Does Omega-3 Make You Bigger? Weight Gain, Muscle and the Trials

Omega-3 does not make you bigger in the sense most people worry about: in a pooled analysis of 21 randomised trials in overweight adults, fish oil changed neither body weight nor BMI, and a second analysis of 11 trials put the difference in weight at exactly zero.^{1, 2} A capsule holds about 9 kcal. For gym-goers the picture is smaller and more specific: one small trial in 60 to 85 year olds found a little more thigh muscle and grip strength after six months at about 3.4 g of EPA and DHA a day, pooled trials in older adults find a modest gain in lower-body strength with no reliable change in muscle mass, and in young adults who lift weights ten weeks of fish oil added no more lean mass than placebo.^{4, 5, 8} Those trial doses, 2 to 5 g a day, are several times what a label supplies and above the level where atrial fibrillation risk rises, so "more for the gym" is not the lesson.¹¹

• Weight: no change in body weight or BMI in pooled trials; a small reduction in waist circumference in some
• Muscle, older adults: small gains in lower-body strength and function at 2 g a day or more for six months; muscle mass itself inconsistent
• Muscle, young adults: resistance training builds it; fish oil did not add to the gain in the one controlled trial
• Soreness: less by under one point on a ten-point scale, below what people notice
• Testosterone: no trial shows omega-3 raises it; the register carries no hormone claim

Sources checked on 29 October 2026.

Does omega-3 increase body weight?

No. The 21-trial analysis in overweight and obese adults found a standardised difference in body weight of -0.07 and in BMI of -0.09; waist circumference fell slightly in people who combined fish oil with a diet and activity programme.¹ The 11-trial analysis found a weighted mean difference in weight of 0.00 kg.² A 2023 network analysis of 16 trials in older adults, including doses above 2.5 g a day, found no change in body weight, fat mass or muscle mass at any dose.³ Fat supplies 9 kcal a gram, so a 1,000 mg capsule of fish or flaxseed oil is about 9 kcal. What are the dangers of omega-6 covers the omega-6 side.

Does omega-3 increase growth?

Not in adults, and in children only in the sense that the essential fats are part of normal growth. The GB register's wording for ALA and linoleic acid is "Essential fatty acids are needed for normal growth and development of children", a maintenance statement conditional on 2 g ALA and 10 g linoleic acid a day; it does not mean more omega-3 makes anyone taller or larger; no trial has found that.¹²

Does fish oil help you get bigger?

A little, in older adults, at high doses; not measurably, in younger adults who train. In 20 healthy young women who had one leg immobilised for two weeks, 5 g a day of omega-3 started four weeks earlier halved the loss of muscle volume, 8% against 14%: a finding about protecting muscle during disuse rather than adding it.⁷ The one controlled trial to ask the gym question gave 21 young men and women ten weeks of resistance training with fish oil or placebo: lean mass rose 3.4% with fish oil and 2.4% without, not a significant difference, with a trend to lower fat mass that did not reach significance either, though bench-press strength rose more with fish oil.⁸ A 2023 pooled analysis of 14 studies in young and older adults found no effect on muscle mass (standardised difference 0.07) and a very small effect on strength (0.12), unrelated to age, dose or whether people trained.⁶

Older adults are where the signal sits. In the 60 to 85 year olds given about 3.4 g of EPA and DHA a day for six months, thigh muscle volume rose 3.6%, handgrip 2.3 kg and one-repetition-maximum strength 4%, with capsules donated by the manufacturer.⁴ Across 16 studies of over-55s, with or without resistance training, omega-3 did not change lean mass but improved lower-body strength (standardised difference 0.54) and sit-to-stand and timed-up-and-go tests, with no effect on walking or upper-body strength.⁵ The network analysis found the strength and function gains concentrated above 2.5 g a day.³ That is a modest, dose-dependent effect on function in people losing muscle with age, not evidence that a capsule adds size.

How much omega-3 for the gym?

There is no gym dose. The muscle and soreness trials used 2 to 5 g of EPA and DHA a day, prescription territory; across the large cardiovascular trials, doses above 1 g a day were associated with about a 50% higher relative risk of new atrial fibrillation, against about 12% at 1 g or less, from a low starting rate.¹¹ EFSA's safety ceiling for supplemental EPA and DHA is 5 g a day, and the register's heart wording applies at 250 mg.^{12, 14} Read the EPA and DHA milligrams on the back of the pack rather than the oil weight on the front: a "1,000 mg fish oil" capsule is about 300 mg of EPA and DHA. How much omega-3, 6 and 9 you need daily has the thresholds, and what are the side effects of omega-3, 6 and 9 has the atrial fibrillation data by dose.

Does omega-3 reduce muscle soreness?

Slightly, and below the level people can feel. Across 12 randomised trials, omega-3 reduced soreness two days after hard exercise by 0.93 points on a ten-point scale, short of the 1.4 points judged the smallest difference that matters, with no effect on strength or range of motion.⁹

Does omega-3 increase testosterone?

No trial shows that it does. The human evidence is a cross-sectional study of 1,679 young Danish men in which fish oil users had higher semen volume, larger testes and lower FSH and LH than non-users; its authors said it needs confirming in randomised trials, and a snapshot cannot show cause.¹⁰ The GB register carries no omega-3 claim about testosterone or any hormone.¹² If you suspect low testosterone, that is a blood test and a GP conversation, not a supplement decision.

