What it's actually good for
Betaine (trimethylglycine, or TMG) is a methyl donor your body already makes from choline and gets from foods like beets and spinach. Supplementally, it does two things with real evidence behind them, and one thing the marketing oversells. It reliably lowers plasma homocysteine — that part is settled biochemistry. It has a plausible, moderately supported case for improving power output during resistance training. What it does not reliably do, despite the pre-workout copy, is change your body composition.
What the research says
Lowering homocysteine (Grade A). Betaine feeds the enzyme BHMT, which converts homocysteine back to methionine. Craig's 2004 review in Am J Clin Nutr lays out the mechanism, and a body of RCTs confirms the effect is dose-dependent and reproducible — this is one of the most predictable things a supplement can do to a blood marker. The catch, and it's a real one: Olthof et al. (2005, PLoS Medicine) found that 6 g/day for six weeks lowered homocysteine but also raised LDL cholesterol by 0.36 mmol/L and triglycerides by 0.14 mmol/L versus placebo — an effect folic acid, the other standard homocysteine-lowering agent, does not share. So "lowers homocysteine" is not automatically "good for your heart"; the lipid trade-off has to be weighed too.
Power output and body composition (Grade B, and softer than the label suggests). Cholewa et al.'s 2018 systematic review and a 2013 RCT by Apicella et al. found that 2.5 g/day over two-plus weeks modestly improved force, power, and anabolic signaling markers in resistance-trained men. But a larger, more recent 2022 meta-analysis (Ashtary-Larky et al., Br J Nutr, 12 RCTs, 369 participants) found no significant effect on body mass, fat mass, fat-free mass, or body fat percentage — directly contradicting the body-composition claim on most betaine labels. Take the performance signal as real but small, and treat "burns fat" or "builds muscle" claims as not supported by the current pooled evidence.
How much, and which form
2.5 g/day, split into one or two doses, is what the performance trials use and is a reasonable default. Homocysteine-lowering studies have used 1.5-6 g/day; higher doses (6 g) are where the LDL effect shows up most clearly, so there's little reason to exceed 2.5-3 g/day outside of a specific medical indication. Betaine anhydrous powder is the standard form — cheaper and easier to dose precisely than pre-workout blends that bury it in a proprietary mix.
Safety & interactions
Well-tolerated up to 6 g/day in trials, with occasional GI upset and a fishy taste/odor as the main complaints. The real caution is the lipid effect above — if you're supplementing long-term, especially at higher doses, get a lipid panel and track it rather than assuming "lowers homocysteine" means "good for cardiovascular risk" outright. Not recommended without medical supervision for people with homocystinuria. This is informational, not medical advice — check with a clinician before starting.
How we picked the brand
A betaine product earns a spot when it's plain betaine anhydrous (no proprietary blend hiding the dose), GMP-certified, and independently tested. NOW Supplements' TMG is ConsumerLab-tested and priced without the pre-workout markup.