Incretin Weight Loss and Body Composition: What the 2026 Evidence Shows

A systematic review lands at the right moment

Semaglutide, tirzepatide, liraglutide, and related incretin therapies can produce substantial changes in scale weight. That number alone does not show how much change came from fat, lean soft tissue, skeletal muscle, fluid, or other compartments. It also does not show whether strength or physical function changed.

A systematic review published in Annals of Internal Medicine (PMID 41996180) addresses that question directly. The review compares body composition outcomes across incretin-based pharmacotherapy and nonpharmacologic weight loss strategies. It arrives alongside a companion meta-analysis in Diabetes, Obesity and Metabolism (PMID 41877354, Eisa N et al., March 2026) that pooled randomised controlled trial data on lean mass changes for incretin therapy versus lifestyle intervention specifically.

The timing matters. A 2026 RCT protocol in BMJ Open (PMID 42020128, Alawadhi AA et al.) describes LEAN-PREP, which will test resistance exercise and protein supplementation in people initiating semaglutide or tirzepatide. The protocol is a study plan, not evidence that either intervention works in this setting.

This article summarises what the 2026 Ann Intern Med review found, puts the numbers in context using the Eisa et al. meta-analysis, flags what a concurrent European Heart Journal paper (PMID 41914150) adds about the cardiovascular implications of losing versus preserving muscle, and explains why the LEAN-PREP trial design is notable.

No dosing guidance is offered here. These medications require a prescribing clinician, and the research reviewed below comes from supervised clinical trials, not self-administration contexts.

What the Ann Intern Med review examined

The Annals systematic review (PMID 41996180) included 35 primary studies from a screen of 8,102 titles and abstracts. Eligible studies were English-language randomized trials in adults reporting body-composition outcomes for liraglutide, semaglutide, tirzepatide, or dulaglutide. The median study duration was 26 weeks, the median sample size was 78 participants, and 42.9% of studies were rated at low risk of bias.

Body composition in most of the included studies was measured by dual-energy X-ray absorptiometry (DXA) or bioelectrical impedance analysis, which distinguishes fat mass from fat-free mass (lean mass). The review captured data on:

• Total body weight change
• Fat mass change (absolute and as a percentage of total weight lost)
• Lean mass change (absolute and as a percentage of total weight lost)
• Bone mineral density where reported

Several points from the review’s framing are important. First, incretin-based therapies tend to produce substantially greater total weight loss than nonpharmacologic methods alone. This means absolute lean mass changes look larger on pharmacotherapy simply because the person is losing more weight overall. The more diagnostic comparison is the proportion of weight lost that comes from lean mass versus fat mass.

Second, among incretin groups the median proportion of total weight loss attributed to muscle-based indices was 28.3% (IQR, 15.9% to 39.9%). Thirteen studies also reported weight loss in lifestyle or placebo comparator groups, but the authors did not pool a simple drug-versus-lifestyle effect because measurements and reporting were heterogeneous.

Third, the review highlights considerable heterogeneity across included trials in how body composition was measured and reported, making direct numerical pooling across all studies difficult. This is not a limitation unique to this review; it reflects a genuine gap in standardisation across the obesity pharmacotherapy literature.

The clinical implication the authors point toward: total kilograms lost is a useful but incomplete metric. Clinicians and researchers need body composition endpoints to evaluate these therapies properly.

How incretin drugs drive weight loss: the mechanism behind the composition question

Understanding why body composition becomes an issue requires a short detour into mechanism.

GLP-1 receptor agonists reduce food intake primarily through two pathways: slowing gastric emptying (which extends satiety after meals) and acting on hypothalamic and brainstem circuits that regulate hunger. The net result is a sustained caloric deficit over weeks and months.

Tirzepatide adds GIP receptor agonism to GLP-1R agonism. Head-to-head SURMOUNT-5 analyses report different weight and metabolic outcomes from semaglutide, but those findings do not isolate the contribution of GIP signaling or establish which tissue compartment caused the difference. A post hoc glycemia analysis (PMID 42002673), for example, is not body-composition evidence.

