Learn everything you need to know to keep your metabolism in check.
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Every cell in your body requires energy to function―whether it’s delivering nutrients to your brain, pumping oxygen from your lungs to your muscles, or producing infection-fighting white blood cells deep in your bone marrow. That energy comes from calories in the food you eat. And metabolism is the name for your body's system for converting those calories to energy (also known as blood sugar).

New Year. New Food. Healthy eating starts here, with the Cooking Light Diet.

Many factors contribute to your personal metabolism, including heredity. You’re born with an internal speedometer that regulates your base metabolic rate (BMR), the pace at which your body uses energy when you’re at rest. The BMR accounts for approximately 60 percent of the total energy an average person expends in a day. (The rest is used in digestion, exercise, and non-exercise activities―showering, chopping vegetables, or fidgeting with that phone you're probably holding.)

“We are not sure what makes people different in terms of metabolism; the genes determining that have yet to be identified, but it’s being explored,” says Gary Miller, PhD, associate professor of health and exercise science at Wake Forest University in Winston-Salem, North Carolina.

However, even if your metabolic rate is governed by genetics, it’s not immutable. “Metabolism can be changed,” Miller says. “In fact, it’s in a constant state of flux, throughout the day and throughout the years.”

How your metabolism changes with age

Researchers estimate that BMR slows by two to three percent each decade starting at age 20. Changes in body composition are a key factor to this slowing effect. As we age, muscle mass tends to decrease. Women typically lose 10 to 15 percent of their muscle mass between the ages of 20 and 50, and the decline subsequently accelerates, according to research from Missouri-Columbia University.

Less frequent exercise. Inactivity is the main culprit in muscle loss. Left unchecked, muscle is replaced by fat. Because muscle is far more active metabolically than fat, the rate at which you burn energy slows. “Muscle burns calories while fat stores them,” says Peter D. Vash, MD, director of medical and scientific affairs at Lindora Medical Clinic in Costa Mesa, California.

General slow-down. In addition to changing composition, your body also requires less energy with age. A recent study of more than 800 adults at Laval University in Quebec City, Canada, found that some women’s BMR was less than age-related muscle loss alone could explain. Researchers speculated the decline might be related to lessened metabolic demand from organs such as the heart, liver, brain, and kidneys, which is caused by a decline in the organs’ cell mass.

Hormone changes. Metabolism can also be slowed by natural changes in levels of hormones, such as estrogen and testosterone, says Nick Flynn, PhD, associate professor of biochemistry at Angelo State University in San Angelo, Texas. He notes changes associated with menopause may cause an increase in body fat and slowed metabolism.

Keep your metabolism in top shape

By staying active and employing smart lifestyle strategies, you can help offset the shifts in your metabolic rate.

Exercise more. “Exercise keeps the engine running,” Miller says. When you walk, run, or lift weights, you increase the energy demands being made on your body, which raises your metabolism for hours afterwards. “It’s not a huge spike, but it makes a difference,” Miller says. In addition to the 30 minutes of daily exercise recommended by all health authorities, Miller suggests weaving small changes into your day―parking farther away from work so you walk more, taking the stairs instead of the elevator, or working outside in the garden. Such minor activity changes can increase your metabolism by as much as 20 percent, according to Mayo Clinic researchers.

Work out with weights. Strength training helps offset age-related loss of muscle mass. It also provides a metabolism boost. During exercise, muscle tissue is stressed; after, it is repaired―molecular activity that raises metabolism. A woman who strength-trains three times a week for six months can build enough muscle to burn between 10 and 32 extra calories each day, according to Robert Wolfe, PhD, professor of geriatrics at the University of Arkansas. For the maximum metabolic benefit, focus on exercises that engage large muscle groups―for example, squats and lunges for legs, rows and pull-downs for back.

Practice portion control. Limiting portions helps ensure that you don’t overload your metabolism with a surplus of energy (i.e. food). “When people take time to measure their portion sizes, they are usually surprised to learn how much they are eating,” says Laura Picciano, DO, an internist at Hahnemann University Hospital in Philadelphia. “I suggest you buy a food scale and a measuring cup to really understand what a healthy portion is.” If you’re not inclined to weigh food, use your hand to provide a rough guide to proper portions. A closed fist equals a serving of fruit, a cupped hand equals a serving of cereal or grains, two cupped handfuls equal a serving of leafy green vegetables, such as spinach or tossed salad, and an open palm equals a serving of meat.

WATCH: 5 Simple Ways to Calculate Portion Size

 Eat less food more often. Some experts recommend grazing―eating five or six smaller meals throughout the day. “Grazing helps normalize blood sugar levels in our bodies rather than producing three large spikes, which is what happens eating three meals a day,” Flynn says. Determine the proper amount of calories for your age, gender, and size using Mypyramid.gov. Then, keep that number in mind as you transition from eating three ordinary-sized meals to five smaller ones. For example, a woman who needs 1,800 calories per day would aim for five meals containing roughly 350 calories apiece.

Power up with help. Eating protein is a proven way to raise metabolism. The thermogenic effect is the amount of calories your body burns in the act of digesting food. Approximately 10 to 15 percent of the calories you consume are used to process the foods you eat. Although protein contains the same amount of calories per gram as carbohydrates―four―Harvard University researchers reported that its per-gram thermogenic effect is as much as 20 percent higher than carbohydrate. “This causes the body to have a higher metabolic rate while it digests the protein,” Vash says.

Sleep tight. Your metabolism ticks around the clock. Researchers at Columbia University in New York City found that people who slept six hours or less per night were 23 percent more likely to be overweight than those who slept seven to nine hours. “It may have to do with two hormones―leptin, which suppresses appetite, and grehlin, which increases food intake and is thought to play a role in regulation of body weight,” says Gerard E. Mullin, MD, of Johns Hopkins Hospital in Baltimore, Maryland. “Lack of sleep lowers leptin and raises levels of grehlin.” Establish a set schedule to obtain at least eight hours of sleep each night.

Laugh it off. Laughter can provide a mini-metabolism boost. Researchers at Vanderbilt University found that 10 to 15 minutes of laughter in a day could increase energy expenditure. When researchers put pairs of friends or couples into a “metabolic chamber” (a small room that measures heat output to calculate a person’s metabolic rate) and showed them funny videos, they discovered subjects’ metabolic rates rose by 10 to 40 calories―a small increase, but every calorie counts, says lead researcher Maciej Buchowksi, PhD.