Semaglutide: body composition at 12 months

A 2026 longitudinal study published in Obesity Facts followed 83 adults with obesity through 12 months of treatment, comparing semaglutide to lifestyle intervention alone. The headline finding is that the drug produced substantially greater weight loss, but a more interesting result sits in the body composition data: absolute muscle mass fell slightly while muscle quality, measured by skeletal muscle index standard deviation scores (SMI-SDS), improved by a clinically meaningful margin^[1].

What the study measured and found

Filippi-Arriaga and colleagues enrolled 44 adults in the semaglutide arm and 39 in a lifestyle-only control arm. Both groups were followed for 12 months at a single centre in routine clinical care. The semaglutide group lost 11.0 kg more body weight on average (p=0.001), with a 4.0 kg/m2 greater reduction in BMI and a 7.0 cm greater reduction in waist circumference^[1]. None of that is surprising; GLP-1 receptor agonist trials have consistently shown weight reductions in this range.

The body composition picture is more textured. Skeletal muscle mass in the semaglutide arm fell from a mean of 29.9 kg to 28.7 kg over 12 months. Fat-free mass declined from 53.8 kg to 52.7 kg. Both changes were statistically significant (p<0.001)^[1]. At the same time, SMI-SDS, a reference-adjusted measure of skeletal muscle index that accounts for age and sex norms, increased by 0.52 in the semaglutide group, compared with 0.09 in the lifestyle group (p<0.001). The interpretation the authors offer: despite losing some absolute muscle mass, semaglutide users moved closer to expected muscle levels for their body size and demographics.

Muscle strength held, and relative strength improved

Absolute handgrip strength was preserved in both groups over the 12-month period. More notably, relative handgrip strength, expressed as grip strength per unit of body weight, improved in the semaglutide group (p=0.003)^[1]. This is a practically meaningful distinction. As body weight falls, if grip strength stays constant, the functional strength-to-weight ratio improves. Whether that translates to real-world physical performance is a question this single-centre study cannot answer, but the signal points away from clinically significant sarcopenia in this population over 12 months.

These findings align with a 2026 systematic review that examined body composition across semaglutide weight-loss trials more broadly. That review concluded semaglutide consistently promotes substantial fat mass reduction with proportionally smaller decreases in lean mass, and that protective mechanisms including anti-inflammatory signalling may contribute to functional muscle preservation, particularly in younger adults.

Adaptive thermogenesis: the semaglutide group suppressed less

Adaptive thermogenesis is the body's tendency to reduce resting energy expenditure beyond what weight loss alone would predict, a compensatory response that works against sustained calorie deficits. Both groups showed a decline in resting energy expenditure over 12 months. The semaglutide group showed a mean adaptive thermogenesis change of -210.2 kcal/day; the lifestyle group showed -373.4 kcal/day^[1]. The between-group difference did not reach statistical significance, so the authors do not claim semaglutide suppresses adaptive thermogenesis. What the numbers suggest is that the semaglutide group's metabolic adaptation was numerically less severe than in the lifestyle-only group, though the study was not powered to confirm this.

Adaptive thermogenesis is one of the mechanisms invoked to explain weight regain after calorie restriction. Whether GLP-1 receptor agonists alter this response in a clinically meaningful way is an open research question. This study adds data but does not settle it.

What this study cannot tell you

This was a single-centre, retrospective observational study with 83 participants. It was not randomised. Assignment to semaglutide or lifestyle intervention was based on clinical judgment, which means the groups may have differed in ways the analysis could not fully account for. The 12-month observation window does not capture what happens to muscle mass or strength if treatment continues for two to five years. And the semaglutide doses used reflected routine clinical practice in Spain, not necessarily the maximum studied dose of 2.4 mg weekly. For a broader view of what long-term semaglutide treatment does to body weight, the STEP programme trials (particularly STEP 5 at two years and the SELECT cardiovascular outcomes trial) remain the primary evidence base.

The current picture

Over 12 months, semaglutide treatment in this cohort produced more weight loss, better muscle quality scores, preserved absolute handgrip strength, improved relative grip strength, and numerically less metabolic suppression than lifestyle intervention alone. The concern about muscle wasting is not entirely without basis, as absolute lean mass did fall, but the functional and reference-adjusted measures moved in a direction most clinicians would consider favourable. Whether that picture holds at 24 or 36 months, and whether resistance training and adequate protein intake change the trajectory of lean mass loss, are the questions the next generation of studies needs to address. If you are considering semaglutide for weight management, review the evidence on the semaglutide overview page and speak with a clinician who can weigh up your individual circumstances.