If your goal is weight loss, then half an hour day of moderately strenuous exercise is the optimal target.
For Weight Loss, Less Exercise May Be More - NYTimes.com
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.