Does fish oil help with weight loss?

No, not in any way you would notice on a scale. The largest meta-analysis, 21 randomized trials in 1,652 overweight or obese adults, found fish oil did not lower body weight against control, with a standardized difference of -0.07 and a confidence interval from -0.21 to 0.07. A second meta-analysis of supplement trials in overweight and obese people put the difference in weight loss at 0 lb (0.00 kg). The one pooled analysis that found any benefit, mixing fish meals with capsules, put the extra loss at 1.3 lb (0.59 kg).

Myth. The idea that fish oil helps you slim is not backed by trials in overweight adults. Taken alone or added to a diet, the capsules did not move body weight in pooled randomized trials.

What the trials found

Adults with overweight or obesity

The best synthesis pooled 21 placebo-controlled trials in 1,652 adults and found no effect on body weight, with a standardized mean difference of -0.07 (95% CI -0.21 to 0.07). The reviewers found low heterogeneity, and no subgroup by dose, EPA to DHA ratio, duration or age changed the null result. The one signal they did find was at the waist. When fish oil was combined with a diet or exercise program, waist circumference fell slightly, with a standardized difference of -0.23 (95% CI -0.40 to -0.06). A standardized difference that size is small, and the authors treat it as a possible benefit that needs longer trials.

A separate meta-analysis of nine trials found a weighted mean difference in weight loss of 0 lb (95% CI -0.93 to 0.95; metric: 0.00 kg, -0.42 to 0.43) between omega-3 and placebo. Its authors call the trials few and of poor quality. A third, which pooled omega-3 taken together with vitamin E in 558 people, found body weight changed by 0.31 lb (95% CI -0.29 to 0.93; metric: 0.14 kg, -0.13 to 0.42), which is no difference either.

The positive outlier is an earlier meta-analysis of randomized trials of fish or fish oil. Participants lost 1.3 lb (0.59 kg) more than controls (95% CI -0.96 to -0.21). It pooled fish meals with capsules, and its abstract does not give the number of trials or people. Even taken at face value, it is about half a kilogram.

Fish oil on top of a diet

In a randomized trial of 128 adults with a BMI of 26 to 40, everyone got diet and exercise counseling and half also took 3 g a day of EPA plus DHA for 24 weeks. The omega-3 group lost 11 lb (95% CI 9.7 to 13; metric: 5.2 kg, 4.4 to 6.0) and the placebo group lost 13 lb (95% CI 11 to 15; metric: 5.8 kg, 5.1 to 6.7). In a smaller double-blind trial of 32 obese adults, fish oil taken with a very-low-energy diet for 4 weeks and then 10 weeks of maintenance had no significant effect on weight loss or on keeping it off.

Women with polycystic ovary syndrome

A meta-analysis of 11 trials in 816 women with PCOS found omega-3 failed to change body weight or BMI. The trials pooled marine and plant sources of omega-3.

Children and teenagers with overweight

Across 12 trials in 1,028 children and teenagers, fish oil lowered BMI by 0.96 kg/m2 (95% CI -1.69 to -0.23) and did not significantly reduce body weight or waist circumference. A BMI that falls while weight stays put may partly reflect children growing taller during the trials. A 2025 meta-analysis of 9 studies in 595 children and teenagers, rated with GRADE, also found omega-3 did not significantly affect weight or BMI z-score, and the reviewers judged 8 of the 9 studies at high risk of bias.

Appetite

Fish oil does not work as an appetite suppressant either. A meta-analysis of 15 controlled trials in 1,504 people found it significantly increased the desire to eat, with a standardized difference of 1.07 (95% CI 0.116 to 2.029). Many of those trials were in people with cancer or other illness, where a better appetite is the goal, so the result does not describe dieters directly.

