The Peter Attia Drive - #353 - AMA #72: Fasting: benefits for body composition and disease prevention, potential risks, and Peter’s updated practical framework

Episode Date: June 16, 2025

View the Show Notes Page for This Episode Become a Member to Receive Exclusive Content Sign Up to Receive Peter’s Weekly Newsletter In this “Ask Me Anything” (AMA) episode, Peter revisits the topic of fasting, sharing how his thinking has evolved over time. The episode explores fasting-related terminology and common types of fasting, as well as the various reasons individuals might choose to fast. Peter explores potential benefits including improvements in body composition, metabolic health, lifespan extension, cardiovascular disease, cancer prevention and treatment, neurodegenerative conditions such as Alzheimer’s and Parkinson’s, and autoimmune symptoms like arthritis. He also examines the risks and potential downsides of fasting and offers a practical framework to help assess when and how fasting may be most appropriately applied. If you’re not a subscriber and are listening on a podcast player, you’ll only be able to hear a preview of the AMA. If you’re a subscriber, you can now listen to this full episode on your private RSS feed or our website at the AMA #72 show notes page. If you are not a subscriber, you can learn more about the subscriber benefits here. We discuss: Why the topic of fasting is being revisited [1:45]; The importance of being open to changing one's opinion in science and medicine as new evidence emerges [4:15]; Defining fasting and how it fits in the framework of dietary restriction, time-restricted feeding, and calorie restriction [7:30]; The most common types of fasting [11:45]; Why someone might choose fasting over continuous caloric restriction [19:00]; The biochemical effects of fasting: fuel partitioning, autophagy, and more [21:00]; Autophagy: a cellular cleanup process stimulated by fasting and energy depletion [26:00]; Comparing the physiological effects of a ketogenic diet to those of long-term fasting [27:45]; How long fasting takes to induce ketosis and boost autophagy, and how autophagy is measured [29:45]; At-home tests for ketones [33:45]; Reducing senescent (“zombie”) cells and inflammation through autophagy [35:00]; Examining the evidence for the long-term health benefits of fasting beyond short-term metabolic effects [37:15]; The common health claims associated with fasting across metabolic, neurological, oncological, autoimmune, and longevity domains [40:30]; The impact of fasting on body weight and body composition [42:00]; Weight regain and potential metabolic consequences of fasting [44:00]; The risk of lean mass loss with fasting and how to mitigate it [47:15]; A practical framework for using fasting as a strategy to improve body composition [52:15]; How extended fasting compares to caloric restriction in improving insulin sensitivity in people with insulin resistance or type 2 diabetes [54:00]; Is there any evidence that fasting can extend lifespan? [55:45]; Examining whether fasting improves cardiovascular disease risk beyond the effects of weight loss and calorie reduction [58:15]; Exploring whether prolonged fasting can reduce cancer risk [59:45]; The potential benefits of fasting or fasting-mimicking diets during cancer treatment: emerging evidence and practical considerations [1:02:15]; Fasting for protection against neurodegenerative diseases: early evidence of therapeutic benefit and open questions [1:10:45]; Various fasting methods for autoimmune diseases: potential symptom relief with cautious application [1:14:00]; Fasting and type 1 diabetes: risks, evidence, and careful dietary alternatives [1:16:45]; Fasting and gut disorders: potential benefits outside of flare-ups [1:18:45]; The primary risks and downsides associated with fasting [1:19:45]; The essential vitamins and minerals to supplement during prolonged fasting [1:21:30]; Who should avoid fasting and why [1:22:15]; Fasting’s role in health: where it shows promise and where it likely offers no distinct advantage over caloric restriction [1:23:45]; and More. Connect With Peter on Twitter, Instagram, Facebook and YouTube

