A new study to be presented at the European Congress on Obesity (ECO 2025) in Spain has found that people using popular weight loss medications like semaglutide and tirzepatide lost mostly fat — and kept almost all of their muscle — after six months of treatment. This is good news, as one concern with weight loss is losing too much muscle along with the fat.
These medications are part of a group called GLP-1 receptor agonists. They were first developed to help people with type 2 diabetes but are now also being used for weight loss. One drug, semaglutide (brand names like Ozempic and Wegovy), targets a hormone called GLP-1.
Another, tirzepatide (brand name Mounjaro), targets both GLP-1 and another hormone called GIP. Both of these hormones help regulate blood sugar and appetite, which can lead to weight loss.
The research was led by Dr. Dinabel Peralta-Reich from the Zucker School of Medicine at Hofstra/Northwell and Lenox Hill Hospital, and Dr. Alexandra Filingeri from New York Weight Wellness Medicine. Their study followed 200 adults with overweight or obesity, aged 18 to 65, for six months. About 60% of the participants used tirzepatide, while 40% used semaglutide.
At the start, the average body mass index (BMI) of participants was 31.4 kg/m². During the study, participants were educated not only on how to take their medication, but also on the importance of eating enough protein and doing resistance training (like lifting weights or using resistance bands). These steps help protect muscle during weight loss.
The team used a device called the InBody 570 to measure body composition. This device uses safe, low-level electrical currents to estimate how much of the body is made up of water, muscle, and fat. They checked each person’s measurements at the start, at three months, and again at six months.
The results showed that weight loss was significant and mostly came from fat, not muscle. On average, women lost about 12% of their body weight — from 156 pounds (71 kg) to 137 pounds (62 kg). Men lost about 13% — from 223 pounds (101 kg) to 193 pounds (88 kg).
When it came to fat loss versus muscle loss, the difference was clear. Women lost an average of 10.8 kg (about 24 pounds) of fat and only 0.63 kg (about 1.4 pounds) of muscle. Men lost 12 kg (about 25 pounds) of fat and just 1 kg (2.4 pounds) of muscle. This means over 90% of the weight lost was fat, which is ideal in any weight loss plan.
The researchers also found that those who reported eating enough protein and doing regular resistance exercises were better at keeping their muscle. Most participants stuck with their medication, with 95% still taking it at three months and 89% at six months. This high level of commitment likely helped the results, too.
While the researchers are still analyzing the specific differences between the two medications, the overall takeaway is clear: with proper medical guidance, it’s possible to lose weight and keep your muscles strong. This is especially important because maintaining muscle helps with strength, energy, metabolism, and overall health.
The researchers also pointed out that more studies are needed to better understand how diet and exercise work together with these medications to protect muscle. But so far, this study provides promising evidence that modern weight loss drugs, when used responsibly and combined with healthy habits, can help people lose fat without losing too much muscle.
If you care about weight loss, please read studies that hop extract could reduce belly fat in overweight people, and early time-restricted eating could help lose weight.
For more health information, please see recent studies that Mediterranean diet can reduce belly fat much better, and Keto diet could help control body weight and blood sugar in diabetes.
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