Where omega-3 adds weight on purpose

In cancer, where losing weight is the danger, omega-3 is used to hold weight on. A network meta-analysis of randomized trials in 1,892 patients with cancer found combined DHA and EPA raised body weight, with a standardized difference of 1.29 (95% CI 0.40 to 2.16). In advanced lung cancer with cachexia, a meta-analysis of 5 clinical trials in 354 patients found a weight gain of 2.7 lb (95% CI 2.3 to 3; metric: 1.22 kg, 1.05 to 1.38) favoring omega-3. These are sick people losing muscle and fat, and the numbers say nothing about healthy dieters, but they make the point that fish oil is no fat burner.

Who should be careful

Across 8 large cardiovascular trials in 83,112 people, omega-3 supplements raised the risk of atrial fibrillation, an irregular heart rhythm, to 4.0% against 3.3% on control, a risk ratio of 1.24 (95% CI 1.11 to 1.38). The authors of that analysis found the risk was dose related, and doses tried for weight loss reach 3 g a day. Bleeding is the other worry people raise. Across 11 trials in 120,643 people, omega-3 did not raise overall bleeding, with a rate ratio of 1.09 (95% CI 0.91 to 1.31). Even so, bleeding is worth raising with a doctor for anyone on blood thinners or heading for surgery, bariatric surgery included.

Not for everyone. If you have had atrial fibrillation, or take a blood thinner, a high-dose fish oil bought to lose weight brings a known rhythm risk and no weight benefit to set against it. That is a conversation with your doctor.

What expert bodies say

We found no expert body that says fish oil helps with weight loss. On safety, EFSA concluded that supplemental EPA and DHA combined at doses up to 5 g a day do not raise safety concerns for adults, and it did not set an upper limit because the data were insufficient. That is a statement about safety. It is not an endorsement of fish oil for anything, weight included.

What to do with this

If you want to lose weight, fish oil capsules are money spent on something that did not work in the trials. Eating fish in place of fattier meat is a different question from taking capsules, and the one analysis that pooled fish meals with fish oil found about half a kilogram. If you take fish oil for another reason, such as high triglycerides, that case stands on its own evidence. More on that: fish oil and triglycerides.

How we searched

What would change this answer

• A large, long trial of fish oil in overweight adults with body fat measured by scan, sized to test the small waist signal. The waist results above come from short trials and standardized differences, and a search of ClinicalTrials.gov found no such trial for fish oil and weight loss.
• The Cochrane adiposity results. Two Cochrane reviews collected body fat outcomes, and we could not read the numbers. If they pointed the other way from the trials above, this page would need to say so.
• A trial that separates eating fish from taking capsules. The only pooled analysis that found any weight benefit mixed the two, so whether the food differs from the supplement has not been tested here.

Sources

1. ev-fowl-01 · Meta-analysis or systematic review · systematic review and meta-analysis of randomized placebo-controlled trials · n = 1,652
   The calculated results indicated that fish oil was not associated with a body weight reduction (SMD = -0.07, 95% CI -0.21 to 0.07, P = 0.31; Fig 2A) compared with controls.
   
   Who: adults 18 years or older with overweight or obesity; fish oil capsules, median 1.92 g/day n-3 PUFA, median 12 weeks
   Effect: body weight SMD -0.07 (95% CI -0.21 to 0.07, P = 0.31), 21 comparisons, I2 25.2%; fish oil alone SMD -0.004 (-0.20 to 0.19)
   Certainty: Best synthesis. Low heterogeneity; no subgroup (dose, EPA:DHA ratio, duration, age) changed the null result. Trials 4 to 24 weeks.
   PLoS One, 2015 · checked 2026-10-07 · we read the fulltext
2. ev-fowl-02 · Meta-analysis or systematic review · systematic review and meta-analysis of randomized placebo-controlled trials · n = 1,652
   However, waist circumference was significantly reduced (SMD = -0.23, 95% CI -0.40 to -0.06, P = 0.008) in those with fish oil supplementation combined with life modification intervention.
   
