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Exercise, Nutrition, and Cancer Prevention: A Candid Conversation with Evolutionary Biologist, Dr Daniel Lieberman
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Released: August 28, 2026

John Marshall
John Marshall, MD
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Exercise, Nutrition, and Cancer Prevention: A Candid Conversation with Evolutionary Biologist, Dr Daniel Lieberman

[00:00:06] John Marshall, MD: Hey, everybody out there, John Marshall for Oncology Unscripted, and I am a very lucky man today that I get to interview somebody who's not only a brilliant writer, but one who has taken his experience and his knowledge and applied it to something that's in everyday use to all of us, frankly, out in the world, but also within the medical profession.

The reason I reached out to Daniel Lieberman is that my wife was reading in one of the national papers an article where he and his wife took on a variety of diets that everyone is convinced is exactly the right diet to be on for a long life. And they tried it for a while, and his description of how they felt with each of these diets really made me smile. And it was at the same time, on the same day, when several of my patients were like, "What should I be doing? How should I be eating? Is there some way to eat my way out of this problem I've got?" And I keep saying, "Well, maybe it's the Mediterranean diet," and that was one of the conclusions that came out of this article.

And today, Dr. Lieberman has published, just released today, a new book called Fed Up, where he really drills down on our nutrition. So, Dr. Lieberman, with that very long preamble, thank you very much for joining us today on Oncology Unscripted.

[00:01:33] Daniel E. Lieberman, PhD: No, it's a pleasure. Thanks for asking me, even though I'm not an expert on cancer.

[00:01:37] John Marshall, MD: You are. You just don't know it yet. I think what we're going to find out is that you know probably more, or what you have learned is probably going to be as impactful on cancer as some of the things that I have learned over my career. But let's start. Your first main book, or one of your many books, was around exercise and the impact of physical activity. And we've had a recent paper that demonstrated that when colon cancer patients were randomized to receive either a pamphlet that said, "Go exercise," or a personal trainer, there was an 8% increase in cure rate among the patients who got the personal trainer.

Talk a little bit about your interest in exercise and health, and how that might feed over to us in the cancer world.

[00:02:26] Daniel E. Lieberman, PhD: I study the evolution of the human body and how and why our bodies are the way they are, and how that's relevant to health. And most of my research has been on the evolution of physical activity, and one of my arguments—the book you're referring to is entitled Exercise, and I titled the book that way because we make people exercised about exercise.

You know, we evolved to be very physically active. But exercise, I define that as discretionary, voluntary physical activity for the sake of health and fitness. And until recently, nobody did that. I mean, it's crazy, right? If you were a very physically active hunter-gatherer or a farmer who had to work hours and hours a day in order to get food, why would you go for a 5-mile run in the morning or lift weights in a gym that you have to pay for, right?

They would think we're mad, right? But the problem is that we live in a world now where people are physically inactive because of our jobs, because machines have now replaced human labor, and we're paying a price for it because physical activity is baked into our physiology, and an absence of physical activity is what we call an evolutionary mismatch.

We're not adapted to a lack of it. But here's the issue, going back to what you mentioned: We evolved to be physically active for two reasons and two reasons only: when it's necessary or rewarding. And just telling a patient to go exercise makes it neither necessary nor rewarding.

It has high efficacy but low effectiveness, in the words of medical science, right? We wrote a piece in JAMA recently, but if we want to help people be physically active, we need to take the time to talk to them about what their barriers are and why they are finding it difficult.

Is it time? Is it stress? Is it the fact that they're overweight, or they're unfit, and they don't enjoy it? How can we help them overcome those barriers? Because it takes time and effort, and the results aren't immediate, and it's not a magic bullet, even though we often advertise it as such.

It just reduces your vulnerability to a wide range of diseases, and, of course, cancer is a major, major one.

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[00:04:25] John Marshall, MD: Coming back to the exercise and now Fed Up with what we're eating, I keep coming back to the final common pathway: some alteration in our microbiome that maybe even the exercise is in some way—how do we translate that into, like, cellular health? And is that through the microbiome? Or what were your conclusions there around exercise and health?

