Have you fallen off the exercise wagon but now are ready to shed those lovely love handles you've acquired? Move that pile of junk off the treadmill and get after it already – just don't run too long or too hard or you might not get the results you crave. A new study on slightly chunky, sedentary young guys who started an exercise program found that those who worked out for about 30 minutes a day, or until they burned 300 calories, lost more weight on average than men who exercised for 60 minutes, or until they burned 600 calories, after 13 weeks. So, not only were these über-exercisers spending precious time busting lung that they could've put toward something far more fun or productive, but they were actually sabotaging their weight-loss efforts.

These results even surprised the University of Copenhagen team behind the study. It predicted the men in the moderate-exercise group would drop fewer pounds than they wound up losing and thought the heavy exercisers would shed more than they did. But lead researcher Mads Rosenkilde has a few theories as to why his team's forecasts were off – and why the 30-minute group actually averaged more weight loss than the 60-minute men.

First, even though the researchers asked the participants to not intentionally alter their eating habits and to track what they ate throughout the 13 weeks, Rosenkilde suspects the heavy exercisers actually ate more food than they owned up to, probably because they were starving after their sweat sessions. He also says the 30-minute workouts likely boosted these guys' energy levels, leading them to move around more throughout the rest of the day, whereas the men who went for a full hour were so bushed that they stayed sedentary after exercising. Another potential problem with over-exercising – and the exhaustion and hunger that come from it – is that you're more apt to grow tired of the routine, fall back off the program, and start re-packing on the pounds you wanted to ditch in the first place.

Rosenkilde points out that everyone responds to exercise differently, so the key is finding the duration and type of workout that makes you feel the burn but doesn't leave you lifeless on the couch afterward and chowing down twice as much for dinner.