HMB is thought to be the active form of leucine - an amino acid that plays a role in regulating protein metabolism. In theory, if you supply HMB as a supplement, you may be able to reduce muscle breakdown during intense exercise.
There is some evidence that HMB reduces muscle catabolism and may protect against muscle damage. For example, creatine kinase, an indicator of muscle damage, is reduced following exercise is subjects consuming HMB. This may indicate a reduced level of muscle damage and could lead to improved muscle function. Research in animals (cattle, pigs and poultry) and humans suggests that HMB can increase muscle mass and strength. Leucine is also a common additive to chicken feed (which is normally low in this amino acid) for the purpose of improving the muscle tissue and providing bigger chicken breasts for dinner
HMB has also been tested by NASA as a dietary approach to preventing the muscle wasting associated with prolonged spaceflight. Supplementation with 1.5-3.0 grams of HMB daily during weight training for 3 weeks increased muscle mass and strength and decreased the rise in exercise induced muscle damage. In one study, untrained subjects lifted weights for 4 weeks with or without 1.5 to 3 grams of HMB per day. The HMB supplements resulted in significant improvements in muscle mass and strength as well as significant decreases in muscle breakdown compared with placebo subjects. Even in trained athletes, HMB supplements of about 3 grams per day resulted in a significant increase in muscle mass and strength as well as a decrease in body fat.
One study looked at the effects of HMB supplements (as the calcium salt) on muscle breakdown (catabolism), strength and body composition during a resistance training program (7 hours per week for 4 weeks). Subjects were 40 experienced weight lifters who received either 3 or 6 grams per day of calcium HMB (or a placebo). Results showed that HMB supplementation resulted in a significant increase in blood levels of HMB, but no significant difference in muscle anabolic/catabolic status, lean body or fat mass, or overall muscle strength. Another study looked at HMB supplements (versus placebo) in 39 men and 36 women (aged 20-40 years). Subjects received 3 grams of HMB per day while training 3 times per week for 4 weeks. In the HMB group, blood levels of creatine phosphokinase (an indicator of muscle damage) were reduced compared to the placebo group, and both upper body strength and fat-free mass were increased. Overall, the study showed that a short-term period of HMB supplementation can increase upper body strength and minimize muscle damage when combined with an exercise program in both men and women.
No side effects have been reported in animal studies (which have used large doses of HMB for several weeks) or in human studies of as much as 4 grams per day.
Athletes trying to minimize protein losses and muscle breakdown may want to consider HMB - particularly during very high intensity periods of training.
Recommended dose depends on training intensity - from 1 gram/day on rest days and easy days to about 3 grams on heavy training days.