What does omega-3 do for body composition, then?

EPA and DHA are built into the membranes of every cell, including muscle; the register's authorised wording for them is about normal heart function at 250 mg a day and normal brain function and vision at 250 mg of DHA, not about weight, fat or muscle; the DHA weight and body-fat claims submitted to the register were rejected.¹² A flaxseed 3-6-9 capsule supplies ALA, the essential plant omega-3, for which the register's cholesterol wording applies at 2 g a day, with linoleic acid, oleic acid and often vitamin E, which contributes to the protection of cells from oxidative stress, and no EPA or DHA, so the EPA and DHA trials above do not read across to it either way.¹² What changes body composition remains progressive resistance work, enough protein and enough sleep. What foods are high in omega-3, 6 and 9 covers the food side, and the Wellgard omega-3 guide covers the family in full.

What we don't know

The muscle trials are small: the trial behind the thigh-muscle figure had 60 participants, 44 of whom finished, and the gym trial had 21.^{4, 8} The pooled analyses disagree on strength, a standardised difference of 0.54 in over-55s against 0.12 across all ages, and no trial has tested whether dose or age explains the gap.^{5, 6} No randomised trial has measured testosterone in healthy men taking omega-3.¹⁰

Frequently asked questions

Does omega-3 cause weight gain?

No. Pooled trials show no change in body weight or BMI, and a capsule is about 9 kcal.^{1, 2}

Can cod liver oil cause weight gain?

No more than any other omega-3 oil; a capsule is about 9 kcal. Cod liver oil's own caution is its vitamin A, which is why the NHS says to avoid fish liver oil in pregnancy.¹³ Is there any danger when taking omega-3 has the detail.

Does fish oil help build muscle?

In older adults, high doses for six months or more gave small gains in strength and function in small trials; in young adults who train, the one controlled trial found no extra lean mass.^{4, 5, 8}

Does omega-3 burn fat?

Not in a way that shows up on the scales: weight and body fat were unchanged in the pooled analyses, with a slightly smaller waist in some.^{1, 2, 3}

Sources

1. Du S, et al. Does fish oil have an anti-obesity effect in overweight/obese adults? PLoS One, 2015. Meta-analysis, 21 trials; no conflicts declared. PubMed 26571503.
2. Zhang YY, et al. Omega-3 polyunsaturated fatty acids in managing overweight and obesity. Journal of Nutrition, Health and Aging, 2017. Meta-analysis, 11 trials, 617 people; funding not stated on PubMed. PubMed 28112774.
3. Tseng PT, et al. Omega-3 polyunsaturated fatty acids in sarcopenia management. Ageing Research Reviews, 2023. Network meta-analysis, 16 trials; no conflicts declared. PubMed 37442370.
4. Smith GI, et al. Fish oil-derived n-3 PUFA therapy and muscle mass and function in older adults. American Journal of Clinical Nutrition, 2015. Randomised controlled trial, 60 adults aged 60 to 85, 6 months; publicly funded (NIH), capsules donated by the manufacturer. PubMed 25994567.
5. Cornish SM, et al. Omega-3 supplementation alone and with resistance exercise on skeletal muscle in older adults. Nutrients, 2022. Meta-analysis, 16 studies, 2,438 adults over 55; funding not stated on PubMed. PubMed 35684018.
6. Santo André HC, et al. n-3PUFA supplementation on muscle strength, mass and function. Advances in Nutrition, 2023. Meta-analysis, 14 studies, 1,443 adults; funding not stated on PubMed. PubMed 36811583.
7. McGlory C, et al. Omega-3 supplementation and muscle disuse atrophy in young women. FASEB Journal, 2019. Randomised controlled trial, 20 women, 2 weeks of immobilisation; funding not stated on PubMed. PubMed 30629458.
8. Heileson JL, et al. Fish oil supplementation and resistance training-induced adaptations. Journal of the International Society of Sports Nutrition, 2023. Randomised placebo-controlled trial, 21 adults, 10 weeks; no conflicts declared. PubMed 36822153.
9. Lv ZT, et al. Omega-3 supplementation for reducing muscle soreness after eccentric exercise. BioMed Research International, 2020. Meta-analysis, 12 trials, 301 people; no conflicts declared. PubMed 32382573.
10. Jensen TK, et al. Fish oil supplement use and testicular function in young men. JAMA Network Open, 2020. Cross-sectional study, 1,679 men; publicly funded; no conflicts declared. PubMed 31951274.
11. Gencer B, et al. Marine omega-3 supplementation and the risk of atrial fibrillation. Circulation, 2021. Meta-analysis, 7 trials, 81,210 people, 4.9 years; publicly funded (NIH). PubMed 34612056.
12. Department of Health and Social Care. Great Britain nutrition and health claims (NHC) register. Page last updated 19 May 2026. gov.uk.
13. NHS. Fish and shellfish: nutrition. Page last reviewed 26 June 2026. nhs.uk.
14. EFSA. EFSA assesses safety of long-chain omega-3 fatty acids. Published 27 July 2012. efsa.europa.eu.

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This article is general information, not medical advice. If you have a heart rhythm disorder, take an anticoagulant or are considering omega-3 above 1 g a day, speak to your GP first.