Yes, you really can speed up your metabolism, though likely not significantly, and certainly not with diet pills, green tea, or any other quick-fix products. There's only real way to make a difference in how your body burns calories: increase your lean muscle mass.
To get why, you have to understand a bit of the science behind metabolism. Your metabolic rate is essentially the speed at which your body expends energy, and it depends on many different factors. Your age, weight, health history, organ function, oxygen capacity, and even your height can all influence how many calories you burn during exercise, but also (and more importantly) during sedentary times of day.
Most of your resting metabolism is taken up by your organs—brain, heart, liver, etc. But the biggest factor affecting your metabolism that you can control is your ratio of body fat to lean muscle mass. "Muscle burns more calories than fat tissue, because muscle requires more energy to maintain," says Harold Gibbons, New York State Director of the National Strength and Conditioning Association. "The more fat you have, the slower your metabolism will be."
Just dropping pounds probably won't help, though, since most weight loss is a combination of fat and muscle tissue. "Usually, when people lose weight, their metabolism actually goes down," says Kim Sasso, a nutritionist at Loyola University Health System. That makes sense; you don’t need as much food to maintain a lower bodyweight. But if your goal is simply to up your metabolism, Sasso says, "it's not until you start decreasing your percentage of body fat and replacing it with increased muscle mass, do you start to see a difference in your resting metabolic rate."
You can see where this is going: To speed up your metabolism, you need to strength-train. And since the two biggest muscles in the body are the glutes and the thighs, lower-body exercises like squats and lunges are a great place to start, according to Sasso.
"Just by increasing their muscle mass on those two areas, you can burn more energy when you're exercising," says Sasso.
Plus, strength workouts have an additional metabolism-boosting benefit. Because this type of anaerobic training involves breaking down and building back up of muscle tissue, the body needs to burn more calories in the 24 to 48 hours after each session—a phenomenon known as excess post-exercise oxygen consumption, EPOC, or, informally, "afterburn.” Researchers, however, are currently debating how intense that afterburn really is.
Cardio is still important, too. "Respiratory function is another big part of metabolism, and to increase that you need to be regularly getting your heart rate up," says Sasso. She recommends extended periods of walking, jogging, or some other form of moderate to intense aerobic activity, as well as body-weight circuit training with no rest between sets. Gibbons points out that cardio workouts generally burn more calories during exercise than strength-training—also important if your overall goal is weight loss or weight maintenance.
As far as foods and supplements go, nothing's been shown to have a meaningful impact on metabolic rate. "Green tea has a reputation for being a metabolism booster because it has compounds like caffeine, but the effect is so tiny, it's negligible," Sasso says. Protein also gets branded as metabolism-boosting, because the body uses more energy to digest it than it does for fat and carbs. But this effect is only temporary and, again, not big enough to make a real difference on its own.
Poor dietary and lifestyle choices can, on the other hand, slow down a healthy metabolism. Specifically, not eating every few hours or consuming too few calories overall, can put your body into starvation mode, so it hangs onto energy (and fat) as long as it can. Not getting enough sleep has also been shown to cause metabolic dysregulation.
Bottom line: Reduce body fat, build lean muscle mass, get plenty of sleep, and don't skip meals, so your body may burn a few more calories at rest. If you're really interested in the numbers, metabolic testing, which gauges your resting metabolic rate, can help you track your progress.