Omega-3 for Weight Loss: What Trials Show

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Omega-3 for weight loss has a thin, mixed evidence base. Pooled analyses of randomized trials disagree on body weight. One, in adults with overweight or obesity, found no difference between omega-3 and placebo, and another found people taking fish or fish oil lost about 0.6 kg (1.3 pounds) more. Both found a slightly smaller waist. Fish oil is not a weight loss drug, and the research does not show that it works as one.

I’m Judy, not a registered dietitian or a medical doctor. I started GLP-1 injections in May 2024, and I review supplements on this site. So this page sticks to what the studies found, who was studied, and where the research runs out. It also covers what omega-3 does have a stronger case for, how much the trials used, and who should check with a clinician first.

Table of Contents

Does omega-3 help with weight loss?

Not reliably. A 2017 meta-analysis of randomized trials in the Journal of Nutrition, Health and Aging pooled 11 trials with 617 adults who had overweight or obesity. Across nine of those trials, fish oil and placebo produced the same weight change (a difference of 0.00 kg, with a confidence interval running from about minus 0.4 kg to plus 0.4 kg). Waist circumference came down slightly more with omega-3, by about half a centimeter, and triglycerides fell. The authors called their results inconclusive because the trials were few and of poor quality.

That is the honest headline. Omega-3 does something for blood fats and maybe waist size in some studies. Its effect on body weight is somewhere between nothing and very small.

What do studies on omega-3 and weight loss actually show?

Pooled trials show a tiny effect at most

A separate 2014 systematic review and meta-analysis in Obesity Reviews looked at fish or fish oil and body composition in randomized trials. People taking fish or fish oil lost about 0.59 kg (roughly 1.3 pounds) more than controls, and their waist was about 0.8 cm smaller. Fat mass and lean mass did not differ between groups. The authors said further research was needed to confirm or refute the findings.

A pound or so over the length of a trial is not what people usually mean when they search for help with weight loss.

A systematic review found no consistent pattern

A 2021 systematic review in Clinical Nutrition ESPEN looked at 20 human studies. Eleven found no effect of omega-3 on weight or body fat and nine found some benefit. Four studies reported lower body weight in women. The authors concluded there is no consistency in the literature that omega-3 helps with weight loss or body fat in humans, partly because the studies differed so much.

One small trial alongside a diet

One small trial that did find a benefit is this 2022 randomized trial in Nutricion Hospitalaria. It had 40 adults aged 30 to 60 with a BMI between 27 and 35. Everyone followed a weight loss diet. Half also took 1,020 mg of omega-3 daily (580 mg EPA and 390 mg DHA) for 12 weeks. Weight, waist and BMI fell in both groups. The omega-3 group lost more abdominal fat mass and percentage than the control group.

That is a result worth knowing about. It is also one trial of 40 people over 12 weeks, so it can’t tell you what would happen to you.

How much omega-3 per day for weight loss?

There is no established dose for weight loss, and I won’t tell you what to take. What I can tell you is what the research used. The trials in the 2017 meta-analysis gave 2 to 6 grams of omega-3 capsules a day, and treatment lasted 3 to 24 weeks. The small 2022 diet trial above used about 1 gram a day.

For safety limits, the NIH Office of Dietary Supplements omega-3 fact sheet says the FDA has concluded that supplements providing no more than 5 g a day of EPA and DHA are safe when used as recommended. It also says the European Food Safety Authority has stated that long-term intakes up to about 5 g a day appear safe. The same fact sheet notes that two large trials found 4 g a day of omega-3 supplements slightly increased the risk of atrial fibrillation in people with cardiovascular disease or at high risk of it.

Whether a supplement is right for you, and at what amount, is a conversation for your clinician or pharmacist.

Does fish oil help you keep muscle while you lose weight?

This one matters if you’re on a GLP-1 medicine and worried about muscle. The evidence is insufficient. A 2026 meta-analysis in Nutrition pooled randomized trials of omega-3 supplements in healthy adults without obesity. These trials were not in people losing weight or taking a GLP-1 medicine, so they cannot answer the question directly. They found no clear effect on fat-free mass (four studies) or muscle strength (eight studies) and rated the certainty of the evidence as very low. If keeping muscle is your worry, my page on amino acids for weight loss covers the protein side.

What is the difference between EPA, DHA and ALA?

The NIH Office of Dietary Supplements fact sheet focuses on three omega-3 fats. It explains that ALA comes from plant oils such as flaxseed, soybean and canola, plus chia seeds and walnuts. EPA and DHA come from fish, fish oil and krill oil.

Your body can turn ALA into EPA and DHA, but the fact sheet says the conversion is very limited, with reported rates under 15%. Fish oil capsules contain EPA and DHA. A flaxseed oil capsule gives you ALA, which is a different thing. For people age 1 and older the fact sheet’s adequate intakes apply only to ALA, and for adult women who are not pregnant or breastfeeding it is 1.1 g a day. The fact sheet says no specific intake recommendations were set for EPA or DHA.

