HMB Increases Fat-Free Mass in Athletes, But Only When Protein Intake Is Low

As you may or may not know, we have a huge guide to the popular supplement HMB here on the site. HMB is a metabolite of the essential amino acid leucine. Having both anti-catabolic and anabolic effects, it has been highly touted as a must-have supplement for athletes. However, most research, with the exception of two anomalous studies from 2014, has failed to find any support for those claims.

Just in time for Christmas, we got a brand new systematic review and meta-analysis of HMB as an early present. This time, the researchers focused on trained athletes and the effects of HMB on body composition, that is body fat and fat free mass.

After discarding the multitude of studies that did not fit the inclusion criteria, the researchers were left with 7 studies assessing body mass, 5 assessing fat mass, and 5 analysing fat-free mass, with some overlap, of course. The studies all featured athletes and lasted for at least 4 weeks.


Much like previous research, this meta-analysis found no significant effects of HMB on body mass or body fat.

However, it did find a positive effect on fat-free mass. According to the research included in the meta-analysis, athletes might see an increase in fat free mass by using HMB.

This effect was significant. It is important to remember that statistically significant does not necessarily mean significant in a real-life scenario, though. It just means that the observed effect likely isn’t just chance. That being said, for high-level athletes, even small effects can make a big difference.

This effect on fat free mass was only apparent with a low protein intake. With a protein intake of 1.6 grams per kilogram and day, the effect of HMB on fat free mass seems to disappear.


If your diet is low in protein, you might benefit from HMB. Athletes seeking to increase muscle mass should not eat low-protein diets anyway, so the benefits of HMB supplementation remain dubious.


Journal of Strength and Conditioning Research, December 20, 2019. Does HMB Enhance Body Composition in Athletes? A Systematic Review and Meta-analysis.

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Andreas Abelsson

Andreas is a certified nutrition coach with over three decades of training experience. He has followed and reported on the research fields of exercise, nutrition, and health for almost as long and is a specialist in metabolic health and nutrition coaching for athletes. Read more about Andreas and StrengthLog by clicking here.