The before and after photos of weight-loss injections are everywhere, and nearly one in five American adults say they have taken a GLP-1 drug. The trials also followed people for a year after they stopped, and that year is worth knowing about.
GLP-1 stands for glucagon-like peptide 1. Cells in your gut release it after a meal. It helps the pancreas send out insulin, the hormone that moves sugar from your blood into your cells. It also slows how fast your stomach empties and cuts how much you eat.1 Medicines that copy it, such as semaglutide and tirzepatide, are used for type 2 diabetes and for weight loss.1
A lot of people take them. In a KFF poll of 1,350 American adults in late October and early November 2025, 18% said they had taken a GLP-1 drug at some point, for weight loss, diabetes or another condition, and 12% were taking one at the time. Of those who had taken one and stopped, 14% said cost was the reason and 13% said side effects.2
Whether to take one is a decision for you and your doctor. What follows is what the trials measured.
STEP 1 was a large trial of semaglutide. It took 1,961 adults with obesity, or overweight plus a weight-related condition, none with diabetes, and gave them a weekly injection of semaglutide or a placebo for 68 weeks. Everyone also had lifestyle counselling. The semaglutide group lost 14.9% of their body weight on average, against 2.4% on placebo.3
A smaller group of 140 people, three in four of them women, also had DEXA scans, an X-ray scan that splits the body into fat, lean mass and bone. On semaglutide their fat mass fell by 19.3% and their lean mass by 9.7%.4 Worked through on the group's average starting body, that puts lean mass at close to 40% of the fat and lean tissue they lost. Because fat fell faster, lean mass made up a bigger share of their body at the end, up 3 percentage points.4
Lean mass is not the same as muscle. It also counts organs, bone, fluid and the water held in fat tissue. A 2024 review found the lean share of weight lost on these medicines ranged from about 15% to 60% across trials. Scans that looked at muscle itself suggest the loss was roughly what you would expect for the amount of weight lost, and the fat inside the muscle went down.5 The review says future studies need to measure strength and function, as well as mass.
At week 68 of STEP 1, the injections and the lifestyle counselling both stopped. Researchers followed 327 participants from Canada, Germany, the UK, the United States and Japan, average age 49, for one more year. The semaglutide group had lost 17.3% of their weight. By the end of the year off treatment they had regained 11.6 percentage points of it, about two-thirds, leaving them 5.6% below where they started. Blood pressure, blood fats and blood sugar measures that had improved moved back towards where they began.6
This follow-up was exploratory and small, and the authors note it had no lifestyle support after week 68. It did not report body composition, so it cannot say how much of the regained weight was fat or lean mass.6
SURMOUNT-4 tested tirzepatide the same way. 670 adults, average age 48, took it for 36 weeks and lost 20.9% of their weight. Half then switched to a placebo without knowing it. Over the next 52 weeks they regained 14.0% of their weight, while those who carried on lost a further 5.5%.7 Both teams concluded that keeping the weight off depended on staying on treatment.6,7
A trial in Copenhagen asked what helps after weight loss. 195 adults with obesity first lost 13.1 kg on an eight-week low-calorie diet. They then spent a year on one of four plans: a supervised exercise programme, liraglutide (an older GLP-1 medicine), both together, or a placebo. Both together worked best, and cut body fat percentage by 3.9 points, about twice either one alone.8
A year after all treatment ended, 109 of them came back to be measured. Those who stopped liraglutide alone regained 6.0 kg more than those who stopped the exercise programme. The exercise group had kept their weight and their body composition.9 Not everyone returned, and the programme was general moderate-to-vigorous exercise, so it does not tell us about strength training alone.
The best evidence for building muscle comes from studies outside these medicines. A review of 49 trials with 1,863 adults found that people who ate more protein while doing resistance training gained more strength and more fat-free mass, which is everything in the body except fat. The extra benefit levelled off at a best estimate of about 1.6 g of protein per kilogram of body weight a day, and the authors suggest up to 2.2 g for people who want to build the most.10 Trials of protein and exercise in people taking GLP-1 medicines are still few.5
Protein also helps with fullness. A review of controlled trials found convincing evidence that higher-protein meals leave people fuller than lower-protein ones.11 In a small study of 20 young women who usually skipped breakfast, a 35 g protein breakfast left them fuller through the day and cut evening snacking on high-fat foods, though their total calories for the day did not change.12
The rest of the plate counts too. When 20 adults lived at a US research hospital for four weeks and ate as much as they liked, they took in about 500 more calories a day on ultra-processed food than on unprocessed food, with the protein on offer matched.13 In 11 adults with type 2 diabetes, eating the chicken and vegetables before the bread and juice cut the blood sugar rise at 30 and 60 minutes by roughly a third.14
Whatever you and your doctor decide about medication, the muscle side is yours to work on. Protein at every meal, a full plate around it, and strength work that suits you, whether that is squats to a chair, step-ups or weights at the gym, all help you keep what you have.
In the trials, some of the weight lost on GLP-1 medicines was lean mass, and in STEP 1 about two-thirds returned within a year. Decisions about weight regain after stopping GLP-1, and about the medication itself, are for you and your doctor. Protein with strength training helps you keep the muscle you have. Eighty Thirty works out your daily protein and keeps count for you, so you can hold on to it. Start with our free macro calculator.
In the STEP 1 trial, people regained about two-thirds of the weight they had lost on semaglutide in the year after stopping. They had lost 17.3% of their weight and ended the year 5.6% below where they started. Lifestyle support stopped too, and the follow-up was small.6
Some of the weight lost is lean mass, which includes muscle. In 140 scanned STEP 1 participants, fat mass fell 19.3% and lean mass 9.7%.4 Lean mass also counts organs, bone and fluid, and scans of muscle itself suggest a loss roughly in line with the weight lost.5
In the SURMOUNT-4 trial, people who switched to a placebo after 36 weeks on tirzepatide regained 14.0% of their weight over the next 52 weeks. Those who carried on lost a further 5.5%.7 The researchers concluded that keeping the weight off depended on staying on treatment.
In one Danish trial it did. A year after all treatment ended, people who stopped liraglutide alone had regained 6.0 kg more than those who stopped a supervised exercise programme. The exercise group kept their weight and body composition.9 Not everyone came back to be measured, though.
There is no figure from GLP-1 trials yet, as trials of protein and exercise on these medicines are few.5 Outside them, a review of 49 trials put the best estimate at about 1.6 g per kilogram a day, with up to 2.2 g suggested for the most muscle.10
Genes, eating less, a slow metabolism, more cardio and eating clean: what the research says about each, and the part they all miss.
27 Oct 2026The same fat came off with cardio or strength training, and the strength group kept more of their lean mass and bone.
27 Oct 2026Eating less takes some muscle with the fat, and strength training and enough protein help you keep it.
29 Sep 2026Our free calculator works out your resting rate, your maintenance calories and your protein target.