NEW YORK • 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.

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

In that study, about 100 overweight, previously inactive young men and women worked out five times a week at a physiology lab, jogging or otherwise sweating until they had burned up to 600 calories a session. After 10 months of this regimen, almost everyone had lost weight. But the extent of their losses fluctuated wildly, even though everyone was doing the same, supervised workouts.

Flummoxed, Dr Willis and his colleagues started brainstorming other possible, perhaps unexpected contributors to the enormous variability to weight loss. They hit upon activity timing.

They sifted through their data again, this time looking at when people in the Midwest trial had shown up at the university lab. They checked weight change against exercise schedules and quickly noticed a consistent pattern.

Those who usually worked out before noon had lost more weight, on average, than the men and women who typically exercised after 3pm.

The early-exercise group also tended to be slightly more active throughout the day and also ate a bit less, although the difference amounted to barely 100 calories a day on average. Yet, they may cumulatively have contributed to the striking differences in how many kilograms people lost, Dr Willis says.

Of course, this study was not large or designed from the start to delve into the chronobiology of exercise and weight. Still, the statistical associations were strong, he says.

But Dr Willis also points out that most of those who worked out later in the day did lose weight, even if not as much as the larkish exercisers, and almost certainly became healthier.

"I would not want anyone to think that it's not worth exercising if you can't do it first thing in the morning," he says. "Any exercise, at any time of day, is going to be better than none."

NYTIMES