Fish oil grew muscle without a single workout in most trials that checked

Omega-3 fatty acids have mostly been studied in athletes and physically active people, where they are pitched as a recovery or performance aid. The more intriguing question is what they do to muscle when there is no training stimulus at all: in sedentary people, or in those whose limbs are immobilised and who are therefore losing muscle by default. If a supplement could support muscle mass in the absence of exercise, that would matter to anyone facing enforced inactivity. This systematic review gathered the randomised trials that tested exactly that scenario.

The authors searched five databases following PRISMA guidelines for randomised controlled trials, at least single blind, conducted in sedentary humans, examining the effect of omega-3 polyunsaturated fatty acids on lean mass and muscle protein synthesis without any physical training. Seven studies covering a total of 192 individuals were included. The outcomes spanned three levels of evidence: skeletal muscle volume and mass, muscle protein expression including the fractional synthesis rate and signalling proteins such as mTOR, Akt and p70s6k, and skeletal muscle gene expression.

Five of the six studies that measured skeletal muscle volume and mass reported higher values with omega-3 supplementation. Both of the two studies that measured muscle protein expression found a beneficial effect on the fractional synthesis rate, the speed at which muscle protein is being built. The mechanistic picture was murkier: no effect was observed on the signalling proteins mTOR and Akt, p70s6k improved in only one study, and the two studies that measured gene expression found no effect of supplementation at all. The muscle appears to respond even while several of the expected molecular switches stay quiet.

This review carves out a genuinely underexplored corner of the omega-3 literature, which has concentrated on physically active populations. The consistency of the muscle mass findings across five of six studies is notable, but the total evidence base of 192 people is small, and the disconnect between the outcome and the usual growth signalling pathways leaves the mechanism unexplained. Open questions include whether the effect holds in larger trials, how it compares with or adds to resistance training, and why gene expression showed nothing at all.

This is a reason to think omega-3 does something real to muscle tissue even without a training stimulus, which is most relevant to periods when training is off the table: injury, immobilisation or stretches of enforced inactivity. It is not evidence that fish oil replaces lifting, and with 192 people spread across all seven trials, the finding is a foundation rather than a conclusion. For someone already training regularly, this review simply cannot say whether supplementation adds anything on top of the work in the gym.