Why one number is not the whole story
Body weight counts fat and everything that is not fat together. The US labels for Wegovy®, Zepbound® and Foundayo™ say the medicine lowers weight with more fat mass lost than lean mass [1][2][3], and the EU product information for Saxenda® describes liraglutide working mainly through loss of fat, with visceral fat falling relatively more than fat under the skin [10]. A scale cannot show any of that.
This is why trials report other measures too. Waist circumference is one of them, and it is the one you can repeat yourself with a tape. Our article on lean mass in GLP-1 trials covers the muscle question in more detail.
Waist size in each weight-management trial
Each weight-management label has a table of changes in waist and cardiometabolic measures. These are group averages from the start to the end of one trial, with a reduced-calorie diet and more activity in every group, and each result below is set against that trial’s own placebo group.
- Wegovy® (Study 2, the STEP 1 trial; 2.4 mg weekly for 68 weeks): waist down 13.5 cm on average, against 4.1 cm with placebo, a difference of 9.4 cm [1].
- Zepbound® (Study 1, the SURMOUNT-1 trial; 72 weeks): down 14.0, 17.7 and 18.5 cm with 5, 10 and 15 mg, against 4.0 cm with placebo [2].
- Foundayo™ (Trial 1, people without diabetes; 72 weeks): down 7.0, 8.2 and 10.0 cm with 5.5, 9 and 17.2 mg daily, against 3.1 cm with placebo [3].
- Saxenda® (Study 1, people without diabetes; 3 mg daily for 56 weeks): down 8.2 cm, against 4.0 cm with placebo [4].
Reading these numbers fairly
The trials differed in length, starting weight, population and design, so these figures cannot be ranked against each other. They are not head-to-head comparisons, and none of them predicts what will happen for one person.
Two patterns inside the labels are worth noticing. Placebo groups, who had the same diet and activity counseling without the medicine, also lost several centimetres on average, so the lifestyle part contributes as well. And within each label, the trial in people with type 2 diabetes reported a smaller average waist change than the trial in people without diabetes [1][2][3][4]. Who took part matters as much as which medicine it was.
After 20 weeks: continuing or switching
The STEP 4 trial gave everyone semaglutide for 20 weeks first, then randomly assigned them to carry on or to switch to placebo for another 48 weeks [5]. In the Wegovy® label’s version of this trial, waist size fell by a further 6.4 cm with continued treatment and rose by 3.3 cm after the switch, a difference of 9.7 cm [1]. Blood pressure and physical functioning scores also favoured continuing [5].
This trial is about stopping or continuing, which is a decision for you and your prescriber. Our article on stopping a GLP-1 describes it in more detail.
What body scans found
The European product information for Wegovy® describes a DEXA scan sub-study of 140 people in STEP 1 and an MRI sub-study of 55 people in a later trial called STEP UP. In both, fat fell more than lean mass compared with placebo, and in STEP 1 visceral fat, the fat around the organs, fell as well [6]. The Mounjaro® product information (the EU brand of tirzepatide) reports the same direction from a DEXA sub-study in SURMOUNT-1 and concludes that most of the weight lost was fat tissue [7].
That SURMOUNT-1 sub-study has since been published in full. Of the 160 people with scans at the start and at week 72, 124 took tirzepatide and 36 placebo. Body weight fell by 21.3% on tirzepatide, fat mass by 33.9% and lean mass by 10.9%; the placebo figures were 5.3%, 8.2% and 2.6%. Visceral fat fell by 40.1% against 7.3%, and waist size by 18.1 cm against 3.4 cm [9].
The authors also looked at the make-up of the loss. In both groups, roughly three quarters of the weight lost was fat and one quarter lean mass, and this share held across most age, sex and weight-loss subgroups [9]. In other words, tirzepatide produced more loss of both fat and lean mass, in about the same proportions as diet and activity alone did in this study.
What these scans can and cannot tell you
On a DXA scan, lean mass is what remains once fat and bone mineral are taken away, which the SURMOUNT-1 authors call lean soft tissue; skeletal muscle is one part of it, not the whole [9]. A fall in lean mass is therefore not the same as an equal fall in muscle, and the scan alone does not measure strength or function.
The published sub-study names its own limits: the subgroups were small, visceral fat was estimated from a region of the scan rather than measured directly, and physical activity, diet and physical function were not assessed. Most authors work for the manufacturer [9]. These are research findings about groups. The labels do not suggest that people on these medicines need a scan; whether one would change anything for you is a question for your clinician.
Measuring your waist at home
The National Heart, Lung, and Blood Institute (NHLBI) describes a simple method: stand up, wrap the tape around your middle just above the hipbones, and read it right after you breathe out [8]. NHLBI adds that fat carried mainly around the waist, rather than the hips, raises the risk of heart disease and type 2 diabetes, and gives the waist sizes in the takeaways above as the point where that risk goes up [8].
- Measure at the same spot each time, just above the hipbones.
- Pick a regular moment, such as the same morning each week, before eating.
- Use the same tape measure each time.
- Write the number down straight away, with the date.
Keeping the measures together
Waist, weight and photos tell more together than apart, especially over months. Looking at all three every few weeks can show a trend that a single day hides, for example a waist that keeps shrinking in a week when the scale did not move.
Bring the series, not a single reading, to your appointment, together with anything that interrupted treatment. Your clinician can put it next to blood pressure and blood tests, which no tape or scale shows.
Wegovy® and Saxenda® are registered trademarks of Novo Nordisk A/S. Zepbound® and Mounjaro® are registered trademarks and Foundayo™ is a trademark of Eli Lilly and Company. PepLabo is not affiliated with or endorsed by Novo Nordisk or Eli Lilly.