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[00:04:52] Daniel E. Lieberman, PhD: So there are a lot of pathways, and one of them, of course, is through the microbiome. Obviously, diet has massive and immediate and huge effects on the microbiome. What you eat can change your microbiome in a matter of hours, actually. And we know that some diets, diets rich in sugar and rich in red meat, cause changes in the microbiome that we know are linked to pathways that are involved in cancer.

For example, carnitine in meat causes the microbiome to produce a molecule called TMAO, which is well known to be carcinogenic, and is one of the reasons why there's a dose-response effect between red meat in the diet and rates of cancer.

But exercise also affects the microbiome. In fact, my colleague here, Rachel Carmody at Harvard, has done this one really neat experiment. I was involved in the experiment, too, but we tracked a bunch of runners who ran a marathon a day across the United States. I was looking at their biomechanics, and a colleague of mine, Aaron Baggish, and I, we looked at their hearts.

We CT scanned them before and after. Rachel studied their poop, right? And compared their microbiomes with those of the people who were accompanying them across the way. Those were the controls as they moved across the country. And physical activity has major effects on the microbiome in ways that appear to be quite beneficial.

When you're physically active, you increase your throughput rate, so you're actually changing what gets to the microbiome. You're changing bile production. All kinds of things are going on that result in shifts to the microbiome. And so it adds another dimension to our knowledge of the microbiome, which is that not only do our diets cause, say, microbiome mismatches, but lack of physical activity also seems to have some effects that have negative health consequences.

When you're physically active, I mean, every runner knows. I've never met a constipated runner. If they exist, maybe, you know, I'm sure there's somebody out there who's constipated, but it's pretty rare, right?

That means that there's faster throughput time, which means that the microbes in the microbiome are getting different food than if you're just sitting in your chair all day long.

Because what gets through the small intestine to the colon is going to be different. So that's just one of many... But, of course, another factor is the immune system, for example. When you're physically active, your bodies produce more natural killer cells and cytotoxic T cells, and we know that those cells are patrolling our bodies 24/7, looking for, among other things, cancerous cells.

And so I think that, and again, we don't have good data on the mechanism, but we have good epidemiological data on lower rates of cancer among people who are physically active versus sedentary. And we have mechanistic data showing that when people are physically active, it raises natural killer and cytotoxic T cells.

You combine epidemiological data with mechanistic data, and you have a pretty good explanation for why people who are physically active have lower rates of a wide range of cancers. So, both are important and in different ways and through multiple mechanisms. Energy is yet another one.

Hormone levels are yet another one. I mean, these are all multifactorial issues, and they have multiple causes, and it's hard to kind of, you know, draw simple one-to-one conclusions.

[00:12:47] John Marshall, MD: Come on. That's where all the best information is. But that brings me to, you know, I'm really nervous about the GLP-1 drugs and how many people are now taking them for weight loss and diabetes control. And I spent the weekend with someone who's on one who had no appetite.

As a person who's watched fads over time and medical interventions, do you have a similar concern about these new drugs, or am I being a little overly nervous?

[00:13:19] Daniel E. Lieberman, PhD: I'm pretty open-minded. Look, I just published a paper with Steven B. Heymsfield, MD, in JAMA on GLP-1s. One of my concerns is, first of all, for some people, they're the first opportunity they've had to overcome hunger. Because the big problem with dieting is hunger, right? When you go on a diet, you get hungry, and it's really hard to overcome hunger.

And now we have, for the first time, a medication that turns off or turns down your hunger, which enables you to actually succeed in the diet. So that's—and it's had some other mitochondrial benefits, et cetera. So there are good things about GLP-1s. But a few things scare me. So one of them is that a lot of people quit them.

So I think the last study I saw showed that about 60% of people tend to quit them after a year or two. So that's a pretty high—That's a pretty typical quit rate, actually, a low adherence rate. And, of course, those individuals, once they quit, their appetite is going to come roaring back, right? Because we know that that appetite has a sort of set or settling point kind of phenomenon.

And so, when they—And here's the other thing that's scary about that, which is that people on GLP-1s, a bit, a bit like, or much like those who have bariatric surgery, tend to lose a lot of muscle, because they're losing so much weight so fast. They're losing a lot of muscle mass, and that's going to put them at risk of sarcopenia as they get older.

