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β-hydroxy β-methylbutyrate (HMB) is a metabolite of the essential amino acid leucine. About 5% of dietary leucine is converted to HMB, which acts on the mTOR and ubiquitin-proteasome pathways to favor muscle protein synthesis and suppress muscle protein breakdown. HMB shines in catabolic states: bed rest, illness, calorie deficit, and aging. Meta-analyses show consistent benefits for lean mass retention and recovery in older adults and clinical patients, with smaller benefits in healthy younger trainees. Standard dose is 3 g per day, split or taken before training.
A leucine metabolite that suppresses muscle breakdown. Strong evidence in older adults, fasted training, and bed rest.
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