Wondering why the pounds you packed on during the pandemic won't go away no matter how hard you workout? Evolutionary anthropologist and GSAS alumnus Herman Pontzer says that the problem is in our brains, not our biceps—specifically, the way we understand the relationship between weight and exercise. The author of the bestselling 2021 book, Burn, Pontzer says that human metabolism evolved over millions of years to defend against weight loss. As a result, the calories we burn every day stay within a fairly narrow range whether we’re gym rats or couch potatoes.
Herman Pontzer is associate professor of evolutionary anthropology and global health at Duke University and has taught at Hunter College and Washington University. One of the world's foremost authorities on metabolism and its development throughout human history, Pontzer received a grant in 2018 from the National Science Foundation for the development and implementation of a global human metabolic physiology database. He received his PhD in biological anthropology from GSAS in 2006.
Note: This transcript has been edited for clarity.
Colloquy: I think, in an interview or maybe an article I was reading about your work, you referred to metabolism as the economics of life. Can you break that metaphor down?
Herman Pontzer: Evolutionary biologists like me, we like to think about energy in terms of a budget. Right? Not so different than the budgets we're all used to using with money. And so energy comes in and gets used to power all of our various systems, all of our 37 trillion cells hard at work all day. And so, the energy that we bring in has to match the energy that we spend.
And so, yeah, we think about it kind of economically. And in the same way that, if I want to know what the priorities are for a business or the priorities are for a country or a state or any sort of big economy, I can understand the priorities for my body by seeing how that energy gets spent. Is it being spent on immune system, on reproduction, on growth? And so, yeah, an evolutionary biologist often takes that kind of economics perspective on energy, because it tells us a lot about what the evolved priorities are for any organism.
Colloquy: There's a lot of research that shows that most folks who lose weight, even if they have a surgical intervention like gastric bypass surgery, end up putting it back on. Why is that?
Pontzer: The simple answer, if it's a simple answer, is that, for a half a billion years of evolution in animals, losing weight's been a really bad sign. Right? If you're losing weight, your economics are not sustainable. Right? You're not bringing in enough to cover your expenses, and that pathway leads to death. And so, your body is built to be really good about defending its body weight. It doesn't want to change weight very easily.
And if it's going to make a mistake one direction or the other, it surely doesn't want to lose weight. And so, your body works really hard to never lose weight. And if you do lose it, to sort of get back to where you were before. So, the word "set point" gets thrown around, I think, too much. I think maybe that's too much stock that gets put into that as some sort of a hard-wired set point, but your body does seem to want to keep itself at its current weight.
For a half a billion years of evolution in animals, losing weight's been a really bad sign…And so, your body is built to be really good about defending its body weight. It doesn't want to change weight very easily.
Colloquy: One traditional assumption about weight loss is that we can speed up our metabolism through exercise, particularly vigorous activity, like high-intensity interval training. But what did your study of the Hadza, a hunter-gatherer tribe in Tanzania, reveal about exercise and metabolism?
Pontzer: For folks who don't know, the Hadza are a traditional hunter-gatherer community in northern Tanzania. So, they don't have any machines or vehicles or electricity or anything like that. Instead, they hunt and gather wild foods each day for the food they need to live. And as you can imagine, that's a really physically active lifestyle. So, they are getting about-- it depends on exactly how you want to measure it, but about 5 to 10 times more physical activity every day than the typical American.
And so, as an evolutionary anthropologist, humans, we all evolved as hunter-gatherers. So, populations that are still hunting and gathering are this kind of really valuable window into what that lifestyle does to our bodies and how any of our bodies would work in that kind of an environment. And so, we wanted to go understand how metabolism works with the Hadza. It seemed really important. And we were sure, going into it, that they would have these really elevated metabolic rates, that they would burn lots of calories every day compared to the typical American because they're so physically active, the Hadza.
