Lessons Learnt from Modern Day Hunter-Gatherers

There aren't many communities left that are still hunting and gathering regularly, and one of the few is the Hadza community in northern Tanzania. So with David Raichlen and Brian Wood and a few others, we organized and got funding for this research project to go and measure total daily energy expenditure with the Hadza — which was just completely surreal, living in a hunter-gatherer camp for a few months. Those initial forays were so eye-opening: what it's like to live like that. We took the same techniques we'd use in a nutrition study in the US or UK, transported them in the back of a Land Rover, and did these measurements with the Hadza. It was amazing — like a big camping trip slash science project. The community is just a bunch of wonderful people. They don't have crops, or cars, or electricity or plumbing or anything like that. They're living in grass houses, hunting wild game, digging up wild plant foods, picking wild berries, hunting with bow and arrow that they make themselves — the men do. It's a very different way of life from how I live day to day. Just a really special opportunity.

Yeah, that was a huge surprise. We had no measurements of what energy expenditure looked like in a hunting-and-gathering group. There were lots of estimates — people had estimated it, and you'll still find those estimates all over the internet: "oh, it's 4,000 calories a day," some huge number, because they're so physically active. And you can imagine, if you're going out every day on foot to get berries and game and dig up wild tubers, you get a lot of activity. The Hadza get more activity in a day than most Americans or British folks or Europeans get in a week. So we knew going in that we were going to see much, much higher energy expenditures. When we put the grants in to the National Science Foundation to fund it — because it's not cheap — it was all premised on this idea that we'd find how many more calories they spend than we do in the West. And that energy gap, that calorie gap, would help explain things like the obesity pandemic in the West. So we spent that initial season, about two or three months with the Hadza getting these measurements, came home, and got them analyzed.

I should say we use an isotope-tracking technique that actually measures how much carbon dioxide your body makes every day. It's a physiological measure, not an estimate of metabolism, because you can't burn calories without making CO2. So it's a really clean, gold-standard measurement of energy per day. We get the results back after the isotope analysis, and — totally shocked. When you compare Hadza men and women to men and women in the US, UK, Europe, it's the same. In fact, because the Hadza are a bit shorter on average than folks in the West, they actually burn fewer calories, because they're just smaller people. If you account for body size, body-fat percentage, lean mass — all the things you have to correct for in an energetics measurement — it's the same. It's indistinguishable. You cannot distinguish Hadza energy expenditure from American.

Yes, that's right.

Well, we have this observation, and the data are solid — we've since seen it in other populations too. Of course, we went out first and tried to replicate it, and we've been able to replicate it not just in other human populations but in other species. So it's a really strong, broadly seen phenomenon: activity levels don't tell us about energy expenditure the way we think they do.

What's the explanation? This is an area of intense research right now. What we think is happening is that the body adjusts the other tasks it does — spending less energy on things like immune function, reproductive function, stress, background activity, the kind of stuff you don't see or pay attention to — and it adjusts those costs to make room for this really high level of activity. So the calories are getting burned by the muscles in activity, but rather than adding on top of the energy budget as extra calories per day, the body instead takes energy away from other systems.

Yeah — I thought you were going to say it sounds like my research isn't replicable and wouldn't hold up, which people have said too. So let me address that first. The first time we had these data back and were looking at them thinking "wow, this is really surprising," we did what any good scientist should do: we assumed we were wrong. We assumed we'd screwed it up. So we did a few things. One is we measured energy expenditure using a completely different technique — heart rate. We had people wearing 24-hour heart-rate monitors, and we'd calibrated their heart rate against a mask-worn respirometry system, so we knew how many calories per beat for each individual. Then we could calculate how many calories they burn every day using that heart-rate technique. And it matched the isotope technique perfectly. So the data for the Hadza are really solid — two techniques, same answer.

Then the question is, well, is it just the Hadza? So we've since looked at other populations and gone back to the literature. A woman named Amy Luke, a fantastic researcher at Loyola, had shown similar results with a group of Nigerian farmers. We'd seen data like this in other populations here and there, so it wasn't crazy to think we were seeing it with the Hadza. And we've since done the exact same experiment with populations in South America — the Shuar and the Tsimane — and we see the same thing. We see it across species. We see it in mice and other rodents in the lab: take them when they're sedentary, get them to exercise more and more and more, and their energy expenditure doesn't go up the way their activity levels go up. Within primates, if you look at populations in zoos versus populations we can measure in the wild, they have the same energy expenditure — even though surely you're more active in the wild than in a zoo. So we assumed something was just wrong and kept trying to show where we'd screwed up, and whenever we go and do that, we replicate the finding rather than crush it. It seems really robust.

