in: Health, Health & Fitness, Podcast

• Last updated: July 2, 2023

Podcast #328: The Pros and Cons of Intermittent Fasting

During the past 10 years or so there’s been a lot of chatter about the health benefits of intermittent fasting — that is, going without food for a short window of time on a regular basis. Some of the touted benefits of intermittent fasting include shredding body fat while maintaining muscle, improving blood sugar levels and insulin sensitivity, increasing longevity, and improving focus. But how many of those purported benefits are real and how many of them are just hype? 

Well, my guest today is a nutrition scientist who has spent the past several years researching and experimenting with intermittent fasting to find out the answers to those questions. His name is John Berardi and he’s the co-owner of Precision Nutrition, an online nutrition coaching company. John has written a free ebook that highlights all the latest research on intermittent fasting as well as his personal experiments with several different IF protocols. Today on the show John cuts through the hype of intermittent fasting and gives us a nuanced look at the benefits and downsides of this diet method. If you’ve been thinking about trying intermittent fasting, you don’t want to miss this show. John breaks down exactly who should use IF and who shouldn’t, and what kinds of results to expect when you fast.

Show Highlights

  • Why John decided to dive deeply into research and experiments with intermittent fasting (IF)
  • Why the idea of IF was controversial when it first popped up
  • The body composition, health, and psychological benefits attributed to IF
  • Why all nutrition research — including that on IF — needs to be looked at with healthy skepticism
  • Can IF cure cancer?
  • The effect of IF on testosterone
  • How your body’s hormones and blood sugar are affected by IF
  • The differences between how men’s and women’s bodies react to IF
  • How to not become “hangry” when fasting
  • How IF can, perhaps surprisingly, boost your energy and focus
  • IF and weight loss
  • The downsides of practicing intermittent fasting
  • Some of the life situations in which you should actually avoid IF
  • Can IF be a tool in your workouts and weight training?
  • How your genetics can inform the effectiveness of your use of IF
  • Why you should be “nutritionally agnostic”
  • Fasting protocols and routines to follow if you want to try it out, including how often the average person should skip meals

Resources/People/Articles Mentioned in Podcast

Experiments with intermittent fasting john berardi, book cover.

If you’re looking for an easy-to-read, but informative breakdown of intermittent fasting, definitely swing by Precision Nutrition to pick up your free PDF copy (you just have to provide your email).

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Read the Transcript

Brett McKay: Welcome to another edition of the Art of Manliness podcast. Well, during the past 10 years or so, there’s been a lot of chatter about the health benefits of intermittent fasting. It is going without food for a short window of time on a regular basis. Some of the touted benefits of intermittent fasting include shredding body fat while maintaining muscle and strength, improving blood sugar levels and insulin sensitivity, increasing longevity, and improving focus, and yes, some of them even say that it can help fight cancer, but how many of these purported benefits are real and how many of them are just hype?

My guest today is a nutrition scientist who has spent the past several years researching and experimenting with intermittent fasting to find out the answers to those questions. His name is John Berardi, and he’s the co-owner of Precision Nutrition and online nutrition coaching company. John has written an ebook that highlights all the latest research on intermittent fasting as well as his personal experiments with several different IF protocols, and it includes all the blood work and everything that went along with that.

Today on the show, John cuts through the hype of intermittent fasting and gives us a nuance to look at the benefits and downsides of this diet method. If you’ve been thinking about trying intermittent fasting, you don’t want to miss this show. John breaks down exactly who should use IF, that’s intermittent fasting, and who shouldn’t, and what kind of results to expect when you fast. After the show’s over, check out the show notes

John Berardi, welcome to the show.

John Berardi: Thank you very much. Thanks for having me. It’s a huge pleasure and honor that you invited me on. As we were talking about, before we started, I have been following the website and the podcast for an embarassingly long period of time, actually. I mean, it was even your site that gave me a lot of motivation and inspiration early on for what we’re doing at Precision Nutrition, the idea that you could just love great content and use that as a vehicle to start a business, like, “Hey, we’re not writing this content for any other reason than to put out fantastic content into the world.”

You were doing this almost before anyone else, and it was a huge inspiration for us to actually do the same. Thank you for that and thanks for all the great stuff over the years.

Brett McKay: Well, thanks for the kind words. It’s very humbling, and I’m honored. Thanks so much. We’re talking about intermittent fasting today. Tell me about your background first. You’re a PhD, but a PhD in what?

John Berardi: Yeah, so I have a PhD in exercise and nutritional biochemistry. My route to what I do today is kind of nonconventional. I was, I guess, probably this story begins most appropriately when I was a very scrawny high school student or very scrawny growing up in general with asthma and allergies and my puffer everywhere that I went, so I wasn’t an athletic kid or anything like that, but identifying very much with the old Charles Atlas, sand kicked in the face by the bully ads.

