The use of weight-loss jabs in the UK has skyrocketed, with an estimated 1.5 million people using them.
Yet drugs like Ozempic and Wegovy don’t just help people lose fat, but potentially muscle too, new research has suggested.
A study that tested weight loss jabs on mice found that although muscle mass changes less than expected, muscles still get weaker and tissues like the liver also shrink.
Weight-loss injections, also known as GLP-1 receptor agonists, work by mimicking the natural hormone which regulates blood sugar, appetite and digestion.
The medications, known as semaglutides, predominantly treat diabetes but are also available on the NHS or via private providers to help adults with a high body mass index (BMI).
For example, although Ozempic is used for type 2 diabetes, it is sometimes prescribed off-label as a weight-loss drug.
The results of the study, published in the journal Cell Metabolism, revealed Ozempic-induced weight loss decreased muscle mass by about 10 per cent.
Most of this lost weight wasn’t from skeletal muscles, which surround bones and joints, but instead from other tissues like the liver, which shrank by nearly half.
However, because the Ozempic was tested on mice, researchers emphasise that more research is needed to determine whether similar changes to organ size occur in humans and whether those changes come with any risks.
“Loss of mass in metabolically active organs, such as the liver, is expected as part of healthy weight loss,” said Dr Ran Hee Choi, research instructor in nutrition and integrative physiology at the University of Utah college of health, and co-first author on the study.
In both mice and humans, weight gain and loss can affect the size of organs like the liver without affecting their function.
“It’s unlikely that the observed lean mass loss represents a serious adverse effect,” added Dr Takuya Karasawa, another co-first author on the study.
Researchers found some skeletal muscles did shrink by about 6 per cent as the mice lost weight, but not enough to explain the overall muscle loss.
However, when someone gains fat, they also tend to gain skeletal muscle. Study authors explain this is because the body needs to work harder to move around.
As a result, losing extra fat can lead to a loss of muscle, which will not affect the person’s overall quality of life.
Researchers also tested the amount of force the mice’s muscles exerted and found that some muscle strength decreased as the mice lost weight, even when the size of the muscle stayed roughly the same.
This potential loss of strength when taking Ozempic could be a particular concern for adults over the age of 60 who are at a higher risk of muscle loss and reduced mobility.
“The loss of physical function is a strong predictor of not just quality of life but longevity,” added Dr Katsu Funai, professor of nutrition at the University of Utah and the senior author on the study.
Dr Funai concluded that further clinical trials of weight loss jabs should check for changes in muscle strength.
A spokesperson for Novo Nordisk, which makes Ozempic, said: “In clinical trial for Wegovy or Ozempic we did not specifically study the medicine’s impact on muscle mass. In a sub-study of 140 patients with a BMI of 40 or less, analysis suggested that treatment with Wegovy was accompanied by reductions in both fat and lean body mass, with a greater reduction in fat mass than lean body mass.
“We recommend that any patients experiencing side effects while taking Wegovy or Ozempic contact their healthcare provider.”
