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When we eat less, our bodies compensate for the lost calories by increasing appetite and lowering metabolic rate. (Annoying, we know.) Researchers set out to discover whether or not exercise produces similar reactions, which may also make it harder to lose weight. A new study from the University of Copenhagen found that 30-minute workouts led to roughly the same amount of weight loss as 60-minute workouts, even though participants were burning fewer calories during the shorter workoutsBody fat loss and compensatory mechanisms in response to different doses of aerobic exercise–a randomized controlled trial in overweight sedentary males. Rosenkilde, M., Auerbach, P., Reichkendler, M.H., et al. Dept. of Biomedical sciences, Univ. of Copenhagen, Copenhagen N, Denmark. American Physiological Society American Journal of Physiology – Regulatory, Integrative and Comparative Physiology, 2012 Sep;303(6):R571-9. Epub 2012 Aug 1.. The researchers suggest working out for longer provides little additional benefit when it comes to losing weight, since the body will end up compensating for those extra minutes spent at the gym.

The Study

Sixty-one sedentary and overweight men were divided into three groups: running and cycling daily for about 30 minutes (burning 300 calories), running and cycling daily for one hour (burning 600 calories), or remaining inactive. Monitor sensors also tracked how active the men were after they hit the gym. They were told to record, but not change, their diets.

The results of this 13-week study were surprising: The group who worked out for only 30 minutes ended up losing about the same amount of both body weight and total fat as the group who worked out for 60 minutes.

Can We Trust It?

Researchers are still unsure why the men who worked out less ended up losing a bit more weight, but they propose the amount of compensation after exercising (increasing appetite and decreasing metabolism) depends on how long we exercise.

Another possibility: Since their food diaries were self-reported, the group burning 600 daily calories could have eaten more than they jotted down. Plus, when looking at the monitor sensors, the group who exercised for longer ended up being less active throughout the day, while those who worked out for 30 minutes spent more time on their feet and theoretically burned more calories outside the gym.

The men working out for 60 minutes could have also gained a bit more muscle. If this study was conducted over a longer period of time, muscle-gain could lead to a revved up metabolism, which might eventually mean more weight loss.

The Takeaway

The study showed that whether burning 300 calories or 600 calories, exercise is an effective way to help lose weight. But keeping exercise levels moderate mayhelp keep energy levels high throughout the day and allow people to burn calories without wanting to replace them in the kitchen. More exercise may cause the body to compensate for the extra calories burned, so remember that a longer bout at the gym doesn’t mean we can turn into a couch potato for the rest of the day!

Think less really is more to help lose weight? Let us know in the comments below, or tweet the author @lschwech.