I saved up my dish washing money when I was 16 and bought a barbell set from sears, and that’s where the passion for working out and strength training came from, and I, over the next couple of years, I think two, two and a half years, I put on about 70 pounds of muscle, went on to compete in some bodybuilding and powerlifting competitions. The discipline I learned in bodybuilding and powerlifting, the nutrition, the regular workouts and stuff really changed my life.

Then I decided to go to community college eventually and get good enough grades to go to university. I did a pre-med undergrad thinking I was going to go to med school, and then realized, I just don’t think I like any of that. I love learning about the body, but I really love exercise and nutrition. Then I did a master’s in exercise physiology, and then a PhD in exercise and nutritional biochemistry, but that route grooms you for academics, to be a professor or a researcher, and I did that for a few years, but my heart just always kept coming back to coaching.

Throughout those years of academics and research, I was on the side working with athletes and recreational exercisers who wanted to look and feel better. At a certain point, I was just like, “You know what, let me just do this,” probably very much like what you did with your website. This is the thing that I think about every moment I’m not thinking about the other things people think I’m supposed to think about, so I’m going to write about it and publish about it.

Over the years, that’s how Precision Nutrition was born with just free articles on the Internet. Thankfully, to date, it’s grown tremendously, and now we have a legit business. Since we started over the last 15 years, we’ve coached over 50,000 people online, and we also have a certification program.

I often like to say, when I talk about nutrition, that, yes, I have an academic pedigree, but it’s not the most important part of what I do or my experience. I think the most important part is having worked with about 50,000 people and having the experience of taking what you might read about in research studies or on the Internet and applying it and actually seeing what works, and then having your whole liveliness based on pairing down all the experiments to the fundamentals that actually do work in the end.

Brett McKay: You actually wrote an article for us in 2012, that’s like almost five years ago, which I can’t believe, it’s, time goes by so fast, about intermittent fasting. Before we get into what intermittent, well, we all know what intermittent fasting is. Fasting is without going without food. Intermittent fasting is doing that on a sort of regular basis, but why have you spend a lot of time researching and experimenting with intermittent fasting?

John Berardi: I mean, I have this weird thing where my profession is also my hobby and passion and all-consuming interest. I coach a lot of people, and I teach other professionals, our system for doing that through certifications and software and things like that, but I mean, I’m just forever interested in physique manipulation, health, body composition, and performance. I’m in my mid-40s now. I compete in masters level of tracks. I’m always experimenting with things on my self. I have academic training, and I have a lot of coaching experience, so I can feel like I can do that successfully without too much risk.

There was a point several years back where I decided, and after years and years of weightlifting and bodybuilding and also powerlifting training, I wanted to make a triumphant return to sport, like competitive sport. I was a runner, a sprinter, a track and field athlete when I was younger, and so I was like, “Oh, I heard about these masters level track meets. Wouldn’t it be cool if I could, in my 40s, run track again?”

The problem was I was just too muscular, and I know this is one of these humble brags, “I was just too muscle-bound to run,” but I weighed too much to be fast, and also I hadn’t done mobility work that would be required or the drills in years, and so I wanted to lose some weight. Right around the same time, and this was 2010, 2011, there was a lot, a lot, a lot of noise on the Internet about intermittent fasting.

Nowadays, we take for granted this particular truth that we’ve learned, but back then, the whole paradigm around food and nutrition was you have to eat small meals frequently through the day for health, and you probably remember this. It was what everyone was talking about. All of a sudden, this radical ban of intermittent fasters were telling people, “No, no, no. It won’t kill your metabolism.” It won’t ruin your health if you went for long stretches without eating in between your meals whether that’s a full day of a fast, periodically, whether that’s just an extended fast each day, so you eat, maybe skip breakfast, and so you have lunch and dinner, and during those times, you make thoughtful choices, so you can get all of your nutritional needs.

At the time, it caused a huge rift. It’s very much like what’s happening now around should we eat meat or no meat and just plants with some of the documentaries that have come out recently. It was the same thing. Everyone was just all frothed up about a different issue, and it was, should you eat frequently small meals or this fasting thing?

When you work in nutrition, there’s about, oh, a million messages a day you get about this kind of a thing, and you get these really heated irrational debates. Being a trained scientist, it’s sort of my mental make up, and I’ve considered it also my vocation to actually take a nonemotional approach to these things and just test them, whether that’s experimentation with small groups of our coaching clients, because we have so many every year that we can do small pilot projects, research projects comparing two groups on two different types of diets, and we’ve done that before, or also just on my self and some of the people who work at Precision Nutrition or some of my friends and colleagues.

