The 30-second summary
- Wegovy and Zepbound were never tested on their own. In the trials they came with diet and exercise counselling, and their US labels approve them for use with a reduced-calorie diet and more physical activity.
- The shot turns appetite down. It does not choose what you eat, and on a small appetite the food you do eat decides how much of the loss is fat rather than muscle.
- In one trial, exercise plus a GLP-1 cut body fat about twice as much as either alone, and kept more weight off after treatment ended.
- In the STEP 1 extension, two-thirds of the weight came back within a year of stopping. Most people do stop at some point.
The drug was never tested on its own
The trial most people's expectations come from is STEP 1, published in the New England Journal of Medicine in 2021. It enrolled 1,961 adults and gave them semaglutide 2.4 mg, the Wegovy dose, or a placebo for 68 weeks. Both groups also got a lifestyle intervention: the paper describes the drug as "an adjunct to lifestyle intervention". The semaglutide group lost 14.9% of their body weight on average, against 2.4% with placebo (Wilding et al., 2021).
That 14.9% is the number on the adverts. The conditions it was measured under are usually left off. The same is true of the labels: in the United States, Wegovy and Zepbound are approved for weight management in combination with a reduced-calorie diet and increased physical activity, not instead of them.
What the shot does, and what it does not
A GLP-1 makes you full sooner and keeps you full longer. That is a very effective way to eat less. It is not a way to eat well, and the difference matters more than it sounds.
When appetite is small, the easiest food to finish is rarely the food your body needs most. Protein is the part that goes missing first, and protein is what tells your body to keep its muscle while the fat comes off. In a real-world study of 396 people on a GLP-1, muscle made up 21.6% of the weight they lost, and those who started leaner lost a larger share.
Muscle is not vanity. It carries your strength, a good share of your daily energy use and much of how well you handle blood sugar. Weight that comes off as muscle is harder to keep off and harder to live in.
Exercise plus the drug beats either one alone
The clearest test of this is a Danish trial published in the New England Journal of Medicine in 2021. After an eight-week diet, 195 adults with obesity were given one of four things for a year: exercise, the GLP-1 liraglutide, both, or neither (Lundgren et al., 2021).
| Body fat change | |
|---|---|
| Exercise alone | down 1.7 percentage points |
| Liraglutide alone | down 1.9 percentage points |
| Exercise and liraglutide | down 3.9 percentage points |
Together, about twice either one alone. Only the combination also improved blood sugar control, insulin sensitivity and fitness.
The same team then followed people for a year after treatment ended. Those who had only taken the drug regained 6.0 kg more than those who had only exercised, and those who had done both kept 5.1 kg more off than those on the drug alone (Jensen et al., 2024).
One honest limit: liraglutide is an older, daily GLP-1, weaker than semaglutide or tirzepatide. Nobody has run the same four-way trial with today's drugs. There is no reason to expect the direction to reverse.
Most people stop, and weight returns
The STEP 1 extension followed 327 people for a year after the trial's 68 weeks ended. They had lost 17.3% of their weight on semaglutide. By week 120 they had regained 11.6 percentage points of it, about two-thirds (Wilding et al., 2022). The drug and the lifestyle advice stopped at the same time.
Stopping is also common. In a study of 201,229 adults with obesity, only 30.7% were still taking their GLP-1 a year after starting. Cost, supply, side effects and reaching a goal all end treatment. The habits built while on it are what stays.
The three things worth adding
1. A protein floor, set by your body rather than your appetite. Most people on a GLP-1 need more protein than their appetite asks for. The calculator sets yours from your weight and your dose and builds three days of food to it.
2. Strength work, twice a week. It does not have to be a gym. Two sessions of resistance work a week is the floor that protecting muscle on a GLP-1 is built on.
3. A plan for after. Decide with your prescriber how treatment will end, and keep the eating and moving habits running before it does. What happens when you stop a GLP-1 covers what the maintenance trials found.
None of this is a reason not to take the medicine. It is the reason the medicine works as well as the headlines say: in the trials, it never worked alone.
Sources
- Wilding JPH, Batterham RL, Calanna S, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity. N Engl J Med 2021. PubMed
- Lundgren JR, Janus C, Jensen SBK, et al. Healthy Weight Loss Maintenance with Exercise, Liraglutide, or Both Combined. N Engl J Med 2021. PubMed
- Jensen SBK, Blond MB, Sandsdal RM, et al. Healthy weight loss maintenance with exercise, GLP-1 receptor agonist, or both combined followed by one year without treatment. eClinicalMedicine 2024. PubMed
- Wilding JPH, Batterham RL, Davies M, et al. Weight regain and cardiometabolic effects after withdrawal of semaglutide: The STEP 1 trial extension. Diabetes Obes Metab 2022. PubMed