Just Move: Scientist Author Debunks Myths About Exercise And Sleep

Jan 21, 2021
Originally published on January 22, 2021 12:02 pm

For much of history, human beings needed to be physically active every day in order to hunt or gather food — or to avoid becoming food themselves. It was an active lifestyle, but one thing it didn't include was any kind of formal exercise.

Daniel Lieberman is a professor in the department of human evolutionary biology at Harvard. He says that the notion of "getting exercise" — movement just for movement's sake — is a relatively new phenomenon in human history.

"Until recently, when energy was limited and people were physically active, doing physical activity that wasn't necessarily rewarding, just didn't happen," Lieberman says. "When I go to these [remote African tribal] villages, I'm the only person who gets up in the morning and goes for a run. And often they laugh at me. They think I'm just absolutely bizarre. ... Why would anybody do something like that?"

Lieberman has spent a lot of time with indigenous hunter-gatherers in Africa and Latin America, cataloging how much time they spend walking, running, lifting, carrying and sitting. He writes about his findings, as well as the importance of exercise and the myths surrounding it in his new book, Exercised.

"If you actually look at what our ancestors do, they walk about 5 miles a day, which turns out to be, for most people, about 10,000 steps," Lieberman says.

Lieberman notes that many people are moving less than they did before the pandemic. He says if 10,000 steps feels out of reach, it's OK to shoot for less — just so long as you're focused on movement. Even fidgeting can keep your muscles engaged.

"The more we study physical activity, the more we realize that it doesn't really matter what you do," Lieberman says. "You don't have to do incredible strength training ... to get some benefits of physical activity. There's all different kinds of physical activity, and it's all good in different ways."


Pantheon

Interview highlights

On the demonizing of sitting as "the new smoking"

When I walk into a village in a remote part of the world where people don't have chairs or a hunter-gatherer camp, people are always sitting. ... Some friends and colleagues of mine actually put some accelerometers on some hunter-gatherers and found that they sit on average about 10 hours a day, which is pretty much the same amount of time Americans like me spend sitting.

So it turns out that I think we've kind of demonized sitting a little falsely. It's not unnatural or strange or weird to sit a lot, but it is problematic if, of course, that's all you do. As I started to explore the literature more, I was fascinated because most of the data that associates sitting a lot with poor health outcomes turns out to be leisure-time sitting. So if you look at how much time people spend sitting at work, it's not really that associated with heart disease or cancers or diabetes. But if you look at how much people sit when they're not at work, well, then the numbers get a little bit scary.

On the importance of "interrupted sitting"

Just getting up every once in a while, every 10 minutes or so just to go to the bathroom or pet your dog or make yourself a cup of tea, even though you're not spending a lot of energy, you're turning on your muscles. - Daniel Lieberman

Just getting up every once in a while, every 10 minutes or so — just to go to the bathroom or pet your dog or make yourself a cup of tea — even though you're not spending a lot of energy, you're turning on your muscles. And your muscles, of course, are the largest organ in your body — and just turning them on turns down inflammation. It uses up fats in your bloodstream and sugars in your bloodstream, and it produces molecules that turn down inflammation. So the evidence is that interrupted sitting is really the best way to sit. In hunter-gatherer camps, people are getting up every few minutes, to take care of the fire or take care of a kid or something like that. And that kind of interrupted sitting, as well as not sitting in a chair that's kind of nestling your body and preventing you from using any muscles, all that kind of keeps your muscles going and turns out to be a much healthier way to sit.

On how chairs with backs have contributed to our back pain

We all think that it's normal for a chair to have a seat back. But until recently, only really rich people — the pope or the king — had a chair with a seat back. Until recently, all human beings pretty much either sat on the ground or, if they did have chairs, they were stools or benches or things like that. ...

The reason it matters for our health is that a seat back essentially makes sitting even more passive than just sitting on a bench or a stool because you lean against the seat back and you're using even fewer muscles, even less effort to stabilize your upper body. And the result is that we end up having very weak backs. So there are a lot of muscles that we use in our backs to hold up our upper body, and those muscles, if we don't use them, just like every other muscle in your body, they atrophy. And weak muscles then make us more prone to back pain. In fact, studies show that the best predictor of whether or not somebody gets lower-back pain — and most of us do get lower-back pain — is whether or not we have weak and, importantly, fatigable backs. I think sitting a lot on chairs with backrests contributes to that.

On the idea that running is bad for your knees

There's this kind of general idea out there that running is like driving your car too much — [that] it's wear and tear, and that running is highly stressful and it just wears away your cartilage, just like driving your car for a long period of time wears out your springs, for example. And that turns out not to be true. Study after study has shown that in terms of "wear" — by which we really mean arthritis, degeneration of the cartilage in your joints — that people who run more are not more likely to get arthritis in their knees. In fact, they're actually slightly less likely to get arthritis, because using your cartilage, using your joints, using your muscles, strength, all the good benefits from physical activity actually turn out to be slightly protective.

That said, it's also true that the most common site of injury for runners is their knees. But a lot of those injuries, I think, are preventable by learning to run properly. We don't treat running as a skill in our culture. We just give people shoes and tell them to head out the door, and some people run really well and some people don't run that well — or their bodies aren't really well-adapted to running, and then they get into trouble. But in terms of wear and tear, I think we can dispel that myth completely.

