What it's actually good for
Betaine (trimethylglycine, or TMG) does two things with reasonable evidence behind them. First, it is a well-established methyl donor that lowers plasma homocysteine — this is biochemically straightforward and confirmed in multiple controlled trials. Whether lowering homocysteine independently reduces cardiovascular risk remains debated, but the metabolic effect itself is real. Second, a growing body of resistance training studies suggests that 2.5 g/day of betaine may modestly improve power output, work capacity, and body composition. The effect sizes are not large — think small but consistent improvements in force production and possibly favorable shifts in lean mass versus fat — and not every trial reaches significance. This puts betaine in solid B territory for performance: real mechanistic rationale, supportive but not overwhelming human data, and no major safety concerns at standard doses.