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New thinking on metabolism suggests a 20-year-old has the same metabolism as a 60-year-old

Some surprising truths about matabolism

We imagine our metabolism as a fire, flaming up when we exercise to torch calories. But that’s just a very small fraction of what it does, according to Herman Pontzer, Ph.D., an evolutionary anthropologist at Duke University, the author of Burn: The Misunderstood Science of Metabolism, and the principal investigator for 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 coauthors, 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.

METABOLISM DOESN’T SLOW DOWN IN MIDLIFE

For years we’ve heard that our metabolism peaks when we’re teenagers and slows significantly as we approach middle age. But the researchers discovered that metabolism crests far earlier and declines much later, and that it has four distinct stages.

Metabolism is at its highest during infancy, 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 we’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!

MEN DON’T HAVE A BETTER 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 he can lose a pound faster than you can). The scientists controlled for these factors and found no difference in metabolic rate.

MILESTONES DON’T CHANGE METABOLISM

It’s a myth that pregnancy and menopause make a big difference. “These major metabolic milestones didn’t affect metabolic rate,” says Jennifer Rood, Ph.D., a coauthor of the study who specializes in research on metabolism and energy expenditure at Louisiana State University’s Pennington Biomedical Research Center. “That was a surprise. But it’s also encouraging. The fact that your rate remains steady through childbirth and into your 60s means 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.”

HAVING MUSCLE HELPS

The types of cells you have affect 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.

Alas, a reality check: There is no proven way to boost metabolism, says Pontzer. 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.”

TIMING MEALS MATTERS

When the day begins to wind down, our body does the opposite—cranking up to be at its calorie-burning peak, according to a study published in Current Biology. “We discovered that you naturally burn about 10 percent more calories in the late afternoon than you do later at night,” says Kirsi-Marja Zitting, Ph.D., an associate neuroscientist at Brigham and Women’s Hospital in Boston, an instructor at Harvard Medical School, and the lead author of the study. That’s about 130 calories that your body is torching without any extra effort on your part.

Researchers suspect that metabolism ebbs and flows according to our circadian rhythms. At night, when we’re asleep, our systems are expending less energy, so our metabolic rate slows. Eat in those wee hours, as night shift workers do, and your body isn’t able to burn off the calories, which could eventually lead to weight gain, says Zitting. During the day, when we’re naturally active, our metabolism rises, until it crests around 5 p.m. “Previous research has shown that hunger spikes in the afternoon as well,” says Zitting. “Since that’s also when we’re burning the most calories, it makes sense that we’re hungriest then, too. Our bodies may be tuned to a late-in-the-day rhythm.”

Syncing our diets to our internal clocks can have health benefits. For general well-being, the most important thing is to maintain a regular schedule, says Zitting. “Wake up and go to bed around the same time every day,” she advises, “and have breakfast, lunch, and dinner at about the same hours as well.” Your body is primed to do things at certain times. Throw its agenda out of whack, and your sleep, health, and weight can suffer.

Also, avoid eating at extreme hours. Let’s say you get home from work at 10 p.m. Don’t consume a big meal then because your body is in slowdown mode and is less likely to burn it off, says Zitting. Instead, have a healthy dinner earlier, before you leave work.

Finally, it may help to eat your carbs early in the day and foods higher in fat at night. “Our research found that you are more prone to burn carbohydrates in the morning and lipids [aka fats] in the evening,” says Zitting. “The difference was small, but it was significant.”

(09/18/2022) Views: 1,074 ⚡AMP