On becoming frail with age

I think one of the most important points about physical activity is that as we age, it becomes not less but more important to be physically active. Muscle atrophy is the perfect example.

I think one of the most important points about physical activity is that as we age, it becomes not less but more important to be physically active. - Daniel Lieberman

We have this notion that as you get older, you retire, you go to Florida, you kick your feet up on the beach or whatever. ... We have plenty of evidence that older individuals in America are less physically active and they do fewer activities that involve strength. And one of the really sort of serious negative consequences of that is that our muscles dwindle, they atrophy. There's a technical term for that, which I think is illuminating — it's called "sarcopenia." Sarco means "flesh" in Greek and penia means "loss" — so "flesh loss." But basically it's frailty. And as we get older and become more frail, a vicious circle sets in because we walk more slowly, it's harder to get out of a chair and that makes us even less likely to be physically active, which keeps that cycle going. So that's the bad news.

But the good news is that it doesn't take a huge amount of physical activity to kind of reverse that, turn it around. Think about Ruth Bader Ginsburg. She was celebrated for her vim and vigor, which meant that a lot of that came from the fact that she kept working out and as she got older, she went to the gym several times a week. Now, she didn't do crazy, "pump iron" stuff. She wasn't trying to be like Arnold Schwarzenegger. But she did a few rounds of weight training every week and that helped keep her marvelously active and vigorous up until her late 80s. And the mechanisms that get turned on when we do a little bit of strength training don't diminish with age. So if you're in your 80s or 90s and you do a little bit of strength training, you'll still get enormous benefits.

On the stress around getting eight hours of sleep a night

It turns out that people who live in places where there is no electricity and there no iPhones and there's no TV, turns out they don't sleep any more than the average American. - Daniel Lieberman

I used to say this to my students, that Thomas Edison robbed us of sleep. We invented electricity, and now we have iPhones and televisions and all these things that keep us up at night and that we didn't used to do. But it turns out that people who live in places where there is no electricity and there no iPhones and there's no TV — turns out they don't sleep any more than the average American. I think the number is 6.7 to 7.1 hours on average at night. And they often don't nap either, by the way, which is something we're also told. If you look at the data, there's no evidence that people [on average] sleep less today than they used to.

And furthermore, to my astonishment, when you look at big epidemiological data sets where you graph how much you sleep on the horizontal axis and your health outcomes on the Y axis, it's a U-shaped curve. And the bottom of that curve is about seven hours. Of course, there's a lot of variation. Some people need more, some people need less.

Daniel Lieberman is a professor in the department of human evolutionary biology at Harvard.
Courtesy of Dan Lieberman

So I think sleep is another one of those examples of how we make people exercise. We make them stressed about what they should be doing, and there's a lot of "virtue signaling" going on. If you tell somebody they're not getting enough sleep and they actually are getting enough sleep, you just make them stressed ... that elevates cortisol. Cortisol is the hormone that's about arousal. Cortisol prevents you from sleeping. And so we get into this kind of vicious circle.

So, while it's true that people who don't get enough sleep, that can be a problem, getting three, four, maybe five hours of sleep a night can be detrimental and it's an issue ... if you're getting six, seven hours of sleep and you feel fine, I think we should all relax and stop being so uncompassionate to each other.

Sam Briger and Seth Kelley produced and edited this interview for broadcast. Bridget Bentz, Molly Seavy-Nesper and Deborah Franklin adapted it for the Web.

Copyright 2021 Fresh Air. To see more, visit Fresh Air.

TERRY GROSS, HOST:

This is FRESH AIR. I'm Terry Gross. The restrictions caused by the pandemic have changed the amount of time many of us spend moving, walking, standing. If you're used to going to work but have spent the better part of the year at home and you can't go to the gym because it's closed or you just don't feel safe doing it, or you don't even have time to take a walk because you're working and have children at home all day because their schools are closed, well, how's that affecting your body and your general health and your mental outlook?

My guest, Daniel Lieberman, writes about the importance of exercise and the myths surrounding it in his new book "Exercised: Why Something We Never Evolved To Do Is Healthy And Rewarding." Some of the questions he addresses include, is sitting the new smoking? Is it bad to slouch? Do you really need eight hours of sleep? Is walking effective for losing weight? Does running ruin your knees? He writes from his perspective as a paleoanthropologist.

He's a professor in the Department of Human Evolutionary Biology at Harvard. In his lab at Harvard, he and his students conduct experiments to study the anatomy, biomechanics and physiology that underlie various exercises and activities. His research has led him to remote villages in Africa and Latin America that have not been industrialized. He studied how much time indigenous people there sit, sleep, walk, forage, dance and run. He's compared their activities, their bodies, their health and longevity to that of Americans.