   Who: overweight or obese adults in randomized placebo-controlled trials
   Effect: waist circumference SMD -0.23 (95% CI -0.40 to -0.06, P = 0.008) with lifestyle program
   Certainty: Standardized mean differences; the authors call the abdominal-fat signal a possible benefit needing longer trials.
   PLoS One, 2015 · checked 2026-10-07 · we read the abstract
3. ev-fowl-03 · Meta-analysis or systematic review · systematic review and meta-analysis of randomized controlled trials · n = 617
   Based on the meta-analysis of nine studies, a statistically nonsignificant difference was revealed in weight loss between n-3 PUFA and placebo (p=0.99; weighted mean difference [WMD]: 0.00; 95% confidence interval [CI] -0.42 to 0.43), whereas n-3 PUFA was superior to placebo in reducing serum triglyceride levels (p=0.0007; standard median difference [Std MD]: -0.59; 95% CI -0.93 to -0.25).
   
   Who: overweight and obese adults in RCTs of n-3 PUFA supplements vs placebo
   Effect: weight WMD 0 lb (95% CI -0.93 to 0.95; metric: 0.00 kg, -0.42 to 0.43), 9 studies
   Certainty: Small number of poor-quality trials per the authors; results called inconclusive.
   J Nutr Health Aging, 2017 · checked 2026-10-07 · we read the abstract
4. ev-fowl-04 · Meta-analysis or systematic review · systematic review and meta-analysis of randomized controlled trials · n = —
   We found evidence that participants taking fish or fish oil lost 0.59 kg more body weight than controls (95% confidence interval [CI]: -0.96 to -0.21).
   
   Who: participants in randomized controlled trials of fish or fish oil intake
   Effect: body weight -1.3 lb (95% CI -2.1 to -0.46; metric: -0.59 kg, -0.96 to -0.21)
   Certainty: Mixed fish meals and capsules; number of trials and people not in the abstract. Even the positive estimate is about half a kilo, and two later meta-analyses of supplements alone found none.
   Obes Rev, 2014 · checked 2026-10-07 · we read the abstract
5. ev-fowl-05 · Randomized controlled trial(s) · single-institution placebo-controlled randomized clinical trial, intention to treat · n = 128
   No significant differences in weight loss were observed between the omega-3 (-5.2 kg; 95% CI: -6.0, -4.4 kg) and placebo (-5.8 kg; 95% CI: -6.7, -5.1 kg) arms.
   
   Who: overweight and obese adults (BMI 26 to 40) receiving dietary and exercise counseling; 5 capsules daily, 3.0 g EPA+DHA at 5:1 vs placebo
   Effect: weight change -11 lb (95% CI -13 to -9.7; metric: -5.2 kg, -6.0 to -4.4) omega-6.6 vs -13 lb (-15 to -11; metric: 3 vs -5.8 kg, -6.7 to -5.1) placebo; difference 1.3 lb (0.61 kg), P = 0.29; 27% dropout
   Certainty: Weight loss was the primary endpoint; 81 of 128 completed.
   Am J Clin Nutr, 2011 · checked 2026-10-07 · we read the abstract
6. ev-fowl-06 · Randomized controlled trial(s) · double-blind randomized controlled trial, two parallel groups · n = 32
   However, it would appear that supplementation with LCn-3PUFA had no significant effect on weight loss or weight maintenance over the 14 weeks.
   
   Who: obese adults on a 4-week very-low-energy diet then 10-week maintenance; 6 g/day capsules (420 mg EPA, 1,620 mg DHA) vs monounsaturated oil
   Effect: no significant effect on weight loss or weight maintenance over 14 weeks
   Certainty: Small trial (n 14 placebo, 18 fish oil at 4 weeks).
   Br J Nutr, 2012 · checked 2026-10-07 · we read the abstract
7. ev-fowl-07 · Meta-analysis or systematic review · systematic review and meta-analysis of randomized controlled trials · n = 816
   However, n-3 PUFA failed to change body weight, body mass index, high-density lipoprotein cholesterol, very low-density lipoprotein cholesterol and hs-CRP in the overall analysis.
   