And so, we went there in 2009 and 2010, my collaborators Dave Raichlen and Brian Wood and I. And we measured total daily energy expenditures, all the calories burned per day, over about a 10-day period with men and women in the Hadza community. And we did it using this technique called "doubly labeled water," which allows us to really precisely measure calories burned. So, people had estimated the calories burned by hunter-gatherers before, but nobody ever measured it before. And so, these are the first real measurements.
And so, we went there and spent a summer collecting all these data and got back and had the data analyzed by one of the leading doubly labeled water labs here in the country. And we were totally shocked. Because even though they are much more physically active than us, the Hadza, they don't burn any more calories than we do.
And in fact, when you account for things like body size, fat percentage, age, no matter how you want to sort of slice it, there's actually no difference at all in daily energy expenditure between Hadza men and women and adults in the US and in Europe and other industrialized populations. So, a total shock. They are 5 to 10 times more physically active, but there is no discernible difference in the total calories burned per day.
Colloquy: So, how is it possible that a group of humans who are so active and fit could burn about the same amount of calories as those who are sedentary? Doesn't that defy the laws of physics?
Pontzer: Yeah. Absolutely. So, we were totally shocked as well. And one thing we did, first of all, is make sure we weren't wrong. So, we actually went and measured it again using a heart rate technique. We calibrated heart rate, so you get the same answers no matter how you measure it. We've been able to show this in other populations, too. So, it's not just the Hadza. Yeah. So, what's going on, and does it sort of defy the laws of physics? Well, let's get back to that economics analogy with energy. Right?
We tend to focus a lot on the energy that we spend on physical activity, especially on exercise. Right? Because we're really aware of that. Our heart rate goes up, and the calories we're burning per minute or per hour are much higher when we're exercising, of course, because we can feel that. And so, we focus on activity when we think about the expenditure. But actually, even if for someone in the Hadza community, most of the calories burned every day are burned on other stuff. They're burned on immune system and on just the basic processes of homeostasis, keeping your cells alive, reproductive system, nervous system.
All of these systems that we aren't even really aware of are actually where you spend the bulk of your calories. And so, what we think is happening is the Hadza and other really physically active populations, they're spending a bit less on those other processes to sort of make room for the physical activity. So, there's no magic here. The laws of physics remain intact, undefeated. [CHUCKLING] It's just that the way that they're spending their energy is different on other tasks.
[The Hadza, a community of hunter-gatherers in Tanzania] are 5 to 10 times more physically active [than the average westerner], but there is no discernible difference in the total calories burned per day.
Colloquy: So, is that good news or bad news for their long-term health?
Pontzer: Well, I think good news. One thing we've been looking into more recently is sort of trying to bring these results home and see what they mean here in the US. When people are more physically active here in the US, we see the same kind of compensation happening. Maybe not as severe as with the Hadza, but we see people, they start a new exercise program, they're not burning as many calories as you'd expect based on the number of calories that were assigned to them in the exercise program. So, we see the same kind of compensation happening here, too.
And what we know is that exercise is really good for us. And one of the reasons it's so good for us is that it does things like lowers our inflammation levels. It lowers our stress reactivity. It gets reproductive hormones in a sort of more healthy range, not the sort of sky-high testosterone and estrogen levels that you can see in sedentary people. And so, those compensations that we see here in the US and we see parts of that as well when we look at other communities, it hasn't been studied as well elsewhere, we think are part of the health benefits of all that activity.
So, when we look at the Hadza, they don't get heart disease. They don't get Type 2 diabetes. They don't have the kind of levels of reproductive cancers, for example, that we see in the US. These other traditional societies, them and other traditional groups, are really healthy that way. And I think that that energy compensation, that sort of re-juggling of the energy budget, is one of the big reasons that they're so healthy compared to us for those non-communicable diseases.
Colloquy: Putting aside the myriad of health benefits of exercise and speaking strictly about weight loss, I mean, is the message here like it doesn't really matter how much activity you have or it doesn't really matter what you eat?