Daniel Lieberman, welcome back to FRESH AIR. Thank you for joining us. So during COVID, I feel like everybody's lives has changed in one way or another. So I'm going to mention a couple of things about my life. I typically, for many years, have worked at the radio station. During COVID, I've been working from home. Simple things that are causing me to take fewer steps in my working life - the bathroom was further away in the office than it is in my small home (laughter). I'd walk every day a few blocks to get lunch and then walk a few blocks back to get it home. I'm not doing that now. I'm just making something really quickly for myself. However, I'm still taking walks. And I'm spending more time making meals because I can't eat out. And I'm spending more time, therefore, washing dishes since I eat three meals at home now instead of none or two since I'm not in the office or eating out. So my first question is, can I count standing and cooking and washing dishes and cleaning as exercise?

DANIEL LIEBERMAN: Well, that's a great question. First of all, I should mention that my step count has also gone down by about 50% for all the same reasons. Well, I think it's important to make a distinction between physical activity, which is just moving - right? - including making, you know, breakfast and lunch and going to the bathroom and all those sorts of things, and exercise, which is physical activity that's sort of discretionary and voluntary for the sake of health and fitness. And so what the pandemic has done for a lot of - almost all of us is to decrease our physical activity, right? I'm not going to work either. I'm working from home. I'm, you know, not making as many trips, you know, to do errands. All those kinds of things are really kind of cutting down on how much I'm sort of walking about. But I'm trying to compensate with exercise. But that's also been challenging. And so the end result is that my overall physical activity levels have definitely decreased regardless of my attempts to compensate with exercise. So not all physical activity is exercise. Exercise is a special kind of physical activity.

GROSS: OK. So you're saying that I can't count washing dishes and washing the floors and standing up, preparing food instead of sitting, eating at a restaurant, that I can't count that as exercise. But I can count that as some form of physical activity, right?

LIEBERMAN: Yeah. Yeah.

GROSS: For those of us in that position, where we're doing more housework and we're doing more cooking and washing dishes, that's still physically better than just sitting and - right?

LIEBERMAN: Absolutely. Yeah. No. It's - what matters is physical activity. It's just that we've created a world in which we don't have to do much work anymore, right? We don't have to be very physically active. And so today, people do something very strange, which is they - you know, they go on a treadmill. Or they, you know, go for a five-mile run in the morning for the sake of just the five-mile run. Or they lift weights whose sole purpose is to be lifted. That's exercise. And that's a very modern behavior. People never did that before because until recently, people had to be much more physically active every day in order to get food or, sometimes, to avoid being somebody else's food.

GROSS: There's a great story you tell in your book. You just mentioned treadmills. And this story has to do with the treadmill. Like I said, you go - done a lot of studying in remote villages where people are still basically foraging and hunting for food. And you're comparing their physical activity with ours and their bodies with ours. So in a remote village in Kenya, you manage to transport there a treadmill (laughter). What was your point in bringing a treadmill to this village?

LIEBERMAN: (Laughter) Well, so - yeah. We've been working in a community up in the - up at the top of the Western Rift Valley in Kenya for almost 12 years now. And the community has no paved roads. There's no electricity. There's no running water. And people do everything by hand, everything. And one of the things that people do is they carry everything, they carry - especially women. Women carry a lot of firewood. Women carry huge sort of jerrycans of water, often long distances, up hills. And we were interested in how people carried because there's - we have some evidence that people can carry - women can carry, you know, huge weights, like 20% of their body mass on their heads with no - essentially no extra cost. And we were curious how they did that. So we have some fancy machines that measure oxygen so we could measure how much energy they're spending. And we thought, the best way to do this would be on a treadmill. So we bought a treadmill and schlepped it up these horrible roads to get it to this area. And we found the closest place where we could plug it in. And then we drove our - we got our, you know, the Jeep - we got our participants to the treadmill.

And it was a total disaster because if you've never been on a - if you're a 40, 50-year-old person and you've never been on a treadmill your entire life, (laughter) it's a very strange and awkward experience. And the women, they got a kick out of it. They sort of enjoyed the treadmill. But they weren't walking normally, the way they normally walked, because it's such a strange, bizarre, flummoxing contraption. And so we actually had to - after all that time and energy and effort to get the treadmill up there, we had to abandon using the treadmill. Instead, I did the experiment outside over ground.

GROSS: Was your conclusion that we walk differently on a treadmill than we do if we're walking on a floor or the street?

LIEBERMAN: Well, yes. I mean, everybody does walk slightly differently on a treadmill. And also, we run slightly differently. But more importantly, it's just that we wanted to make sure people were walking the same way they normally walk when they're carrying things. And, you know, you change how you walk. And if you're not used to a treadmill, you know, it's - you walk very awkwardly. It's kind of, you know, it's a bizarre experience to kind of walk and not get anywhere.

GROSS: I've always wondered, like, is there more tension in your body when you're walking on a treadmill because it's slightly unnatural and also because your brain always knows that you have to walk in place at the speed of the treadmill and it's a kind of narrow area?

