Losing weight is often seen as a success, but what happens if the weight keeps coming back?
A new study suggests that repeated cycles of weight loss and weight gain may quietly reduce muscle in the body, even if a person’s final weight changes very little.
The research was carried out by scientists at the University of California, San Francisco and published in Radiology.
Muscle is much more than strength. Healthy muscles support walking, climbing stairs, protecting joints, controlling blood sugar, and maintaining independence as people age. Losing too much muscle can make everyday life more difficult and may increase the risk of injury.
To better understand how changing body weight affects muscles, researchers studied 1,433 middle-aged adults enrolled in the Osteoarthritis Initiative.
The participants were followed for four years, allowing scientists to compare people whose weight stayed stable with those whose weight repeatedly went up and down.
The research team used MRI scans together with artificial intelligence to calculate thigh muscle volume. They also measured fat inside muscles and around the knee to build a clearer picture of body composition rather than relying only on body weight.
The study found that participants who repeatedly lost and regained weight experienced almost four times more thigh muscle loss than those whose weight remained stable. Their average muscle loss reached about 3.7%, compared with around 1% among people with stable weight.
One surprising finding was that both groups ended the study at nearly the same overall body weight as when they started. This shows that the number on the scale does not always reflect important changes happening inside the body.
The researchers believe this information is becoming increasingly important as more people use highly effective weight-loss medicines. Although the study did not directly examine people taking GLP-1 drugs, it raises important questions about what happens if treatment is stopped and weight returns.
The scientists adjusted their results for physical activity, diet, age, sex, and several other health factors. Even after considering these influences, weight cycling remained strongly associated with greater muscle loss.
The study encourages people to think beyond body weight alone. Preserving muscle during weight loss may be just as important as reducing body fat because muscles play a key role in long-term health and physical function.
Like all observational studies, this research has limitations. It cannot prove that weight cycling directly caused the muscle loss, and the results may not apply equally to every population. Even so, the advanced imaging methods and large number of participants strengthen confidence in the findings.
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