Stepping on the scale has become a ritual for many people on treatment with a GLP-1 medication. The number goes down, and it feels like progress. But that number alone says little about what’s really changing in your body. It adds everything up: fat, muscle, bone, organs and water.

In this article, you’ll learn what body composition is, why it matters during weight loss, how it’s usually measured and what’s worth tracking besides weight.

What body composition is

Body composition is a way of looking at the body divided into parts. The most common split separates fat mass from lean mass, which includes muscle, bone, organs and the body’s water.

Two people with the same weight and height can have very different compositions. One may have more muscle; the other, more fat. And that makes a difference for health, for strength and for everyday life.

Why it matters during treatment

When weight comes off, not all of it is fat. The large trials that led to the approval of semaglutide (STEP 1) and tirzepatide (SURMOUNT-1) for treating obesity measured body composition in some of the participants. In both, most of the weight lost was fat, but a meaningful share was lean mass.

That’s why looking only at the scale can hide part of the story. Two months with the same weight loss can mean different things, depending on how much of that loss came from fat and how much from muscle. It’s also why adequate protein and strength training show up so often in the recommendations for people on this kind of treatment.

The limits of BMI

Body mass index (BMI), calculated from weight and height, is widely used, but it has limits. In 2025, an international commission published in The Lancet Diabetes & Endocrinology proposed new criteria for diagnosing obesity and pointed out that BMI can both underestimate and overestimate how much fat a person has.

The commission recommends using BMI as a screening tool and for population studies, not as a measure of individual health. To confirm excess fat, it suggests adding at least one body measurement, such as waist circumference, waist-to-height ratio or waist-to-hip ratio, or a direct measurement of body fat when available.

How body composition is measured

There are several methods, each with pros and cons:

DXA (whole-body densitometry). An imaging scan that separates fat, lean mass and bone. It’s considered accurate, but it isn’t always available and it costs money.
Bioelectrical impedance. Scales and devices that send a weak electrical current through the body and estimate composition from it. They’re practical and common in gyms and clinics, but they give an indirect estimate based on equations.
Tape measure. Measuring your waist and hips is simple, cheap and easy to repeat at home.

A 2016 review of body composition assessment in adults with more severe obesity identified five practical considerations, including the body’s hydration, the person’s position during the test and equipment limits. The same review concluded that, in this group, bioelectrical impedance tended to overestimate lean mass, while DXA gave more reliable measurements.

In practice, that means the number from a bioimpedance scale should be seen as an estimate. If you use one, always compare under the same conditions: same device, same time of day, ideally in the same state of eating and hydration. And bring the results to the people following your treatment to interpret.

What to track besides weight

The scale is still useful, but it gives a fuller picture alongside other signs:

Tape-measure numbers. Waist and hips, measured the same way every time, every few weeks.
How your clothes fit. Pants that button more easily tell a story the scale doesn’t always show.
Everyday strength. Climbing stairs without stopping, carrying groceries, getting up from a chair more easily.
Progress in your workouts. More reps, more load, a stronger band.
Tests ordered by your doctor. When it makes sense, they can order a body composition assessment.

A fuller picture

The goal of treatment isn’t just to see a smaller number on the scale. It’s to gain health, preserve strength and live better. Looking at body composition helps keep the focus on what matters: losing fat while looking after lean mass.

Logging weight, measurements and strength wins over the weeks, in a notebook or an app, creates a much fuller picture of your progress. And that picture is great to bring to your appointment.

Sources

Rubino F, et al. Definition and diagnostic criteria of clinical obesity. The Lancet Diabetes & Endocrinology, 2025.
Johnson Stoklossa CA, et al. Practical Considerations for Body Composition Assessment of Adults with Class II/III Obesity Using Bioelectrical Impedance Analysis or Dual-Energy X-Ray Absorptiometry. Current Obesity Reports, 2016.
Wilding JPH, et al. Once-weekly semaglutide in adults with overweight or obesity (STEP 1). New England Journal of Medicine, 2021.
Jastreboff AM, et al. Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1). New England Journal of Medicine, 2022.

Body composition: why the scale doesn't tell the whole story