The body composition challenge is rooted in physiology: any sustained caloric deficit triggers both fat and lean tissue catabolism. The body does not exclusively burn adipose stores. Muscle protein turns over constantly, and when energy availability is chronically low, muscle protein breakdown accelerates relative to synthesis. The degree to which this occurs depends on:

1. Protein intake relative to lean mass targets
2. Mechanical loading (resistance exercise signals mTOR to preserve muscle protein synthesis)
3. Rate of weight loss (faster loss correlates with higher lean mass fraction lost)
4. Baseline body composition (individuals with higher adiposity lose proportionally less lean mass)

GLP-1R / GIP-R agonism
          |
          v
changes in appetite, intake, and gastric emptying
          |
          v
sustained energy deficit
       /        \
      v          v
fat-mass      lean-tissue change
change             |
                   v
       interpreted with exercise, diet,
       baseline composition, and function

The flowchart above shows why body-composition outcomes are not determined by the drug alone. Energy balance is one input; diet, activity, baseline composition, illness, age, and measurement method can also affect the observed result.

Numbers from the Eisa et al. meta-analysis

The Eisa N et al. meta-analysis (PMID 41877354, Diabetes, Obesity and Metabolism, March 24, 2026) pooled data from randomised controlled trials comparing lean mass outcomes in incretin therapy versus lifestyle intervention. It provides the most direct numerical comparison currently available.

Key quantitative findings from that meta-analysis:

• Lean mass as a proportion of total weight lost: across incretin therapy trials, approximately 25-39% of weight lost was lean mass. Lifestyle-only arms showed a similar range, roughly 20-35%, depending heavily on whether resistance training was included.
• Absolute lean mass loss: because incretin therapies produce larger total weight loss, the absolute kilograms of lean tissue lost were greater in pharmacotherapy arms even when the proportional loss was comparable.
• Exercise context matters: pooled lifestyle interventions that included resistance training had a lean-mass proportion of 17.5% (95% CI, 14.2-20.8), but this does not establish the effect of adding resistance training to every incretin regimen.

The meta-analysis authors note that most included trials did not prospectively control protein intake, and that protein targets varied widely. This is a confounder that complicates interpretation: some of the lean mass differences between arms may reflect diet composition rather than the drug effect itself.

The meta-analysis found broadly comparable proportional lean-mass loss between incretin pharmacotherapy and lifestyle interventions. That result argues against treating the drug class as uniquely muscle-wasting, but it does not prove that all agents, populations, or intervention designs have the same effect.

This is the crux of the Ann Intern Med review’s contribution: it frames the body composition question not as “drugs vs. lifestyle” but as “caloric deficit magnitude and rate vs. structural supports for lean mass preservation.”

Cardiovascular implications of the lean mass question

A companion perspective published in European Heart Journal (PMID 41914150, Khan MS et al., March 31, 2026) addresses why the lean-versus-fat distinction matters beyond aesthetics or athletic performance.

The paper focuses on the relationship between body composition and cardiovascular disease (CVD) risk reduction. Several findings from that analysis are relevant here:

Fat mass and cardiovascular outcomes are related but not interchangeable. The SELECT trial reported fewer major cardiovascular events with semaglutide than placebo in participants with cardiovascular disease and overweight or obesity without diabetes.[5] That outcome trial does not, by itself, prove that a particular change in visceral fat mediated the benefit. Compartment-specific mediation requires dedicated measurement and analysis.

Lean mass preservation has independent cardiovascular relevance. Skeletal muscle is the primary site of glucose disposal after meals. Low muscle mass is associated with insulin resistance, higher fasting glucose, and elevated CVD risk even after adjusting for total body weight. Sarcopenia (clinically meaningful muscle mass and function loss) is an independent predictor of heart failure hospitalisation and cardiovascular mortality in older adults.

The evidence gap in older populations. The Annals review included studies with mean ages ranging from 20 to 63.7 years. This limits inference for older adults at higher baseline risk of sarcopenia or frailty. The European Heart Journal review argues that future trials should measure muscle quality and function rather than relying on scale weight or lean mass alone.

Read together, these papers support measuring fat, lean tissue, and function separately. They do not provide a universal monitoring schedule or an individualized exercise or nutrition prescription.

The LEAN-PREP trial: the field’s next step

The LEAN-PREP study protocol (PMID 42020128, Alawadhi AA et al., BMJ Open, 2026) directly addresses gaps identified in the review literature.

What LEAN-PREP is testing. The trial is a randomised controlled study designed to determine whether a structured resistance exercise program combined with individualised protein intake targets can preserve lean body mass in people prescribed semaglutide or tirzepatide. It is one of the first trials with this specific intervention structure in GLP-1/GIP-RA pharmacotherapy users.

Why the design is notable.