Hey everyone, welcome to a sneak peek, ask me anything or AMA episode of the Drive Podcast. I'm your host, Peter Atiya. At the end of this short episode, I'll explain how you can access the AMA episodes in full, along with a ton of other membership benefits we've created. Or you can learn more now by going to peteratiamd.com forward slash subscribe. So without further delay, here's today's sneak peek of the Ask Me Anything episode. Welcome to Ask Me Anything AMA episode 72. For today's AMA, we focus on a topic we initially covered all the way back in AMA 11

in January, 2020, which is all things relating to fasting. This is a topic in which my thinking has evolved greatly, as well as one that we get an almost infinite number of questions about from our subscribers. In this episode, we discuss the various types of fasting, which oftentimes can be used interchangeably, but actually can mean very different things. We talk about the reasons why people might fast

and its potential benefits focusing on body composition, metabolic health, overall lifespan extension, cardiovascular disease, cancer prevention, cancer treatment, Alzheimer's and Parkinson's disease, along with various autoimmune symptoms, including arthritis and more. We talk about the potential negative aspects of fasting that everyone should be aware of. And finally, we wrap it up with a framework to help you think about this for yourself. If you're a subscriber and you want to watch the full video of this podcast, you can find it on the show notes page. If you're

not a subscriber, you can still watch a sneak peek of this video on our YouTube page. So without further delay, I hope you enjoy AMA number 72. Peter, welcome to another AMA. How are you doing today? Oh, I'm doing well. Thank you. That's good. It's great because today we are going to talk about a topic that I actually looked back and looked. And do you know the first time we covered this topic on an AMA? Any idea when it was? Is the topic F1? No, but it does start with an F. Oh gosh. No, I don't know. So for those listening, it is fasting today.

And the first time we talked about it was AMA 11, released January 2020. All right. Considering this is AMA 72, we've talked a lot about stuff in between then, before we've dedicated a full episode to it. Since then, there's been a lot of things that have changed on how you think about this. Your opinions evolved.

We've kind of talked about it here and there, but it's also a topic we get asked about so much and there's so many people who talk about this in different ways. And I think it creates some confusion and you've written about it in the book. You do it with yourself. You have patients who do it. So we kind of gathered all these questions and said, all right, let's dedicate a full episode to this. And in it, we'll first start by just identifying the different terms.

I think sometimes what causes confusion is people may say the term fasting or different versions of fasting and mean it in different ways. So once we identify that, we're then going to focus a lot of it on what are the reasons that people may fast? What are the reasons that they see benefits? Anything from body comp to metabolic health to just general lifespan hopes to cardiovascular disease, cancer prevention, treatment, neuro, Alzheimer's,

Parkinson's, autoimmune symptoms, arthritis and more. So a lot of different things that we'll talk about the possible benefits. We'll also look at some possible negative consequences that people should be aware of. And then hopefully wrap it up in kind of a nice framework for people to apply it to themselves and understand how you apply it in your own life.

And also how you apply it with patients. So all that said, anything you wanna add before we get rolling? No, I don't think so. I mean, obviously, I'd love to spend some time talking about Formula One, but I don't think we're going to have time with that aggressive agenda. Do Formula One drivers do long-term fasting? Do you know? I don't know any who do.

Well, then that might be the extent of F1 in this podcast. Also, have you been to your kid's school recently? Any elementary kids have strong opinions on fasting that you've heard of recently? No, but I know who to ask, so. There you go. Something tells me that kid's got some opinions on this. I would love to hear what they are.

Rolling into it, I always like to kind of do this every now and then because one thing about you is you're not afraid to change your opinion. So when new evidence comes out, you're not afraid to be like, okay, how does that fit in the scaffolding of how I thought about this before and where does that change? And we usually do it in major episodes like episode 200, 300, called Strong Convictions Loosely Held on what you've changed your opinion on. And this is a topic where I think your opinion has evolved a little bit as new

evidence has come out, as you've seen it in practice. And I think it's always important for people to understand why you feel so strongly about changing your opinion and the ability to change your opinion. Because sometimes in the space, people get really wed to what they think and they'll dig their heels in. And when people change opinions, you'll see comments like, well, how could you say this if you used to say this? And I think it's always helpful for people to hear from you as we set the stage.

Why do you think it's so important for scientists, doctors to be able to change their opinion? Well, honestly, I think it's important for everybody to be able to change their mind or change their opinion in light of new information. I also realize that we live in a world today where that's quite difficult. There's a reason I would never be able to be a politician. I think politics is an area where it's very difficult to change your mind and maintain credibility.

It seems that the harder you dig in on your position, the more credible you seem. So again, to be clear, I think this should be viewed as a feature, not a bug in life, but it should especially be viewed as a feature in the field of science and medicine where there's no such thing as absolute certainty. So people have heard me say this before, but everything in science is probabilistic and there are certain things where the probability is so high that it is effectively a certainty. So, if I were to drop a bowling ball while standing on planet earth, I can say with essentially near certainty that it will accelerate towards the ground at 9.8 meters per second squared.

