Chances are, you think of your metabolism as a fire, flaming up when you exercise to torch calories. But that’s just a very small fraction of what it does, according to Herman Pontzer, PhD, an evolutionary anthropologist at Duke University, the author of Burn, and the principal investigator of a game-changing new study on metabolism.

“Metabolism is the work your cells do every minute of every day,” says Pontzer. “You’ve got 37 million of them, and each cell is like a little factory churning out everything needed to keep your body functioning.” Some calories do get burned during exercise, but most of what you eat is used to fuel the continuous work your cells do.

The new research, published in Science by Pontzer and more than 80 co-authors, looked at metabolism more precisely than had ever been done before. It measured metabolism in almost 6,500 people around the world, from newborns to 95-year-olds.

Once the scientists controlled for variables that affect energy expenditure, like a person’s body size and percentage of fat, they got “a clear road map of metabolism over our life span,” says Pontzer. Here’s what it means for you.

Metabolism Doesn’t Slow Significantly as You Age

For years you’ve heard that your metabolism peaks when you’re a teenager and slows significantly as you approach middle age. But the researchers discovered that metabolism crests far earlier and declines much later, and that it has four distinct stages. From infancy to age 1, metabolism is at its highest, and a baby’s metabolic rate is 50 percent higher than an adult’s.

From ages 1 to 20, metabolism drops about 3 percent a year. Then from ages 20 to 60, metabolism holds steady. After age 60 it slowly starts to decline (0.7 percent a year). That means for 40 years, you’re burning calories at a steady rate, about 2,500 a day on average, says Pontzer. And a 60-year-old has the same metabolism as a 20-year-old.

Sex Doesn’t Affect Metabolism

“There’s nothing special about the male metabolism,” says Pontzer. “Men tend to be bigger, and their bodies consist of more lean muscle and less fat.” Muscle uses more energy than fat, which accounts for the difference (the reason why men may be able to lose a pound faster than women).

The scientists controlled for these factors and found no difference in metabolic rate.

Pregnancy and Menopause Don’t Slow You Down

“These major metabolic milestones didn’t affect metabolic rate,” says Jennifer Rood, PhD, a co-author of the study who specialises in research on metabolism and energy expenditure at Pennington Biomedical Research Center at Louisiana State University.

“That was a surprise. But it’s also encouraging. The fact that your rate remains steady through childbirth and into your 60s means that you don’t have to work harder to maintain a healthy weight, as long as you follow a healthy diet and get the recommended amount of exercise and rest.”

Muscle Mass Matters

The types of cells you have affects the amount of work they do and the energy they burn, says Pontzer. “A cell in your fat is not as busy as a cell in your muscles,” he says. “If you have a lot of lean mass, you’re going to burn calories more efficiently than someone who has more fat mass.” That’s why exercise, particularly muscle-building strength training, can be beneficial.

So, Can You Increase Metabolism?

First, the reality check: There is no proven way to boost metabolism, says Pontzer. Exercise can push it up a little, but your daily energy expenditure is the same, the study found. Metabolism stays steady.

But exercise and diet do make a difference. “Think of it this way: You’re burning a set number of calories each day, but you get to decide how to burn them,” says Pontzer.

“If you expend them on exercise, you’re going to be a lot healthier and have less inflammation than someone who doesn’t. The same is true with food. You decide how to fuel your body. That’s where a healthy diet factors in.”

This story first appeared on www.shape.com

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