A recent placebo controlled trial, published in the Journal of Strength and Conditioning Research, examined the effect of 15 days of betain supplementation on peak concentric and eccentric force during isokinetic bench press exercise. Subjects received either betain or placebo over 15 days and during this time were asked to visit the lab 5 times (separated by 72 hours between visits) for exercise testing sessions. After each exercise session subjects were asked to rate their muscle fatigue and soreness. At the end of 15 days subjects were asked to visit the lab once a week for 4 weeks without betain or placebo. After this washout period, subjects were given the opposite treatment (betain or placebo) for 15 days and the exercise testing protocol was repeated. The researchers reported no significant effect of betain versus placebo on maximum or concentric or eccentric force. However subjects reported a significant reduction in fatigue with betain supplementation.
Hoffman JR, Ratamess NA, Kang J, Gonzalez AM, Beller NA, Craig SA. Effect of 15 days of betaine ingestion on concentric and eccentric force outputs during isokinetic exercise. J Strength Cond Res. 2011 Aug;25(8):2235-41.