LIEBERMAN: Absolutely. I mean, I find - I - you know, I put people on treadmills for a living. And I sometimes use a treadmill to exercise. But I find them unpleasant. And I - you know, for me to go on a treadmill, I need to listen to a podcast or something to keep it from being too monotonous. And, you know, there's - you know, you're often in - you're indoors, so you're not getting any fresh air. And it's just not a great experience. It's kind of like, you know, taking cod liver oil. I mean, it's just - for most people, treadmills aren't that much fun. And I found it kind of interesting and sort of deliciously ironic that the sort of modern treadmill was invented in the Victorian period to torture prisoners. So Oscar Wilde, for example, was condemned to tread on a - you know, trudge on a treadmill for, like, five, six hours a day to prevent him from enjoying himself.

GROSS: He probably had great lower body strength, though, after that (laughter).

LIEBERMAN: Yeah.

GROSS: Good cardio (laughter). One of the themes of your book is that we never evolved to exercise. But nowadays, we need to exercise. And you write - you've said, insufficient regular exercise is abnormal and pathological. So let's start with, how can you say that we didn't evolve to exercise? What do you mean by that?

LIEBERMAN: Well, exercise is discretionary, planned, voluntary physical activity for the sake of health and fitness, right? It's like what I did this morning. I went for a five-mile run - right? - solely for the sake of the fact that it's good for my health and both mental as well as physical. But in the past, nobody did that. In the past, people every day were physically active. They would go out and walk or sometimes run and dig things and carry things and do all kinds of other activities that were necessary to get food.

But they struggled to get enough energy. And they were already pretty physically active. And so spending extra energy to do something that had no benefit is, actually, a problem, right? It's a cost. It takes energy away from other functions that are important, like taking care of your body or reproducing. So until recently, when energy was limited and people were physically active, you know, doing physical activity that wasn't necessary or rewarding just didn't happen. When I go to these villages, I'm the only person who gets up in the morning and goes for a run. And often, they laugh at me. They think I'm just absolutely bizarre. They think it's pretty funny. You know, why would anybody do something like that?

GROSS: So another reason why you say we haven't evolved to exercise is that we evolved to conserve calories. We evolved to do the hunting and gathering and taking care of children and all that, and then to conserve calories after that because there might not be enough food. And so expand on that, if you would.

LIEBERMAN: Yeah. So humans are incredibly energetically expensive creature. We are very energetically intense. We spend a lot of energy on our bodies. And - but we also struggle to get all that energy because we - you know, we have an energetically intensive reproductive strategy. We have an energetically intensive physical activity strategy. You know, we spend a lot of energy. Here's an interesting fact. The average sedentary American is still more active than the typical wild chimpanzee - you know, chimpanzees will walk a few miles a day, two to three miles a day, but even a sedentary American does about that or a little bit more - because chimpanzees spend most of their time just eating. They just put food in their mouth and mostly rest. We also - but we not only spend a lot of energy just going around doing things, but also, we have a very energetically intensive reproductive strategy. Humans, we have babies at twice the rate as our cousins, the chimpanzees. We spend a lot longer growing up. So we have to invest more energy in growing our bodies. And the list goes on. So until recently, people struggled to get enough food. But they had a very energetically intensive strategy. And so when that occurs, you have tradeoffs, right? You can't - you know, every time you spend energy on one thing, you can't spend it on something else. And so energy spent on unnecessary physical activity was detrimental. So we evolved to spend energy when it was necessary or rewarding. And otherwise, it makes sense to take it easy. So, you know, when people aren't working in, you know, non-Western, non-industrial environments, they're tending - they tend to rest and take it easy and not spend unnecessary calories unless there's some benefit, like dancing, for example, which is obviously very beneficial for social reasons and mental health reasons, and for play. Play is, obviously, very important. But other than that, they take it easy.

GROSS: There's a lot of myths about exercising that you write about. And I want to get to those. But first, we have to take a short break. If you're just joining us, my guest is Daniel Lieberman. His new book is called "Exercised: Why Something We Never Evolved To Do Is Healthy And Rewarding." We'll be right back after a break. This is FRESH AIR.

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GROSS: This is FRESH AIR. Let's get back to my interview with Daniel Lieberman, author of the new book "Exercised: Why Something We Never Evolved To Do Is Healthy And Rewarding." He's a professor of evolutionary biology at Harvard.

So you write that we really didn't evolve to exercise because the way humans evolved, we were conserving our energy to forage for food and to hunt and raise children. We also didn't evolve to sit in chairs all day. So a question you ask in your book - and I want to ask you your thoughts about this - is, are our chairs killing us? (Laughter) Is it true that sitting is the new smoking, that sitting for long periods is as damaging as smoking is?

LIEBERMAN: Well, I decided to start the book with a discussion about physical inactivity because if you want to understand physical activity, we also have to understand inactivity, which is what we actually spend most of the time doing, whether you're a modern, you know, desk-chair-bound professor like me or a hunter-gatherer or a subsistence farmer. And recently, we've sort of demonized sitting, right? Sitting is the new smoking. And you can regularly pick up the newspaper or click on websites which tell you that, you know, every hour you sit, two hours of your life walk away, you know? And there's all kinds of scary statistics about sitting.

