Portal et al. (Portal. 2011) investigated the effect of 3g HMB/day "on body composition, muscle strength, anaerobic and aerobic capacity, anabolic/catabolic hormones and inﬂammatory mediators in elite, national team level adolescent volleyball players (13.5–18 years, 14 males, 14 females, Tanner stage 4–5) during the ﬁrst 7 weeks of the training season". The results were unequivocal, yet not overtly impressive:
HMB led to a significant greater increase in FFM by skinfold thickness (56.4 ± 10.2 to 56.3 ± 8.6 vs. 59.3 ± 11.3 to 61.6 ± 11.3 kg in the control and HMB group, respectively, p < 0.001). HMB led to a significant greater increase in both dominant and non-dominant knee flexion isokinetic force/FFM, measured at fast (180°/sec) and slow (60°/sec) angle speeds, but had no significant effect on knee extension and elbow flexion and extension. HMB led to a significant greater increase in peak and mean anaerobic power determined by the Wingate anaerobic test (peak power: 15.5 ± 1.6 to 16.2 ± 1.2 vs. 15.4 ± 1.6 to 17.2 ± 1.2 watts/FFM, mean power: 10.6 ± 0.9 to 10.8 ± 1.1 vs. 10.7 ± 0.8 to 11.8 ± 1.0 watts/FFM in control and HMB group, respectively, p < 0.01), with no effect on fatigue index.With no measurable effect on aerobic fitness, anabolic hormone levels or inflammatory mediators, the use of HMB appears to be advisable only in power and strength athletes.
If you are a bodybuilder on a high protein diet, I would however doubt that you would see any benefits from additional HMB. A diet rich in quality protein (high BCAA and particularly leucine content) should provide you with adequate amounts of leucine to keep endogenous HMB levels elevated.