Dieting revs up your metabolism
Exhibit A to dispel this myth: The Biggest Loser. In order to lose weight, you need to burn more calories than you're taking in, but it's important not to completely deprive yourself by cutting too many calories out of your diet. "That can change your metabolism for the worse," warns Dr. Robert Lustig, professor of pediatrics in the division of endocrinology at UCSF and president/co-founder of the nonprofit Institute for Responsible Nutrition.
The key is a hormone called leptin, which tells your brain when you've consumed enough calories. If you're overweight, there's a good chance you have high insulin levels, and insulin blocks leptin, explains Dr. Lustig. Your brain doesn't "see" the leptin, so it thinks you're hungry, causing you to eat more, driving weight gain. "In order to fix your metabolism then, you have to fix the leptin issue," he says. It's not just a matter of cutting your calories, in other words; you have to reset your hormones over time. In fact, calorie restriction is associated with a slower metabolic rate, so extreme diets tend to move your RMR in the other direction. With slower metabolic rates, you'd need to decrease your calorie count even more to continue losing, or even to maintain weight loss. Hence the Biggest Loser conundrum.