So I was curious about that. And because one of - my experiences are that when I walk into a village in a remote part of the world where people don't have chairs or, you know, like, you know, a hunter-gatherer camp, people are always sitting. And it turns out - so some friends and colleagues of mine actually put some accelerometers on some hunter-gatherers and found that they sit, on average, about 10 hours a day, which is pretty much the same amount of time Americans like me spend sitting. So it turns out that I think we've kind of demonized sitting a little falsely. It's not unnatural or strange or weird or - you know, to sit all the time or sit a lot. But it is problematic if, of course, that's all you do. And as I started to explore the literature more, I was fascinated because most of the data that associates sitting a lot with poor health outcomes turns out to be leisure time sitting. So if you look at how much time people spend sitting at work, it's not really that associated with heart disease or cancers or diabetes. But if you look at how much people sit when they're not at work, well, then the numbers get a little bit scary.

GROSS: So some people say that if you sit too long, too frequently, that it can harm your immune system, it could trigger your immune system and create inflammation. Do you think that's true? And if so, what is the connection between sitting and inflammation?

LIEBERMAN: Well, it turns out that what - that it's true that sitting too much does lead to chronic inflammation. But it turns out that there are two factors. The first is that it's really leisure time sitting which is mostly associated with that. And that's - because that means if you because if you sit at work and then you go home and you said all the time as well, that means you're probably not getting any physical activity, which means you're probably gaining weight, which - and gaining weight is associated with higher levels of inflammation.

But the other thing that's really fascinating is that just getting up every once in a while, every few - every 10 minutes or so, just to, you know, go to the bathroom or pet your dog or make yourself a cup of tea, you don't - even though you're not spending a lot of energy, you're turning on your muscles. And your muscles, of course, are the largest organ in your body. And they're - and just turning them on turns down inflammation. It uses up fats in your bloodstream and sugars in your bloodstream. And it produces molecules that turn down inflammation.

So the evidence is that interrupted sitting is really the best way to sit. So, you know, in hunter-gatherer camps, you know, people are getting up every few minutes, you know, to take care of the fire or take care of a kid or something like that. And that kind of interrupted sitting, as well as sitting when you're not sitting in a chair that's kind of nestling your body and preventing you from using any muscles, all that kind of keeps your muscles going and turns out to be a much healthier way to sit.

GROSS: So you challenge the common idea that back support is really good when we're sitting in chairs. And we've established we all sit in chairs a lot, or at least many of us, myself included, sit in chairs a lot - for myself, like, way too much of the time. So why do you think back support is not necessarily essential and may actually be counterproductive?

LIEBERMAN: Well, when you're sitting in a chair with a back - with a seat back, you know, we all think that's normal for a chair to have a seat back. But until recently, only really rich people, you know, the pope or the king had a chair with a seat back. Until recently, all human beings pretty much either sat on the ground, or if they didn't have chairs, they were stools or benches or things like that.

I mean, if you were - even if you were a courtier in the court of, you know, the king back in the 15th century, unless you were really high up, you would be sitting on a bench or a stool. And the reason it matters for our health is that a seatback essentially makes sitting even more passive than just sitting on a bench or a stool because you lean against the seat back and you're using even fewer muscles, even less effort to stabilize your upper body. And the result is that we end up having very weak backs.

So, you know, there are a lot of muscles that we use in our backs to hold up our upper body. And those muscles, if we don't use them, just like every other muscle in your body, they atrophy. And weak muscles then make us more prone to back pain. In fact, studies show that the most - the best predictor of whether or not somebody gets lower back pain - and most of us do get lower back pain - is whether or not we have weak and, importantly, fatigable backs. And I think back, you know, sitting a lot on chairs with back rests contributes to that.

GROSS: You say that there's actually no good evidence that slouching causes back pain.

LIEBERMAN: Yeah. I think this is an example of where we have confused cause and effect. And I was really fascinated by this because as I started looking into the literature about slouching - because, you know, the idea that we shouldn't slouch really began in the 19th century with German orthopedics, who were, you know - the common chair that we have, a chair with a back only, became available to people during the Industrial Revolution.

In fact, it was not until the 1850s that, you know, that cafe chair, that kind of - what we see everywhere in cafes and all over the place, that was the first mass-produced industrial chair with a back. And there wasn't - before that that many people, you know, who weren't wealthy had chairs with backs available to them. And when that happened, you know, German anatomists and others were - you know, Victorians were kind of appalled by all the sitting that people were doing. And it was just opined that you ought to sit in the same way that you stand, your posture should be the same. And there was no evidence, actually, to support that.

And today, even today, people, you know, if you're slouching, somebody will tell you to stop slouching. It's not good for you. And what the evidence tells us, actually, is that if you have a weak back, you're more prone to slouching, but it's not the slouching itself that causes the back pain. It's actually the weak back. So the tendency to slouch is really more of a marker of your - the fact that you don't have a particularly strong back rather than the cause of the back pain itself. So we've confused cause with effect.

GROSS: If you're just joining us, my guest is Daniel Lieberman, author of the new book, "Exercised: Why Something We Never Evolved To Do Is Healthy And Rewarding." He's a professor of evolutionary biology at Harvard. We'll be back after we take a short break. I'm Terry Gross, and this is FRESH AIR.