I was just really fascinated, and the timing came perfect because this big debate was up. I was wondering what all the hype was about, and I also wanted to lose about 20 pounds for track and field, so I decided I would try a whole bunch of different intermittent fasting type interventions in my life and document everything to see how it would turn out.

That was kind of genesis of the experiment. It was part professional curiosity, part personal goal setting, and then also part, just a deep interest in this particular emotional debate of the day, like what would happen? Unlike the debate today around whether meats are causing cancer, and we should all eat plant-based or whatever, which will take 30 years to resolve, the intermittent fasting thing, you could figure out, at least in terms of body composition and blood markers pretty quickly, in like 12 or 16 weeks. You try it, you measure stuff carefully, you see what turns out. It was the perfect short term experiment at the time.

Brett McKay: As you said, there’s a lot of, there was a lot of hype, and there still is a lot of hype and a lot of talk on the web about intermittent fasting. What are some of the touted benefits that you seen out there that would tribute to intermittent fasting?

John Berardi: Yeah, I mean, some of the benefits are … I mean, I like to break things down into different categories. We could say there are body composition benefits. We could say there are proposed health benefits, and then we can say there are proposed physiological benefits.

If we look at the body composition benefits, well, people say that you can lose a lot of body fat without the traditional pain of dieting. Historically, when people went on a diet, in particular, if they were active, and let’s say they went to a gym and they hired a personal trainer or a diet coach or whatever, they would get that message, small meals eaten frequently throughout the day, maybe cut out carbohydrates. It’s pretty restrictive, and it’s pretty challenging, difficult, mentally, emotionally, even physiologically. Workouts start to suffer and stuff like that.

A lot of people are proposing with intermittent fasting, you don’t have to deprive yourself so much, really, if you’re doing, let’s say, an extended fast, which some people call 16:8 fasting where you have 16 hours of the day where you don’t eat meals, and eight, during an 8-hour window, you do. During those eight hours, you can indiscriminately, but there’s a lot more latitude in your choices because you’ve gone through this extended fasting period. That’s one of the proposed benefits. It’s an easier way to lose body fat if you can get the initial phase of really being hungry in the morning at breakfast. You know, body comp benefits.

Then there’s proposed health benefits, which are better regulation of blood sugar, lower blood lipids, and a whole host of other things associated with the things we might be able to measure with a blood test, and things from lowering insulin and lowering inflammation and stuff like that. Some of that does bear out in the literature, just not quite in the way that people on the Internet are saying.

Then third, the psychological benefits. Now, for a lot of people, hunger is an emergency. It’s like when it’s time to eat, if you don’t eat, you have this sort of preloaded notion that it’s an emergency, and if you don’t get food, bad things are going to happen like you’re going to get hypoglycemic or you’re going to get really irritable. Some people call it hangery, like hunger and anger mixed. I think that’s part psychological, but it’s also part physiology. It’s that you are so trained not to have periods of hunger in your life that you get maladapted type of response when you don’t eat.

One of the sort of proposed benefits, and I saw this personally with doing intermittent fasting is that you learn that hunger isn’t an emergency. You can actually go quite a long period of time without food and be very serene about it if two things happens. One, you know that it’s okay, it’s not an emergency, and two, you train your body into being what is called metabolically flexible.

For people who are used to eating frequently throughout the day, or even just breakfast, lunch, and dinner, what tends to happen is there’s a neurotransmitter/hormone called ghrelin, and it’s an anticipatory hormone. It’s a hormone that is released about 30 minutes before your habitual meal time, so if you always eat lunch at 12:30, it’s going to start coming on around 11:30 or 12:00, and it’s what makes your stomach start to grumble. It’s what starts to make you feel hungry before there’s even food around. It’s your body saying, “Oh yeah, I know Brett eats usually around 12:30, so let’s start getting ready now.” Ghrelin’s trainable, which means if you change your meal time after about 14 days, it’ll start being released at the 30 minutes prior to the new meal time.

What tends to happen with intermittent fasting is you start to manipulate ghrelin and make it a bit more flexible, and then you also make your bodies use the fuel more flexible. People who, let’s say, have a hard time losing body fat in some cases, aren’t usually well adapted at using whatever fuel’s available whenever it’s available including the fuel that’s already stored on your body.

Unless you’re 2% body fat, you have lots of meals stored on you right now, so you don’t need breakfast to feed your body the energy that’s required. It should just be able to use the meals that are on you, all that stored energy that you through glycogen, through body fat, even through your free amino acid pool, which is the breakdown products of protein.

