Slow? Fast? Important? Irrelevant? We break down the facts on the ultimate weight-loss buzzword: metabolism.

By Gail O’Connor
January 23, 2019

Your metabolism is a complex system. It determines how quickly and efficiently your body burns calories and how much you can eat in a day without putting on weight. Scientists are still gaining new insight into the factors that make your metabolism run. But they know for sure that becoming a healthier, stronger woman can fire yours up—prolonging good health, improving mood, slowing the effects of aging, and maybe even helping you lose some weight along the way.

Experts share the latest findings, plus what you need to know to make all that good stuff happen.

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Some parts of your metabolism are beyond your control.

Although many of us talk about “metabolism” as if it’s a single bodily process, there are actually three types, each of which expends energy (or calories) at a different rate. Your resting metabolic rate determines the amount of energy your organs use to stay functional when you’re just sitting around. It makes up the largest piece of the metabolism pie (around 60 to 75 percent) on a regular workday with minimal activity, and there’s very little you can do about it. In fact, contrary to what you may have heard, thin people don’t have faster resting metabolic rates. “The bigger you are—regardless of whether that weight comes from muscle or fat—the higher your resting metabolic rate will be,” says Martica Heaner, PhD, adjunct associate professor of nutrition at Hunter College in New York City. Your active metabolism—which accounts for about 10 to 15 percent of the calories you burn in a day—dictates the energy you use up when you’re walking, running, exercising, even fidgeting (for some people). This is the type you have more control over, to some degree, since the more you move, the more calories you burn. Finally, there’s diet-induced thermogenesis—the energy your body uses to consume and digest food. Yep, you get a bonus burn—8 to 12 percent of your daily calorie use—for eating!

TRY THIS TRICK: Believe it or not, spicy food and green tea can fire up diet-induced thermogenesis a bit. So brew some tea or pour a little hot sauce on dinner. “You’ll get a teeny increase in your metabolic rate—we’re talking maybe a bump of 1 percent for an hour. But these little changes add up over time,” says exercise physiologist Polly de Mille, clinical director of the Tisch Sports Performance Center at the Hospital for Special Surgery in New York City.

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Your muscles are in charge.

A pound of muscle burns seven to 10 calories a day, while a pound of fat burns just two or three. We all lose muscle as we age, starting in our 20s, and as it vanishes, so does our calorie-burning power. “By your 70s, your resting metabolism may be 15 percent slower than it was in your 20s,” says de Mille. “That’s 15 percent less food you can eat without gaining weight.” While building new muscle can help counteract this trend, it’s even more important to engage the muscle you already have, says Wayne L. Westcott, PhD, professor of exercise science at Quincy College in Quincy, Massachusetts. Every time you challenge your muscles by strength training, they go through a breakdown-and-repair process, or remodeling. This means you burn calories while working out and continue to burn them after you put the weights away. Keep up the practice, and you’ll increase the speed of your resting metabolism, even if your muscles stay about the same size.

TRY THIS TRICK: Do two or three 20-minute sessions of resistance training (12 to 20 sets of exercises) each week. In three months, your resting metabolism will be about 6 percent faster. When you exercise, focus on major muscle groups. And don’t shy away from heavy weights. Start with one that’s about half as heavy as the largest weight you can lift. As you become more proficient, switch to weights that are 60 to 75 percent of your maximum lift.

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A lack of protein can slow your metabolism.

If you’re not already on the protein bandwagon, get on board. Although the USDA suggests consuming 5 ounces of a protein source per day as part of a 1,600-calorie diet, many experts say that recommendation is conservative or even on the low side, particularly for healthy adults over 50. Your body needs amino acids—the building blocks of protein—to stay functional. “If you don’t eat a diet rich enough in them, your body’s forced to tap your muscles, which have a great reservoir,” says Wayne W. Campbell, PhD, professor of nutrition science at Purdue University in West Lafayette, Indiana. When you lose valuable muscle, your resting metabolism pays the price.

TRY THIS TRICK: Make sure you’re putting protein in every meal and snack—starting the day with 15 grams (about two eggs) is a great idea. And don’t overlook whey, one of two proteins found in milk. It’s rich in the amino acids muscles thirst for and can aid recovery after workouts.

Dieting is the enemy.

Any weight-loss diet—yes, even one that seems sensible—will leave your metabolism slower than it was when you weighed more. That’s partly because each time you shed pounds, you lose fat and muscle, but when you ditch your diet and regain weight, the pounds come back as fat. And since smaller people have slower metabolisms than bigger people, you’ll have to eat even fewer calories than you did at the start of your diet to maintain your new weight. More annoying news: The part of your brain that manages your metabolism cares little about whether you ever fit back into your favorite jeans, and cares very much about whether you have the energy you need to survive. Try to cheat your body out of the calories it’s come to rely on, and it will immediately start robbing your muscles of fuel and directing that energy to your vital organs—causing your metabolism to dip lower.

TRY THIS TRICK: If your goal is to lose a significant amount of weight, take it slow. “It’s best to lose about 10 percent of your body weight, maintain that weight for three to six months, then lose more if you desire,” says Laura J. Kruskall, PhD, director of the UNLV Dietetic Internship & Nutrition Center. “This gives your body time to adjust to physiological adaptations, like a slower metabolism, and gives you time to learn healthy weight-maintenance behaviors.” Also, never eat fewer calories than your resting metabolism requires. The easiest way to determine that magic number: Take your body weight in pounds and multiply by 10.

Your metabolism likes sleep.

A single night of sleep deprivation can alter your metabolism and trigger weight gain, according to recent research from Uppsala University in Sweden. Lack of sleep tends to slow people’s metabolism, in part because that’s when your body repairs itself, which burns calories, says de Mille.

TRY THIS TRICK: Debating between an extra hour of sleep or working out? Do both! If you sleep in and then squeeze in 10 minute bouts of strength training throughout your day, you’ll give your metabolism an optimal shot at burning calories.

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