HMB Side Effects: What the Research Shows
HMB, short for beta-hydroxy-beta-methylbutyrate, is a metabolite of the amino acid leucine and has been sold as a muscle-preserving supplement since the 1990s. Debate about its effectiveness continues, but its safety record has received less attention from consumers. Here the editorial team looks at what trials have actually recorded about adverse effects.
What HMB is and how it is used
When the body breaks down leucine, a small fraction, roughly 5 per cent, is converted to HMB. Researchers led by Steven Nissen in the 1990s proposed that supplying HMB directly could reduce muscle protein breakdown, and early studies reported gains in strength and lean mass in untrained people.
The standard amount used in studies is 3 g per day, usually split into doses with meals. It is available as calcium HMB, the original and most studied form, and as the free acid, which is absorbed more quickly.
Later research has been less impressive. A meta-analysis by Rowlands and Thomson found small benefits in untrained individuals but trivial effects in trained lifters, and a well-controlled 2019 trial by Jakubowski and colleagues found no advantage of HMB over leucine in young men.
HMB is also used in clinical nutrition, particularly in products for older adults at risk of muscle loss. This clinical use has generated additional safety data in more vulnerable groups, which is helpful when judging its risk profile.
Side effects reported in trials
Across dozens of trials, HMB at around 3 g per day has been remarkably well tolerated. A 2000 analysis by Nissen and colleagues pooled data from nine studies and found no adverse effects on a wide range of safety markers, and no increase in reported side effects compared with placebo.
Where complaints do occur, they are usually mild and digestive. Users occasionally report:
- bloating or a feeling of fullness;
- mild nausea, especially when taken on an empty stomach;
- loose stools or constipation;
- an unpleasant taste with some free acid preparations.
The International Society of Sports Nutrition position stand, published in 2013, concluded that HMB is safe in both young and older populations at the studied doses, while acknowledging that long-term data beyond a year are limited.
A year-long study by Baier and colleagues in older adults, using a mixture of HMB, arginine and lysine, also reported no meaningful safety concerns, which offers some reassurance about extended use, though in combination with other ingredients.
Effects on blood tests, liver and kidneys
Because HMB is a metabolite and is partly excreted in urine, people sometimes worry about the liver and kidneys. Studies that measured these organs directly have been reassuring.
Gallagher and colleagues examined haematology, liver and kidney markers in young men taking either 38 or 76 mg/kg per day of HMB for eight weeks during resistance training. They found no adverse changes in liver enzymes, kidney function, blood counts or lipids.
The pooled Nissen analysis even reported small reductions in total and LDL cholesterol and in systolic blood pressure in the HMB groups. These findings were secondary and should not be read as a reason to take HMB for heart health.
| Marker | Findings in published studies |
|---|---|
| Liver enzymes (ALT, AST) | No adverse changes at studied doses |
| Kidney function (creatinine, urea) | No adverse changes |
| Blood count | No adverse changes |
| Cholesterol and blood pressure | Neutral or slight improvement |
| Glucose and insulin | Mostly neutral; limited data |
Some animal studies have raised questions about insulin sensitivity with HMB, and a few human data are mixed. This is not established as a clinical problem, but people with diabetes may wish to monitor their glucose if they start taking it.
Forms, product quality and hidden risks
Calcium HMB provides a small amount of calcium, roughly 400–500 mg in a 3 g daily intake depending on the salt. For most people this is harmless, but those with a history of kidney stones or high blood calcium, or who already take calcium supplements, should take it into account.
Free acid HMB, sometimes sold as HMB-FA, is more acidic and may cause more stomach irritation in sensitive users. Claims that it produces dramatically greater effects have not been consistently replicated.
The bigger risk with any sports supplement is often not the ingredient itself but what else is in the tub. Multi-ingredient products may combine HMB with stimulants or other compounds, and contamination with substances banned in sport has been documented in the supplement market generally.
In Great Britain, HMB is sold as a food supplement. EFSA, whose opinions still shape the UK approach, did not accept proposed health claims for HMB relating to muscle, so marketing promises of muscle growth are not authorised claims. Athletes subject to testing should prefer batch-tested products.
Who should avoid HMB or seek advice
Although HMB looks safe for healthy adults, certain groups should not use it without medical input. Pregnant and breastfeeding women should avoid it because there are no safety data in these groups.
Children and adolescents have no demonstrated need for HMB, and it has not been studied adequately in them. A balanced diet with sufficient protein is a better approach for young athletes.
People with significant kidney or liver disease, or with disorders of calcium metabolism, should consult their specialist first. The same applies to anyone on several regular medicines, simply because interactions have not been well studied.
Finally, older adults considering HMB as part of a clinical nutrition product should discuss it with their GP or dietitian, who can also address protein intake, vitamin D and exercise, which have stronger evidence for preserving muscle.
Conclusions of the editorial team
HMB has one of the cleaner safety records among sports supplements. At around 3 g per day, trials report few side effects beyond occasional digestive upset.
The more pressing question is not whether HMB is harmful but whether it is worth taking, as benefits in trained people appear small.
Choose single-ingredient, tested products, factor in the calcium content and seek advice if you belong to one of the cautious groups.
For further reading, the editorial team recommends our articles on leucine and muscle protein synthesis, on preventing muscle loss with age, and on independent testing schemes for sports supplements.
References
- Wilson JM, Fitschen PJ, Campbell B, et al. International Society of Sports Nutrition Position Stand: beta-hydroxy-beta-methylbutyrate (HMB). J Int Soc Sports Nutr. 2013;10(1):6.
- Nissen S, Sharp RL, Panton L, Vukovich M, Trappe S, Fuller JC Jr. Beta-hydroxy-beta-methylbutyrate (HMB) supplementation in humans is safe and may decrease cardiovascular risk factors. J Nutr. 2000;130(8):1937–1945.
- Gallagher PM, Carrithers JA, Godard MP, Schulze KE, Trappe SW. Beta-hydroxy-beta-methylbutyrate ingestion, part II: effects on hematology, hepatic and renal function. Med Sci Sports Exerc. 2000;32(12):2116–2119.
- Rowlands DS, Thomson JS. Effects of beta-hydroxy-beta-methylbutyrate supplementation during resistance training on strength, body composition, and muscle damage in trained and untrained young men: a meta-analysis. J Strength Cond Res. 2009;23(3):836–846.
- Jakubowski JS, Wong EPT, Nunes EA, et al. Equivalent hypertrophy and strength gains in β-hydroxy-β-methylbutyrate- or leucine-supplemented men. Med Sci Sports Exerc. 2019;51(1):65–74.
- Baier S, Johannsen D, Abumrad N, Rathmacher JA, Nissen S, Flakoll P. Year-long changes in protein metabolism in elderly men and women supplemented with a nutrition cocktail of beta-hydroxy-beta-methylbutyrate (HMB), L-arginine, and L-lysine. JPEN J Parenter Enteral Nutr. 2009;33(1):71–82.
- Nissen S, Sharp R, Ray M, et al. Effect of leucine metabolite beta-hydroxy-beta-methylbutyrate on muscle metabolism during resistance-exercise training. J Appl Physiol. 1996;81(5):2095–2104.
