HMB

What it is

HMB (beta-hydroxy-beta-methylbutyrate) is a compound your body makes in small amounts when it breaks down leucine, an essential amino acid found in protein foods. Supplements provide far more than the body produces on its own.

How it works

HMB is a metabolite of leucine, the amino acid most associated with switching on muscle building. It's thought to work mainly by slowing muscle protein breakdown and, to a lesser degree, supporting muscle protein synthesis. That profile may explain why it seems most helpful when muscle is under threat — during aging, illness, or inactivity — and less so in healthy athletes whose muscles are already well supported by training and diet.

What it's been studied for

Commonly taken to preserve muscle during aging, illness, or bed rest, and to boost strength and recovery from training.

What the evidence shows

Modest for muscle preservation, weak for performance. Meta-analyses in older adults suggest HMB helps preserve muscle mass, especially during inactivity like bed rest, though effects on strength and physical function are less consistent; in younger, trained athletes, most meta-analyses find small or no benefits beyond training alone. The key limitation is that many trials are small, some combine HMB with other nutrients, and headline results from a handful of athlete studies haven't been consistently replicated.

Who should consider it

Older adults facing a stretch of reduced activity (such as recovery from illness, surgery, or a hospital stay), or those with age-related muscle loss, might reasonably discuss it with their provider, ideally alongside adequate protein and resistance exercise. Young, healthy people who train regularly and eat enough protein probably won't notice much difference. It's not a substitute for exercise, which remains the most powerful tool for keeping muscle.

Amounts commonly studied

Studies typically used 3 grams per day, usually split into two or three doses; trials in older adults ran from several weeks to a year.

How to take it

Studies typically used 3 grams per day, often divided into two or three doses taken with meals or around workouts. Benefits in older adults were seen over several weeks to months, and it seems to work best paired with sufficient protein and some form of resistance or physical training. If you're recovering from illness or surgery, talk to your provider or a dietitian about your overall nutrition plan.

Forms & absorption

Calcium HMB (HMB-Ca) is the most studied form; a free-acid form (HMB-FA) is absorbed faster but hasn't clearly outperformed it. HMB also appears in some medical nutrition drinks designed for older adults.

Food sources

None — HMB occurs only in trace amounts in foods (such as catfish, avocado, grapefruit, and cauliflower), and the body makes a little from dietary leucine, so studied doses require a supplement.

Safety & interactions

Generally well tolerated at studied amounts, including trials lasting up to a year in older adults; mild digestive upset is occasionally reported. Safety data in pregnancy, breastfeeding, and children are lacking, so it's best avoided there. People with kidney or liver disease should talk to their provider first, and unexplained muscle or weight loss deserves a medical evaluation rather than a supplement.

Common questions

Will HMB help me build muscle at the gym?

If you're young, healthy, and already training, probably not much. Meta-analyses in trained athletes generally find small or no benefits for strength or muscle beyond what training and adequate protein provide.

Is HMB useful for older adults?

That's where the evidence looks best. Trials suggest it may help older adults preserve muscle, particularly during bed rest or reduced activity, though improvements in strength and function are less consistent. It works best alongside enough protein and exercise, and your provider can help you judge whether it fits your situation.

Is HMB safe?

At studied amounts of about 3 grams a day, it has been well tolerated in trials lasting up to a year. Data beyond that, and in pregnancy or children, are limited.