But we shouldn't lose sight of the fact that there's actually a probability function that is telling us that. I've been, as you note, I think, or alluded to, I've had people get angry at me on social media when I've posted a changing position on something, most notably or most recently discussions around GLP-1 agonists. And so I think on a recent post I quoted John Maynard Keyes when he was asked why he changed his mind on something and he responded, and I love this, when the facts change, I change my mind. What do you do, sir? I love that quote. I think the best way I could say it is the way I

try to think on every topic is being in a constant state of evolution. I'm not wed to being right. I'm wed to knowing what is right. And I think that's probably the single most important piece of my mental scaffolding that if someone asked me, Hey, Peter, what's a good way to orient myself to information? I would say that is it. I'm not suggesting that that's unique to me. I think a lot of people do that, but that's what I would recommend to people it. I'm not suggesting that that's unique to me. I think a lot of people do that but that's what I would recommend to people is orient yourself towards knowing what is right and Less around being right then you'll constantly find yourself going in the right direction

Thank you for that level set. All right getting started Fasting I think it'd be helpful just to define the different terms around fasting. When you've talked about nutrition in the past and you've talked about the need to make sure you watch how many calories you eat, you've talked about a DR, TR, CR framework. So do you just want to spend a minute defining these terms as we'll talk about them today? So when we mention different things, people understand what specifically we're talking about. Yeah, so taking a step way back, people criticize me for not being as dogmatic as maybe I once was or as others are when it comes to nutrition. But the truth of the matter is, all the available evidence basically says the following.

You need to be in energy balance, so too much and too little is bad. You need adequate amounts of protein. That seems to be the one constant. You have to be able to maintain lean mass, but the actual amount of carbohydrates and fats, provided you're an energy balance, doesn't seem to matter that much. You obviously need to have sufficient amount of micronutrients and you need to avoid toxins. And once you sort of get through that, it's an open canvas as to what you want to do. Now, for most of us, I certainly include myself in this position, maintaining energy balance requires some effort. What that means is I can't just eat whatever I want whenever I want in whatever quantity I want. If I did

that, I would be out of energy balance. I would be metabolically unhealthy and I would deal with all the sequelae of that. So I can't give you a number as to what people are in my bucket, but experientially through taking care of patients for more than a decade, it appears to be more than 80% of people are in my category where they have to pay some attention. And so the framework that I devised around this was this idea of DR, TR, and CR. And you have to be doing generally at least one of those things. So DR means dietary restriction.

So you deliberately restrict some element within the diet. The most common applications of this are typically certain macronutrients. So there are people who restrict carbohydrates. There are people who restrict fat. There are people who restrict animal products. There are people who restrict carbohydrates. There are people who restrict fat. There are people who restrict animal products. There are people who restrict maybe processed foods. There's no right way to do this. There are lots of ways to do it.

And any form of DR that is significant enough generally produces the desired effect. The second area is TR or time restriction. This is what people loosely refer to as intermittent fasting or daily time restriction where you just say, Hey, look, I'm not really going to pay attention or dramatically change what I eat or even pay attention to how much I'm eating when I'm eating, but I'm really going to restrict the period of time in which I eat. The narrower I make my eating window, the more I'm going to probably restrict overall calories. Finally, there's CR, which says, actually, what I'm going to do is just focus on minimizing my total caloric intake over the course of the day.

And I'm going to do this not necessarily by paying attention to what I'm eating or when I'm eating, but just always focusing on eating less. Obviously, one can choose to mix these. When I was at my absolute lightest and leanest, which was necessary when I was racing a bicycle, I was basically going very hardcore on DR and TR. So I was highly restrictive as to when I ate. I was basically eating one snack and one meal a day,

and I was being insanely restrictive on what I was eating. And not surprisingly, I weighed 16, 17 pounds less than I do today. So lots of different ways to do this. Now the problem as it pertains to our discussion today, which is about fasting, is that we don't really have clean definitions of this. The term fasting applies to a diverse set

