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Oct. 10, 2000 -- Beta-hydroxy-beta-methylbutyrate -- with a name like that, you'd expect it to do something scientific. Better known as HMB, this substance, which is found in many protein supplements downed by gym rats, even has the OK of the folks on the International Olympic Committee. They categorize it as a legal substance that does not infringe on existing doping laws, and say it appears to have no harmful effects.

"Well, the International Olympic Committee tends to allow things that haven't been shown to work," Mark Juhn, DO, says with a laugh. "So I think their attitude is, 'Well, if you want to take something that hasn't been proven to work, go ahead.'" Juhn is a family and sports medicine doctor at the University of Washington in Seattle.

But does HMB, which is touted as being able to enhance gains in strength and lean muscle mass, really work? A review of studies of the supplement, recently published in the journal Sports Medicine, indicates that it may have a small effect -- but only on untrained athletes just starting a strength-training program. And many experts, including Juhn, are skeptical of its effectiveness.

"HMB has not been regarded as garnering much acceptance in the medical literature and amongst sports medicine professionals as being very convincing [for muscle building]," Juhn tells WebMD. "It definitely does not have very convincing data."

Juhn explains that HMB is a substance formed naturally when the body breaks down the amino acid leucine. Amino acids are the building blocks of protein, and proteins are the building blocks of muscle. "To make muscle, your body has to undergo what is called protein synthesis," Juhn says.

"The idea is that, theoretically, if you ingest your amino acids, you'll be able to increase protein and increase muscle mass," Juhn says. "But the other theory with it is that you can also help reduce the amount of muscle breakdown [caused by strength training] if you take the amino acids." This theory is problematic, though, because building new muscle requires some breakdown of protein within the muscle tissue, he says.

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