Yeah, I think it is. If you really believe that the calories you burn every day are just a simple matter of how active you are — and, I suppose, how big you are — and that it's really that simple... a lot of diet and nutrition advice is predicated on that. A lot of medical advice is predicated on that. The online calculators you go on to figure out how many calories — all predicated on that very simple way of thinking. Then yes, this is very much a frame shift in how you should think about metabolism.

I will say, one of my big learning processes in this — coming at it from the outside, because I'm not a nutritionist, I'm an evolutionary anthropologist — was having to find and learn from and network with people who'd been measuring calories in people for decades and did it better than me: people like Dale Schoeller and John Speakman and Bill Wong and Amy Luke. When you talk to them, they'd say, "Oh yeah, you're right, we know it doesn't work as simply as everybody says, and we're not really that surprised by the Hadza results. Sure, it's a little eye-opening, but if you knew the literature like we know the literature, you'd know it's not that shocking." So for people who really are in this world, there's so much knowledge that it's not a frame shift for them. I think it's a frame shift for everybody who's been getting the filtered, simplistic explanations that aren't very helpful.

That's a great question. We're seeing, for example, that the Hadza population's average expenditure is the same as the US population's average. But around those averages — like every other biological measurement — there's variation. Some people burn more, some burn less. We have no idea if that's genetically determined. Probably it is, but again, most traits are a combination of environment and genetics working together. Early-life experiences could shape it. We don't know.

As a species, we burn a certain number of calories for a given body size, and that has changed over evolutionary time. Species averages reflect the genome of the species; they get moved around by evolution and parked in different places. So homeostasis, if you think about it as an evolutionary biologist, just describes where we've been parked by evolution. There's nothing special about it. Our homeostasis isn't the same as a chimp's, or an iguana's, or a starfish's — it's a product of our evolutionary journey. And I think the calories we burn per day as humans are part of that suite of traits: this is what's normal for humans, and our homeostatic processes defend that parking spot. But it might be different from another species.

Oh, they'd say two things. First, that I'm wrong on the data — that if you look at a lot of exercise-intervention studies, you do see people's energy expenditure increase when you take a sedentary person and put them in an exercise program. I'd say yes, but only for a couple of months. If you look at the long-range studies — the ones that run 10, 12, 18 months — the body adjusts. At one or two months in, energy expenditure is much higher, and that actually corresponds to when people experience weight loss the most from a new exercise program. But go out 10, 12, 16 months and you almost always see — I can't think of a study where you don't — much lower energy expenditure than you'd expect from the exercise workload. You dose this person with 300 calories of exercise a day, you measure them a year later, and their daily expenditure is only up 100 or 200 calories — for a lot of people, up zero calories, even though they're doing the 300 calories of exercise. That's a very common outcome. On average, everybody's lower than they should be for the workload.

But it's fair to say we don't really understand the process of that adjustment — how long it takes. We could use a lot more studies on these long-term changes to exercise interventions. That's a different question from the one we asked with the Hadza, where you've got whole-lifestyle differences. So there are a lot of things different between us and the Hadza in terms of lifestyle — that's a different question from "if I start an exercise program tomorrow, what's going to happen to my body?" So that's a fair point. I still think we see compensation in those exercise-intervention studies pretty strongly, but I'd agree we need more work on it.

The other thing they'd say is: even if you're right, you're doing a disservice to the public by pointing this out, because one of the main reasons people exercise is to lose weight. That's one of the easiest ways to get people into a gym. We know exercise is really important for health, and I totally agree — there's nothing in my report that says exercise isn't important. In fact, all that adjustment I think the body's doing is probably part of why exercise is so good for you. But the critique — and I've had people say this to me — is that you can't tell people their metabolism adjusts, because they'll see it as a reason not to exercise, and it's hard enough to get people to exercise. So just basically shut up, stop saying it, because we don't want people to stop.

And I'm sympathetic to that, because it's clear exercise is really important for health — totally agree. Humans are irrational, evolved creatures; we're not robots. You can't just give us a list of all the good things about exercise and expect us to go and do it. You need a psychologically compelling reason, and for a lot of people weight loss is that reason. So I understand it. But my response would be: if you lie to people, it's worse — because when they find they're not losing weight, when they see the evidence that weight loss doesn't usually come durably from exercise, that's worse. It's worse to be factually wrong on the public messaging. That's my view.

