Benefits
Athletic performance: strength, mainly lower body
A pooled analysis of 17 placebo-controlled trials (317 participants) found that betaine taken for at least 7 days moderately improved maximal strength (effect size 0.47), mainly in the lower body, where it only narrowly reached significance. Vertical jump improved only after one low-quality study was excluded, a tentative finding. Upper-body strength, sprint or throw power, and muscular endurance did not improve.
Body composition improvement
In a 6-week randomized trial of 23 strength-trained men, body fat percent, fat mass, and lean body mass improved significantly within the betaine (2.5 g/day) group but not the placebo group. The trial did not report a significant between-group difference in these body-composition measures, so the result should be read cautiously. A proposed creatine-synthesis mechanism is unconfirmed in humans.
Homocysteine reduction (a cardiovascular risk marker)
Betaine helps maintain healthy homocysteine levels by donating a methyl group that converts homocysteine to methionine via the enzyme BHMT, a pathway independent of B vitamins. In a 6-week placebo-controlled trial in healthy adults, 1.5, 3 and 6 g/day lowered fasting homocysteine by 12, 15 and 20 percent versus placebo. This is a blood-marker change: no trial has shown that betaine reduces heart attacks, strokes, or other cardiovascular events.
Methylation support and fatty-liver evidence (mixed)
Betaine is a methyl donor used in phosphatidylcholine synthesis and SAMe regeneration in the liver. Human evidence in fatty liver disease is mixed: a 12-month randomized placebo-controlled trial of high-dose betaine (20 g/day) in 55 NASH patients did not improve steatosis, fibrosis, or the NAFLD activity score versus placebo. More recent pilot trials at lower doses (2 to 8 g/day) reported reductions in ALT and other blood markers of liver injury, but these were small, mostly within-group comparisons rather than placebo-controlled results. Benefit on hepatic fat is not established.
Mechanism of action
BHMT-mediated homocysteine remethylation
Betaine donates a methyl group to homocysteine via betaine-homocysteine methyltransferase (BHMT) in the liver and kidneys, converting potentially harmful homocysteine back to methionine. This B-vitamin-independent remethylation pathway complements the folate/B12-dependent methionine synthase route, providing a backup mechanism particularly important when folate or B12 are insufficient.
Cellular osmolyte protection
Betaine accumulates intracellularly as an organic osmolyte, protecting cells from hypertonic stress by balancing water activity without disrupting protein structure. In muscle, this may help maintain cell hydration and protein stability during intense exercise, one proposed explanation for the modest strength effects seen in some trials.
Creatine synthesis substrate provision
Betaine indirectly supplies methyl groups (via methionine and SAMe) for guanidinoacetate methylation, the final step in creatine synthesis. This has been proposed as one route to its ergogenic effects, but in a 10-day randomized placebo-controlled trial in untrained people, betaine (2 g/day) did not raise muscle phosphocreatine while creatine did, so the mechanism is unconfirmed in humans.
Clinical trials
Randomized, double-blind, placebo-controlled crossover study in 12 recreationally trained men examining 14 days of betaine supplementation (1.25 g twice daily, 2.5 g total) on strength and power outcomes including bench press throw power, isometric bench press force, vertical jump power, and isometric squat force. (Lee et al. 2010, J Int Soc Sports Nutr)
12 recreationally trained men (mean age 21). 14-day intervention, within-subject design.
Bench throw power and isometric bench press force rose versus both pre-supplementation and placebo values; vertical jump power and isometric squat force rose versus pre-supplementation values only. No change in jump squat power or rep counts. Effects were more apparent in upper-body movements. The sample was small (n=12), and a later meta-analysis found betaine's strength benefit is mainly in lower-body movements.
Randomized, placebo-controlled trial in 23 resistance-trained men receiving betaine (2.5 g/day, n=11) or placebo (n=12) during a 6-week periodized training program. Outcomes: body composition (body fat %, lean body mass, fat mass), arm and thigh size, bench press and back squat 1RM, vertical jump, training volume, urinary homocysteine thiolactone (HCTL). (Cholewa et al. 2013, J Int Soc Sports Nutr)
23 resistance-trained men. 6-week intervention.
Body composition (body fat percent, fat mass, lean body mass) and arm size improved significantly within the betaine group but not the placebo group, and bench press work capacity was greater in betaine at two of three microcycles. Vertical jump power showed only a trend (p=0.07). Betaine attenuated the rise in urinary homocysteine thiolactone. There were no significant between-group differences in maximal strength (1RM bench press or squat).