Measuring progress beyond the scale: BMI limits, waist circumference, DXA and BIA

The scale measures mass. On a GLP-1 the mass that leaves is mostly fat but not entirely, and the mass that matters for health is not evenly distributed. Three other measures fill in what the scale cannot.

BMI: useful for populations, limited for you

BMI is weight in kilograms divided by height in metres squared. The WHO cut points (18.5, 25, 30, 35, 40) are conventions for population statistics. The Wegovy and Zepbound labels use 30, or 27 with a weight-related condition, to describe the trial populations. During treatment BMI falls in proportion to weight, so it adds nothing the scale does not already say. What it cannot see:

• Composition. A kilogram of muscle and a kilogram of fat count the same.
• Distribution. Fat around the organs carries more metabolic risk than fat under the skin; BMI is blind to where fat sits.
• Change in composition under treatment. If you gain muscle while losing fat, BMI understates progress.

The FormBlends Calculators site has a BMI tool that shows the weights at each cut point for your height; it is context, not a target.

Waist circumference: the cheapest measure of the fat that matters

A 2020 consensus statement from the International Atherosclerosis Society and the International Chair on Cardiometabolic Risk reviewed the evidence and concluded that waist circumference predicts cardiometabolic risk independently of BMI, that it should be measured routinely as a vital sign, and that it declines during weight loss in a way that tracks visceral fat (PubMed 32020062).

Method, from the same statement: stand, breathe out normally, measure at the top of the iliac crest (the hip bone) or at the midpoint between the lowest rib and the iliac crest, tape snug but not compressing, parallel to the floor. Use the same landmark every time; the two landmarks give different numbers. Once a month is enough; the change over months is the signal.

Thresholds commonly used in guidelines are 102 cm (40 in) for men and 88 cm (35 in) for women, with lower population-specific values for South Asian, Chinese and Japanese adults among others. The consensus statement argues these should be refined by BMI category, and that a reduction in waist at a stable weight still indicates lost visceral fat.

In the SURMOUNT-1 DXA substudy, visceral fat fell along with total fat, and the reduction in fat mass was about three times the reduction in lean mass in percentage terms (33.9 versus 10.9 percent; PubMed 39996356). A falling waist is that fat leaving.

DXA: the reference the trials used

Dual-energy X-ray absorptiometry is what STEP 1 and SURMOUNT-1 used to measure fat, lean and bone. It is available in many hospitals and some sports-science and private clinics at modest cost, uses a very low radiation dose, and takes about ten minutes. It reports total and regional fat mass, lean soft tissue and bone mineral content. Two scans on the same machine, months apart, tell you what share of your loss was lean. The 2025 joint advisory lists body composition and muscle strength assessment among the baseline screening items for someone starting GLP-1 therapy (PubMed 40445127), which is an argument for a scan before or early in treatment, not only after.

Bioimpedance scales: read with care

Home scales that report body fat percentage pass a small current through the body and estimate composition from resistance. The estimate depends on hydration, recent food, exercise, skin temperature and the equation the manufacturer chose. On a GLP-1, hydration is exactly the thing that varies, because intake drops and gastrointestinal effects shift fluid. A single reading is unreliable; a consistent morning reading on the same device, averaged over weeks, tracks direction reasonably. Do not compare its absolute number with a DXA result.

Strength as a measure

Grip strength, the number of sit-to-stands in 30 seconds, and the weight you can lift for five repetitions are all body-composition measures in disguise: they track muscle function directly. They cost nothing and change in weeks. The resistance training guide suggests keeping a log of a few lifts for exactly this reason. If the scale is falling and your lifts are steady or rising, you are keeping what matters.

Putting it together

Weekly: weight, averaged (see how to weigh yourself). Monthly: waist at one landmark. Every training block: the same few lifts. Once, and again after six to twelve months if you can: DXA. That set answers the question the scale alone cannot: how much of what left was fat.