You are what you eat.

That’s apparently true even for people who do rigorous exercise regularly.

In a study published today in Current Biology, researchers say the human body tends to adjust to long-term exercise regiments.

The result is active people burn about the same amount of calories per day as people who are more sedentary.

The reason is biological evolution. The body is trying to make sure it doesn’t starve to death.

“More activity does not necessarily mean more calorie expenditure,” said Herman Pontzer, one of the study authors and an associate professor of anthropology at Hunter College and City University of New York.

Pontzer told Healthline he is always quick to point out that people should still exercise. It’s good for your cardiovascular health as well as your muscles and bones.

However, if you want to lose weight, you’re better off focusing on your diet than the number of miles you run or the minutes you log at Zumba class.

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Tribe Sparks Interest in Research

Pontzer became interested in this specific research when he was studying the Hadza tribe in northern Tanzania.

The hunter-gatherers are highly active, walking long distances, and doing hard physical labor every day.

However, Pontzer and his colleagues discovered the tribe dwellers expended similar energy levels to more sedentary people living modern lifestyles in the United States and Europe.

“That was a big surprise,” said Pontzer. “What we’ve heard about how people expend calories seemed to be wrong. It was one of those light bulb moments.”

So, Pontzer’s team analyzed the daily activity level and energy expenditures of more than 300 people with modern lifestyles.

What they found was people with moderately active lives burned only about 200 calories a day more than people who were sedentary.

In addition, people with high activity levels burned the same amount of calories as those with moderate lifestyles.

The researchers concluded there may be a “sweet spot” for exercise routines. Do too little and you’re out of shape. Do too much and your body adapts, and you end up spinning your caloric wheels.

Dr. Michael Roizen, the chief wellness officer at the Cleveland Clinic, told Healthline that Pontzer’s study backs up other past research.

“We’ve known for a long time that there is a balance between activity and calories,” Roizen said.

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Why Does the Body Do This?

The adaption by the body goes back to our primitive ancestors.

Pontzer said if a human, or any species for that matter, is burning a lot of calories but not ingesting extra food, the body starts to think food is getting difficult to find. So, it adapts by burning fewer calories so it won’t starve.

“It makes good evolutionary sense,” Pontzer said.

That might explain why people tend to steadily lose weight during the first few weeks of an exercise program and hit a plateau of sorts after that.

“That’s when your body says ‘Whoa, I need to start preserving some calories,’” Roizen said.

He noted there are four categories of exercise that people can focus on for weight loss.

One is doing the equivalent of 10,000 steps, or 100 minutes of walking, a day. It doesn’t matter if the exercise is walking, running, or swimming.

Another is 30 minutes a week of resistance training.

A third is the equivalent of 40 jumps a day.

The fourth is 20 minutes a day of cardiovascular exercise that brings the heart rate up to 80 percent of the age-adjusted maximum heart rate.

Roizen said once you get above those levels, the efficiency of your exercise starts to dwindle.

Most people, he notes, don’t have to worry about exceeding these limits.

“We are so low on the exercise scale,” he said.

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It’s All About Food

Given all this, Pontzer and Roizen say our wellness approach should focus more on diet than exercise when it comes to shedding pounds.

“The best way to lose weight is to avoid extra calories,” Roizen said.

Pontzer said his team wants to do more study to see if there are any variations, such as marathon training or other extreme exercise.

When it comes to extreme exercise, Roizen added, the body may adjust for calorie intake and output so that the athlete’s weight stays about the same throughout their rigorous training schedule.

In addition, Pontzer’s team wants to study how the body changes when it starts to adapt to calorie expenditure. They hope to explain how the body can adapt to great physical demands without burning extra calories.

Roizen said it may be that the body’s immune system signals our gut bacteria to change the ratio of calories it extracts from our food. However, that is just a theory at this point.

“There all kinds of things we don’t understand,” Roizen said. “What we do know is that this [the body adjustment] happens.”