It's the sum of the energy-producing systems of the body, explains endocrinologist Steven Petak, M.D., of the Texas Institute for Reproductive Medicine and Endocrinology in Houston. "Every cell in the body uses fuel—glucose or something that can be turned into glucose," he explains. Your thyroid hormones determine how quickly your body burns that fuel—and that is your metabolic rate.

Unfortunately for many people, 70 to 80 percent of metabolism is probably genetic, says Dr. Petak. While it is possible to rev your metabolism by taking various substances—including amphetamines, phentermine, caffeine, and ephedrine—it's not worth the main side effect: a tendency to make you wired or jittery. Smoking also tends to speed up the metabolism, which is one reason many teenage girls begin smoking. "Smoking should in no way, shape, or form should be used to treat weight problems. The health risks greatly exceed any possible weight loss benefit," says Dr. Petak.

Fortunately, at least some of your metabolic rate is determined by learned behaviors, like eating when you're down instead of going for a run. The good news is that you can change those behaviors, simply through awareness or with therapy. The approach Dr. Petak recommends to keep that thyroid gland pumping and your weight in check is to smarten your eating habits. Following a low-glycemic-index diet (like Sugar Busters!) in order to keep insulin levels from going up too much may help some people protect against weight gain in the midsection. "However, while a diet like this may be quite helpful for many people, it's not useful for an endurance athlete, who needs to be carbo-loading," says Dr. Petak. And eating breakfast may help get your metabolism rolling after an all-night fast. Unfortunately, there's no one-size-fits-all diet plan that will speed up metabolism. "Some people say you should have three meals a day, while others say to eat only when you're hungry. We're all different—nothing works for everyone," says Dr. Petak.

Whichever eating plan you choose, it's important to start developing good eating and exercise habits as soon as possible, because metabolically, it's all downhill from here. Most of us gain weight as we age because our metabolism no longer works as efficiently at burning fuel. The metabolic rate tends to drop so much over time that our weight may go up 10 to 15 pounds each decade after we hit 30 as we begin to lose muscle. On the bright side, says Dr. Petak, "Your metabolic rate might not drop so much if your activity level doesn't drop. Usually, though, older people just don't burn as much fuel. But a proper diet coupled with exercise should keep it under control." Both cardio and weight training have their place in a metabolism-boosting regimen. 

Cardiovascular exercise will burn more calories in the short term, while adding muscle mass will help raise the overall metabolic rate slightly because muscle burns more calories than fat does. "Even though the resting metabolic rate may only increase a small amount over a few days," says Declan Connolly, Ph.D., director of the University of Vermont's Human Performance Laboratory, "over the course of months or years, it can make quite a difference."