People who exercise in the morning seem to lose more weight than people completing the same workouts later in the day, according to a new study of workouts and waistlines. The findings help shed light on the vexing issue of why some people shed considerable weight with exercise and others almost none, and the study adds to the growing body of science suggesting that the timing of various activities, including exercise, could affect how those activities affect us.

The relationship between exercise and body weight is somewhat befuddling. Multiple past studies show that a majority of people who take up exercise to lose less weight drop fewer pounds than would be expected, given how many calories they are burning during their workouts. Some gain weight.

But a few respond quite well, shedding pound after pound with the same exercise regimen that prompts others to add inches.

This variability interests and puzzles Erik Willis, a data analyst with the Center for Health Promotion and Disease Prevention at the University of North Carolina at Chapel Hill. For almost a decade, he and colleagues at the University of Kansas, the University of Colorado Denver and other institutions have overseen the Midwest Exercise Trial 2, an extensive examination of how regular, supervised exercise influences body weight.