Look, I'm an evolutionary anthropologist — we have whole conferences devoted to people arguing about the shape of a fossil and what it means about the social behavior of a long-extinct species. So in my mind, the nutrition space is full of hard data. But it's not physics, is it? It's not chemistry, it's not mathematics — you can't just prove it with certainty. And you're right.

I think that's one of the fun things about being an anthropologist coming into this space. I'm used to thinking about us as a whole organism, full of strange evolved psychology along with strange evolved physiology, and how the diversity of humans plays out. In so many research trials the goal is to eliminate diversity, so you can focus on one particular thing — and I understand why you'd do that from a design point of view. But the diversity is the point. That's what it means to be human: to be diverse. So there's going to be confusion from that, inevitably. I'd also say a lot of the confusion out there is completely intentionally ginned up by people who benefit from it. In the nutrition space, the loudest voices on Twitter are not the researchers. The people with the most followers are the charlatans — it just is, if you look at the number of followers. So it's partly intentional, but also just the nature of the science.

Yeah. I'm just impressed at how optimistic and generous they are. Not just the Hadza — I've had the chance to work in all different corners of the world, Africa, Eurasia, South America, and it's one of the things I love about my job. What I'd say is that the communities that seem to have the least are often the most grounded and the most generous. On paper you might expect the exact opposite — that people who have it most comfortably would be the most generous. It's not what happens. The Hadza are incredibly generous with their time, and they always share — with each other, and with you. I've had lots of good honey and tubers and berry snacks, and even meat, that they worked very hard to get and were happy to share. We feed ourselves out there — it's like a big camping trip — but you end up sharing a lot of time with people.

I think that grounding comes from a real connection to each other. It comes from the fact that they don't wonder what they're going to do with their lives. There's none of this existential "gosh, who am I?" They know who they are. There's a confidence and a calmness that comes from that. They're not trying to be anything they're not. There's not a lot of inequality — there's not a lot to own, period, so people don't own ten times as much as somebody else. It doesn't happen. There's none of that social pressure. It's very egalitarian; there's nobody telling you what to do. This ethos of sharing, and growing up with the same social network you're born into — I think a lot of that gives them psychological strength. It's so hard to emulate any of it back here. You get off the plane back in the US and you're kind of back at it the way you left. But you do try to hold onto it.

I'm glad you asked that, because I think we can dispel that one quickly. I don't think there's a group alive today that doesn't know about the outside world. That's a romantic myth we tell ourselves in the West — we want to believe there are these unspoiled groups out there. It's not true. It's a really connected world. You're not going to meet a Hadza guy who hasn't seen an iPhone. It's not going to happen. Now, the Hadza take the bits they like and leave the rest. It's a bit like — here in the United States we have communities called the Amish or Mennonite outside Philadelphia, a major city. Of course they know about the outside world. But they don't have cars; they ride horse and buggy, they farm, they use old traditional practices. They've held onto that even as the world changed around them, and the Hadza are basically the same. There's less intrusion in Hadza land, but you don't have to worry about spoiling them, because they understand the outside world.

Now, there is a real responsibility not to muck things up, and we take that really seriously. You don't want to just go in there, extract your science, and leave them with nothing. We try to be partners. We've set up things like the Hadza Fund — people can check that out — to work with the community, keep them healthy, and give back. So of course you're careful about how you work with these groups, but it isn't like they're trapped in amber. That's just not true.

I think it's so personal. The way I'm talking with you is the way I'd talk with anybody — it's the way I'd talk to people at a bar, and it's the way I try to write. One of the best pieces of advice I got when I started writing for magazines, before the book, was: try to write like you talk. And if you're excited about the work you're doing, like I am — I don't have to work to get excited about what I do, because I love it, I think it's so cool, I feel so lucky to do it — that energy is going to come through in the way you talk. And if you can write honestly, it'll come through in the way you write.

The problem becomes two things. One is that people get afraid — you're trained to be afraid to be candid. In peer-reviewed scientific writing that's understandable: you have to be really careful and clear about exactly what you did, exactly what you measured, and then be very clear about the limitations of how you interpret it. Totally understandable — but you have to be that clear and precise until it ends up being really dry. What's missing in that kind of writing is the "why": why did you bother to do it at all, why is it interesting? Because you're writing for people who are already interested. The researcher reading that dry science report already gets why it's a big deal; you don't have to explain it. Whereas when you're writing a magazine piece or a book, you've got to start at the big, interesting question of why we care about this stuff at all, and be open about it. There's an openness, maybe a vulnerability, you have to have — to be willing to say, "this is why I think this is so cool." You'd never say that in a scientific article, but you have to start there with good writing. The best writers I've read make it really obvious why they love it, and if they love it, you'll love it.