   Who: women with polycystic ovary syndrome in RCTs of n-3 PUFA vs control
   Effect: body weight and BMI no change in the overall analysis
   Certainty: Marine and plant sources pooled; effects larger with more than 8 weeks of use.
   J Ovarian Res, 2023 · checked 2026-10-07 · we read the abstract
8. ev-fowl-08 · Meta-analysis or systematic review · systematic review and meta-analysis of randomized controlled trials · n = 1,028
   Compared to control, fish oil supplementation significantly reduced body mass index [BMI, mean difference (MD): -0.96 kg/m2, 95% confidence interval (CI): -1.69 to -0.23, P = 0.01] but did not significantly reduce body weight or waist circumference (P = 0.68 and 0.76).
   
   Who: overweight or obese children and adolescents in RCTs of fish oil vs control
   Effect: BMI MD -0.96 kg/m2 (95% CI -1.69 to -0.23); body weight P = 0.68; waist circumference P = 0.76
   Certainty: BMI falling without weight falling may partly reflect growth in height over the trials.
   Front Pediatr, 2021 · checked 2026-10-07 · we read the abstract
9. ev-fowl-09 · Meta-analysis or systematic review · systematic review and meta-analysis with GRADE · n = 595
   However, ɷ-3 PUFA supplementation did not significantly affect weight, BMI-Z score, Fasting blood sugar (FBS), insulin, Total cholesterol (TC), Low-density lipoprotein- cholesterol (LDL-C), and High-density lipoprotein-cholesterol (HDL-C).
   
   Who: obese or overweight children and adolescents in trials of omega-3 PUFA supplementation
   Effect: weight and BMI z-score no significant change; 8 of 9 studies high risk of bias
   Certainty: Newer than the corpus snapshot; confirms the pediatric pattern of ev-fowl-08.
   Nutrition & Metabolism, 2025 (omega-3 and cardiometabolic risk factors in obese/overweight children and adolescents) · checked 2026-10-07 · we read the abstract
10. ev-fowl-10 · Meta-analysis or systematic review · systematic review and network meta-analysis of RCTs (CINeMA) · n = 1,892
    Among 25 studies assessing body weight, the DHA&EPA supplementation group showed statistically significant improvements compared to the control group (standardized mean difference [SMD] = 1.29, 95% confidence interval [CI]: 0.40-2.16, P < 0.05), with a surface under the cumulative ranking (SUCRA) score of 48, suggesting modest superiority among compared interventions.
    
    Who: cancer patients in RCTs of EPA alone or DHA plus EPA vs control
    Effect: body weight SMD 1.29 (95% CI 0.40 to 2.16), moderate confidence; BMI SMD 0.53 (0.04 to 0.92), high confidence; lean body mass no effect
    Certainty: Used to counter cancer weight loss, not to slim; shows omega-3 is not a fat burner.
    Support Care Cancer, 2025 · checked 2026-10-07 · we read the abstract
11. ev-fowl-11 · Randomized controlled trial(s) · meta-analysis of clinical trials · n = 354
    There is a significant difference in change in weight (mean difference [MD]: 1.22, 95% CI: 1.05-1.38, P < .01) and HRQoL scores (Global Health [MD: 14.40, 95% CI: 9.22-19.59, P < .01] and Physical Functioning [MD: 10.38, 95% CI: 8.50-12.27, P < .01] subscales) favoring the omega-3 fatty acids group.
    
    Who: patients with advanced non-small cell lung cancer and cancer cachexia
    Effect: weight MD +2.7 lb (95% CI 2.3 to 3; metric: 1.22 kg, 1.05 to 1.38); lean body or skeletal mass MD 2.05 (-0.55 to 4.66), not significant
    Certainty: Few trials; the abstract calls the included designs clinical trials and does not state that they were randomized, so rated B, not A.
    Integr Cancer Ther, 2024 · checked 2026-10-07 · we read the abstract
12. ev-fowl-12 · Meta-analysis or systematic review · systematic review and meta-analysis of controlled clinical trials · n = 1,504
    However, the n-3 PUFA supplementation significantly increased the desire to eat (SMD = 1.07, 95% CI 0.116, 2.029, P = 0.02) compared to control.
    
