What Exactly Is Metabolism—and Can You Change It?

Many of us only have a vague answer to the question of ‘what is metabolism?’ Turns out (spoiler!), there’s more to it than simply how much food you can eat without gaining weight. “Metabolism refers to a range of biological processes taking place within your body’s cells and how many calories it takes to do that work,” explains Herman Pontzer, Ph.D., an associate professor of evolutionary anthropology and global health at Duke University and author ofBurn.

Scientifically, metabolism refers to all of the chemical reactions that happen within your body—processes like breathing, muscle functioning, building and maintaining bones, digesting food and turning it into energy, and even thinking. All of this requires energy—aka calories—so we measure metabolism as the number of calories used to keep the engine going. Sloths, for example, have the slowest metabolism of all mammals—as much as 1.5 times slower than the average human’s. Find out more about why some people have faster metabolisms, how to estimate your metabolism and if there’s anything you can do to speed it up.

Can Losing Weight Slow Your Metabolism?

Do some people naturally have a faster metabolism than others?

Yes, some people’s engines use more energy (what we call a fast metabolism) than others. You and a friend may be the same age and size, but she can indulge in a nightly scoop of ice cream and never gain weight, while the same habit would send the scale trending upward for you.

Each of these three components of metabolism play out differently from person to person, for reasons that are only partially understood. While it’s easy to see why a Peloton instructor may burn more calories per day than a computer programmer, there’s still quite a lot of variability. All things being equal (age, sex, size, etc.), research shows that range is plus or minus about 20%—meaning that you may require 2,400 calories a day to meet your body’s energy needs, while another person needs 2,800 or just 2,000.

Researchers don’t know exactly what’s behind these differences, but how much lean muscle you have (as opposed to body fat) appears to play a large role because muscle and fat have different energy requirements (more on that in a minute). Other factors appear to be genetics, age and size (a taller body uses more energy than a petite one, for example).

How do I find out how fast mine is?

Dina Metti, M.S., who teaches nutrition at San Diego State University, recommends the Mifflin-St. Jeor formula, which uses your weight, height and age to find a rough estimate of your RMR. We say rough because it assumes that biological sex, age, height and weight are the only factors in the equation, which is not true. (And, of course, resting metabolic rate is only one of the three parts of metabolism—although it does account for the largest percentage of it.)

For scientists like Pontzer, the gold standard for measuringtotaldaily energy expenditure—including physical activity and the thermic effect of food, in addition to resting metabolic rate—is via the “doubly labeled water” method. Study participants drink water that contains special hydrogen and oxygen isotopes and researchers then test their urine for up to a week so they can determine how quickly those components are flushed out of their systems, which indicates how fast or slow someone’s metabolism is.

There are calculators that do the math for you, too, if the “doubly labeled water” method isn’t realistic for your. These calculators use your RMR as your baseline metabolism. From there, you can add in your activity level to get an idea of the number of calories you require each day, so you know how many you need to eat to lose, gain or maintain your body weight. Try this one from theBaylor College of Medicine. Some fitness centers also offer handheld metabolism-measuring devices, but Metti says skip them: they can be pricy, and an online calculator is just as accurate-and free.

Can I increase my metabolism?

Can certain foods increase my metabolism?

Research has also found that capsaicin (the compound that makes hot peppers hot) can give your metabolism a temporary boost. The catch is that most studies on the topic have been small or only done on animals. (Diet trials are hard to do well because the way to get the highest-quality evidence is to closely control every aspect of the subjects' diets. Easy to do in mice; not so easy in humans.) Plus, the added calorie-burn isn’t exactly impressive. For instance, one study found that people who ate 1 gram of red cayenne pepper (about 1⁄4 teaspoon of the dried stuff ) burned about 10 extra calories over the next four hours.

Lakatos is also a fan of green tea. It has less caffeine than coffee; however, it also contains an antioxidant called EGCG, which appears to increase brown fat. “Think of all these foods as bonuses,” says Lakatos. “They’re not going to make a major difference, but little things can add up.”

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