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How People Really Burn Calories, and What That Means for Weight Loss: Shots

Vigorous physical activity may not be as helpful in losing weight as you might hope.

Catherine Falls / Getty Images


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Catherine Falls / Getty Images


Vigorous physical activity may not be as helpful in losing weight as you might hope.

Catherine Falls / Getty Images

It’s an everlasting question: what diet is best for losing weight? Or what should we eat (or avoid) to stay healthy?

Paleo or keto followers will put off your ears as to why their diet is the most sensible one. People who opt for a vegan diet (no animal products, including dairy products) speak for both personal and global Health.

Hermann Pontzer, a evolutionary anthropologist at Duke University, argues that human metabolism has advanced to the point that How we eat and use our calories is more important than all of our collective obsession with What Eat.

In his new book, Burn: New research blows the lid off how we can really burn calories, stay healthy, and lose weight, Pontzer breaks up the science of metabolism and tells stories from his work on calorie consumption in hunter-gatherer societies.

One of the most amazing realizations is the idea of limited daily energy consumption. This is the idea that human metabolism adapts to our activity level to keep our daily calorie expenditure in a surprisingly narrow range – no matter how hard you exercise. But don’t let this depressing fact stop you from going to the gym – it is still important to keep exercising daily to maintain the weight and overall health.

This interview with Pontzer is adapted from one interview for Tulsa’s Public Radio Medical Monday Program and was edited for length and clarity.

In your book, you debunk the common metaphor we use for calorie expenditure – an engine or a machine. They say it would be more accurate to compare it to running a business. Why this?

The engine view does a few things right. We give our bodies fuel in the form of food. And we burn it up in all the tasks our body does, like an engine burns fuel.

But an engine, like the engine in your car, cannot decide How it burns the fuel. When it comes to energy consumption in a car, it all depends on how hard you step on the accelerator. Your body is not like that. Your body is more of a business because, like any business, it has an overarching goal. The ultimate goal of your body is to survive and reproduce, because that is what every organism has evolved to do. But there are many parts and parts and departments that serve that overall goal.

In a company, you have finances, sales, people and security and everything else. It’s the same with your body. You have all of these different organ systems that all work together. And like in a business, you can juggle things on a low income. So you spend less on this or that task. And when things go well, you can increase the amount of energy you spend on various tasks. And this type of juggling or prioritizing that companies do is the same that your body can do with the consumption of calories.

One fallacy with the calorie burning engine model is that we think, OK, I need to burn more calories than I consume, either by eating less or exercising more, or both. But as you point out, the metabolism adapts and it becomes more difficult to lose weight. While exercise isn’t a really good weight loss strategy, it’s still very important to your overall health, right?

That is exactly right. After all, when you are more physically active, you stop burning calories each day, but you do change the way your calories are spent. When you spend your calories on exercise, it means you are spending fewer calories on other tasks.

And for most of us this is a really good thing because if we spend less energy on inflammation, for example, we reduce our inflammation levels. For example, if we spend less energy on stress responses, our cortisol levels do not rise as high and our adrenaline levels do not rise so high, we achieve a lower stress response. And it seems that this movement could also help keep testosterone levels in men or estrogen levels in women at slightly healthier levels. This adjustment, this metabolic adjustment that we make, is one of the reasons exercise is so good for us.

They have done extensive research with modern day hunters and gatherers like the Hadza in Tanzania to better understand how human metabolism works. What did you learn?

To date, the Hadza have no domesticated plants, animals, machines, weapons, electricity or the like. They live in grasshouses in the open savannah in northern Tanzania. And every morning they wake up and the women get plant foods like berries and tubers. The men go hunting for game with bows and arrows.

For someone like me who studies how humans evolved, such a community is simply an invaluable way of asking what hunting and foraging does to our bodies. Because we humans have evolved over millennia as hunting and collecting animals. And yes – in such a population, food can sometimes be scarce. And you always expend a lot of energy on physical activity. So your body needs to be really good at prioritizing how it uses its calories.

