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 of Burn.
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.
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.
The first and largest component to your metabolism is your resting metabolic rate (RMR), which accounts for a whopping 60 to 75% of your total daily energy expenditure. And it's pretty much what it sounds like: the number of calories your body uses when you're just sitting around doing nothing. Then there's the thermic effect of food, a fancy term for the amount of calories you burn by chewing and digesting what you eat and absorbing and storing those nutrients. It accounts for about 10% of your total daily energy expenditure. (This phenomenon explains why "negative calorie" foods—which contain fewer calories than required to break them down—might exist, though there's no solid proof that munching celery will somehow subtract calories from your body.) Lastly, there's the thermic effect of physical activity, meaning how much energy you use by being active. That comprises about 15 to 30% of the amount of energy you burn through every day—and it includes everything you do, including gym sessions, walking the dog and washing dishes. Seemingly insignificant motions—typing, tapping your toes, fidgeting—count too.
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.)
Things like genetics, the functioning of your thyroid and body composition also play key roles. "Metabolism is largely determined by how heavy you are—more specifically, how much muscle and body fat you have," says Corby Martin, Ph.D., professor and director of the Ingestive Behavior Laboratory at Pennington Biomedical Research Center in Baton Rouge, Louisiana, which studies food intake and metabolism to develop novel weight-management strategies. It may seem counterintuitive, but the bigger you are, the more cells your body has to fuel, and the more energy you'll burn. Even the size of your organs matters significantly: research published in PLOS One found that up to 43% of the variation in metabolic rate between individuals can be attributed to differences in the size of their kidneys, muscles, brain and liver. Age matters too. As you getup there in years, your cells aren't as metabolically active—though it probably doesn't happen as early as you think it does.
For scientists like Pontzer, the gold standard for measuring total daily 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 the Baylor 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?
Yes—to a point—with strength training. A review published in Current Sports Medicine Reports found that, on average, lifting weights increases RMR by about 7%, or an average of about 100 calories a day. This is because even at rest, muscle uses more energy than fat, burning about three times more calories—an estimated 6 calories per pound per day versus 2 for fat. Sure, 100 calories isn't huge, but it can make a difference for your weight over time. Think of strength training as one of the best ways to optimize the metabolism you were born with. If your goal is to shed some pounds, rather than trying to overhaul your body's internal furnace, Pontzer says the more effective strategy would be to focus on, "How do I get my diet to match my energy expenditure?" Yep, he's talking about the old calories-in versus calories-out math.
Can certain foods increase my metabolism?
You've probably seen a million stories on foods that supposedly stoke your metabolism. And although there is some science there, many experts, including Pontzer, don't believe dietary tweaks have any impact. Yet others, like Mercedes Carnethon, Ph.D., vice chair of the department of preventive medicine at Northwestern University's Feinberg School of Medicine, are more optimistic. She points to research—albeit mostly from small, preliminary studies—that suggests some foods can give your metabolism a kick, though she notes that they won't "overcome a plate of nachos consumed at midnight." Chief among them: upping your protein consumption, assuming you're currently falling short, as many older adults are. (If you're already eating plenty—like most other Americans—don't eat extra or you could overtax your kidneys.) That's because the thermic effect of digesting protein is much higher than that of fat or carbs. "An estimated 25 to 30% of the calories consumed in protein are used to break down and process it," says Lyssie Lakatos, RDN, CDN, CFT, a dietitian, personal trainer and co-author of Fire Up Your Metabolism. To find out how much protein you need, multiply your weight in pounds by 0.36 to get the number of grams to aim for.
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.
Capsaicin appears to work—at least a little—by heating you up from the inside, says Lakatos. (As body temperature rises, so does metabolism.) It might also help by increasing the amount of so-called brown fat in your body or making the brown fat you already have more active, so it burns more calories. This type of fat sits primarily in your neck and contains far more mitochondria—the energy powerhouses of cells— than white fat.
Then there's caffeine. While you don't want to go overboard—no one needs jitters or insomnia—this pick-me-up does seem to increase metabolism in a few ways. First, it's a stimulant, so it literally revs up energy use. It might also help by increasing the metabolic activity of brown fat, according to a small 2019 study at the University of Nottingham, a public research institution in the United Kingdom. Studies have shown that caffeine can increase your resting metabolic rate by anywhere from 3 to 11%. In one trial, adults who consumed 100 milligrams of caffeine (roughly the amount in an 8-ounce cup of coffee) several times a day burned an extra 79 to 150 calories. But other research suggests that to get the most benefit, you'd have to slug way more java than that. (Not a great idea.)
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."
Some reporting from EatingWell Magazine, January/February 2022 and EatingWell Magazine, September 2019.