HMB is often marketed as a magical muscle building supplement. Specifically, HMB is claimed to have anabolic properties that can increase muscle growth. It is often promoted as a quick fix to build muscle easily. But does the evidence support the hype? Should athletes start supplementing with HMB? This blog provides an update on a previous blog to outline whether HMB does actually help to build muscle.
What is HMB?
Many athletes turn to supplements to promote training adaptations and/or improve competition performance. Some supplements, such as creatine, have been shown to enhance training adaptations and are popular among athletes. While other, less evidence-based, supplements are also used. ß-hydroxy-ß-methylbutyrate, or HMB, is a metabolite of the essential amino acid leucine. That is, when we consume leucine, a possible by-product is HMB. Given the important role of leucine in muscle building processes, HMB gained popularity as a possible anabolic compound. As a supplement, HMB is available in two forms: free acid or calcium bound. But does HMB increase muscle growth? Can HMB activate muscle building processes?
What is the rationale for HMB?
Processes, such as muscle growth, are regulated by signals, and a series of underlying events (1). As subtle increases or decreases in muscle growth can be difficult to detect, researchers will often use techniques to estimate changes in muscle growth. These signals and events change in response to certain stimuli, such as the consumption of anabolic compounds and/or strength training, to “turn on” muscle building processes. Put simply, signals act as a green light for the body to start increasing the synthesis of new muscle proteins, which can lead to muscle growth over weeks, months, and years.
HMB has been shown to increase anabolic signalling and protein synthesis in cells. In order to build muscle, we must be in a positive protein balance. That is, we are synthesising more muscle proteins than we are breaking down (2). Therefore, under controlled conditions (e.g., in a petri dish) HMB shows promise for muscle growth. But does this lead to increased muscle building in humans?
Can HMB increase muscle building processes?
To answer this question, we must first look at the processes underlying muscle growth. Just because a supplement can stimulate muscle building processes, does not mean this will lead to greater muscle growth, as we discussed previously with BCAAs. There can often be a mismatch between underlying processes and outcomes athletes are interested in, such as building muscle and increasing strength. Even if anabolic compounds stimulate anabolic signalling, sufficient substrate is required to promote muscle building. And even if proteins are being formed, they also need to be stable and not broken down. Consider the following scenario: you are trying to build a wall. You have bought the best equipment, and the best workers, but you only have a few bricks. No matter how good your cement is, or how efficient your workers are, if you don’t have enough bricks at the right time and in the right place, the building of your wall will be slower. In the context of muscle growth, signalling needs to be activated, but there must be enough bricks (i.e., amino acids) available to build your wall (i.e., new muscle proteins).
Just because a supplement can stimulate muscle building processes, does not mean this will lead to greater muscle growth.
With this in mind, we turn to a few studies that have looked at the short-term response to HMB. At a dose of ~2.5 g of free acid HMB, Wilkinson et al (3) found HMB increases muscle building processes in humans. That is, HMB increased anabolic signalling and muscle protein synthesis, with minimal effects on muscle protein breakdown. The same group confirmed these findings with the calcium bound form of HMB, showing ~3 g of HMB also increases anabolic signalling and muscle protein synthesis (4). However, the authors found leucine alone increased muscle building processes greater than HMB (3). Considering the anabolic response to isolated compounds, such as leucine or BCAAs, is typically lower than that compared to intact protein sources providing all essential amino acids, this begs the question: is HMB more effective than protein alone? Many studies compared HMB with no HMB (and no other protein or amino acids source). Does HMB offer any additional benefit to muscle growth beyond that seen with sufficient protein intake?
Can HMB increase muscle growth?
As aforementioned, HMB may increase muscle building processes in the short-term (3, 4), but there is minimal evidence to suggest an effect on long-term muscle growth. If we consider all the literature and the average effects of HMB on muscle mass, the data will be skewed with some studies showing extraordinary gains in muscle mass. Indeed, some authors have observed anabolic steroid-like gains in muscle mass, with no plausible explanation as to why HMB could have such an affect. In the previous blog, we discussed these extraordinary findings in the context of the bulk of evidence around HMB as a performance enhancing supplement (e.g. for body mass, fat mass loss, muscle strength gain). In terms of muscle growth, the majority of intervention studies show no effect, or a small effect of HMB compared to placebo (5). Thus, there is currently insufficient evidence to recommend HMB as a supplement to increase muscle growth in athletes.
