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Lean mass and body composition: what the DXA substudies measured

How much of the weight lost on semaglutide and tirzepatide was fat and how much was lean tissue, what DXA lean mass actually includes, how that compares with weight loss by other means, and what is known about protecting muscle.

By FormBlends editorial teamUpdated September 4, 2026Educational, not medical advice

"You lose muscle on these drugs" has become a fixed belief. The trials did measure body composition, in substudies, and the numbers are more specific than the belief.

What the substudies found

SURMOUNT-1. In a DXA substudy of 160 participants, total fat mass fell by 33.9% on tirzepatide (pooled doses) versus 8.2% on placebo, and lean mass fell by 10.9% versus 2.6%. The paper reports that about three quarters of the weight lost was fat and one quarter lean, and that the ratio was similar to the placebo group's smaller loss (PubMed 35658024).

STEP 1. In a DXA substudy of 140 participants, semaglutide reduced both fat mass and lean mass, with the fat loss larger in absolute and relative terms, so that lean mass as a proportion of total body mass rose (PubMed 33567185). Visceral fat fell as well.

SURPASS-3 MRI. In people with type 2 diabetes, tirzepatide reduced liver fat by 8.1 percentage points at the 15 mg dose versus 3.4 with insulin degludec, and reduced visceral and abdominal subcutaneous fat volumes (PubMed 35468325). This is fat quality as well as quantity: the depots most linked to metabolic harm shrank.

Fat and lean mass changes on tirzepatide versus placebo, and what lean mass includesLeft panel: four downward bars showing percent change. Tirzepatide fat mass minus 33.9 percent, placebo fat mass minus 8.2 percent, tirzepatide lean mass minus 10.9 percent, placebo lean mass minus 2.6 percent. A note says about three quarters of weight lost on tirzepatide was fat. Right panel: a stacked block labelled DXA lean mass split into skeletal muscle, water, organs and connective tissue, with a note that DXA cannot separate muscle from the other components.Percent change from baseline, 72 weeks0-10%-20%-30%-33.9%-8.2%-10.9%-2.6%Fat massLean masstirzepatide, pooled dosesplaceboabout 3/4 of weight lost was fatWhat DXA calls lean massSkeletal muscleBody waterOrgansConnective tissueDXA reports the sum. It cannot say how muchof a lean-mass change is muscle.block heights are illustrative, not proportions
Figure 8. Composition of weight lost in the SURMOUNT-1 DXA substudy, drawn to the reported percent changes, and what DXA lean mass contains. Left bars: percent change in fat mass and lean mass by arm (PubMed 35658024). Right: the components DXA reports as lean.

What "lean mass" is

DXA divides the body into fat, bone mineral and everything else, and calls the everything else lean mass. Skeletal muscle is the largest part of it, but so is body water, and organ tissue, blood volume and the connective scaffolding that held the lost fat all count. When a person loses 20% of body weight, some lean loss is structural: less tissue to carry, less blood to supply it, less water. Cava, Yeat and Mittendorfer's review puts the usual lean share of diet-induced weight loss at around 20 to 30% without resistance training (PubMed 28507015), which is the range the DXA substudies fall in. Nothing in the trial data shows a lean loss out of proportion to the weight loss.

What the data cannot show is muscle quality or strength. Neither substudy measured muscle cross-sectional area, strength or physical function, and the honest position is that the muscle question is open, not answered in either direction.

What is known about protecting it

The evidence is borrowed from weight loss in general. Villareal and colleagues randomised older adults with obesity to diet plus aerobic, resistance or combined exercise: lean mass fell least in the resistance and combined groups, and physical function improved most with the combination (PubMed 28514618). Leidy and colleagues reviewed protein intake during energy restriction and found higher intakes, in the range of 1.2 to 1.6 g per kg per day, associated with better lean retention and satiety (PubMed 25926512). Whether these interventions change outcomes for people on GLP-1 medicines specifically is the subject of ongoing trials; extrapolating from general weight loss is reasonable but is extrapolation.

What is debated

Whether the lean lost on treatment is regained as fat after stopping, leaving a worse composition than baseline, is a concern raised from the general weight-cycling literature and not yet measured in a GLP-1 withdrawal trial. Whether drugs that add a myostatin or activin pathway blocker will preserve muscle during GLP-1 treatment is being tested and is not established.

Where to go next

Questions people ask

Is losing lean mass the same as losing muscle?

Not exactly. DXA lean mass is everything that is not fat or bone: skeletal muscle, but also water, organ tissue and the connective tissue that supported the lost fat. Some lean loss during weight loss is expected and not harmful. Whether the muscle component falls more than it would with the same weight loss by other means has not been shown.

What is known about protecting muscle during treatment?

The evidence comes from weight loss in general, not from GLP-1 trials. Resistance training preserved lean mass better than aerobic exercise alone in older adults losing weight (PubMed 28514618), and higher protein intake is associated with better lean retention during energy restriction (PubMed 25926512). Whether these change outcomes on GLP-1 medicines specifically is being tested, not proven.

Sources

  1. Jastreboff AM, Aronne LJ, Ahmad NN, et al. Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1). N Engl J Med 2022. PubMed 35658024 Accessed September 4, 2026.
  2. Wilding JPH, Batterham RL, Calanna S, et al. Once-weekly semaglutide in adults with overweight or obesity (STEP 1). N Engl J Med 2021. PubMed 33567185 Accessed September 4, 2026.
  3. Cava E, Yeat NC, Mittendorfer B. Preserving healthy muscle during weight loss. Adv Nutr 2017. PubMed 28507015 Accessed September 4, 2026.
  4. Villareal DT, Aguirre L, Gurney AB, et al. Aerobic or resistance exercise, or both, in dieting obese older adults. N Engl J Med 2017. PubMed 28514618 Accessed September 4, 2026.
  5. Leidy HJ, Clifton PM, Astrup A, et al. The role of protein in weight loss and maintenance. Am J Clin Nutr 2015. PubMed 25926512 Accessed September 4, 2026.
  6. Gastaldelli A, Cusi K, Fernández Landó L, et al. Effect of tirzepatide versus insulin degludec on liver fat content and abdominal adipose tissue in people with type 2 diabetes (SURPASS-3 MRI). Lancet Diabetes Endocrinol 2022. PubMed 35468325 Accessed September 4, 2026.

Canonical URL: https://formblendsscience.com/mechanisms/lean-mass-and-body-composition. Written by the FormBlends editorial team. This page is educational and is not medical advice; see the medical disclaimer.