What is HMB?
Beta-hydroxy-beta-methylbutyrate, or HMB, is a metabolite of the amino acid Leucine. Your body produces HMB from dietary leucine and some studies have shown that production is correlated with leucine intake (1). While you can get increase it through leucine supplementation, this is an inefficient process as only 5% of leucine is converted to HMB (2). With touted benefits such as increased lean body mass, improved strength, and improved body composition, let’s dive in and see exactly what the science says about HMB.
How Does HMB Work?
Molecular Mechanisms
Since HMB is a small molecule, it is important to explore what molecular mechanisms it is involved in. Firstly, it is well documented that HMB activates mTOR (3, 4, 5), a key component of muscle hypertrophy and also activates P70S6K, another critical component in muscle protein synthesis (5).
In addition to increasing signals for muscle growth, HMB also appears to reduce proteasome activity, which is responsible for muscle-protein breakdown (6, 7).
Does HMB Work?
Molecular mechanisms are one thing; results in the real world are another. Does HMB deliver any actual benefit? Let’s take a look at the research as a whole, and draw some conclusions.
Hypertrophy
Unlike many supplements, HMB has been tested in a variety of individuals: trained, untrained, male, female, old, and young. It has also been tested in different training modalities, with and without dietary intervention. This does 2 things: 1) give us the ability to draw more generalized conclusions, 2) makes data interpretation a nightmare and requires substantial thought and insight. Despite the heterogeneity of the studies, we can make some good conclusions from the data available.
In Untrained Individuals.
The research in untrained individuals has been conducted primarily for period of 4 and 8 weeks. In the studies that lasted 4 weeks, individuals who took HMB had bigger increases in fat-free mass (FFM) than those who took the placebo. When you look at studies that last 8 weeks, this effect is almost gone. Why is this? It is likely due to a problem with the research methodology.
Most of the 8 week studies did not include any type of periodization, which may have halted the growth of the HMB group which “adapted” sooner. They may have reached the same point because the stimulus for the HMB group at 4 weeks was similar to the placebo by 8 weeks (that is my conjecture). Also, in studies with untrained people, we see a dose response curve where increasing the dose from 1.5 g to 3 g led to greater increases in FFM. Currently, no research has systematically explored higher levels.
In Trained Individuals
Interestingly, we see the opposite effect of HMB with respect to time relating to the trained athletes, where it does not elicit greater improvements in the short-term, but does in the long-term.
There have been a host of poorly-controlled studies which give almost useless data for drawing conclusions, so I will focus on the well-controlled ones. A short-term (4 week) study that examined collegiate football players found no improvements in strength or lean mass with supplementation. In a longer-term study (7 weeks) using trained individuals, HMB supplementation led to a 4.5 kg (9.9 lb.) increase in the bench press, and a 3.2 kg (7 lb.) increase in their squat, when compared to placebo. Additionally, a recent study showed that HMB in a 12 week cycle showed clear benefits, and that it may be due to its effect during “over-reaching” phases. (8)
These studies led me to the conclusion illustrated below.
In Caloric Restriction
The idea of preserving muscle mass in a state of caloric restriction has given rise to numerous supplements, with BCAAs being the leader (although its efficacy here is questionable). Since HMB has been shown to reduce muscle catabolism, it would stand to reason that it might be effective in preserving muscle mass during times of caloric restriction. Unfortunately, only one study has been done to date (using a population of trained judo athletes), and they found that 3g of HMB supplementation had no real effect on lean mass (-1.6% in control versus -0.5% in the HMB group), and only slightly prevented decreases in power output (-11% in control versus -5% in the HMB group).
When you look at the actual data, the variations pretty much “cover the spread” to suggest no meaningful difference. Now, it is important to note that the study only lasted 3 days, which makes gleaning any real useable information difficult.
Conclusion
HMB appears to be most effective at augmenting gains in lean mass during early stages of training in untrained individuals and at improving strength over longer training periods in trained individuals. The evidence gathered thus far is preliminary, and while there is data to suggest it may be beneficial, more studies need to be done in a well-controlled manner to determine how much of an effect it has and whether it is repeatable in well-controlled conditions.