Short answer: HMB is made from leucine; only about 5% of leucine becomes HMB.1 NIH says HMB generally helps recovery from muscle-damaging exercise, while leucine or other BCAAs alone have not consistently improved performance or muscle.1 In older adults, HMB raised fat-free mass and leucine alone did not, in separate meta-analyses.23
| HMB | Leucine | |
|---|---|---|
| What it is | A product of leucine; about 5% of leucine is turned into HMB1 | An essential amino acid and BCAA1 |
| NIH summary | General agreement it speeds recovery from muscle-damaging exercise; other benefits less certain1 | BCAAs or leucine alone have not consistently improved performance or muscle1 |
| Older adults | More fat-free mass (9 trials, 448 people); no added effect with exercise2 | Alone: no effect on muscle mass, grip or leg press (17 trials)3 |
| Study amounts | 1.5 to 6 g a day, most often 3 g1 | Found in protein foods; whey has more than casein or soy1 |
| Safety data | 3 g a day for 3 to 8 weeks well tolerated1 | Suggested upper safe intake about 500 mg/kg a day in men14 |
HMB: NIH says getting 3 g a day from food would take more than 600 g of protein, so supplements are the only practical source. Studies varied so much that benefits are hard to predict, there is no expert consensus on months of use, and the Australian Institute of Sport does not recommend it outside research.1 In older adults, a 2021 meta-analysis found more fat-free mass with HMB alone, but no added effect when combined with exercise.2
Leucine: NIH says leucine might stimulate protein building in exercised muscle, but supplements of BCAAs or leucine alone have not consistently helped.1 A 2022 meta-analysis of 17 trials in 1,418 older adults found leucine alone did not improve muscle mass or strength, while combinations with vitamin D slightly improved grip and walking speed.3
We found no trial comparing HMB with leucine directly. These are study protocols, not personalized advice; confirm any dosing with your prescriber.
HMB as a calcium salt is about 13% calcium, so 3 g a day adds about 400 mg of calcium; the free acid form has none. It has not been studied in teenagers.1
For leucine, studies suggest an upper safe intake of 500 mg per kg a day in healthy men, where blood ammonia began to rise; a more cautious figure is about 24.5 g a day for a 70 kg older man.4 More detail: Whey vs soy protein: which is better?, and the HMB (Beta-Hydroxy-Beta-Methylbutyrate) and Leucine (L-Leucine) guides.