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GROSS: This is FRESH AIR. I'm Terry Gross. Let's get back to my interview with Daniel Lieberman, author of the new book "Exercised: Why Something We Never Evolved To Do Is Healthy And Rewarding." He's a paleoanthropologist and a professor of evolutionary biology at Harvard. He studied why humans did not evolve to want to exercise, but why exercise is nevertheless important for good health and well-being. In his lab at Harvard, he conducts experiments to study the anatomy, biomechanics and physiology of movement and exercise. His research has also taken him to the Arctic, Africa and Latin America to study the activities of indigenous people in remote villages with no electricity or running water, to study their physical activities and see how their bodies compare to ours. His research has led him to challenge a lot of myths about exercising.

Let's move on to running. And you ask the question, is it true that running is bad for your knees? What does the research have to say?

LIEBERMAN: Yeah. There's, you know, this kind of general idea out there that running is like, you know, driving your car too much. It's, like, wear and tear, right? And that running is highly stressful, and it just wears away your cartilage, just like driving your car for a long period of time, you know, wears out your springs, for example. And that turns out not to be true (laughter).

Study after study has shown that in terms of wear, by which we really mean arthritis - right? - degeneration of the cartilage in your joints - that people who run more are not more likely to get arthritis in their knees. In fact, they're actually slightly less likely to get arthritis because, you know, using your cartilage, using your joints, using your muscles, strength - all the good benefits from physical activity actually turn out to be slightly protective.

That said, it's also true that the most common site of injury for runners is their knees. But a lot of those injuries, I think, are preventable by learning to run properly. And I think, you know, we don't treat running as a skill in our culture. We just, you know, give people shoes and tell them to head out the door. And some people run really well and some people don't run that well, or they're not really well - you know, their bodies aren't really well-adapted to running, and then they get into trouble. But in terms of wear and tear, I think we can dispel that myth completely.

GROSS: You went to a foot race of indigenous people who are famous for their running in Mexico. Tell us about the people in the race.

LIEBERMAN: Oh, so I've had the chance to do fieldwork with the Tarahumara, who live in - they call themselves the Raramuri - who live in northern Mexico. And they're a Native American group that's, among other things, very famous for their running. And they have these beautiful races. There's a women's race called the ariweta, where women - it's often about a 25-mile race where women flick this kind of cloth loop with a stick, and then they chase it and they flick it again and they chase it. And the men have a race that's sort of similar, but with a ball - like, a wooden ball - which they kick and then they chase and then they kick and then they chase. And there are two teams.

And it turns out that that race is - those races are a form of prayer. They're a way of honoring their god. And it's a sort of, for them, a spiritual event. And it's an absolutely wonderful, beautiful example of the kinds of running traditions that used to be omnipresent throughout the new world. Pretty much every Native American group we know of had running traditions that often had sort of spiritual - important spiritual and social overtones. They're also a time for people to get together. People bet on the races, so there's money exchanged or, you know, corn and clothes and other goods. It's just a wonderful, joyous, delightful experience.

GROSS: Does everybody participate in the running, or is it only, like, the really, really good runners?

LIEBERMAN: So only the really good runners do the whole thing. So there are teams of usually about five individuals on each team, both for the men's and the women's races. And they're the ones who stick it out through the whole race. But what's fun is that during the race - they're often on these kind of loops - right? - sort of like a 5-kilometer sort of there-and-back kind of loop - people will just join in, you know - anybody who wants to. And you follow behind the runners, and you shout and cheer things. And I've done this on numerous occasions and had a blast. And people will run, you know, a few miles with the runners. And so it's a very communal event.

But only - you know, we have this idea - I call it the myth of the athletic savage - that, you know, people who are uncontaminated by civilization are just these incredible endurance runners, and they can do it easily. I can tell you it's hard for these runners, just like it would be hard for us, right? They struggle. They have nausea. They have cramps. It's really challenging for them. And they need support and help. And they have their own form of Gatorade. But they do it because it's meaningful for them, and it's important to them, even though it is challenging, just like it's challenging for ultrarunners to do, you know, 100-mile races in the United States. Humans are humans, no matter where they are.

GROSS: Let's talk about muscle atrophy because that's part of the problem. Like, if you don't exercise, you literally lose muscle strength - the muscles stop working. What happens when muscles atrophy and why do they atrophy?

LIEBERMAN: Yeah. I think one of the most important points about physical activity is that as we age, it becomes not less, but more important to be physically active. And muscle atrophy is the perfect example. So, you know, we have this sort of notion that as you get older, you, you know, go to - you retire, you go to Florida, you kick your feet up on the beach or, you know, whatever.

But what happens is that when you do that - and we have plenty of evidence that older individuals in America are less physically active, and they do less - fewer activities that involve strength. And one of the really sort of serious negative consequences of that is that our muscles dwindle, they atrophy. There's a technical term for that, which I think is illuminating. It's called sarcopenia. Sarco means flesh in Greek, and penia means loss, so it's flesh loss. But basically, it's frailty. And as we get older and become more frail, a vicious circle sets in because we, you know, walk more slowly, it's harder to get out of a chair. And that makes us even less likely to be physically active, which keeps that cycle going. So that's the bad news.

