beta-hydroxy beta-methylbutyric acid (HMB) is a metabolite of the essential amino acid leucine and is synthesized in the human body . Beta-hydroxy beta-methylbutyrate is one of many dietary supplements promoted to enhance lean body mass and strength gains associated with resistance training. HMB is usually supplied as a calcium salt, which means the HMB molecule is bound to calcium.
Manufacturers typically recommend doses from 2-6 grams daily. Similar doses are administered in clinical trials.
HMB for Bodybuilders
Among bodybuilders, beta-hydroxy beta-methylbutyric acid has been extensively used as ergogenic aid to enhance exercise performance and muscle hypertrophy . Several studies have indicated that HMB supplementation may have ergogenic benefits, including anticatabolic , anabolic , and lipolytic effects (breakdown of fat) .
A study conducted on untrained males examined the effects of HMB (0, 1.5, or 3 grams daily) on muscle metabolism and performance during resistance-exercise. Results of the study have shown decreased muscle damage and protein degradation (depending on the dose ranging from 20-60%). Total weight lifted also increased depending on the dosage of HMB (8, 13, and 18.4% for 0, 1.5, and 3 grams).
The efficacy of HMB has also been tested on experienced athletes. Nissen et al.  investigated HMB supplementation on strength and body composition in trained and untrained males in combination with resistance training. Bench press performance increased up to 55%, there was also greater decreases in body fat and increases in lean body mass with beta-hydroxy beta-methylbutyric acid supplementation.
There are also quite a few studies that do not support the efficacy of beta-hydroxy beta-methylbutyric acid.[8-11]
Hoffman at al.  suggested that studies contrast each other be cause supplementing with beta-hydroxy beta-methylbutyric acid may be maximized when muscular damage is heightened. In support to these hypothesis one study  also suggested that HMB supplementation may only be effective with increase in muscle catabolism. This concept can be also be applied to Kreider et al.  study, where subject were instructed to maintain their same training routine and volume they had prior to HMB supplementation. No significant decreases in muscle damage, fat mass, increases in lean body mass or 1 RM performance in lifts were measured .
However, most studies have been done by authors who profit from HMB sales and therefore, may not be trustworthy. Most of them are done by Steve Nissen (chairman of MTI, who holds the patent on HMB) and Scott Connelly (owner of MET-Rx, which sells products that contain HMB).
HMB Free Acid
The first study with HMB free acid  reported promising results as 3 grams of HMB-FA daily administered to highly trained individuals attenuated serum indices of muscle damage (as evident by reduction of muscle-specific metabolite, 3-MH) and increased an athlete’s mental perception of physical preparedness to train following high-volume, muscle-damaging resistance exercise.
A very well-designed study in 2014  reported astonishing results utilizing HMB free acid (HMB-FA). After 12 weeks, HMB-FA supplementation resulted increased total strength (77.1 ± 18.4 vs. 25.3 ± 22.0 kg, p < 0.001), and increased lean body mass gain (7.4 ± 4.2 vs. 2.1 ± 6.1 kg, p < 0.001). During overtraining cycle HMB-FA also attenuated increases in creatine kinase and cortisol. It seem highly unlikely that one could gain over 7 kg of lean body mass in 12 weeks, especially highly trained individual. This would suggest that HMB-FA supplementation is comparable to 600 mg testosterone weekly . Therefore, further research should be done.
Another study in 2016 reported both additive and synergistic effects when combining HMB-FA (3 g/day) and Adenosine-5-triphosphate – ATP (400 mg/day) supplementation . So, athletes who are faced with greater training frequencies (such as elite athletes or military personnel) or individuals undergoing resistance training with an overreach training component may benefit from supplementing HMB-FA + ATP as they may not only prevent typical declines in performance but may also result in additional gains in strength.
There is a lot of evidence that support HMB’s effectiveness but many are flawed and untrustworthy. All HMB-FA studies were (at least in part) funded by patent holder Metabolic Technologies, Inc. Considering all the information available, no definitive conclusion can be drawn.
Safety and Side Effects
No potential adverse side effects have been found when supplementing with beta-hydroxy beta-methylbutyric acid in humans consuming 3–6 grams daily [10,13].
