New Fitness-Tracker Data Show Patients on GLP-1 Drugs Are Moving Less, Not More

I have written before about why I now prescribe resistance training alongside every GLP-1 or bariatric treatment plan, largely because of the well-documented risk that these drugs cause loss of lean muscle mass along with fat. A new study presented at ENDO 2026, the Endocrine Society's annual meeting, adds an important and somewhat counterintuitive piece to that picture: not only do patients lose muscle on these medications, many of them also become measurably less physically active while taking them.

Objective data, not self-report

Most of what we know about lifestyle behavior during GLP-1 treatment comes from patient questionnaires, which are notoriously unreliable for physical activity — people tend to overreport how much they move. This study took a different approach, linking electronic health records from the National Institutes of Health's All of Us Research Program with objective Fitbit wearable data. Researchers identified 1,950 adults with obesity who started a GLP-1 receptor agonist, of whom 753 had sufficient continuous wearable data before and after starting the medication to analyze activity trends directly. The cohort was 78.6 percent female with an average age of 52.7 years — reasonably representative of the population most commonly prescribed these drugs in the United States today.

What the trackers showed

After starting GLP-1 therapy, average daily step counts fell from 5,047 to 4,487 steps per day, and time spent in moderate-to-vigorous physical activity dropped from 28 to 22 minutes daily. Both changes are statistically real declines, not noise, and they moved in the same direction: less walking, and notably less of the higher-intensity activity that does the most for cardiovascular fitness and long-term muscle preservation. The decline was most pronounced in men and in patients reporting joint or muscle pain, while age, heart failure, and prior stroke did not meaingfully predict the pattern — suggesting this is not simply explained by patients becoming too frail to move, but by something more behavioral.

Why this likely happens

There are a few plausible, non-mutually-exclusive explanations, and this study cannot fully distinguish between them. Rapid, substantial weight loss can itself cause fatigue in the initial weeks. Nausea and gastrointestinal side effects, especially early in treatment or after dose increases, understandably reduce motivation to exercise. And there is a subtler possibility worth taking seriously: patients may unconsciously treat the medication's powerful effect on appetite and weight as a substitute for effort elsewhere, easing off the walking or gym routine because "the drug is doing the work now." Whatever the mix of causes, the practical result is the same: a treatment intended to improve long-term metabolic health is, for a meaningful subset of patients, being accompanied by a decline in the one behavior most protective against the muscle loss these same drugs can cause.

What this changes about how I counsel patients

This is the first large study to document this pattern using objective wearable data rather than self-report, and it changes how I frame the conversation when starting someone on a GLP-1 medication. I no longer assume that appetite suppression and weight loss will naturally translate into more activity as patients feel lighter and more capable — the data suggest the opposite tendency is at least as common. I now build a specific, concrete activity plan into the first visit, not the third or fourth: a step-count target, or ideally a structured resistance-training referral, discussed before the first dose increase rather than after a patient has already noticed muscle loss on the scale.

The takeaway for patients

If you are starting or already taking a GLP-1 medication, do not assume the drug's effect on your weight will automatically translate into a more active lifestyle — for many people, it appears to work in the opposite direction, especially if joint pain or fatigue is present. Building deliberate physical activity, particularly resistance exercise, into your routine from day one is not an optional add-on to this treatment; based on this data and what we already knew about muscle loss on these drugs, I now consider it a core part of it.