Fitbit data from 1,950 adults in the NIH All of Us program shows daily steps and exercise time fell after starting a drug like Ozempic or Wegovy, putting muscle loss and the prescribing model on the same table as the weight.
The pounds came off. The steps did not.
Fitbit data from 1,950 adults with obesity in the NIH All of Us Research Program shows that daily step counts and exercise time fell after participants started a GLP-1 medication, according to a ScienceDaily summary of an ENDO 2026 presentation from Saturday in Chicago. The weight loss was real. So was the slowdown, and the two findings are colliding in a way that the standard "miracle drug" framing does not capture.
GLP-1 agonists, the drug class that includes Ozempic and Wegovy (semaglutide) along with Mounjaro and Zepbound (tirzepatide), are now prescribed to millions of Americans for weight and blood-sugar control. The drugs are documented to reduce both fat mass and lean mass, the technical term for everything in the body that is not fat: skeletal muscle, organ tissue, water, and connective tissue. That second effect has lived mostly in the fine print of body-composition studies. The ENDO 2026 analysis brings it into the daylight by pairing it with movement data, and the combined picture is harder to spin.
Researchers used Fitbit-derived step counts and self-logged exercise from participants in All of Us, a federal precision-medicine cohort designed to reflect the diversity of the U.S. population. After starting a GLP-1, the cohort moved less, even as body weight continued to drop. The activity decline is not a verdict that the drugs "do not work." On the weight outcome being measured, they did. The decline is a verdict on a prescribing model that treats the injection as the whole plan.
Two things are happening at once. The drug is reducing appetite and producing a calorie deficit large enough to drive meaningful weight loss. It is also reducing lean mass, the tissue that determines how strong, mobile, and metabolically resilient a person stays through middle age and beyond. When daily activity falls at the same time, the fraction of weight lost as muscle tends to climb rather than fall, and the patient ends up lighter on the scale and more depleted in the gym, on the stairs, and in daily living. That is the under-discussed cost of an otherwise celebrated therapy.
The ENDO 2026 finding is conference-grade rather than peer-reviewed. The underlying study, the authors' names, and the exact magnitude of the step-count drop were not spelled out in the ScienceDaily release this story draws on, and the activity signal comes from consumer wearables and self-reports, which are useful for trend but not a clinical performance measure. Patients and clinicians reading the headline should treat the direction, not the number, as the news. The bigger question is what to do about it.
The constructive lane is a paired protocol: pharmacotherapy plus deliberate resistance training and a daily movement floor, co-owned by patient and prescriber from the first prescription. Two or three short resistance sessions a week, a step target that survives the appetite loss, and protein intake that protects lean mass are not lifestyle garnish. They are part of the prescription. The muscle-preservation question belongs in the room when the pen hits the pad, not six months later when a patient wonders why they feel frailer even though the scale looks better.
That shift is also the open research question. Body-composition literature outside this conference already documents the lean-mass cost of GLP-1 therapy; pairing that evidence with activity data is the new move. Future work will need to track whether patients who lift weights and walk more hold onto muscle and function better than those who rely on the injection alone. The Fitbit signal is a prompt for that trial, not the answer.
For now, the reader-facing version of the ENDO 2026 finding is simple. The weight-loss era has a muscle problem, and the activity decline is the first population-scale hint that the problem is showing up in daily life. The drugs are working. The protocol around them is not.