GLP-1 Weight Loss Is Mostly Driven by Fat Loss, Not Muscle

Key Takeaways:

• GLP-1 receptor agonists and dual GLP-1/GIP therapies lead to significant weight loss, primarily through reductions in fat mass rather than muscle.
  
• Improvements in body composition, including reductions in visceral fat, occur as early as three months into treatment.
  
• Although some lean body mass loss is observed, it is relatively modest compared with overall weight loss, suggesting a favourable pattern of change.
  
  
  

GLP-1 therapies in the context of obesity care

A recent study published in the International Journal of Obesity examined how glucagon-like peptide 1 receptor agonists and dual GLP-1/glucose-dependent insulinotropic polypeptide agonists affect body weight and composition in adults living with overweight or obesity.

The global prevalence of obesity has risen substantially in recent decades. This condition is closely linked to a range of cardiometabolic complications that can reduce both quality of life and life expectancy. As a result, effective obesity management typically requires a personalised, multidisciplinary approach. This may include behavioural support, dietary changes, physical activity, pharmacological treatments, and, in some cases, surgical intervention.

GLP-1 receptor agonists and related incretin-based therapies have emerged as an important pharmacological option. These treatments are known to support sustained weight loss and improve obesity-related comorbidities. They have also demonstrated cardioprotective effects. However, while these therapies predominantly reduce fat body mass, they may also lead to some loss of lean body mass, which has raised concerns, particularly for older adults and people at risk of frailty.

Study design and methods

To better understand these effects, researchers conducted a comprehensive evaluation of clinical studies assessing GLP-1-based therapies and their impact on body composition.

Databases including Web of Science, PubMed, and Scopus were systematically searched for relevant studies involving adults with overweight or obesity, with or without type 2 diabetes. Following removal of duplicate records, studies were screened based on titles, abstracts, and full texts to determine eligibility.

The methodological quality of the included studies was assessed using established tools. Meta-analyses were then performed on studies that provided numerical data on changes in body composition and anthropometric measures. Statistical analyses used random-effects models, with subgroup analyses based on drug type and treatment duration. Publication bias was evaluated using Egger’s test.

In total, 36 studies were included in the qualitative review, with 24 contributing to the meta-analyses. Most studies were conducted in Europe and Asia, and many reported outcomes at six months. Twenty-four studies included people living with type 2 diabetes. The most frequently studied medications were liraglutide and semaglutide. Body composition was commonly assessed using bioelectrical impedance analysis and dual-energy X-ray absorptiometry.

Changes in weight and body composition

Early effects at three months

After three months of treatment, participants experienced a significant reduction in body mass of approximately 9 percent. This effect was particularly notable among those receiving beinaglutide.

Substantial improvements in body composition were also observed. Visceral adipose tissue area decreased by 29.25 cm², while fat body mass was reduced by 17 percent. Lean body mass showed a smaller but statistically significant reduction of 2 percent.

In addition, body mass index decreased by 2.96 kg/m² and waist circumference by 9.6 cm.

Outcomes at six months

At six months, body mass remained significantly reduced, with an average decrease of 5 percent.

Both fat body mass and lean body mass declined further, by 6 percent and 1 percent respectively. Skeletal muscle mass showed a modest reduction of 3 percent. Visceral adipose tissue continued to decrease, with a reduction of 32.31 cm².

Body mass index fell by 2.40 kg/m², while waist circumference decreased by 2.3 cm.

Longer-term results at twelve months

At twelve months, body mass was reduced by 4 percent, with continued decreases in body mass index of 1.74 kg/m² and waist circumference of 3.2 cm.

Both fat body mass and lean body mass were reduced by 4 percent. The most pronounced reductions were reported in a study involving liraglutide, although the authors noted considerable variability between studies and advised caution when comparing specific agents.

Egger’s test indicated some publication bias in outcomes reported at three months, but no significant bias was observed at later time points.

Interpreting fat loss and muscle preservation

Overall, the findings demonstrate that GLP-1-based therapies produce meaningful improvements in key measures associated with obesity, including body mass, body mass index, and waist circumference, across multiple time points.

The most rapid and substantial changes occurred within the first three months of treatment. Across all timeframes, reductions in fat mass, particularly visceral fat, were more pronounced than reductions in lean mass.

The authors described this pattern as “quality” weight loss, characterised by a predominance of fat mass reduction with relative preservation of lean tissue. Importantly, no single GLP-1 receptor agonist was found to be superior in preserving lean mass.

Implications for clinical practice and future research

These findings have important implications for clinical care. While some degree of lean mass loss occurs with GLP-1-based therapies, the overall pattern of weight loss appears favourable, particularly given the substantial reductions in fat mass and visceral adiposity.

Future research should focus on optimising treatment strategies that combine pharmacotherapy with lifestyle interventions. In particular, there is a need to explore approaches that support the preservation of lean mass, such as targeted nutritional strategies and resistance training programmes.

Such considerations are especially important for people at higher risk of sarcopenia, including older adults and those with existing muscle loss.

CCH insights:

These results are encouraging, but it is important not to become complacent about lean mass loss during weight loss. The studies analysed here involved average weight losses of less than 10% of body weight, but some people can lose 15-20% of body weight on GLP-1 therapy – do they lose similar proportions of body fat and lean tissue? All patients on GLP-1 therapy should receive advice on lifestyle measures to help minimise lean tissue loss.