Beta-hydroxy-beta-methylbutyrate, or HMB, is a by-product of breakdown of the amino acid leucine and has been studied mainly for its effects on protein metabolism and muscle growth. HMB appears to exert its effects on muscle by modifying cholesterol metabolism.¹ One report on nine clinical studies concluded 3 grams of HMB per day for three to eight weeks resulted in an average drop in total cholesterol levels of 3.7% and a drop in LDL-cholesterol levels of 5.7%.²

HMB (beta hydroxy-beta-methylbutyrate) is a metabolite (breakdown product) of leucine, one of the essential branched-chain amino acids.  Biochemical and animal research show that HMB has a role in protein synthesis and might, therefore, improve muscle growth and overall body composition when given as a supplement. However, double-blind human research suggests that HMB may only be effective when combined with an exercise program in people who are not already highly trained athletes. Double-blind trials found no effect of 3 to 6 grams per day of HMB on body weight, body fat, or overall body composition in weight-training football players or other trained athletes.^3,4,5,6,7 However, one double-blind study found that 3 grams per day of HMB increased the amount of body fat lost by 70-year old adults who were participating in a strength-training program for the first time.⁸ A double-blind study of young men with no strength-training experience reported greater improvements in muscle mass (but not in percentage body fat) when HMB was used in the amount of 17 mg per pound of body weight per day.⁹ However, another group of men in the same study given twice as much HMB did not experience any changes in body composition.

  

The combination of glutamine, arginine, and the amino acid derivative, hydroxymethylbutyrate (HMB), may prevent loss of lean body mass in people with AIDS-associated wasting. In a double-blind trial, AIDS patients who had lost 5% of their body weight in the previous three months received either placebo or a nutrient mixture containing 1.5 grams of HMB, 7 grams of L-glutamine, and 7 grams of L-arginine twice daily for eight weeks.¹⁰ Those supplemented with placebo gained an average of 0.37 pounds, mostly fat, but lost lean body mass. Those taking the nutrient mixture gained an average of 3 pounds, 85% of which was lean body weight.