What is HMB?
HMB (beta-hydroxy beta-methylbutyrate) is a compound your body produces in small amounts when it breaks down the amino acid leucine. Leucine is the key trigger for muscle protein synthesis, and HMB is one of its byproducts. As a supplement it is sold to reduce muscle breakdown and protect muscle during hard training or dieting.
How it works
HMB is thought to work mainly by reducing muscle protein breakdown rather than dramatically increasing growth. This anti-catabolic effect is why it is most often discussed in the context of preserving muscle during a calorie deficit or in people new to training whose bodies are highly responsive.
What the research says
- Shows the clearest benefit in untrained beginners starting a program
- May help preserve muscle during an aggressive calorie deficit
- Limited benefit for trained lifters who already eat enough protein
- Some early studies showed dramatic effects that later research could not replicate
- For most experienced trainees, adequate protein makes HMB largely redundant
Effective dose
3g per day is the standard dose, often split into smaller amounts through the day. It comes in two forms, calcium HMB and a free acid form, with the free acid claimed to absorb faster. For most people eating enough protein, the practical benefit is small either way.
What to check on the label
Check for a full 3g daily dose. Be aware HMB is relatively expensive for what the evidence supports in trained people. If you are an experienced lifter hitting your protein target, your money is almost always better spent on the basics like creatine and protein.
Safety
HMB appears safe at the standard 3g dose with no significant side effects reported in studies. The main downside is cost relative to benefit for most users rather than any safety concern.