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Most people who start working out in hopes of shedding pounds wind up disappointed, a lamentable circumstance familiar to both exercisers and scientists. Multiple studies, many of them covered in this column, have found that without major changes to diet, exercise typically results in only modest weight loss at best (although it generally makes people much healthier). Quite a few exercisers lose no weight. Some gain.

But there is encouraging news about physical activity and weight loss in a new study by researchers at the University of Copenhagen. It found that exercise does seem to contribute to waist-tightening, provided that the amount of exercise is neither too little nor, more strikingly, too much.

To reach that conclusion, the Danish scientists rounded up a group of pudgy and sedentary young men, a segment of the population increasingly common in Denmark, as elsewhere in the world. The volunteers, most in their 20s or early 30s, visited the scientists’ lab to undergo baseline measurements of their aerobic fitness, body fat, metabolic rates and general health. None had diabetes, high blood pressure or heart disease and, while heavy, they were not obese.

The men were then randomly assigned to exercise or not. The non-exercisers, who served as controls, returned to their former routines, with no change to their diets or sedentary ways.

A second group began 13 weeks of almost daily moderate workouts, consisting of jogging, cycling or otherwise sweating for about 30 minutes, or until each man had burned 300 calories (based on his individual metabolic rate).

A third group tackled a more strenuous routine of almost hourlong workouts, during which each man burned 600 calories.

The men were asked not to consciously change their diets, either by eating more or less, and to keep detailed daily food diaries throughout the 13 weeks.

On certain designated days, they also were asked to don sophisticated motion sensors that would measure how active they were in the hours before and after exercise.

At the end of the 13 weeks, the members of the control group weighed the same as they had at the start, and their body fat percentages were unchanged, which is hardly surprising.

On the other hand, the men who had exercised the most, working out for 60 minutes a day, had managed to drop some flab, losing an average of five pounds each. The scientists calculated that that weight loss, while by no means negligible, was still about 20 percent less than would have been expected given the number of calories the men were expending each day during exercise, if food intake and other aspects of their life had held steady.

Meanwhile, the volunteers who’d worked out for only 30 minutes a day did considerably better, shedding about seven pounds each, a total that, given the smaller number of calories that they were burning during exercise, represents a hefty 83 percent “bonus” beyond what would have been expected, says Mads Rosenkilde, a Ph.D. candidate at the University of Copenhagen who led the study.

That impressive weight-loss windfall for the light-duty exercisers “was a bit of a shock,” he says.

And it’s not completely clear from the experiment’s additional data just why participants in that group were so much more successful at dropping pounds than the other men.

But there are hints, Mr. Rosenkilde says. Food diaries for the group burning 600 calories a day reveal that they subsequently were increasing the size of their meals and snacks, although the additional caloric intake wasn’t enough to explain the difference in their results. “They probably were eating more” than they jotted down, Mr. Rosenkilde speculates.

They also were resolutely inactive in the hours outside of exercise, the motion sensors show. When they weren’t working out, they were, for the most part, sitting. “I think they were fatigued,” Mr. Rosenkilde says.

The men exercising half as much, however, seemed to grow energized and inspired. Their motion sensors show that, compared with the men in the other two groups, they were active in the time apart from exercise. “It looks like they were taking the stairs now, not the elevators, and just moving around more,” Mr. Rosenkilde says. “It was little things, but they add up.”

The overall message, he says, is that the shorter exercise sessions seem to have allowed the men “to burn calories without wanting to replace them so much.” The hourlong sessions were more draining and prompted a stronger and largely unconscious desire to replenish the lost energy stores.

Of course, the study involved only young men, whose metabolisms and weight-loss motivations may be quite different from those of other groups, including women.

The study also was short-term, and the results might shift over the course of, say, a year of continued exercise, Mr. Rosenkilde says. The men working out for 60 minutes were, after all, packing on some muscle, while the 30-minute exercisers were not. That extra muscle offset some of the vigorous exercisers’ weight loss in the short term — they sloughed off fat but added muscle, decreasing their net loss — but over the longer term it could amp up their metabolism, aiding in weight control.

Still, if the relationship between working out and losing weight remains complicated and tangled, one point is unequivocal. The men who were sedentary “lost no weight at all,” Mr. Rosenkilde says, so if you hope to shed pounds, “any amount of exercise is better than none.”