• Prior trials that added exercise to GLP-1 therapy were not primarily powered to detect body composition differences. LEAN-PREP is.
• Protein targets are prospectively defined and monitored, addressing the confounder the Eisa meta-analysis identified.
• Both semaglutide and tirzepatide arms are included, which will allow a secondary comparison of whether the dual-agonist’s greater total weight loss translates to different lean mass outcomes under the same exercise and nutrition protocol.

What it will not answer. LEAN-PREP is a protocol paper; results are not yet available. The trial will answer whether resistance exercise plus protein protects lean mass in this context, but it will not resolve questions about optimal exercise volume, protein dose, or how outcomes differ by age, sex, or baseline body composition. Those questions will require further trials.

For researchers following body-composition outcomes, LEAN-PREP is useful because it prespecifies an intervention, a comparator structure, MRI muscle area, DXA composition, strength, and physical-function endpoints. Those design choices are more informative than assuming that a change in DXA lean mass is equivalent to a change in muscle performance.

Nonpharmacologic approaches: what the comparison data show

The Ann Intern Med review’s nonpharmacologic comparator arm is worth examining on its own terms. Not everyone taking an incretin drug will have access to structured exercise programs. And for some individuals, nonpharmacologic weight loss is the primary or only approach.

The review found that nonpharmacologic interventions produce a wide range of body composition outcomes depending on their design:

Caloric restriction only (matched to a pharmacotherapy-level deficit) produces lean mass loss proportions similar to incretin monotherapy, roughly 25-35% of weight lost. This is consistent with decades of caloric restriction literature and underlines that the drug is not uniquely muscle-wasting; the deficit is the primary driver.

Caloric restriction plus resistance training consistently produces the most favorable body composition ratios. In the best-designed trials, lean mass loss falls to 10-20% of total weight lost, with some protocols showing net lean mass preservation or gain while fat mass declines. This is the benchmark the LEAN-PREP trial aims to replicate in a pharmacotherapy context.

Aerobic exercise without dietary restriction produces modest total weight loss but good fat-to-lean ratios. The absolute fat mass lost is smaller than with restriction, but lean mass is largely preserved. This approach underperforms on total weight loss but performs well on body composition quality.

More intensive energy restriction can change both the magnitude and composition of weight loss. Cross-trial comparisons remain vulnerable to differences in duration, baseline adiposity, measurement method, diet composition, and exercise exposure.

Taken together, the nonpharmacologic data show that intervention design matters. They do not establish that behavior is the only determinant of body composition or that pooled results predict an individual’s response.

Limitations of the current evidence base

The Ann Intern Med review is a valuable synthesis, but it is also an honest accounting of how incomplete the current body composition literature is.

Measurement heterogeneity. DXA is a common reference method for body composition in clinical research, but included studies also used bioelectrical impedance analysis (BIA), which is sensitive to hydration and model assumptions. Comparing DXA-derived lean mass changes with BIA-derived estimates across trials introduces uncertainty that pooling cannot fully remove.

Limited follow-up for a long-term question. Follow-up in the Annals review had a median of 26 weeks, while weight-management therapy can continue much longer. Whether compartment changes plateau, continue, or partly reverse over longer periods is not well characterized by those pooled data.

Under-representation of older adults. The Khan et al. European Heart Journal paper (PMID 41914150) specifically flags that trial populations skew younger and healthier than the real-world population initiating incretin therapy. Lean mass loss consequences are most clinically significant in older adults, yet this is the group least represented in the trials the review could pool.

Protein intake not controlled. As the Eisa meta-analysis notes, protein intake was rarely standardized across trial arms. Because dietary intake can affect lean-tissue outcomes during an energy deficit, this is an important confounder.

No trials with muscle function primary endpoints. Body composition (lean mass in kilograms) and muscle function (strength, power, physical performance) are related but not identical. An individual can lose lean mass without losing grip strength, and vice versa. None of the trials reviewed used muscle function as a primary endpoint, which limits clinical interpretation.

These limitations are not reasons to dismiss the review’s findings. They are a roadmap for the next generation of trials, of which LEAN-PREP is one.

Common questions

Is the lean mass loss on GLP-1 drugs a reason to avoid them?

The review does not answer an individual’s treatment decision. It shows consistent fat and visceral-adiposity reduction alongside variable muscle-related losses, with no objective physical-function outcomes in the included studies. Benefits, risks, alternatives, and monitoring depend on the indication and patient context and belong in a clinical discussion.

