours

What’s in ours

Omega-3: Fish oil concentrate, 1,000 mg per serving, from the label

Side effects, medicines, and who should ask first

Side effects are usually mild: an unpleasant taste, bad breath, heartburn, nausea, stomach discomfort, diarrhea, headache and smelly sweat.^1,4,5

Two large trials found that 4 g a day for several years slightly raised the risk of atrial fibrillation, an irregular heart rhythm, in people with or at high risk of heart disease. Across seven large trials, the rise was larger above 1 g a day than at 1 g or less.^1,8

No tolerable upper intake level has been set for EPA and DHA. FDA has concluded that supplements providing no more than 5 g a day of EPA and DHA are safe when used as recommended, and the European Food Safety Authority considers long-term intakes of up to about 5 g a day to be safe.^1,4

Your doctor may want to check on you

High doses of omega-3s may cause bleeding problems when taken with warfarin or other anticoagulants, and fish oil can have antiplatelet effects at high doses. Fish oil might also prolong clotting time, measured as INR, in people taking warfarin. The label of a prescription omega-3 medicine says people who also take an anticoagulant or an antiplatelet medicine should be monitored periodically.^1,4,6

On the medicine's label

Ask first if you are allergic to fish or shellfish
Ask first if you take warfarin or another blood thinner, or an antiplatelet medicine such as aspirin
Ask first if you have atrial fibrillation, another heart rhythm problem or a bleeding disorder
Ask first if you have been prescribed omega-3 medicine for high triglycerides, because a supplement is not a substitute
Ask first if you are pregnant or nursing, or under 18

Questions people ask

What does omega-3 do for the brain?

DHA is a building block of cell membranes in the brain and the retina. But in trials of healthy older adults, memory and thinking scores did not differ between omega-3 supplements and placebo, so no brain claim is made here.¹

What does omega-3 do for skin?

Skin cells, like all cells, carry omega-3s in their membranes, and a true shortage causes rough, scaly skin with an itchy rash. That shortage is very rare in the US. The sources reviewed here include no trial of fish oil on the skin of people who already get enough, so no skin claim is made.⁴

What does omega-3 do for men?

The research does not single out men. VITAL and the dose-response trial behind the claims above enrolled both men and women, and the same two claims apply to both.^1,2

Can food give as much as a supplement?

Yes, if you eat fatty fish. Three ounces of farmed Atlantic salmon has about 1.8 g of EPA plus DHA, about as much as six typical softgels. People who eat little fish get about 90 mg a day.¹

Can omega-3 cause bleeding?

At high doses fish oil can make platelets less sticky and lengthen bleeding time, and NIH says high doses may cause bleeding problems in people taking warfarin or other anticoagulants. Ask your doctor before combining fish oil with a blood thinner.^1,4

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Sources

*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.

This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.