But the good news is that it doesn't take a huge amount of physical activity to kind of reverse that, turn it around. I mean, think about Ruth Bader Ginsburg, who - you know, she was celebrated for her vim and vigor, which a lot of that came from the fact that she kept working out as she got older. She went to the gym several times a week. She didn't crazy pump iron stuff. She didn't - you know, wasn't trying to be like Arnold Schwarzenegger, but she did, you know, a few rounds of weight training every week. And that helped keep her, you know, marvelously active and vigorous up until her late 80s. And the mechanisms that get turned on when we do a little bit of strength training don't diminish with age. So if you're in your 80s or 90s and you do a little bit of strength training, you'll still get enormous benefits.

GROSS: Let me reintroduce you here. If you're just joining us, my guest is Daniel Lieberman. And he's the author of the new book "Exercised: Why Something We Never Evolved To Do Is Healthy And Rewarding." We'll be right back after we take a short break. This is FRESH AIR.

(SOUNDBITE OF LIZZO'S "JUICE (INSTRUMENTAL)")

GROSS: This is FRESH AIR. And if you're just joining us, my guest is Daniel Lieberman, author of the new book "Exercised: Why Something We Never Evolved To Do Is Healthy And Rewarding." And he writes about the importance of exercise and myths surrounding it. He's a professor of evolutionary biology at Harvard.

One of the issues that we face now is that we're living longer, but more people are suffering from chronic, long-term diseases, living with them or dying as a result of these chronic, long-term diseases. A good example of that is diabetes. And you write about how our sweet tooth evolved and how that's contributed to diabetes. So how did our sweet tooth evolve?

LIEBERMAN: Well, we evolved to want energy. I mean, the basic equation of life, if you want to be really reductionist about it, is, you know, energy in and babies out. That's what we evolved to do. And any way to get more energy is good. And any way to spend less energy is also good because it enables you to have more babies, which is sadly the only thing natural selection cares about. So we evolved to love sweet things. It turns out, the favorite food of hunter-gatherers in Africa - and we were - at some point, all our ancestors were hunter-gatherers in Africa. But if you ask hunter-gatherers in Africa what they like the most, they'll tell you it's honey.

And I've been - I've gone on, you know, hunting trips with hunters in Tanzania, and they often end up being honey-collecting tips. And they go from hive to hive to hive and just eat unbelievable quantities of honey that would make, you know, any dentist gasp. But - because we love sweet things. And sweet things, of course, if you're physically inactive, cause you to add weight. And when you get - when you add fat, especially in your in your abdomen - you get, you know, belly fat or what's called visceral fat, that fat is very inflammatory. It turns on your immune system at this kind of slow burn, and that leads to diseases like diabetes.

And so one of the just enormous basic fundamental benefits of physical activity is that it counters that obesity, and it counters that inflammation. The major tissues that produce the anti-inflammatory molecules that keep us from being inflamed are muscles. So when you exercise, your muscles are producing all these molecules that are called myokines that actually turn down inflammation and keep us healthy, preventing us from getting diabetes, prevent us from getting cancer, prevent us from getting Alzheimer's, prevent us from getting atherosclerosis, you know, hardening of the arteries, prevent us - you know, it's just - it's extraordinary how many benefits you get just from moderate levels of physical activity, just turning on your muscles.

GROSS: In terms of getting diabetes, is there a difference between honey and sugar and in terms of inflammation in general in your body?

LIEBERMAN: Not really. I mean, I think - we talk - we think so much about, you know, whether it's, you know, high fructose corn syrup or sucrose or whatever. I mean, sugar is sugar basically. I mean, there are different kinds. And some sugars - fructose, for example - are handled more by your liver, and some are handled more by insulin. But the bottom line is, if you eat a lot of sugar, no matter whatever kind of sugar it is, you're going to add weight.

And when you're - and when you add weight, when you add fat, you don't add cells. The cells just swell. And as those cells swell, they become like broken balloons, and they start to misfunction (ph). And that swelling causes inflammation, so your immune system comes in and starts producing all these molecules to try to kind of deal with the kind of local injury. And that's what's so pernicious. So it's really about weight gain.

So if you eat a reasonable amount of - you know, if you're hunter-gatherers eating sugar but you're also very physically active, you're not going to run into the same kind of trouble as if you're an American who's eating a lot of sugar and then, you know, sits in a chair all day long.

GROSS: So the Africans who were hunter-gatherers and were gathering a whole lot of honey and eating it, they were not getting diabetes.

LIEBERMAN: There's - if there's any cases of diabetes in this population, I've never seen it or heard it. I mean, diabetes, heart disease is just almost unknown in these populations. There's a recent study that was done by some colleagues, Hilly Kaplan and Mike Gurven, on a population in the Amazon called the Tsimane. And they actually managed to get a large sample of these folks into CT scanners and measured their hearts and found that they have - even though they eat a lot of carbohydrates in their diet, which is basically sugar - they have - there's no evidence for even the - there's just no traces of even heart disease developing in older individuals in the society. So these really are modern Western diseases that are caused by a combination of diet and physical inactivity.

