Researchers in the US have identified an antibody which could greatly reduce the loss of lean body mass in people taking weight-loss medications.

Lean body mass – much of which is skeletal muscle – has been shown to account for up to 25 to 40 per cent of the weight lost by people taking GLP-1 medications, but taking apitegromab alongside the injections could halve the rate of lean body mass loss.

Apitegromab is an antibody that selectively inhibits myostatin activation and is capable of increasing muscle mass.

The study involved 102 participants. Half took apitegromab alongside their weight-loss injections and half received a placebo.

After 24 weeks, total weight loss was almost identical across both groups, but the apitegromab group retained 55 per cent more lean body mass than the placebo group.

Those receiving the placebo lost an average of 3.5kg of lean body mass, while those taking apitegromab lost 1.6kg. This represented a 30.2 per cent reduction in lean body mass in the placebo group, compared with 14.6 per cent in the apitegromab group.

Lean body mass includes connective tissue, water, muscle and the organs.

Body composition

Changes in body composition, including visceral fat, subcutaneous fat and trunk fat, were similar between the two groups.

Although apitegromab significantly preserved lean body mass, participants showed no significant advantage over the placebo group in changes in physical function or strength, including grip strength, during the 24-week study.

Participants were assessed again eight weeks after the trial ended and the difference in lean body mass between the apitegromab and placebo groups remained significant. However, there were still no notable differences in cardiometabolic measures or physical function.

The findings suggest that preserving lean body mass alone is not enough to maintain physical function and that exercise and resistance training are likely to remain essential alongside weight-loss medication.

Further research will now be needed to establish whether apitegromab is safe and effective in larger populations and whether it could become a practical addition to GLP-1 therapy.

Published in Nature Medicine, the proof-of-concept study – Apitegromab for lean mass preservation during tirzepatide-induced weight loss: a randomized, double-blind, placebo-controlled phase 2 trial – was carried out across a number of specialised clinical research centres.

The researchers say future studies should investigate the impact of apitegromab alongside interventions such as resistance exercise, increased protein intake, sleep and physical activity.