What proportion of weight lost is lean mass on semaglutide?

The Eisa et al. meta-analysis (PMID 41877354) found roughly 25-39% of total weight lost was lean mass across incretin therapy arms. This range overlaps with caloric restriction-only arms, suggesting it is the deficit rather than the drug that determines the ratio. Trials incorporating resistance exercise and adequate protein pushed the proportion toward 15-20%.

Should clinicians order body composition testing for people on GLP-1 therapy?

The Ann Intern Med review does not make a specific clinical recommendation on testing frequency. The authors do call for body composition endpoints to be included in future trials as standard. In clinical practice, the appropriateness of DXA monitoring will depend on individual patient risk (age, baseline muscle mass, functional status) and is a conversation for the prescribing clinician.

Do the reviews establish a medication strategy for preserving lean mass?

No. The European Heart Journal article discusses investigational approaches, while LEAN-PREP is testing non-drug interventions. Neither source supplies a validated medication protocol for preserving lean mass during incretin therapy.

Does tirzepatide preserve lean mass better than semaglutide?

The LEAN-PREP publication (PMID 42020128) is a trial protocol, not a results paper. It cannot establish that one agent preserves lean tissue better. A valid comparison needs reported compartment and function outcomes under a prespecified head-to-head design; receptor-level reasoning alone is insufficient.

Bottom line

The 2026 Annals systematic review (PMID 41996180) found that incretin-based interventions produced substantial fat-mass reduction and also reduced skeletal-muscle mass or fat-free mass. Proportional lean-tissue change was not consistently greater than with nonpharmacologic comparators, but heterogeneity and limited function data prevent a universal estimate.

Three papers published within the same six-week window reinforce the review’s framing:

1. Eisa N et al. (PMID 41877354, Diabetes Obes Metab, March 2026): meta-analysis quantifying lean-mass proportions across randomized trials; resistance-exercise co-intervention was associated with a lower pooled proportion, but that comparison is not proof of an effect for every regimen.
2. Khan MS et al. (PMID 41914150, Eur Heart J, March 2026): establishes that lean mass preservation has cardiovascular relevance, with particular concern for older adults.
3. Alawadhi AA et al. (PMID 42020128, BMJ Open, 2026): LEAN-PREP trial protocol, a randomized study designed with muscle area and function endpoints in semaglutide and tirzepatide users.

The practical research takeaway is that scale weight should not be the only endpoint. Trials need standardized body-composition methods, prespecified muscle measures, adverse-event reporting, and objective strength or physical-function outcomes.

LEAN-PREP and similar trials may clarify whether changes in muscle area, lean mass, strength, and function move together. Until results are available, the protocol should not be cited as proof of benefit.

What about retatrutide?

Retatrutide is relevant to future body-composition research but was not one of the four agents eligible for the 2026 Annals systematic review. It should therefore not inherit the pooled semaglutide, tirzepatide, liraglutide, or dulaglutide estimates. Retatrutide studies need prospectively defined DXA or imaging outcomes plus strength and physical-function measures before its fat-to-lean pattern can be compared reliably. See the retatrutide research guide for compound-specific evidence.

References

1. Batsis JA, et al. Effect of incretin-based and nonpharmacologic weight loss on body composition: a systematic review. Annals of Internal Medicine. 2026. https://pubmed.ncbi.nlm.nih.gov/41996180/
2. Eisa N, et al. Lean mass changes with incretin therapy versus lifestyle intervention: a systematic review and meta-analysis of randomized controlled trials. Diabetes, Obesity and Metabolism. 2026. https://pubmed.ncbi.nlm.nih.gov/41877354/
3. Khan MS, et al. Fat, muscle, and anti-obesity medications in cardiovascular disease prevention. European Heart Journal. 2026. https://pubmed.ncbi.nlm.nih.gov/41914150/
4. Alawadhi AA, et al. LEAN mass Preservation with Resistance Exercise and Protein during semaglutide and tirzepatide therapy (LEAN-PREP): protocol for a randomized controlled trial. BMJ Open. 2026. https://pubmed.ncbi.nlm.nih.gov/42020128/
5. Lincoff AM, et al. Semaglutide and cardiovascular outcomes in obesity without diabetes. New England Journal of Medicine. 2023. https://pubmed.ncbi.nlm.nih.gov/37952131/

This article is a synthesis of publicly available research. It does not constitute medical advice. Decisions about weight loss pharmacotherapy should be made with a qualified healthcare provider.