Lean mass makes up 28% of weight lost on GLP-1 and SGLT2 drugs

Drugs with powerful weight loss effects, notably GLP-1 receptor agonists and SGLT2 inhibitors, are now routinely prescribed for obesity and chronic disease. Weight lost on any regime is never purely fat, and lean body mass, which includes muscle, matters for strength, function and long-term health. Whether these medicines cause a meaningful loss of lean tissue, and how large that loss is relative to the total, has been an increasingly urgent question as prescriptions climb. This review set out to pool the randomised trial evidence on exactly that.

The authors searched PubMed, MEDLINE, the Cochrane Library and CINAHL from inception to October 2022 for randomised controlled trials of GLP-1 receptor agonists or SGLT2 inhibitors against placebo or an active comparator, with change in lean body mass as the primary outcome. Thirty-six studies qualified, 21 for GLP-1 receptor agonists and 15 for SGLT2 inhibitors. The trial populations spanned obesity (8 studies), type 2 diabetes (20 studies), type 1 diabetes (5 studies) and polycystic ovary syndrome (3 studies).

Pooling the trials showed lean body mass fell in the intervention groups: by 1.51 kg on average with GLP-1 receptor agonists and by 1.04 kg with SGLT2 inhibitors. Sub-group analyses were largely consistent whichever body composition outcome was used, whatever the measurement technique and whatever the underlying condition, and the results were not modified by sex. A further pooled analysis found that lean mass accounted for 28% of the overall weight lost on these drugs, with a plausible range from 22% to 34%. In other words, the lean loss appears proportionate to the total loss rather than an outsized penalty.

The result confirms a widely voiced concern that pharmacological weight loss takes lean tissue with it, while also sizing that concern: the loss appears congruent with overall weight loss rather than exceeding it. The authors suggest monitoring of body composition and consideration of combined therapy to preserve lean mass as these prescriptions expand. What the review cannot answer is whether the lean tissue lost translates into reduced strength or physical function, or whether training and diet during treatment can offset it. Those remain live questions for future trials.

If you are taking, or considering, one of these medicines, this is a reason to think of the number on the scale as a blend: on average, 28% of what disappears is lean tissue rather than fat. That looks proportionate to the overall loss rather than an extra penalty of the drugs themselves, but it is worth knowing when judging progress by weight alone. It is also a reason to pay attention to body composition, not just the scale, during treatment.