of feeding patterns, but it's a general definition. It can range from anything like only consuming water for days at a time to what I just described a moment ago, which was time restriction, which is, hey, I'm just going to not eat for 16 or 18 hours a day. I think as we go through this discussion, Nick, we'll certainly shed more light on the different types of protocols and maybe the pros and cons of each. Yeah, and maybe kind of as a follow-up from there, let's just cover what are the most common types of fasting. So when people are talking about

fasting, do you just kind of want to list out what those are? And I think we actually have a graph that we look at internally that we can throw up, which would be helpful here. Sure. So as I'm talking, maybe look at what's sitting over my shoulder here. So intermittent fasting, I kind of referred to that a moment ago. That's the intermittent abstinence of food from anywhere. Depends on who you read. I typically refer to it as sort of 12 to 24 hours. Some of the literature says actually it could be 12 to 36 hours.

I think it depends on whether you're referring to time restricted eating or TRE, during which period of time food intake is limited to short periods of time within a day or something called ADF, which is alternative day fasting, where you alternate a day of regular feeding and a day of not feeding. So again, if you look at this graph or this figure, you can see what an ADF protocol might look like. And again, the pink squares on this over the course of a seven day week show you days of either complete or significant caloric restriction.

So you could see in the ADF, you've got a blue day on day one, three, five, and seven. So just normal ad libitum feeding. And then day two, four, six, you've got anywhere from zero to 25% caloric intake, depending on how aggressive you want to be. Contrast that with periodic fasting, where you have just a period of normal ad libitum days, followed by a couple of successive days, could be more than one, of course, of complete caloric restriction. Time restricted eating on the other hand, each day has a period of caloric restriction. In my view, it's very hard to call it TRE if it's less than 16 hours, although I guess

technically people might consider it TRE if it's less than 12, but I would say sort of 16 to 18 is what most people are doing there. And then finally, long-term fasting where you just have a number of days stacked up. So again, that's probably a helpful way to orient people to fasting. Just one interesting historical piece of context, my book, I think it has two chapters on nutrition. There was originally a section in there that I thought was still in there. And when we were preparing for this podcast, I went back to look at it and I'm like, where is it?

Where is it? Where is it? And I realized we cut the entire thing out. In the part of the book that I wrote about fasting, there was an entire section I had written about the world's longest recorded fast, which was by this guy named Angus Barberi, who was a Scottish fellow in the late 60s, dramatically overweight, probably in the ballpark of about 450 pounds, but did a medically supervised fast, forget this, 382 days. So 382 days, he was hospitalized, consuming nothing but tea, coffee, sparkling water, vitamins and yeast extract. Over that period of time, over a year, he lost 276 pounds, taking him down to a body

weight at discharge from the hospital, if my memory serves me correctly, of about 170, 175 pounds. What was really interesting is there was an article written about in a medical journal, it might've even been the New England Journal of Medicine, roughly six or seven years post discharge from the hospital, and he still maintained a normal weight. He might've been about 10 pounds heavier than he was at the time of his discharge, which was a completely normal weight for him. So again, it was just a very interesting story, and of course speaks to the body's remarkable

resilience provided there's enough fat to metabolize. I do love the idea of if we asked you how many chapters were in your book and you had a list out, you would probably list out 30. Because you'd be like, oh, we talked about this, talked about this, talked about this. So it does speak to the good job that the editor did in editing that guy down. But I do love the concept of you telling a patient, hey, go look at this chapter in Outlive and them just scrolling through and never seeing it. They're like, ah, there's no such thing. Yeah. Yeah. And you'd be like, hey, go look at this chapter in Outlive and them just scrolling through and never seeing it. Right, they're like, ah, there's no such thing.

Yeah. Yeah. And you'd be like, no way. I definitely wrote about that. What's really sad, by the way, and I don't know how this happened, but I can't even find the Google Docs of the 80 to 100,000 words that were discarded. I'm sure they exist somewhere. I went to look for them to see if I could pull up the story on Angus and I couldn't find him.

I do. I am sure there is a graveyard of Google Docs around that book that are somewhere in the ether. You've done a lot of these yourself. So just out of curiosity, how many of those different types of fasting protocols that we talked about have you personally done at one point? Thank you for listening to today's sneak peek AMA episode of The Drive. If you're interested in hearing the complete version of this AMA, you'll want to become a premium member. It's extremely important to me to provide all of this content without relying on paid ads.

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