Story by

Susanne Rust

Get this: Although you may be just sitting at your desk or planted on your couch while reading this, you are burning the same number of calories as the hardest-working hunter-gatherer in East Africa.

Indeed, the fact that you get from one place to another in your car, on a train or on a bus; that you ride an escalator or elevator to go up and down floors; and that you move only when you absolutely must makes no difference.

You still are expending the same amount of energy as the Hadza, who generally walk between five and seven miles a day to find food.

And this finding indicates that our Western propensity for obesity is not so much related to a sedentary lifestyle, but easy access to high-calorie and processed foods.

This finding is the result of new research, which appears today in the Public Library of Science journal.

“This is one of those great ‘a-ha’ moments in science,” said Brian Wood, who was a postdoctoral fellow at Stanford University while doing the research, but now is an assistant professor of anthropology at Yale University. “Where we went in and did some methodical testing, and came out with a finding that really challenged the way we’d seen and explained things before.”

Wood and study co-author Herman Pontzer, however, stressed that this research does not suggest that physical activity has no benefits.

“Physical activity is related to all kinds of good things that make people healthy,” said Pontzer, an anthropologist at Hunter College of the City University of New York. “It’s just unlikely the underlying cause of obesity in Western populations.”

To understand the relationship between physical activity and daily energy expenditure, the researchers wanted to see how much energy the Hadza, a hunter-gatherer group living in East Africa, used.

For years, many scientists had suggested that the global obesity pandemic was the result of a sedentary lifestyle. Humans, they argued, evolved in hunter-gatherer-type societies, where we were physically active all day long. When people started to become more sedentary, without the pressure or need to search constantly for food, we became fat.

So Pontzer, Wood and a team from universities in Tanzania, England, Arizona and Missouri decided to see just how much energy hunter-gatherers, such as the Hadza, expend in a day and compare that with people living in developed nations and farming cultures.

The researchers had the Hadza wear GPS wristbands so they could see how far they traveled each day. They also collected urine samples from the Hadza that provided them with information about the amount of energy the Hadza used.

And what the researchers found – when they compared this with samples collected from people in developed areas, such as North America and Europe – was there was no difference. People expend roughly the same amount of energy, no matter how different in terms of their physical activity. 

"At the end of the day, we know humans respond to their environments," Wood said. "I think one of the things our work is showing is that there are certain ranges of adaptability. Maybe we are more constrained by our evolutionary history than we thought."

The authors speculated that our limited range of energy expenditure is a trait our species adapted over millions of years of evolution, in response to a complex web of environmental signals and food availability. They pointed to other studies on different mammal species that show similar results.

The point being that no matter what you do, no matter how physically active or inactive you are, it is very difficult, if not impossible, to budge out of that constraint. And it may be why diets so often fail – your body will keep coming back to this limited range of calorie burning.

Curiously, the scientists did find that farmers living in a non-developed area of Bolivia used slightly more energy than either the hunter-gatherers or those living a “modern” lifestyle.

Pontzer said more research needs to be done to understand why, but it does throw some water on the idea that some societies abandoned hunting-gathering for farming because it required less work, or energy.

“This idea that farming is somehow better than hunter and gathering, that it is an easier lifestyle, turns out not to be true,” he said.

The researchers would like to expand their work to other populations and learn more about the physiology of the Hadza and others, to better understand how physical activity is related to energy expenditure.

But for now, if you’re trying to lose weight, the best solution might be to cut back on the energy you consume, because the energy you expend isn’t likely to change much.