Intense physical activity may not be as helpful in losing weight as you may hope. Catherine Falls/Getty Images
It's an eternal question: What diet is best for weight loss? Or, what should we eat (or avoid) to stay healthy?
Devotees of paleo or keto will talk your ear off about why their diet is the most sensible. People choosing vegan diets (no animal products, including dairy) make a compelling case for both personal and global health.
Herman Pontzer, an evolutionary anthropologist at Duke University, argues that human metabolism has evolved to the point where how we eat and expend our calories is more important than all of our collective obsession with what to eat.
In his new book, Burn: New Research Blows the Lid Off How We Really Burn Calories, Stay Healthy and Lose Weight, Pontzer breaks down the science of metabolism and shares tales from his work studying caloric expenditure among hunter-gatherer societies.
One of the most startling findings is the notion of constrained daily energy expenditure. This is the idea that the human metabolism adapts to our activity levels to keep our daily calorie burn in a surprisingly narrow range — no matter how hard you work out. But don't let that depressing fact hold you back from the gym — it's crucial that you still get daily exercise for weight maintenance and overall health.
This interview with Pontzer is adapted from an interview for Public Radio Tulsa's Medical Monday program and has been edited for length and clarity.
In your book you debunk the common metaphor we use for caloric expenditure — an engine or a machine. You say it would be more accurate to compare it to running a business. Why is that?
The engine view gets a few things right. We put fuel into our bodies in the form of food. And we do burn it off in all the tasks that our body does, the way that an engine burns fuel.
But an engine, like the engine in your car, doesn't get to decide how it burns the fuel. A car's energy burn is all about how hard you step on the gas pedal. Your body isn't like that. Your body is more like a business, as it has an overall goal like any business does. The overall goal of your body is to survive and reproduce, because that's what every organism has evolved to do. But there are many parts and pieces and departments that are in the service of that overall goal.
In a business you have finance, sales, human resources and security and everything else. It's the same with your body. You've got all these different organ systems that all work together. And like a business, when income is low, you can juggle things around. So you spend less on this or that task. And when things are good, you can ramp up the energy that you spend on different tasks. And so that kind of juggling or prioritization that businesses do is the same that your body can do with how it spends calories.
One fallacy with the engine model of calorie burning is we think, OK, I've got to burn more calories than I take in, either by eating less or exercising more or both. But as you point out, the metabolism adjusts, and it becomes harder to lose weight. So even though exercise isn't really a great weight-loss strategy, it's still very important for your overall health, right?
That's exactly right. If you're more physically active, eventually you don't burn more calories a day, but you change the way your calories are spent. If you spend your calories on exercise, what that means is you're spending fewer calories on other tasks.
And for most of us, that's a really good thing, because if we spend less energy, for example, on inflammation, we reduce our inflammation levels. If we spend less energy on stress reactivity, for example, our cortisol levels don't go up as high and our adrenaline levels don't go up as high, we achieve lower levels of stress response. And it seems that that exercise might also help keep testosterone for men or estrogen levels for women at a slightly healthier level. So that adjustment, that metabolic adjustment that we make is one of the reasons exercise is so good for us.
You've done extensive research with modern-day hunter-gatherers, like the Hadza people of Tanzania to better understand how human metabolism works. What did you learn?
The Hadza, to this day, don't have any domesticated crops or animals or machines or guns or electricity or anything like that. They live in grass houses in the open savanna in northern Tanzania. And every morning they wake up and women are off to get plant foods, such as berries and tubers. The men go off to hunt for a wild game using bow and arrow.
For somebody like me who studies how humans evolved, a community like that is just an invaluable way to ask what hunting and gathering does to our bodies. Because we humans evolved over millennia as a hunting and gathering species. And yes — in a population like that, food can be scarce sometimes. And you're always spending lots of energy on physical activity. So your body really has to be good at prioritizing how it spends its calories.