GROSS: Sleep is a problem for a lot of people, too. It's hard for a lot of people to fall asleep. It's hard for a lot of people to stay asleep, and it's hard for a lot of people to find enough hours to sleep, especially if the model that you're using is, oh, you're supposed to get eight hours of sleep every night. And it's hard for a lot of people to find eight hours when they can be in bed uninterrupted. What do you think the research has to say about eight hours of sleep being essential?

LIEBERMAN: Well, I was astonished by this. So I actually started to study sleep as part of writing the book because I was curious about the - you know, we're exhorted all the time to sit less but then to sleep more. And I was like, curious about that. So I started to look into the literature. And it - you know, a common story out there, like, I learned, and I used to say this to my students is that, you know, Thomas Edison robbed us of sleep. You know, we invented, you know, electricity, and now we have iPhones and televisions and all these things that keep us up at night that we didn't used to do.

And - but it turns out that people who live in places where there is no electricity and there are no iPhones and there's no TV, turns out they don't sleep any more than the average American. They sleep - I think the number is 6.7 to 7.1 hours is on average a night. And they often don't nap either, by the way, which is something we're also told.

So there's actually - if you look at the data, there's no evidence that people used to sleep - people sleep less today than they used to. And furthermore (laughter), to my astonishment, when you look at big epidemiological data sets where you graph how much you sleep on the horizontal axis and your health outcomes on the Y axis, it's a U-shaped curve. And the bottom of that curve is about seven hours. So people who - of course, there's a lot of variation. Some people need more. Some people need less. But it turns out that people who sleep seven hours a night tend to be healthier on average than people who sleep eight hours a night. And that's even after you factor in that some people might be sick and because they're sick, they're sleeping more.

So I think sleep is another one of those examples of how we make people exercise, right? We make them stressed, you know, about what they should be doing. And we kind of - there's a lot of virtue signaling going on. And, you know, if you tell somebody they're not getting enough sleep and they actually are getting enough sleep, you just make them stressed - psychosocially stressed - that elevates cortisol. Cortisol is the hormone that's about arousal. Cortisol prevents you from sleeping. And so we get into this kind of vicious circle.

So while it's true that people who don't get enough sleep are - you know, that can be a problem, right? You know, getting three, four, maybe five hours of sleep a night can be detrimental, and it's an issue. You know, if you're fine - if you're getting six, seven hours of sleep and you feel fine, I think we should all relax and stop being so, you know, uncompassionate to each other.

GROSS: You are not by nature an athletic person. When you were in school, you once hid in the closet during gym (laughter).

LIEBERMAN: (Laughter).

GROSS: You say you were always, like, the last picked on teams. So how did exercise become the thing that you study?

LIEBERMAN: Yeah, I'm still astonished at the turn in my life. I would never have predicted a bunch of years ago that I'd be writing about this stuff. I mean, I was the proverbial nerdy kid. I mean, it's a cliche, but yeah, I was picked last for most teams. I was ashamed of my body. I was - I felt, you know, deeply insecure about how bad an athlete I was.

But I got in - but my mother was a - she was really my hero in this regard because my mother was a professor at the University of Connecticut. And in the late '60s, the university built a fancy gym. And they wouldn't let women use the gym, even though federal funds were used to build it. So she was so mad that she started running in order to liberate the gym. And she couldn't even run a quarter of a mile at first. But that got her hooked. And she eventually became a runner. This is in the late '60s, before the running boom.

So I kind of just thought it was normal to run. And, you know, my father started running with my mother. And I just kind of imitated them. And I learned that it kind of kept me from being too hyper, was kind of a form of medicine. So I'd go running occasionally. But I wasn't like, you know, a really serious runner. I didn't really go very far.

And it wasn't until I started studying the evolution of running because I was interested in head stabilization - because I was a head guy - I was studying skulls - that I got really interested in this idea that humans evolved to run long distances. And we published a paper in Nature in 2004 entitled "Born To Run." Then, I started really upping my running. And then, I started getting invited to marathons. And then, I started running marathons.

You know, but I'm not a great athlete. If you looked - if you met me, you would not think I was a great athlete. I'm not particularly fast. But I've discovered that just even normal, ordinary human beings are able to be, you know, extraordinary athletes compared to what we think we could be. I'm never going to win a race. But it's been - it's turned out to be a kind of a wonderful part of my life.

But I've also, I think because of my background - you know, because I'm not a jock, and I didn't grow up being very athletic - I kind of - I think I have - I understand the fact that most people don't love exercise. And they're sick of exercists (ph) - I call them exercists - people who nag and brag about exercise. I mean, it's just irritating to have people, you know, tell you, you know, how much they run or how much they lift and how much you ought to do it. You know, a lot of us recoil from that. And I don't think that that's been particularly helpful.

GROSS: Daniel Lieberman, a pleasure to talk with you again. Thank you so much, and stay healthy, stay well.

LIEBERMAN: Thank you so much. I really enjoyed it, too.

GROSS: Daniel Lieberman is a Harvard professor and the author of the new book "Exercised." After we take a short break, jazz critic Kevin Whitehead will review a new album by soprano saxophonist Jane Ira Bloom and bassist Mark Helias, recorded remotely during the pandemic. This is FRESH AIR.

(SOUNDBITE OF MUSIC) Transcript provided by NPR, Copyright NPR.