Learn

Losing weight without losing muscle: tracking lean mass, including on GLP-1 medicines

The scales tell you how much weight you have lost, but not what it was made of. Some of it will be fat and some will be lean mass: muscle, and other tissue that isn't fat. That second part is worth keeping track of, and weight loss medicines have made it a common question.

Last reviewed

Lean mass, fat-free mass and muscle

Your body weight can be split into two parts: fat, and everything else. The “everything else” is usually called lean mass or fat-free mass, and it is worked out like this:

lean mass = weight × (1 − body fat %)

Someone who weighs 100 kg at 35% body fat is carrying 35 kg of fat and 65 kg of lean mass.

Lean mass is not the same as muscle. It includes muscle, but also bone, organs, connective tissue and water. The water part changes day to day and moves quickly at the start of a diet. So a fall in lean mass is not automatically a loss of muscle, although muscle is usually part of it.

What the medicine trials measured

The two large trials behind the current weight-loss injections each scanned a subgroup of participants with DXA, which measures fat and lean mass separately:

  • Semaglutide (STEP 1). Of an average 13.6 kg lost, 8.3 kg (62%) was fat and 5.3 kg (38%) was lean mass. Fat fell by about 19% and lean mass by about 10%, so the share of the body that was lean still went up.
  • Tirzepatide (SURMOUNT-1). Participants on the medicine lost about 21% of their body weight, including 34% of their fat and 11% of their lean mass. About three-quarters of the loss was fat and a quarter was lean mass. The same split, 75:25, was seen in the placebo group, who lost much less weight in total.

That last point matters. A quarter to a third of weight lost being lean mass is not unusual, and it is not special to these medicines. It is roughly what weight loss by diet alone produces too. What is different with the medicines is that people lose much more weight, so the lean mass lost is larger in kilograms.

“A quarter of weight lost is muscle” is only a rule of thumb

The idea that about one-quarter of any weight loss is fat-free mass is widely quoted. A 2014 review that examined it concluded it is “at best an approximation”. The real share depends on how much fat you start with (people with more fat tend to lose proportionally less lean mass), on how fast you lose weight, on diet, and on whether you are doing exercise that loads the muscles.

What published guidance says

In May 2025 four US organisations — the American College of Lifestyle Medicine, the American Society for Nutrition, the Obesity Medicine Association and The Obesity Society — published a joint advisory on nutrition for people taking GLP-1 medicines. It emphasises “adequate protein intake and strength training to preserve lean mass”, and “preserving muscle and bone mass through resistance training and appropriate diet”.

How much protein and what kind of training suits you depends on your health, your other medicines and your kidney function. That is a conversation for whoever prescribes your treatment, or a registered dietitian. This page does not give targets.

Following your own lean mass at home

You cannot get a DXA scan every month, but you can follow a trend. Here is the same person as above, some months on:

  • Before 100 kg at 35% body fat → 65.0 kg lean, 35.0 kg fat
  • After 88 kg at 30% body fat → 61.6 kg lean, 26.4 kg fat

Of the 12 kg lost, 8.6 kg was fat and 3.4 kg (28%) was lean mass, close to what the trials found. The catch is how much this depends on the body-fat figure. If the second reading had come out at 33% instead of 30%, the same calculation would say 6.0 kg of lean mass lost. At 27%, it would say 0.8 kg. A three-point error in body fat, well within what home methods produce, changes the answer completely.

So the things that make a home trend worth having are:

  • One method, used consistently. The same smart scale, the same caliper sites, or the same tape measurements — never a mix. Different methods disagree with each other by more than the change you are looking for.
  • The same conditions. Same time of day, same point in your routine. Scales that estimate body fat from an electrical current are affected by how much water you are carrying.
  • Several readings, not two. Look at the direction over weeks. A single surprising reading is more likely measurement noise than a real change.
  • Know your method's blind spots. Tape-measure formulas are slow to show short-term change: a 2023 study found they underestimated the fall in body fat over eight weeks of training in men and missed it entirely in women (see theUS Navy method).

Strength is a useful cross-check that needs no equipment. If the weights you lift, or the everyday tasks you do, are holding steady while your weight falls, that is reassuring alongside the numbers.

Sources

  • Wilding JPH, et al. “Once-weekly semaglutide in adults with overweight or obesity” (STEP 1).New England Journal of Medicine, 2021;384:989–1002 — DXA substudy.
  • Look M, et al. “Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study of adults with obesity or overweight.” Diabetes, Obesity and Metabolism, 2025. doi:10.1111/dom.16275.
  • Heymsfield SB, et al. “Weight loss composition is one-fourth fat-free mass: a critical review and critique of this widely cited rule.” Obesity Reviews, 2014;15(4):310–321.
  • Mozaffarian D, et al. “Nutritional priorities to support GLP-1 therapy for obesity: a joint Advisory from the American College of Lifestyle Medicine, the American Society for Nutrition, the Obesity Medicine Association, and The Obesity Society.” Published 30 May 2025 inObesity (doi:10.1002/oby.24336) and three other journals.
  • Foulis SA, et al. “Body composition changes during 8 weeks of military training are not accurately captured by circumference-based assessments.” Frontiers in Physiology, 2023;14:1183836.

This page is reference information about how a measurement is taken and what the research says about it. It is not medical advice, and no figure on it is a diagnosis. Talk to a doctor, nurse or registered dietitian about your own health.