Beta-hydroxy-beta-methylbutyrate, or HMB, is a metabolite of the branched-chain amino acid (BCAA) leucine. Both of these compounds have been shown in scientific studies to offer muscle-building benefits to at least some lifters, putting them among the elite of performance supplements.

But is one better than the other? In particular, is supplemental HMB better for a muscle growth supplement stack than taking leucine by itself.

As we’ll see, the answer may depend on your goals and current situation.

Why Leucine and HMB Might Help You

Leucine is an essential amino acid, meaning that your body requires it but cannot make it from other nutrients. You MUST consume leucine directly from the foods you eat, with good sources including fish, poultry, egg whites, and lean meat.

Scientists discovered decades ago that leucine in supplemental form can enhance the rate of protein synthesis in mammals, and it has also been shown to limit proteolysis, or muscle breakdown, particularly in traumatic situations. Subsequent studies found that the proteolysis effect required the use more leucine than was required for accelerated muscle synthesis. This led to speculation that slowing proteolysis through leucine supplementation is due to one or more metabolites of leucine — byproducts of the protein synthesis mechanisms, perhaps.

Armed with this hypothesis, scientists began studying HMB, one of those metabolites.

Researchers from Iowa State found in a 1996 study that HMB did, indeed, reduce the extent of proteolysis experienced after weight training sessions when compared to the use of a placebo. Later studies, like this one from the United Kingdom in 2004, backed up that initial finding: HMB seems to stem proteolysis caused by muscle trauma.

As study into HMB continued, some researchers reported that there were also seeing enhanced muscle protein synthesis from its use, even when extra leucine was not included. In a 2008 literature review, scientists from the University of Illinois reported that multiple studies pointed to HMB as a compound capable of both limiting proteolysis AND boosting muscle synthesis. The net effect was a gain in lean body mass for the subjects in those studies.

Is HMB Better than Leucine?

Since HMB works in your favor on both sides  of the nitrogen-balance equation, you might conclude that it is a better supplement for inducing muscle growth than leucine by itself.

However, multiple studies, even some in that 2008 review mentioned above, have found NO positive results from taking HMB. Speculation about why that is so covers a range of reasons, but it seems likely that it’s due to the nature of HMB’s actions in the body.

HMB is at its best when you are primed to “burn” protein, either for energy or from the trauma of training or  disease. In particular, HMB is most potent at easing the extent of proteolysis under those conditions, helping you keep more of the proteins — muscle — you already have. As long as you are training really hard, or if you’re beginning a new routine, then HMB will probably help you.

If, on the other hand, you are in more of a maintenance mode, you may not be doing enough damage for HMB to make a difference. In that case, leucine may be a better bet.

In the end, it’s not a matter of HMB or leucine being better than the other, just different in terms of their actions.

Of course, before you introduce either supplement into your routine, you need to visit your doctor to ensure that your plans won’t jeopardize your health. If you check out fine, though, this duo just might help you get bigger and stronger.