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Your metabolism doesn't tragically sink the day you turn 30 — nor is it uniquely speedy in your 20s. If you're gaining weight, you're probably doing too many things that decrease your metabolism and not enough that boost it. Take a look.

Decreases Your Metabolism

Sitting. The less you move, the fewer calories your body needs. As a result, your metabolism temporarily slows.

Dieting. Cutting back on calories makes your body slow down and hold on to fat (in the event of famine, for instance!). Outsmart nature: Trimming only 250 to 300 calories a day from your diet urges your body to shed fat without scaring it into thinking you're never going to eat again.

Aging. Metabolism falls about 1 percent per decade, says Susan B. Roberts, Ph.D., at the Energy Metabolism Laboratory at Tufts University in Boston. So every year, you need about a measly 10 fewer calories a day to maintain your weight.

No Effect

Caffeine. "Caffeine increases heart rate, which could help burn calories," says Dan Bernadot, Ph.D., of Georgia State University in Atlanta. "But it's a small and fleeting increase and won't really boost metabolism."

Increases Your Metabolism

Protein. Protein helps build muscle, and the more muscle you have, the faster your metabolism. Aim to get 20 to 25 percent of your calories from protein.

Snacking. Eating gives your metabolism a brief spike because you've given it something to do (burn calories), so eating four or five mini meals a day is better for your metabolism than having two large ones.

Cardio. Exercise that gets your heart rate up boosts your metabolism while you're doing it, and it makes you burn more calories in the hours after your workout — even when you're resting. In fact, regular workouts can bump up metabolism enough to cancel the drop that you get when you diet, says Kathy McManus, M.S., R.D., director of the department of nutrition at Brigham and Women's Hospital in Boston.

Resistance training. This builds muscles, which burn calories even when you're sitting around. For every pound of muscle you gain, you can burn about 50 extra calories per day. You can gain a pound of muscle in a month if you work out with weights two to three times a week.

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