Pontzer: Yeah. If we're strictly looking at weight loss, then this is just more evidence—and it's not the only line of evidence—but it's just more evidence that exercise alone is a pretty poor tool for weight loss. If you absolutely push it as hard as you can for as long as you can, you might see some weight loss, at least initially until your body kind of rights the ship. But the long-term expected weight loss from exercise alone is something like 2 kilograms. Right? Less than five pounds.
And we're talking like a year after you start your exercise program. You can be diligent about it every day or every week. And you can expect to have lost, at the end of the year, about five pounds. So, it's not a great tool for weight loss. It is really good for all the other things, as you say. Now, weight gain or weight loss, again, it's about that energy budget. Calories in versus calories out.
And what our data are showing is that the calories out is really hard to budge. Your body doesn't want to change that very much. And so, what that says is that then weight gain, obesity, overweight, these issues that we struggle with in the US, it is about the energy coming in. Right? It's about the energy in part of the equation, so it is about diet.
Colloquy: But are all calories created equal?
Pontzer: Yeah, they are. So, whether you get your calories from carbohydrates or from fats or even from proteins, although you get a few of them from proteins in a typical diet, they all will have the same effect on your weight in terms of the calories that you store in your body is weight. Now, that doesn't mean that all foods are created equal. Right? I mean, a cupcake and a bowl of broccoli might have the same number of calories, but they'll have different effects on your health. So, that's not the argument of sort of calories are being equal. But in terms of just pure weight gain and weight maintenance, yes, calories are calories.
Colloquy: So, no “carbohydrates are the bad guy” here?
Pontzer: No. I think that's really been oversold. So, simple sugars, added sugars in our processed foods that we eat are one way that we end up eating more calories than we planned to or we can trick our brains a bit into over consuming because your brain is pretty good at matching energy intake and expenditure. So, all those extra empty sugar calories are one way that you can push it over the edge. So, in that sense, yeah, carbs aren't great because sugars are carbs.
But added oils are no good either. Right? And they're another way that we can trick our brains and overconsume. And oils are fats. When we look across different populations, like the Hadza, for example, the Hadza eat more carbs than we do. Right? And they don't have any issues with their weight. If carbohydrates were really driving the bus when it came to weight gain, the Hadza should all be overweight. But of course, they're not.
Colloquy: I've asked you a lot about our ability or lack thereof to speed up our metabolism. Can we slow it down if we do the wrong things?
Pontzer: The short answer is yes. The easiest way to slow down your metabolism is to starve yourself. Right? And a lot of what people recommend in terms of diets is essentially that. Right? I mean, you cut half of your calories out of your normal daily intake, your body will sense that as starvation. And so, again, you are an evolved organism. We all are. And humans and other animals will slow their metabolisms down to sort of try to get through this starvation period. Right? It's an evolved response.
And so, that's one of the problems with sort of crash diets and other really severe changes to intake. Your body will actually fight that, because it senses it as, oh, my gosh. There's no food in the environment. We need to take care of that. We need to get through this period and get to the other side.
Colloquy: What does the science say about aging and our metabolic rate?
Pontzer: Yeah. That's been a real sort of surprise as well that's come out of some recent work from a big global collaboration that I'm part of. The labs who study energy expenditure sort of pooled all our data together, and we got a data set of 6,400 and some people from age 8 days up into their 90s and looked at energy expenditure over the lifespan.
And look, I'm in my 40s, so my body doesn't feel the same as it did when I was in my 20s either. And yet, actually, when you look at the data, it's not there. So, when you look at energy expenditure per day, calories burned per day, from your 20s into your 60s, it's really steady. The big thing is driving how many calories you burn is your body size, basically, and how much of that is lean mass. Right? Muscle and organs, which burn more calories, versus fat which burns fewer calories.
So in any of these analyses, we have to control for body size. We have to control for fat percentage. But once you do that, there's no difference in metabolism. Your cells are just as busy in your 30s and 40s and 50s as they were in your 20s. So, that was a real surprise. We don't see a change with menopause, for example. If we did, we'd see that showing up in sort of late 40s, early 50s with women. We don't see that. We don't see any obvious changes at all until you get sort of into your 60s.