The Hadza walk everywhere they go, and compared to us, are seldom sedentary. I'd assume they burn significantly more calories than we do in a day. Yet surprisingly, your work shows that their metabolism isn't all that different from the average American.
About 10 years ago, we went and measured how many calories men and women in the Hadza community burn every day. The Hadza are so physically active, we'd expect that their total calories burned every day would be much higher than we see in the U.S. and Europe and other industrialized populations. And instead, what we found was that actually, even though men are getting 19,000 steps today, women are getting 13,000 steps a day on top of all the other work they do, they aren't burning more total calories every day than we are in the West.
Physical activity ends up being another one of those things that the body can juggle and adjust. And so in the same way that your body can adjust to changes in your food environment, your body can adjust to changes in your physical activity. So for the Hadza, their "metabolic business" has adjusted so that they spend less on other body systems to make room for that big physical activity workload that they have.
What does this mean for someone who is trying to lose weight today?
If you or I started an exercise program tomorrow, we will burn extra calories from that exercise for a while. But after a couple of months, our bodies will adjust so that we're spending about the same energy every day as we were before we started the exercise. Your body adjusts how it spends its energy to keep the total calories burned every day within a relatively narrow range. It just speaks to how adaptable and flexible our bodies are and how we're not really in charge of our metabolisms the way we think.
You include a section in the book about the TV show The Biggest Loser in which contestants competed to see who could lose the most weight. What was the problem with that?
Contestants went on this show and were put under a brutal routine of intense exercise, coupled with near starvation. You can lose a lot of weight that way. But it's not sustainable. Your body pushes back hard by slashing its metabolic rate. Some of those contestants have been followed for years afterward. The folks that have been able to keep the weight off still have lowered metabolic rates from what they went through. A lot of the contestants gained the weight back.
It goes to show you the way to fix the obesity crisis societally or [to lose and keep weight off] individually is not some big, drastic crash approach. You've got to go more sustainably than that because the body will just push back if you push too hard.
So if your goal is to lose weight, nutrition will offer the bigger impact than exercise. But for maintenance of healthy weight, that's where exercise is essential?
That's right. Let's rethink what exercise is doing. I call it the rhythm section of your body. Exercise keeps everything on the same page, on the same beat, and it helps regulate how your body works. And so once you get to a healthier weight, once you are able to lose weight and get to a set point where you want to be, exercise is really key in keeping yourself there. Exercise changes the way that 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 hunter-gatherers, we ate a certain way, and we didn't have problems with obesity or Type 2 diabetes or high blood pressure. But based on your study of the Hadza, what is it that the paleo folks get wrong?
If you go out and have a chance to live with a group like the Hadza, you realize that a lot of the stories we tell ourselves about the past, including things like the paleo diet, just kind of fall apart. So there's this idea in the paleo diet world that there's one sort of single natural human diet, and that diet was very meat heavy, hardly any carbs at all and certainly no sugars.
[In reality] the Hadza have a mix of plants and animals in their diets. It changes day to day and year to year, but about half of the calories are coming from plants. And not only that but actually something like 10[%] to 20% of their calories every day comes from wild honey, which is just sugar and water, you know, which it would not be on any paleo diet person's menu. Another big part of their diet is the starchy tubers and these root vegetables, which you often aren't allowed to eat on some version of the paleo diet.
One last thing that stunned me from your book: You write about the metabolic cost of pregnancy — comparing pregnant women to Tour de France riders.
You can push the body as in the Tour de France, where riders burn 7,000 or 8,000 calories a day for three weeks. But it also makes sense that pregnancy is pushing the same metabolic limits as something like the Tour de France. They both run your body's metabolic machinery at full blast for as long as it can keep it up. It just speaks to how taxing pregnancy is, for one thing, but it also speaks to how these things are all connected. Our energetic machinery gets co-opted into these different tasks and makes connections that unite all of these different experiences.
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