Hydroxymethylbutyrate (HMB), a metabolite of the amino acid leucine, has been investigated for its role in muscle preservation and body composition, particularly in the context of weight loss. The primary scientific rationale for using HMB in weight loss is its purported ability to reduce muscle protein breakdown, potentially helping to preserve lean muscle mass during periods of caloric restriction or intense exercise. The preservation of lean mass is important because it helps maintain metabolic rate as body weight decreases.

Clinical studies on HMB supplementation in overweight or obese individuals show mixed results. Some randomized controlled trials (RCTs) have demonstrated that HMB may help reduce the loss of lean body mass during weight loss interventions, especially when combined with resistance training. However, the effect of HMB on actual fat loss or total weight reduction is less clear, with most studies indicating only modest or non-significant impacts compared to placebo. A 2015 meta-analysis concluded that HMB supplementation has a small effect on body composition, primarily by decreasing fat mass and preserving muscle in trained individuals, but the evidence is not robust or consistent.

HMB is not traditionally used for weight loss; its use is based on modern scientific investigation. The current scientific evidence supporting its use for weight loss—specifically for fat loss rather than muscle preservation—is limited and not definitive. Therefore, while there is some scientific basis for its use in body composition and muscle preservation during weight loss, the overall evidence for significant weight loss effects is weak (rated 2/5).