Research in the American Journal of Physiology found that Danish men lost more weight with a half hour of daily exercise than they did with a full hour; their body fat remained the same in both cases. Moderation wins again.

PROBLEM: Creating a negative energy balance -- burning off more than you take in -- is the not-so-secret key to successful weight loss. Dieters, however, often find that eating less triggers compensatory mechanisms, such as increased appetite and a slowed metabolism, that make this balance difficult to maintain. Looking at the other side of this equation, is there an infinitely positive relationship between amount of exercise and pounds shed? Or is there a certain point where the compensatory mechanisms kick in, so that extra time on the treadmill ceases to affect weight loss?

METHODOLOGY: Sixtymoderately overweight Danish men were randomly assigned to either a moderate or high-dose aerobic routine of running, biking, or rowing. The moderate exercisers burned 300 calories per day, which took about 30 minutes, while the high-dose group burned 600 calories, which, predictably, took about twice as long. The subjects' body composition was monitored throughout the 13-week experiment, as were their compensatory behaviors (food intake and non-exercise physical activity). Their accumulated energy balance was calculated from their changes in body composition.

RESULTS: The group that practiced moderate exercise lost an average of 7.9 lbs in body weight, while the group that worked harder only lost an average of 6.0 lbs. Both saw similar losses in fat mass (about 8.8 lbs in the moderate group, and 8.3 pounds in the high-dose group). Researchers measured no significant difference in caloric intake or non-exercise energy expenditure.

CONCLUSION: Comparingmen who exercised for half an hour each day to men who worked out for twice as long, less exercise was actually associated with greater weight loss, and no significant difference infat loss. So the moderate exercisers got a lot more for their effort.

IMPLICATIONS: "Lose more weight in half the time" sounds like the kind of pitch that can get you into trouble. And the researchers aren't sure exactly why they got the results that they did. They suggest that less exercise may be associated with a greater willingness to engage in other forms of physical activity throughout the day that they did not measure, or that the more intense workouts may have lead to more compensatory food intake (though, within the limits of this study, they measured no difference). This calls to light the interplay of all the variables that go into weight loss and gain, and how, when factored in the bigger picture of life and physiology and behavioral psychology and taco rewards, more exercise isn't always better.

The full study, "Body fat loss and compensatory mechanisms in response to different doses of aerobic exercise -- a randomized controlled trial in overweight sedentary males," is published in the American Journal of Physiology.

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Study: For Body Fat, 30 Minutes of Exercise as Good as 60