3 g/day divided into doses.
Helps preserve muscle in situations of catabolism.
View the study (RCT) →Little effect in people who are already trained and well nourished.
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HMB, or beta-hydroxy-beta-methylbutyrate, is a metabolite of leucine (the branched-chain amino acid with the greatest capacity to activate muscle protein synthesis) produced endogenously from the oxidation of dietary leucine in modest amounts: approximately 5 percent of ingested leucine is converted to HMB in the liver and muscle; the biological effects of HMB are distinct from those of the parent leucine and are characterized mainly by a significant reduction in muscle protein catabolism (anticatabolic) combined with modest effects on protein synthesis and membrane repair, a pharmacological combination that positions it as a supplement with a specific profile for preserving muscle mass in catabolic contexts.
Unlike leucine (which acts mainly by activating the mTOR pathway of protein synthesis), HMB acts preferentially by reducing protein degradation through inhibition of the ubiquitin-proteasome system and the autophagy-lysosomal system when they are overactivated; this mechanistic specificity makes HMB a more useful supplement for situations of elevated catabolism (athletes with extreme loads, older adults with sarcopenia, patients in convalescence) than for healthy people with moderate training, in whom the benefit on body composition is more modest.
The indication with the strongest clinical support is the prevention and treatment of sarcopenia and loss of muscle mass in older adults: several randomized trials have documented significant improvements in muscle mass, strength, and function with supplementation at 3 grams daily of HMB for 8-12 weeks, combined with exercise or not, an indication recognized in some geriatric guidelines as a nutritional adjunct in people at risk of or with established sarcopenia.
A meta-analysis published in 2015 with 7 trials in older adults confirmed significant improvements in lean mass and strength with HMB supplementation for 8 weeks or more, modest magnitudes but clinically relevant for preserving function in a population with a natural tendency toward progressive muscle loss.
In situations of prolonged immobilization (bed rest, postoperative recovery, fractures in a cast) that cause accelerated loss of muscle mass, several studies have documented that HMB supplementation significantly reduces muscle atrophy, an indication with clinical support in rehabilitation medicine and clinical nutrition for preserving muscle mass during episodes of forced immobilization.
In athletes doing strength training, the effects of HMB are modest but replicable: several meta-analyses have documented small improvements in body composition, strength, and reduced markers of muscle damage after intense exercise at doses of 3 g/day for 4-12 weeks; the improvements are quantitatively smaller than with creatine (the best-supported ergogenic supplement) but consistent with an anticatabolic profile that has a place in multimodal supplementation regimens for athletes with high loads.
In cancer cachexia and chronic diseases with progressive muscle loss (advanced heart failure, COPD with malnutrition), several trials have documented improvements in body composition and quality of life with supplementation over months, an indication with moderate clinical support in integrative oncology and palliative medicine.
The dose studied and shown to be effective is 3 grams daily, divided into 2-3 doses with or without food; the two main commercial forms are HMB-Ca (calcium salt, the original form and the one most used in research) and HMB-FA (free acid, with a slightly faster pharmacokinetic profile and better acute bioavailability); the effect on muscle mass and strength appears gradually and requires at least 4-8 weeks of continued use for clinical evaluation; combining it with strength training is clearly synergistic and necessary to obtain the full effects in people able to train.
Generally very well tolerated at the doses studied; the adverse effects described are rare and mild (mild digestive discomfort, occasionally headache); no significant toxicities have been described with prolonged use at usual doses; people with serious kidney disease should be cautious because HMB is excreted by the kidneys; in pregnancy and breastfeeding, specific data are limited and avoidance is recommended as a precaution; in sports with doping controls, HMB is not prohibited and is widely used; people who already consume adequate complete protein with good daytime distribution (1.6-2.2 g/kg/day divided into 4-5 doses) obtain less additional benefit from HMB than people with suboptimal protein intake; cheap products may have variable purity, and it is advisable to choose products with third-party verification for athletes subject to doping controls.
Information, not medical advice. Scientific evidence reflects the quality of the studies.