The Hadza go everywhere they go and, compared to us, they are rarely sedentary. I would assume they burn a lot more calories than we do in a day. Surprisingly, however, your work shows that their metabolism isn’t all that different from the average American.

About 10 years ago we measured how many calories men and women in the Hadza community burn each day. The Hadza are so physically active that we would expect their daily calorie consumption to be much higher than in the US and Europe and other industrialized populations. And instead, we found that even though men take 19,000 steps today, women take 13,000 steps a day on top of all their other work, they no longer burn total Calories every day when we’re in the west.

After all, physical activity is one of those things that the body can juggle and adapt to. Just as your body can adapt to changes in your eating environment, so can your body adapt to changes in your physical activity. For the Hadza, their “metabolic business” has adapted so that they spend less on other body systems to make room for the large physical workload they have.

What does this mean for someone who wants to lose weight today?

If you or I started an exercise program tomorrow, we become burn extra calories from this exercise for a while. But after a few months, our bodies will adjust so that we are using roughly the same amount of energy every day as we were before starting the exercise. Your body adjusts its energy input in order to keep the total calories burned daily within a relatively narrow range. It only speaks for how adaptable and flexible our body is and how we are not really responsible for our metabolism as we think.

You add a section to the book about the TV show The biggest loser where participants competed against each other to see who could lose the most weight. What was the problem with that?

Participants took part in this show and were subjected to a brutal routine of intense exercises associated with starvation. So you can lose a lot. But it’s not sustainable. Your body is pushing back hard by reducing its metabolism. Some of these candidates were persecuted years later. The people who were able to keep the weight off still lowered their metabolic rates from what they went through. Many of the participants gained the weight back.

It shows you the way to socially resolve the obesity crisis or [to lose and keep weight off] individually is not a big, drastic crash approach. You have to be more sustainable because the body just pushes back if you push too hard.

So if your goal is to lose weight, diet will have a greater effect than exercise. But is exercise essential here to maintain a healthy weight?

That’s true. Let’s rethink what exercise does. I call it the rhythm section of your body. Movement keeps everything in sync, in sync, and helps regulate how your body works. So once you have reached a healthier weight, once you are able to lose weight and have reached a certain point where you want to be, exercise is really the key to keeping yourself there. Exercise changes the way your body regulates how hungry you feel, or how full you feel.

The Paleo Diet is based on the idea that when we were all hunters and gatherers, we had a specific diet and no problems with obesity or type 2 diabetes or high blood pressure. But based on your study of the Hadza, what are the Paleo people doing wrong?

When you go out and get the chance to live with a group like the Hadza, you find that many of the stories we tell each other about the past, including things like the Paleo Diet, just kind of fall apart. So in the world of the paleo diet there is the idea that there is a single type of natural human diet, and that diet was very meat heavy, hardly any carbohydrates and most certainly no sugar.

[In reality] the Hadza have a mixture of plants and animals in their diet. It changes from day to day and year to year, but about half of the calories come from plants. And not only that, but actually something like 10[%] Up to 20% of their calories per day come from wild honey, which consists only of sugar and water, which would not be on a paleo dieter’s menu. Another big part of your diet is the starchy tubers and those root vegetables, which you are often not allowed to eat on some versions of the Paleo diet.

One last thing that amazed me in your book: you write about the metabolic costs of pregnancy – you compare pregnant women to Tour de France riders.

You can push the body like in the Tour de France, where the riders burn 7,000 or 8,000 calories a day for three weeks. But it also makes sense that pregnancy would hit the same metabolic limits as the Tour de France, for example. Both keep your body’s metabolic machinery running at full speed as long as it can keep it going. For one thing, it only speaks for how stressful pregnancy is, but it also speaks for how these things are all connected. Our energetic machinery is incorporated into these different tasks and creates connections that unite all these different experiences.


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