If you're on a GLP-1 medication like Wegovy or Mounjaro and the scale is moving, that's not necessarily the whole story. A significant share of what you're losing isn't fat. It's muscle — and the standard ways people track their progress won't show you that.
Why these drugs cause muscle loss
In the clinical trials the manufacturers had to run to get these drugs approved, around 40% of the weight participants lost on semaglutide was lean mass, not fat. Tirzepatide trials showed a similar pattern: roughly a quarter to a third of total weight lost came from muscle and other lean tissue. Some lean loss is expected with any weight loss, including plain dieting. But the speed of these drugs, and the appetite suppression that often crowds out protein, can tip the ratio in the wrong direction.
That matters because muscle isn't just for lifting things. It's metabolically active tissue: it stores glucose, keeps your insulin sensitivity high, and protects your strength and independence as you age. Losing it in your 40s and 50s sets up problems that don't show up until much later - and don't reverse easily when they do.
How to actually track what you're losing
Most people track weight loss with a bathroom scale. Some use the smart scales that send a mild electrical current through your feet, or the body composition pods and scanners you find in gyms. These tools aren't useless, but they're imprecise in ways that matter. Smart scales estimate body fat using electrical impedance, which is sensitive to hydration levels, time of day, and how recently you've eaten. Two scans taken on the same morning can return meaningfully different numbers. Gym scanners vary in quality and calibration. None of them give you the granular data you'd need to make confident decisions about what's actually happening inside your body.
A DEXA scan: "dual-energy X-ray absorptiometry" is different in kind, not just degree. It uses two low-dose X-ray beams to distinguish between fat, lean muscle, and bone across different regions of your body. It can tell you not just how much fat and muscle you carry overall, but where. It's the same technology used in clinical research and osteoporosis diagnosis.
A session takes around 10 minutes, the radiation dose is negligible (roughly equivalent to a short flight), and the output is precise enough to detect meaningful changes between scans months apart. For anyone using a GLP-1, that precision matters a great deal.
Arthur Lidgey at Scanletics runs these scans, and the picture he's building across his clients is consistent with the trial data. Here's how he put it when we spoke to him:
"I've seen quite variable results with GLP-1s because I've seen people losing weight so rapidly that they're coming back and they've lost maybe a small amount of fat but a large majority of muscle.
This is then going to lead to much greater problems later down the line as they're getting older as well.With a DEXA scan, particularly if people are considering using GLP-1s or already using them, we can course correct over time if they're losing weight healthily or if they're just sacrificing valuable muscle mass as well. It is not a miracle pill; it's not a miracle drug to be taken. It is something to be used alongside a nutrition programme and a healthy exercise programme."
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How to prevent muscle loss while you're on the drug
Three things make the difference between using a GLP-1 well and using it as a shortcut.
Eat enough protein. Appetite drops sharply on these medications, so protein often gets squeezed out first. Anchor every meal around it, even when you're not hungry. It's the main lever you have for holding muscle while the weight comes off.
Build resistance training into the routine now, while you're on the drug. Two or three sessions a week changes the composition outcome significantly — your body holds onto muscle when you give it a reason to. It also builds the habit structure that continues to pay out long after the prescription ends.
Get a DEXA scan at the start, midway through, and when you're tapering off. That sequence gives you a clear picture of whether the weight you're losing is the weight worth losing, and lets you adjust protein and training before the damage compounds.
Planning the way off
There's a harder conversation that isn't happening enough around these medications: what comes next.
In a follow-up study run after one of the main semaglutide trials, participants who stopped the drug regained two thirds of their lost weight within a year. The drug suppresses appetite. Without it, appetite returns. If the habits aren't in place to fill the gap, the weight comes back and takes more muscle with it on the next cycle.
Staying on indefinitely isn't a solution either. The long-term effects are still being studied, the cost is prohibitive for most people, and dependence on a drug to maintain a basic body composition isn't a sustainable place to land.
The right model is closer to rehab than it is to long-term medication. You use the drug to create a window: reduced appetite, lower weight, some breathing room — and you spend that window building the habits that will hold when the prescription ends. That means learning what to eat, training your body to use glucose well, and developing a relationship with your own signals rather than outsourcing them to a drug.
Used that way, with a clear point at which the habits take over, these medications can be the start of something that lasts. Used as a destination, they're an expensive loan with a steep repayment schedule.
What does the off-ramp look like for you?