And sarcopenia, of course, is a vicious cycle, right? Because once you become frail, you become less physically active. That increases the frailty. It's a major issue as people age. And so I'm kind of concerned that people who are on GLP-1s, if they're not doing strength training, they're at increased risk.

But other than that, I don't know. I think we're—It's an experiment that's being conducted in real time.

[00:15:00] John Marshall, MD: Real time. And actually, those people with access, right? So this is a US-focused experiment.

[00:15:06] Daniel E. Lieberman, PhD: Yeah, so—

[00:15:07] John Marshall, MD: Not everyone has access like we do.

[00:15:09] Daniel E. Lieberman, PhD: So we'll see. I don't know. I mean, there are benefits—everything has trade-offs. As an evolutionary biologist, that's my mantra, really. I mean, it's what my lab studies, and there are benefits and costs to GLP-1s. For some people, the benefits may surely outweigh the costs, but I think we need to be sensible about how we use them.

I think titrating the doses better may help some people with the nausea or the complete, you know, anhedonia and the complete loss of appetite. But also, here's the final thing. GLP-1s don't tell you what to eat. You still have to figure out what to eat. So you might still be eating junk food, but just less, right?

Although I gather that some people on GLP-1s just lose their craving for really energy-rich foods. So that can be—they're less desirous to eat junk food. But we still need to solve the problem of—because, you know, when you and I walk into a supermarket in the United States, there are 40,000 different kinds of foods in the average supermarket.

40,000 foods. That's an astonishing number, right? And we never evolved to cope with that. I mean, our ancestors—this is the start of my book, Fed Up—but our ancestors evolved to eat what they hunted and gathered. They didn't, they didn't get... It's like you didn't, like, 20,000 years ago, be like, "Hmm, gee, shall I have kudu today, or shall I have ostrich?"

You know? If they managed to kill a kudu, they'd eat kudu. If they managed to kill an ostrich, they'd kill an ostrich. But most of the time, they brought home plants, right? And then, once we invented farming, farmers don't say, like, "Gee, you know, should I plant artichoke this year so that in the spring I'll be able to have lots of art..."

I mean, they just, you know, they have a bunch of crops that work, and they ate what they hunt, what they grew. Now we're in this very weird world where we don't have to know anything about how to get our food. We have to know how to choose our food. And when there are 40,000 products in the supermarket and 60% to 70% of them are ultra-processed, the fiber has been removed, and that's another issue for cancer, obviously.

So fiber has an important role in cancer prevention. And they've got all kinds of, you know, emulsifiers and preservatives and whatever, and they're loaded with sugar and loaded with saturated fat. And, you know, guess what? We get into trouble. And so GLP-1s don't help us solve that particular problem, and we need to figure out in our country how to improve our food system and also give people more information.

So I do a lot of work in England, and in England, when you shop in a supermarket, a lot of the foods have a little green, yellow, or red label on them. It's like super simple. Green means it's probably very healthy. Red means it's probably not very healthy. Yellow means, don't eat too much. It's super simple.

You might have debates about what's green, yellow, or red, but why can't we do that in the United States? Just give people...

[00:17:56] John Marshall, MD: Yeah.

[00:17:57] Daniel E. Lieberman, PhD: Because we're bamboozled by the all-natural labels and the this and the, you know, if loaded with vitamin C and all that kind of stuff. Give people a little bit of information to help them make what...

Because most people want to eat healthy foods, they just don't know what to eat.

[00:18:13] John Marshall, MD: I have to go exercise, I have to feed my microbiome appropriately, which is what we're talking about here: what's the right mix of things to have a nice, healthy microbiome and lower our risks of diseases in general.

I'm just hoping, as a guy from Kentucky, that bourbon is good for my microbiome. I'm kind of counting on it. I don't even want your opinion on that, because it'll just ruin my day. But nonetheless, I have to thank you very much for giving us this much time, Dr. Daniel Lieberman, author of many books, the most recent one launched today, Fed Up. Thank you very much for joining us on Oncology Unscripted.

[00:18:51] Daniel E. Lieberman, PhD: My pleasure. Thank you so much for inviting me.

This transcript was generated by AI and lightly edited for clarity.