There is currently insufficient evidence to recommend HMB as a supplement to increase muscle growth in athletes.
HMB has also been suggested as a possible countermeasure to age-related muscle loss. In an umbrella review (i.e., a review of reviews) by Prof Stuart Phillips (6), the authors found large heterogeneity among studies, leading to the conclusion that “most reviews reported no effect [of HMB on muscle mass] or insufficient data to reach a definitive conclusion”. Therefore, at current, consuming sufficient dietary protein appears to be more important for muscle growth. Although HMB may be an anabolic compound, there is minimal evidence to suggest its superiority to intact protein.
What is the take-home message for HMB?
HMB can stimulate muscle building processes. This has been shown in cells as well as in humans. However, there is minimal evidence suggesting HMB can promote long-term changes in muscle growth. Even if there was an effect, HMB does not appear superior to other strategies of increasing muscle growth (e.g., through consuming sufficient dietary protein). Therefore, the evidence does not support the hype for HMB as a magical muscle building supplement. Instead, athletes can obtain benefits to muscle growth consuming sufficient dietary protein in combination with training.
Fujita S, Dreyer HC, Drummond MJ, Glynn EL, Cadenas JG, Yoshizawa F, Volpi E, Rasmussen BB. Nutrient signalling in the regulation of human muscle protein synthesis. J Physiol. 2007 Jul 15;582(Pt 2):813-23. doi: 10.1113/jphysiol.2007.134593. Epub 2007 May 3.
Biolo G, Tipton KD, Klein S, Wolfe RR. An abundant supply of amino acids enhances the metabolic effect of exercise on muscle protein. Am J Physiol. 1997 Jul;273(1 Pt 1):E122-9. doi: 10.1152/ajpendo.1997.273.1.E122.
Wilkinson DJ, Hossain T, Hill DS, Phillips BE, Crossland H, Williams J, Loughna P, Churchward-Venne TA, Breen L, Phillips SM, Etheridge T, Rathmacher JA, Smith K, Szewczyk NJ, Atherton PJ. Effects of leucine and its metabolite β-hydroxy-β-methylbutyrate on human skeletal muscle protein metabolism. J Physiol. 2013 Jun 1;591(11):2911-23. doi: 10.1113/jphysiol.2013.253203. Epub 2013 Apr 3.
Wilkinson DJ, Hossain T, Limb MC, Phillips BE, Lund J, Williams JP, Brook MS, Cegielski J, Philp A, Ashcroft S, Rathmacher JA, Szewczyk NJ, Smith K, Atherton PJ. Impact of the calcium form of β-hydroxy-β-methylbutyrate upon human skeletal muscle protein metabolism. Clin Nutr. 2018 Dec;37(6 Pt A):2068-2075. doi: 10.1016/j.clnu.2017.09.024. Epub 2017 Oct 6.
Jakubowski JS, Nunes EA, Teixeira FJ, Vescio V, Morton RW, Banfield L, Phillips SM. Supplementation with the Leucine Metabolite β-hydroxy-β-methylbutyrate (HMB) does not Improve Resistance Exercise-Induced Changes in Body Composition or Strength in Young Subjects: A Systematic Review and Meta-Analysis. Nutrients. 2020 May 23;12(5):1523. doi: 10.3390/nu12051523.
Phillips SM, Lau KJ, D'Souza AC, Nunes EA. An umbrella review of systematic reviews of β-hydroxy-β-methyl butyrate supplementation in ageing and clinical practice. J Cachexia Sarcopenia Muscle. 2022 Oct;13(5):2265-2275. doi: 10.1002/jcsm.13030. Epub 2022 Jul 12.