I was so worried. It's one of the things I was most worried about — that my colleagues would say, "oh, that's junk, it's like junk food, why are you doing that?" And the truth is, even in a 300-page book you can't get to every little detail and nuance — you just can't. So you're giving a more broad-brush view, and you still want to be careful about the science, but it has to be readable. It's a different job from writing science articles. I was afraid people wouldn't see that and would say, "how come it's not as careful and nuanced? Who do you think you are to speak for our field, to write this stuff up?" And of course you're afraid you might have got details wrong. All these things go through your mind.

But in fact the response has been awesome. The people I respect the most, and was most worried about, have been really supportive. They understand — "yeah, I wouldn't have written it that way, but that's your view," or "sure, you've simplified this piece because otherwise you get into the weeds and people stop reading." They get it. So the response has really been fantastic, and the thing I was worried about — my colleagues being dismissive — just didn't happen.

Yeah, that's the tension throughout the whole book as you're writing it: how much detail do I let myself get into without writing a textbook? Because it can't be a textbook. I have to say, my editors at Avery, Penguin Random House, were fantastic — they never pressured me to write a particular way. One thing I did do, before it was published, in the draft stages, was send it to colleagues I really respected, who knew their science backwards and forwards and were leaders in it — I'm lucky to be colleagues and friends with some of these folks — and they gave me their feedback. That was really helpful: "you really can't miss that detail, it's too important, make sure you put it in," or "this is kind of in the weeds, you could probably simplify it." So I never got pressured, but I think people do — it depends on the editor. So it's important to find the right people to work with.

Yeah. One is I've always followed my nose and done what I really enjoyed — and I'm so lucky to be able to say that. In research, the people who love it will do more of it, and you'll get rewarded for the work eventually, if the world's fair. Sometimes it's not. But I've always found myself thinking about this stuff, doing the mental work even when I'm at the gym or on a run or hanging out with my kids — my mind will drift: "oh gosh, maybe we can put it together this way, maybe this is a way to ask that question." If your mind does that anyway, then academia is a great fit for you. It is for me. So I've always been able to follow what I love, and because of that I think I've been more productive and more successful.

The other thing — and I should say these are things you don't have a lot of control over, so I don't know if it's really advice — is that I've worked very hard to have a life outside of academia. I've got a family I love, I like to get outside into the woods, I like climbing and running and biking and playing music, working in my woodshop. Those experiences are grounding, and they give you some wins and some perspective — that it isn't all about this paper that just got rejected harshly by reviewers who weren't fair. If that's all you've got, if that's all your life is, it's very easy to get wrapped up in it, and then you get more defensive, less creative, less flow. For me, having the outside stuff to ground myself and say, "yeah, you know what, it's just a paper, it's just a grant rejection, happens all the time, we'll do another one" — that gives you the confidence to aim big and try big things, rather than just doing the small things you know will work.

The second one is easier, because I've been reading a lot of that recently. The Hungry Brain by Stephan Guyenet is fantastic on diet and how it affects our weight. Right next to that would be Alex Hutchinson's Endure, which is fantastic on the limits of endurance and expenditure — not really focused on calories, but on the other ways our bodies regulate activity. So The Hungry Brain and Endure, fantastic books.

And for evolutionary biology — I'm a harsh critic there, because that's really home for me, so it's hard for me to say "oh yeah." But how about Survival of the Friendliest? That's one you probably haven't heard of. It's a really nice take on the evolutionary shaping of our social brain, and why we have this tendency to fall into in-group-versus-out-group behaviors. It's written by a colleague of mine, Brian Hare — I nearly said Brian Wood, who I work with at the Hadza — and his wife and writing partner, Vanessa Woods. Full disclosure, they're also good friends of mine. But it's a fantastic book, not read enough, and a nice perspective on how our evolutionary past shapes our social present.

Yeah. Diet and exercise are two essential tools, but they have two different jobs. Diet is for weight; exercise is for everything else. And you can't mix and match — you don't get to exercise today and eat junk. You've got to do both. I'm sorry to tell you, but it has to be both.