Simply put, your metabolism is the incredibly complex process the body uses to convert food into energy. It's also, of course, one of the first things we scapegoat when weight loss stalls: We blame our obstinate metabolism. Then we worry that this is just an excuse—calorie cutting, not metabolism, determines weight-loss success.
Of course, there's nothing simple about the way we burn, and conserve, calories. Humans vary wildly in their metabolic rates.
"If you looked at 10 different women," says Florence Comite, MD, an endocrinologist in New York City, "each one would have a different resting metabolic rate—it could be as much of a difference as 1,000 calories per person." Resting metabolic rate—the calories you use when you're not active—can account for about two-thirds of the calories a body burns daily. Even women of similar height and weight can have different rates. Comite recommends having your doctor test your resting metabolic rate so you know your baseline. This can help you work with an expert to better understand your daily calorie requirements.
The way we use energy is greatly affected by factors beyond our control, such as age, gender, hormones, body size, and genetics.
Does that mean our metabolism is fixed, steady and unchangeable? No, it's actually in constant flux, affected by lifestyle and health factors, including medications and sleep quality.
Activity also plays a role. You can, at least temporarily, rev up your metabolism. A combination of aerobic exercise in intervals (walking, running, biking, dancing around your living room) and weight lifting to build lean muscle mass can boost your burn rate. In other words, a proper exercise program not only burns calories, but it may also increase your ability to burn calories at rest.
The quantity of food, and its quality, also affect metabolism. You have to consume enough calories of good foods, Comite says. "A lot of people don't have enough lean protein or healthy carbs in their diet. Starve yourself, and you can rebound and actually hold on to calories."