HMB

Meta-analyses find small benefits in untrained subjects and clear anti-catabolic effects during bed rest, illness, and ageing. In trained athletes the effect is small to absent, and the largest positive trials have been methodologically criticised.

HMB is a case study in why "who was in the study" matters more than "did the study find something".

The compound is a leucine metabolite, and its distinctive property is that it works on the other side of the muscle balance sheet. Where leucine and protein drive synthesis, HMB mostly slows breakdown. That immediately tells you where it should help: any situation where muscle is being lost rather than built.

And there, the evidence is decent. Bed rest, immobilisation after injury, illness, cancer cachexia, and age-related muscle loss are the settings where HMB has produced its most convincing results — preserving lean mass that would otherwise disappear. For an older adult recovering from a hip fracture, this is a genuinely reasonable intervention.

The problem is that HMB is sold almost exclusively to the group it helps least. In trained lifters, meta-analyses put the effect at small-to-nothing. The intuitive explanation is that anti-catabolic support is not the binding constraint for a well-fed lifter eating 2 g/kg of protein — breakdown was never the limiting factor.

The marketing rests heavily on a small number of trials reporting spectacular results in trained athletes, including gains that would outclass anabolic steroids. Those studies came predominantly from one research group, were never independently replicated, and have drawn sustained methodological criticism. Treat them as the outliers they are.

If you fit the profile where it helps — returning from injury, immobilised, over sixty and losing muscle, or dieting hard on low protein — 3 g daily split into three doses is the studied protocol, and it is safe and well tolerated. If you are a healthy trained lifter eating enough protein, the honest recommendation is to spend the money on food instead. Leucine from a normal protein intake already provides everything HMB is derived from.

How it works

HMB is a downstream metabolite of leucine — roughly 5% of dietary leucine converts to it. Rather than driving muscle protein synthesis the way leucine does, its main documented action is anti-catabolic: it suppresses the ubiquitin-proteasome and autophagy pathways that break muscle protein down.