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

Omega-3 does not make you bigger in the sense most people searching this 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 daily serving of capsules holds about 10 kcal. For gym-goers the picture is smaller and more specific: in healthy 60 to 85 year olds, about 3.4 g of EPA and DHA a day for six months increased thigh muscle volume by 3.6% and handgrip strength by 2.3 kg, 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 supplement label supplies and above the level at which the risk of atrial fibrillation rises, so "more for the gym" is not the lesson.¹¹ Omega-3 is a fat the body builds into cell membranes, not a bulking agent or a hormone.

• 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

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 the same for BMI; 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 fish oil capsule is about 9 kcal and a two-capsule flaxseed 3-6-9 serving holding 1,386 mg of oil about 12 kcal. An earlier version of this page blamed omega-6 for weight gain; there is no evidence for that, and what are the dangers of omega-6 sets out what the trials actually found.

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, and 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 that asked the gym question directly gave 21 young men and women ten weeks of progressive resistance training with fish oil or placebo: lean mass rose 3.4% with fish oil and 2.4% without, a difference that was not significant, with a trend to lower fat mass that did not reach significance either.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%; the trial had 60 participants and 44 finished it.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, found in small trials, 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 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 in one table, and what are the side effects of omega-3, 6 and 9 covers 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 the finding needs confirming in randomised trials, and a snapshot of that kind 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 is a different product from the oils in these trials: it 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 trials above, all of EPA and DHA, 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 omega-3 family in full.

Frequently asked questions

Does omega-3 cause weight gain?

No. Pooled trials show no change in body weight or BMI, and a serving of capsules is about 10 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? A meta-analysis of randomized controlled trials. PLoS One, 2015. PubMed 26571503.
2. Zhang YY, et al. Efficacy of omega-3 polyunsaturated fatty acids supplementation in managing overweight and obesity: a meta-analysis of randomized clinical trials. Journal of Nutrition, Health and Aging, 2017. PubMed 28112774.
3. Tseng PT, et al. Omega-3 polyunsaturated fatty acids in sarcopenia management: a network meta-analysis of randomized controlled trials. Ageing Research Reviews, 2023. PubMed 37442370.
4. Smith GI, et al. Fish oil-derived n-3 PUFA therapy increases muscle mass and function in healthy older adults. American Journal of Clinical Nutrition, 2015. PubMed 25994567.
5. Cornish SM, et al. Effects of omega-3 supplementation alone and combined with resistance exercise on skeletal muscle in older adults: a systematic review and meta-analysis. Nutrients, 2022. PubMed 35684018.
6. Santo André HC, et al. The influence of n-3PUFA supplementation on muscle strength, mass, and function: a systematic review and meta-analysis. Advances in Nutrition, 2023. PubMed 36811583.
7. McGlory C, et al. Omega-3 fatty acid supplementation attenuates skeletal muscle disuse atrophy during two weeks of unilateral leg immobilization in healthy young women. FASEB Journal, 2019. PubMed 30629458.
8. Heileson JL, et al. The effect of fish oil supplementation on resistance training-induced adaptations. Journal of the International Society of Sports Nutrition, 2023. PubMed 36822153.
9. Lv ZT, et al. Omega-3 polyunsaturated fatty acid supplementation for reducing muscle soreness after eccentric exercise: a systematic review and meta-analysis of randomized controlled trials. BioMed Research International, 2020. PubMed 32382573.
10. Jensen TK, et al. Associations of fish oil supplement use with testicular function in young men. JAMA Network Open, 2020. PubMed 31951274.
11. Gencer B, et al. Effect of long-term marine omega-3 fatty acids supplementation on the risk of atrial fibrillation in randomized controlled trials of cardiovascular outcomes: a systematic review and meta-analysis. Circulation, 2021. PubMed 34612056.
12. Department of Health and Social Care. Great Britain nutrition and health claims (NHC) register. gov.uk.
13. NHS. Fish and shellfish: nutrition. nhs.uk.

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.