Ask someone six weeks into a GLP-1 whether they are moving more or less than before and most will say more, because they weigh less and it feels easier. Their watch says otherwise.
What happened
At ENDO 2026, the Endocrine Society's annual meeting, researchers presented wearable-device data from the National Institutes of Health's All of Us Research Program. Sajana Maharjan and colleagues at HSHS St. John's Hospital in Springfield, Illinois started from 1,950 adults with obesity who began a GLP-1, and analysed the 753 who had enough device data to measure before and after.
Daily steps fell from 5,047 to 4,487. Moderate to vigorous activity fell from 28 minutes a day to 22. The cohort was 78.6% female with a mean age of 52.7.
This is observational, and a step count is not a controlled experiment. People's activity drifts with the seasons, with work and with age, and a study like this cannot separate the medicine from the year. What it can do is contradict the assumption, which it does.
Why a fifth of your activity is worth more than it sounds
Six minutes of vigorous activity and 560 steps are small numbers on their own. They matter here because of what else is happening at the same time.
Weight lost on a GLP-1 is not all fat. Somewhere between a quarter and a third of it is lean tissue, and the two things that hold that proportion down are enough protein and enough load through the muscle. As the researchers put it, these medicines reduce not only fat but also lean muscle mass, which makes physical activity essential for preserving strength and long-term health.
So the activity drop lands at precisely the moment the body can least afford it. Less food, less protein in absolute terms, and now less stimulus telling the muscle it is still needed.
There is a plausible reason it happens, too. Eating much less leaves less to spend, and the body economises quietly: fewer fidgets, fewer stairs, a shorter walk to the shop. Nobody decides to do this. It shows up on a wrist rather than in a memory.
What it means for you
Do not trust your impression of how much you are moving. Look at the number your phone or watch already records, compare this month against the month before you started, and treat a fall as information rather than a failure.
Then defend two things specifically. Resistance work twice a week, because that is the signal that keeps muscle, and a floor under your daily steps that you do not negotiate with. Neither has to be ambitious. Both have to be actual.
What to do with this
Protein is the other half of holding on to muscle, and the amount you need does not fall when your appetite does. The calculator sets your floor from your weight and your dose.