β-羥基－β-甲基丁酸(β-hydroxy-β-methylbutyrate, HMB)為白胺酸代謝物，未受訓練者及高齡者接受阻力訓練與HMB 3g/d，可增加除脂體重與肌力，對良好訓練的運動員無顯著影響。癌症、愛滋病及類風濕性關節炎患者，體內蛋白質分解作用增加，導致肌肉流失，補充HMB可增加除脂體重。HMB可能的作用機轉為抑制泛素蛋白酶體的蛋白質分解作用，提高mTOR蛋白質合成作用，此外HMB可能代謝為HMG-CoA，作為膽固醇合成的碳骨架，膽固醇可維持細胞功能與完整性。選手限制飲食控制體重，可能造成肌肉流失，需進一步研究補充HMB是否能夠緩減肌肉流失，維持正常運動表現，並探討適用時機及作用機轉。

β-hydroxy-β-methylbutyrate (HMB) is a leucine metabolite. It has been reported that HMB supplementation of 3g/d orally could help to increase lean body mass and strength in resistance-trained adults and elderly, while this effect does not seem to occur in well-trained athletes. Cancer, AIDS, and rheumatoid cachexia are characterized by increased muscle protein breakdown and muscle wasting. Moreover, patients with muscle atrophy in catabolic conditions can be ameliorated by HMB treatment. The possible mechanism on declining muscle wasting may decrease muscle proteolysis via inhibiting ubiquitin-proteasome and increase protein synthesis via upregulating mammalian target of rapamycin (mTOR). In addition, the hypothesis is proposed that HMB could be converted to β-hydroxy-β-methylglutaryl CoA (HMG-CoA) as carbon source for cholesterol synthesis. Sufficient cholesterol provided by HMB possibly can maintain cellular function and integrity in tissues. Since short- or long-term energy restriction may sometimes result in the decline in fat-free mass and exercise performance. Further investigations with possibility of HMB supplementation to ameliorate muscle wasting and improve exercise performance are warranted in well-trained athletes during energy restriction.