Muscle growth drug ‘could reduce loss of lean tissue’ when using slimming jabs

A drug that promotes muscle growth could significantly reduce the loss of lean body mass when using slimming jabs, research suggests.

While GLP-1 based jabs such as Wegovy and Mounjaro have proved highly effective at helping people who are overweight or obese, experts have warned it is not only fat that is lost. Studies suggest 25-40% of total weight loss is down to a reduction in lean body mass – non-fat components of the body, including muscle.

The authors of the study said this mattered because lean body mass was important for physical strength and overall health. It burns more calories than fat tissue and is linked to a lower risk of type 2 diabetes.

Now a small trial has suggested the use of a monoclonal antibody called apitegromab can help retain lean body mass when loosing weight with tirzepatide – the drug found in Mounjaro. Apitegromab works by blocking myostatin, a protein involved in inhibiting muscle growth.

Prof Alexander Miras, an obesity expert at Ulster University who was not involved in the work, described the findings as very important, noting that while GLP-1s had been associated with improved “functionality” – meaning it was easier to carry out everyday activities – they had also been associated with a loss in muscle mass and strength.

“This means that [people] may be less able to lift heavy weights, for example, or walk up a hill,” Miras said. “This new medication may help reduce the effects of GLP-1-based drugs on muscle strength and therefore improve functionality even further compared to someone not on the new medication who is just taking tirzepatide.”

Writing in the journal Nature Medicine, researchers in the US reported how they randomly divided 102 participants into two groups, with 51 given apitegromab alongside tirzepatide, and the other 51 given a placebo with tirzepatide.

The trial, which was funded by the apitegromab producer Scholar Rock, revealed after 24 weeks that total weight loss was similar for the two groups.

However, participants given apitegromab alongside tirzepatide lost on average 1.6kg (3.5lb) of lean mass (corresponding to 14.6% of total weight loss) while those given apitegromab alongside a placebo lost on average 3.5kg of lean mass. In other words, the use of apitegromab was associated with a 55% greater retention of lean mass relative to placebo.

The team added that the number of people experiencing side-effects were similar between the two groups, with most deemed to be mild. However, the study has limitations, including that most participants were women and the study was small and short in duration.

Prof Naveed Sattar, a cardiometabolic medicine expert at the University of Glasgow, said far larger-scale and longer trials were now needed, not least to check safety.

“This is an early-stage trial that suggests novel drugs can help mitigate muscle mass loss with prescribed tirzepatide. However, it’s too early to say whether this actually benefits peoples health or ability to move or function better,” he said.

“In the meantime, people prescribed these drugs should be supported to increase their physical activity, as this can help maintain muscle mass in a physiological way. Importantly, activity should also be framed as something enjoyable and sustainable, rather than purely as a medical add‑on,” he added.