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It's no wonder you think you need to do this to lose weight: Many cardio machines tell you when you're above and below the zone. But this reason for sticking to low-intensity exercise has been completely debunked. "Because fat takes longer than carbs to be converted to energy, you burn a higher percentage of it when you're sitting or walking than when you're running. So the old thinking was that with low-intensity exercise you could torch body fat and lose weight," Porcari explains. But the theory didn't work in practice. "In one study, we had people walk or run for half an hour. On average, the walkers burned 240 calories, 44 percent of which were fat, so they burned 108 fat calories. The runners burned 450 calories, 24 percent of which were fat, so they burned 120 fat calories. Whether you look at total calories or fat calories, the runners clearly came out ahead," Porcari says.
The Solution: There's nothing wrong with low-intensity exercise, particularly if you have joint problems. "But to lose weight, you'll probably need to do it for longer than half an hour. Just for general health, the recommendation is 30 minutes five days a week," Porcari says.