Is fish oil better than eating fatty fish?

The research I cited doesn’t compare them for weight, so I can’t say one is better for that. What the NIH fact sheet does say is that cold-water fatty fish such as salmon, mackerel, tuna, herring and sardines contain high amounts of long-chain omega-3s. Cod, tilapia and shellfish contain less. It also reports that the American Heart Association advises one to two servings of seafood a week for heart health, and does not recommend omega-3 supplements for people who don’t have a high cardiovascular risk.

The FDA’s advice about eating fish notes that the Dietary Guidelines for Americans recommend at least 8 ounces of seafood per week based on a 2,000 calorie diet, and less for children. If you are pregnant or breastfeeding, that same FDA page has its own guidance on which fish to choose because of mercury.

What are the side effects and risks of fish oil?

The ODS fact sheet says commonly reported side effects are usually mild. They include an unpleasant taste, bad breath, heartburn, nausea, stomach discomfort, diarrhea, headache and smelly sweat.

Two points deserve more attention. First, atrial fibrillation: as mentioned, the larger trials at 4 g a day found a slight increase in people with cardiovascular disease or high cardiovascular risk. Second, blood thinners. The fact sheet says that doses of 2 to 15 g a day of EPA and DHA might increase bleeding time. It says most research shows 3 to 6 g a day of fish oil does not significantly affect the anticoagulant status of people taking warfarin. Even so, it says people taking blood thinners and other regular medicines should discuss omega-3 with their health care providers, and it adds that the package inserts of prescription omega-3 products say patients taking anticoagulants should be monitored periodically.

None of the studies described here report testing fish oil in people taking a GLP-1 medicine, so they can’t tell you whether it adds anything on top of one. If you take a GLP-1, a blood thinner or any other medicine you take regularly, or if you’re pregnant or nursing, ask your prescriber or pharmacist before starting fish oil. See my GLP-1 weight loss guide for how those medicines work.

What does omega-3 have better evidence for?

High triglycerides. A 2017 meta-analysis found triglycerides fell more with omega-3 than placebo in adults with overweight and obesity. The NIH fact sheet reports that the American Heart Association concluded 4 g a day of prescription omega-3 lowers triglycerides, used alone or alongside other lipid-lowering medicine. The ODS fact sheet describes that finding as applying to high-dose prescription products used to manage high triglyceride levels. It is not the same thing as an over-the-counter capsule bought for weight loss.

Where does omega-3 fit among supplements for women over 40?

For weight, the evidence for omega-3 is weak. Fish oil fits better in a conversation about heart health and diet, and the NIH fact sheet describes conflicting trial results there too. If you want a plain look at which supplements have evidence for what, start with best supplements for women over 40, which does not cover omega-3. For another supplement, read berberine for weight loss, which has its own evidence and its own cautions.

Frequently Asked Questions

Can omega-3 help with belly fat?

Maybe a little. Two reviews found waist circumference about half a centimeter to 0.8 cm smaller with omega-3, but waist size is not a direct measure of belly fat, and the 2014 review found no difference in fat mass. One small 12 week diet trial found a bigger drop in abdominal fat percentage. The effects were small and the trials were small, so treat it as unproven.

How fast do you see results from omega-3 for weight loss?

The studies don’t show a reliable result to wait for. The trials pooled in the 2017 meta-analysis ran 3 to 24 weeks and found no difference in body weight between omega-3 and placebo. I wouldn’t expect the scale to move because of fish oil.

Does fish oil make you gain weight?

In the 2017 meta-analysis, weight change with omega-3 did not differ from placebo (within about 0.4 kg either way, over 3 to 24 weeks), so those trials did not show weight gain from it.

Should I take omega-3 while on Ozempic, Wegovy or another GLP-1?

That is a question for your prescriber. None of the studies described here report testing fish oil alongside a GLP-1, so they give no answer either way. Bring the exact product to your pharmacist so they can check it against your other medicines.

Is flaxseed oil as good as fish oil?

They are different omega-3s. Flaxseed oil provides ALA, and the NIH fact sheet reports that conversion of ALA to EPA and DHA is very limited, with rates under 15%. This page does not review any flaxseed oil weight study.

Do I need a supplement if I eat salmon?

The NIH fact sheet says the AHA advises one to two servings of seafood a week and does not recommend omega-3 supplements for people who aren’t at high cardiovascular risk. Your own diet, risk and medicines decide the answer, so it is worth a short talk with your clinician.

A note before you buy anything

I’m not a registered dietitian or a medical doctor, and nothing here is medical advice. Please talk with your doctor or pharmacist before adding fish oil, especially if you’re taking a GLP-1 medicine, a blood thinner, or any medicine you take regularly, or if you’re pregnant or nursing.

Sources

Omega-3 for Weight Loss: What the Trials Actually Show