Using the inverse variance weighting method, MR-Egger regression, and weighted median method, we evaluated the causal relationship between four fitness supplements and their related metabolites with myasthenia gravis risk. The inverse variance weighting analysis showed that genetically predicted glutamine levels were significantly inversely associated with late-onset myasthenia gravis risk (OR=0.64, 95% CI: 0.46 to 0.87, P=0.005), indicating that each standard deviation increase in glutamine levels was associated with a 36% reduction in the risk of late-onset myasthenia gravis. The weighted median method (OR=0.63, 95% CI: 0.42 to 0.94, P=0.024) and MR-Egger regression yielded effect estimates consistent in direction.