Key takeaways
In the STEP 1 trial, semaglutide users lost about 15% of body weight, and part of that loss was lean mass
In a 2021 NEJM trial, combining exercise with a GLP-1 drug lowered body-fat percentage about twice as much as either alone
A 2026 wearable study found daily steps and exercise minutes fell after people started a GLP-1 drug
GLP-1 receptor agonists such as semaglutide (Ozempic, Wegovy) and liraglutide have changed obesity treatment. They are trending again because a study presented at the Endocrine Society's ENDO 2026 meeting reported that people tend to move less after starting them, reviving the debate about muscle and fitness on these drugs. Here is what the research shows, and how to keep your fitness while the scale goes down.
This article is for information only and is not medical advice. Talk to your doctor before starting, stopping or changing any medication.
How much weight, and what kind?
In the STEP 1 trial (Wilding et al., 2021), 1,961 adults with overweight or obesity took weekly semaglutide 2.4 mg or placebo for 68 weeks, alongside lifestyle advice. The semaglutide group lost about 14.9% of body weight, compared with about 2.4% on placebo.
A subgroup of 140 participants had DXA body scans. Fat mass fell by about 19% and visceral fat by about 27%, but lean mass also fell, by roughly 10%. Because fat fell faster, the proportion of lean mass in the body went up. Still, a substantial share of the weight lost was lean tissue. Lean mass is not all muscle, but muscle matters for strength, blood sugar control and VO2max.
Relative vs absolute VO2max
VO2max is usually expressed per kilogram (ml/kg/min). If you lose weight and nothing else changes, that number goes up — simply because you are dividing by fewer kilograms. Your absolute VO2max (litres of oxygen per minute) depends on your heart, blood and muscle, and the drug does not train any of these. If you lose muscle and stop exercising, absolute fitness can fall. More on this in VO2max and body weight.
| Measure | What weight loss alone does | What training adds |
|---|---|---|
| Relative VO2max (ml/kg/min) | Usually rises as body mass falls | Rises further |
| Absolute VO2max (L/min) | Unchanged or may fall | Rises |
| Lean and muscle mass | Falls along with fat | Better preserved |
| Strength | May fall | Maintained or improved |
The best evidence: drug plus exercise
Lundgren et al. (2021) tested what helps people keep weight off. After an 8-week low-calorie diet in which participants lost about 13 kg, 195 adults were randomised for one year to placebo, a supervised exercise programme, liraglutide 3.0 mg, or exercise plus liraglutide.
- All active treatments maintained weight loss better than placebo.
- The combination gave the largest weight loss (significantly more than exercise alone, but not more than the drug alone) and roughly twice the reduction in body-fat percentage of either treatment alone.
- Only the combination group showed clear improvements in cardiorespiratory fitness, blood sugar control and insulin sensitivity.
The message is that the drug and exercise do different jobs, and they work best together.
The 2026 warning sign: people move less
At ENDO 2026, Maharjan and colleagues used Fitbit data linked to health records in the NIH All of Us programme. Among 753 adults with obesity who started a GLP-1 drug and had enough wearable data, daily steps fell from about 5,050 to 4,490, and moderate-to-vigorous activity fell from about 28 to 22 minutes a day. The largest drops were in men and in people with joint or muscle pain.
This is an observational conference abstract, not yet a peer-reviewed paper, and it cannot show that the drugs cause people to move less. But it suggests that exercise does not take care of itself on these drugs.
How to protect muscle and fitness
- Strength train at least twice a week. Work all major muscle groups and progress the load over time. Our strength training guide covers the basics for endurance-minded people.
- Keep aerobic work going. Build an easy base with zone 2 training, and once you are comfortable add one interval session a week, such as the Norwegian 4×4.
- Eat enough protein. Smaller appetites make this harder, so ask your doctor or a dietitian for a target that suits you and spread it across meals.
- Track fitness, not just weight. Check your estimate with the VO2max calculator and your fitness age every few months. A rising number on a stable or falling body weight is a good sign.
- Plan for the long term. Fitness fades within weeks of stopping training; see how fast you lose VO2max.
If you have heart disease, diabetes, or have been inactive for a long time, talk to your doctor before starting vigorous exercise. Feeling faint, unusually tired or dizzy during training on a GLP-1 drug is a reason to stop and seek medical advice.
The bottom line
GLP-1 drugs reduce body weight, which raises VO2max per kilogram, but fitness is built by training. Combining the medication with strength work, aerobic exercise and enough protein gives you the best chance of losing fat while keeping the muscle and heart fitness that matter for long-term health.
Frequently asked questions
Does Ozempic raise your VO2max?
Your VO2max per kilogram can rise because you weigh less, but the drug does not train your heart or muscles. Your absolute fitness, in litres of oxygen per minute, only improves if you exercise.
Do GLP-1 drugs make you lose muscle?
In the STEP 1 body-composition substudy, most of the weight lost was fat, but lean mass also fell. Strength training and enough protein are the best-studied ways to limit this.
What exercise should I do on a GLP-1 drug?
Aim for strength training at least twice a week plus regular aerobic work such as brisk walking, cycling or intervals. Talk to your doctor before starting, especially if you have heart disease or diabetes.
Sources
- Wilding JPH, et al. Once-weekly semaglutide in adults with overweight or obesity. N Engl J Med, 2021.
- Lundgren JR, et al. Healthy weight loss maintenance with exercise, liraglutide, or both combined. N Engl J Med, 2021.
- Endocrine Society. Exercise decreases among people taking GLP-1 medication (Maharjan S, et al., presented at ENDO 2026). Endocrine Society press release, 2026.