Children have really high metabolisms, which is another interesting piece of the data. But, yeah, there's no middle age slowdown. So, all those changes that we feel—right? Because like I said, I don't feel the same as I did in my 20s. But those can't be metabolism. We'd see that. But they must be other changes.
Exercise is really good for us. And one of the reasons it's so good for us is that it does things like lower our inflammation levels. It lowers our stress reactivity. It gets reproductive hormones in a sort of more healthy range, not the sort of sky-high testosterone and estrogen levels that you can see in sedentary people.
Colloquy: And then when you hit 60, I mean, does it drop off a table or is it a slow decline?
Pontzer: Right. It's a slow decline, and there's a lot of variation. So, our data is cross-sectional, right? We didn't follow somebody for 90 years. We took snapshots of different people over the lifespan. And so, there's variation there. Some people remain at a high metabolic rate. Some are a bit lower. But the average is you decline about almost a percentage a year. It's 0.7% a year, so you can say 7% every decade your metabolism goes down.
And what do we mean by that? Well, what we're saying is that your cells are basically getting less busy. Right? They're becoming less active as you age. And now, exactly why that is is a real puzzle, I think, and something we're excited about trying to figure out next.
Colloquy: To wrap up, I wonder if you could talk about your current research. You're looking at the variability in the metabolic rates of the very young and those over 60. What's the question that you're trying to answer, and how will you go about it?
Pontzer: Yeah. Right. So, there's these big changes that happen on either end of the life course. Right? Like you say. So, children, you're actually born with a metabolic rate that's not so different than mom's. Right? So, you're born like a small adult, which is sort of strange. As a sidebar, small animals tend to have inherently fast metabolisms, so strange that you would be born with sort of an old person's metabolism. But you are. Anyway, that quickly ramps up.
And by one year of age, your metabolic rate is 50% higher than it should be for your size. OK. So, your cells are 50% more active than they would be if they were the cells of an adult. And generally speaking, we know why that is. It's all the developmental costs. It's the growth costs. Right? So, children are busy doing all sorts of things. Their bodies are growing and developing. So, in some general sense, we think we know why that sort of skyrocketing metabolism is happening.
What we don't know is, well, OK. There's variation among children. Some peak higher than others. Some rocket up faster than others. Could we look at that metabolism variation as a way of saying, “Oh, this child is developing a little bit faster. This is a child developing a little bit slower. This child is encountering some kind of struggle, some kind of challenge that they have to ramp up their metabolic rate even more.”? Right?
This could go a lot of different directions and could kind of help us understand why it is that each one follows its own unique schedule. They'll hit the milestones around the same time. But I've got two children, and they all hit-- the teeth come in at different times. They learn to walk at different times. Parents will know this. Can we see that happening? Can we get a window into that variation of development by looking at metabolism? So, we're going to try to focus on that, starting with that one-year-old peak because that seems to be the most obvious place to start.
On the other side of the life course, you've got people who get into their 60s and 70s and their metabolism is coming down. Well, is that because of disease processes that are kicking in and the cells are just sort of accumulating damage and they can't kind of maintain themselves at that same level they were when they were younger? So, is it sort of reflecting the effects of aging, or perhaps is it kind of predictive? You have a slow metabolism in your 60s and 70s and that predicts-- well, I can tell you that that's going to predict perhaps worse outcomes, worse aging outcomes, faster decline for you.
So, it could be causal. It could be just another biomarker of aging. Either way, it could be really useful, because we could get a metabolic rate for you, a person in their 60s or 70s, and maybe be able to say something helpful and useful about, look, here's where you're at and maybe you don't feel the effects of aging yet. But we're going to try to do x, y, or z. That's down the road but to try to help you age better. And so, right now, we're trying to look at that variation in metabolic rate as whether it tells us anything about how people age well or age poorly.