    Who: participants in controlled clinical trials of n-3 PUFA supplementation vs control
    Effect: overall appetite SMD 0.458 (95% CI -0.327 to 1.242, P = 0.25); desire to eat SMD 1.07 (0.116 to 2.029, P = 0.02)
    Certainty: Many trials in cancer or ill patients, where higher appetite is a goal; not specific to dieters.
    Syst Rev, 2024 · checked 2026-10-07 · we read the abstract
13. ev-fowl-13 · Meta-analysis or systematic review · systematic review and meta-analysis of randomized controlled trials · n = 558
    There was no significant effect of omega-3 and vitamin E co-supplementation on body weight (BW) (WMD=0.14 kg; 95% CI: -0.13 to 0.42; p=0.297), and body mass index (BMI) (WMD=0.08, 95% CI: -0.01 to 0.16, p=0.073).
    
    Who: adults in RCTs of omega-3 1,000 to 4,000 mg/day plus vitamin E 400 IU vs control
    Effect: body weight WMD 0.31 lb (95% CI -0.29 to 0.93; metric: 0.14 kg, -0.13 to 0.42); BMI WMD 0.08 (-0.01 to 0.16); BMI rose in some subgroups (over 8 weeks, women over 50)
    Certainty: Fixed-effects model; co-supplement design, so not pure fish oil.
    Int J Vitam Nutr Res, 2023 · checked 2026-10-07 · we read the abstract
14. ev-fowl-14 · Meta-analysis or systematic review · systematic review and meta-analysis of randomized controlled trials with meta-regression · n = 83112
    In meta-analysis, a significant association was noted between n-3 FA treatment and risk of AF (4.0% vs 3.3%; RR 1.24, 95% CI 1.11-1.38, p = 0.0002).
    
    Who: participants in cardiovascular outcome RCTs with at least 1000 people and at least 1 year of follow-up
    Effect: AF 4.0% vs 3.3%, RR 1.24 (95% CI 1.11 to 1.38); <=1 g/d RR 1.12 (1.04 to 1.21); >1 g/d RR 1.51 (1.26 to 1.80)
    Certainty: Dose-related; relevant to anyone taking high doses for weight in hope of an effect.
    Cardiovasc Drugs Ther, 2021 · checked 2026-10-07 · we read the abstract
15. ev-fowl-15 · Meta-analysis or systematic review · systematic review and meta-analysis of randomized clinical trials · n = 120643
    There was no difference in the pooled meta-analytic events of bleeding among patients receiving omega-3 PUFAs and those in the control group (rate ratio [RR], 1.09 [95% CI, 0.91-1.31]; P=0.34).
    
    Who: patients in randomized clinical trials receiving omega-3 PUFAs
    Effect: bleeding RR 1.09 (95% CI 0.91 to 1.31); high-dose purified EPA: 50% relative, 0.6% absolute increase
    Certainty: Relevant for people on blood thinners or before surgery, including bariatric surgery.
    J Am Heart Assoc, 2024 · checked 2026-10-07 · we read the abstract
16. ev-fowl-16 · Position of an expert body · agency scientific opinion (news summary) · n = —
    It concludes that supplemental intakes of EPA and DHA combined at doses up to 5g a day do not raise safety concerns for adults.
    
    Who: adults in the EU
    Effect: EPA+DHA up to 5 g/day supplemental: no safety concern; data insufficient to set a UL
    Certainty: Doses tried for weight (about 2 to 3 g/d) sit below this level.
    EFSA, 2012, news: EFSA assesses safety of long-chain omega-3 fatty acids (Scientific Opinion EFSA Journal 2012;10(7):2815) · checked 2026-10-07 · we read the section