(Other common names: Hydroxymethyl Butyrate, B-Hydroxy B-Methylbutyrate Monohydrate, Calcium B-Hydroxy B-Methylbutyrate Monohydrate, BetaTOR, Free acid beta-hydroxy-beta-methylbutyrate, Beta-Hydroxy-Beta-Methylbutyric Acid, Beta-hydroxy-beta-methylbutyrate, B-Hydroxy B-Methylbutyrate, Hydroxymethylbutyrate)
- Slater, Gary J., and David Jenkins. “beta-Hydroxy-F128b-Methylbutyrate Supplementation and the Promotion of Muscle Growth and Strength.” Sports medicine 30.2 (2000): 105-116.
Kreider, Richard B. “Dietary supplements and the promotion of muscle growth with resistance exercise.” Sports medicine 27.2 (1999): 97-110.
Knitter, A. E., et al. “Effects of β-hydroxy-β-methylbutyrate on muscle damage after a prolonged run.” Journal of Applied Physiology 89.4 (2000): 1340-1344.
Jówko, Ewa, et al. “Creatine and β-hydroxy-β-methylbutyrate (HMB) additively increase lean body mass and muscle strength during a weight-training program.” Nutrition 17.7 (2001): 558-566.
- Gallagher PM, Carrithers JA, et al. “β-hydroxy-β-methylbutyrate ingestion, part I: Effects on strength and fat free mass.” Med Sci Sports Exerc 2000, 32:2109-2115.
Nissen, S., et al. “Effect of leucine metabolite β-hydroxy-β-methylbutyrate on muscle metabolism during resistance-exercise training.” Journal of Applied Physiology 81.5 (1996): 2095-2104.
Nissen, S., et al. “Effect of beta-hydroxy-beta-methylbutyrate supplementation on strength and body composition of trained and untrained males undergoing intense resistance training.” FASEB J 10.3 (1996): A287.
Slater, G., et al. “Beta-hydroxy-beta-methylbutyrate supplementation does not affect changes in strength or body composition during resistance training in trained men.” International journal of sport nutrition and exercise metabolism 11.3 (2001): 384.
Paddon-Jones, Douglas, Andrew Keech, and David Jenkins. “Short-term beta-hydroxy-beta-methylbutyrate supplementation does not reduce symptoms of eccentric muscle damage.” International journal of sport nutrition and exercise metabolism 11.4 (2001): 442.
Kreider, R. B., et al. “Effects of calcium b-HMB supplementation during training on markers of catabolism, body composition, strength and sprint performance.” JEPonline 3.4 (2000): 48-59.
- Hoffman Jay R, Cooper Joshua, Wendell Michael, Im Joohee, Kang Jie: Effects of β-Hydroxy β-Methylbutyrate on Power Performance and Indices of Muscle Damage and Stress During High-Intensity Training. The Journal of Strength and Conditioning Research 2004, 18(4):747-752.
- Nissen SL, Panton L, Fuller J, Rice D, Ray M, Sharp R: Effect of feeding β-hydroxy-β-methylbutyrate on body composition and strength of women. FASEB J 1997, 11:A150.
- Gallagher PM, Carrithers JA, Goodard MP, Schulze KE, Trappe SW “β-Hydroxy-β-methylbutyrate ingestion, Part II: Effects on hematology, hepatic and renal function.” Med Sci Sports Exerc 2000, 32:2116-2119.
- http://www.bodybuilding.com/fun/issa73.htm Retrieved 14. Feb 2013
Wilson, Jacob M., et al. “The effects of 12 weeks of beta-hydroxy-beta-methylbutyrate free acid supplementation on muscle mass, strength, and power in resistance-trained individuals: a randomized, double-blind, placebo-controlled study.” European journal of applied physiology (2014): 1-11.
Bhasin, Shalender, et al. “The effects of supraphysiologic doses of testosterone on muscle size and strength in normal men.” New England Journal of Medicine 335.1 (1996): 1-7.
- Wilson, Jacob M., et al. “β-Hydroxy-β-methylbutyrate free acid reduces markers of exercise-induced muscle damage and improves recovery in resistance-trained men.” British Journal of Nutrition 110.03 (2013): 538-544.
- Lowery, Ryan P., et al. “Interaction of beta-hydroxy-beta-methylbutyrate free acid and adenosine triphosphate on muscle mass, strength, and power in resistance trained individuals.” The Journal of Strength & Conditioning Research 30.7 (2016): 1843-1854.