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Thomson, JS, Watson, PE, and Rowlands, DS. Effects of nine weeks of β-hydroxy-β-methylbutyrate supplementation on strength and body composition in resistance trained men. J Strength Cond Res 23(3): 827-835, 2009-The dietary supplement β-hydroxy-β-methylbutyrate (HMB) is claimed to increase strength, lean body mass, and decrease fat mass when used in conjunction with resistance training. Although there is some support for these claims, the evidence is not conclusive, and it is even less so for resistance trained individuals. Therefore, we aimed to further elucidate the effects of HMB supplementation in trained men. A randomized, double-blind, controlled study design was used to investigate the effects of supplementing 22 resistance trained men with 3 g·d-1 of HMB or corn starch placebo for 9 weeks with resistance training. The effect of HMB on strength was determined using the 1-repetition maximum (1RM) method for the lower body (leg extension) and upper body (bench press, bicep preacher curl) at baseline and after the supplementation period. Body composition was assessed by skinfolds and bioelectrical impedance analysis (BIA). Overall, 9 weeks' HMB supplementation resulted in a clear-cut, trivial increase in combined averaged strength measures of 1.6% (90% confidence limits: ±4.3%). When considered in isolation, however, leg extension 1RM increased by a substantial 9.1% (90% confidence limits: ±7.5%), but the effect on upper-body strength was inconclusive (bench press: −1.9 ± 9.3%; bicep curl: −1.7 ± 4.7%). Based on BIA estimates, HMB had a decreasing (although inconclusive) influence on fat mass of −9 ± 14%, but it had a clear, trivial effect on fat-free mass of 0.2 ± 2.2%. The magnitude of change in body mass was trivial, but the probability of substantial reductions in skinfold thicknesses ranged from negligible to likely. In previously trained men, supplementation of HMB in conjunction with resistance training provides a substantial benefit to lower-body strength, but it has negligible effects on body composition.