We run for all sorts of reasons. Some of us run because we like how it makes us feel. Others run to scratch a competitive itch.
For those who run to manage their weight, however, a bit of bad news: running may burn some calories, but it will not “boost” your metabolism—at least not in the long run.
Science has shown that, at a population level, people vary in how many calories they burn every day. What has remained unknown is whether that variation is due to people burning a ton of calories one day and only a few the next, or if it’s because certain groups of people have consistently fast or slow metabolisms.
A recent study published in Nature Communications has helped answer the question of if metabolism is a static trait or not. The researchers showed that individuals’ metabolisms vary by 20 percent or more from person to person regardless of activity level or body composition. (So, no, being on the skinnier side is not evidence that you have a faster metabolism.) However, your individual metabolic rate—fast or slow—doesn’t vary over time.
“Total energy expenditure, which we call the total calories you burn every day, is a repeatable measure like height or other things that are stable for you,” said Herman Pontzer, Ph.D., an associate professor of evolutionary anthropology at Duke University, author of the book Burn on human metabolism, and one of the study’s lead researchers.
To come to this conclusion, Pontzer and his coauthors drew from a database of studies that measured metabolism using a technique called doubly labeled water. Participants in these studies drank a glass of water containing isotopes of hydrogen and oxygen, which scientists can track in the body. How quickly the subjects lost each of those isotopes—to things like pee, sweat, and breathing—revealed how many calories they had burned in that time. Ergo, “total energy expenditure,” i.e., metabolism.
Pontzer’s research team looked at more than 300 adults in the database whose total energy expenditures had been measured at two time points, ranging from two weeks to more than eight years. They then adjusted for body size and composition—because more cells burn more calories, and different types of cells (e.g., fat vs. muscle) burn calories at different rates—and determined whether someone had a “fast” or “slow” metabolism by seeing whether they burned more or fewer calories than would be expected for someone with those attributes.
Finally, when they looked at people’s daily energy expenditures over time, they found that these numbers didn’t really change.
“If you’re a high-metabolism person, you’ll have that high metabolism for your body size today, and you’ll have it again when we measure you again in a few months or even a few years,” Pontzer said. “And it works on the other side too: if you’re a slow-metabolism person today, you’ll have a slow metabolism in a couple months or a couple years.”
Does a slow metabolism mean I’ll gain weight?
This finding led to the researchers’ second question: if you have a fast metabolism, are you protected against weight gain? And conversely, if you have a slow metabolism, are you doomed to gain weight?
“That is the premise of every exercise program in the world,” Pontzer said, “and it’s wrong.”
The researchers found that having a slow metabolism did not make people more likely to gain weight and having a fast metabolism didn’t make people less likely. Pontzer said that this is due to the human brain’s outstanding ability to match, over the long run, calories consumed to calories burned.
As humans, we match our energy needs with greater than 99 percent accuracy. The problem is that, due to the food environment in which most of us live, that 1 percent mismatch is consistently in the same direction: overconsumption.
Running is not a magic “metabolism booster”
Now, the seemingly obvious caveat is that the people in this study were not specifically runners. However, Pontzer says that most runners aren’t that different in how much energy they expend, over the course of time, compared to regularly active individuals.
“We think of exercise as putting our foot on the gas of our metabolism. That’s not the right way to think about it,” said Pontzer.
Exercise is complex: it affects how our body regulates all of its cells, and that in turn affects things like feelings of hunger and fullness, immune function, and even behavior. Ultimately, Pontzer said, while runners will obviously see day-to-day fluctuations in the energy they burn—more on long-run days than, say, recovery days—our bodies adjust how much energy we’re expending during our non-exercise hours to keep the total daily caloric burn within a fairly narrow range. You only need to look at how seriously elite athletes take their rest to see this balancing act in action.
“I don’t think there’s a researcher out there who’s looked at the data and doesn’t understand how important exercise is for health,” Pontzer said. “But there’s an academic debate as to why it’s important for health. There are people who really don’t want to give up the idea that there’s this weight control effect, but I just don’t see the data there. Maybe on the margins, but not as a big factor.”
Ultimately, Pontzer points to diet as the biggest lever people have to pull when it comes to managing weight. Of course, that doesn’t mean it’s easy. “The food environments that we’ve built for ourselves really make it hard for people to maintain a healthy bodyweight,” he said.
The good news is that physical activity does seem to have an effect on regulating consumption—those who exercise are, broadly speaking, less likely to overeat than those who don’t. So if you’re running to manage weight, don’t ditch your trainers.