You should be able to eat the meals that are on you. The problem is, if you’re not used to going through little periods of a longer fast, your body may have a hard time eating the food that’s on you. It may not release the correct balance of hormones in the right amounts so that you could easily use that fuel. One thing that intermittent fasting does is it trains you to be more metabolically flexible. In other words, if you have to skip lunch and dinner one day for whatever reason, your body will just easily eat the meals that’s on itself without feeling aggravated and angry and hypoglycemic or whatever the feelings that people describe that they may have.

Really, people break it down in three buckets in term for potential benefits. One is the body composition one, perhaps lose fat more easily. Two is a health one, perhaps fix a bunch of blood markers for health, and then three is a physiological/psychological one, or at least where the two interface, making you more easily able to use the fuel that’s on you if you’re not eating a meal, and also making you, I guess, less of a jerk when you don’t eat a meal.

Brett McKay: Let’s break this down a little bit. Let’s talk about these, the health benefits, the blood marker benefits. I see a lot about this, and you see these studies, I think they’re done on, they’re usually done animals, mice, and you’re like, yeah, intermittent fasting can kill cancer or intermittent fasting can increase longevity and make you live longer. Is there any credence to any to these research? Will people get those benefits or should they look at it with a bit of healthy skepticism?

John Berardi: Yeah, I mean, I think the latter. With nutrition and physiology and biology and health, it is to everyone’s advantage to assume that the truth is way more nuanced than anything you read than even the experts of the day know. If you go into any biological discussion thinking it should be settled or cut and dry or someone on the earth knows the answer to it, you would be wrong.

The truth is, especially when it comes to nutrition, we did an infographic for our website called Why Nutrition Science is so Confusing, and what we did is we compared the nutrition, I guess the history of nutritional science to the history of chemistry. Really, the history of chemistry started being chronicled thousands of years, B.C., and for the first several hundred years, it was still in the alchemy phase so they were trying to change metals.

Nutrition as a science really began in the late 1800s, so if we want to liken it to chemistry, and chemistry’s had thousands and thousands of years to learn stuff, and it obviously still doesn’t know everything, nutrition is still in the alchemy days. We are still in the phase where chemistry had tried to start and had not begun with anything relevant or useful yet. I mean, I’m being a little facetious here, but the truth is, nutrition science is so young, so if anything tells you they know, they probably don’t. It takes a much more nuanced view.

If we look at your question in a very nuanced way, does intermittent fasting have the potential to treat and cure cancer? Well, that seems non-nuanced to me. A nuance there is, in rodent models, which we know aren’t identical to human models, certain types of intermittent fasting may do a couple of things.

With a particular type of tumor, it may actually produce a small regression, or … And what’s been shown way more often is that when treating cancer, whether it’s radiation or chemotherapy, the animals can tolerate a higher dose without side effects, which, in many causes, is really useful because a higher dose may be required to shrink the tumor mass.

We see, again, it’s subtle things like that. Now, what ends up happening? Well, what ends up happening is people go the Internet and say, “Hey, look, intermittent fasting cures cancer.” No, but … I mean, there’s all this intriguing data. The fun thing is when I wrote our original book, Experiments with Intermittent Fasting, which I think is what got us on your radar, to do the article on your site, we … I said in the next five to seven years, the researcher will really hit its stride, and we’ll really figure out what intermittent fasting is doing.

Well, as you reminded me in this scary way, it was five years ago since I wrote the book and we did the article on your site, and the research hasn’t yet hit its stride. We don’t know a lot more now than we did then, so, I mean, again, in the book, we review some of the research but there’s a little data to suggest favorable improvements in certain blood markers, and some of these cancer outcomes that we saw, but the notion that intermittent fasting is this panacea or cure-all is very false.

As much as it benefited me, and you saw the results of my experiments and stuff, I don’t actually do it anymore. There’s a whole group of people that we coach that we actively dissuade from trying it until a certain point in their own health and fitness journey. In women, we’re very, very careful because a whole host of things, how intermittent fasting may affect a female body differently than a male body.

I think with everything else, there needs to be some nuance, and there needs to be some understanding of the conditional if-then statements. If you want to try intermittent fasting and you’re a women, then something. If you want to try intermittent fasting and you’re a man, then it may be something different. If you want to try intermittent fasting and you’re a man who is young and single with a robust and flexible metabolism, it may be one thing. If you want to try intermittent fasting and you’re a man who is middle aged, who doesn’t exercise, who doesn’t have a flexible metabolism and has a stressful life, it may be something totally different.

That’s how I think through things. It’s really sort of a conditional type of way. If this and this, then what, and the then is different. It’s not, if you want to try intermittent fasting, try it because it’s good because that is a false statement.

Brett McKay: Right. There’s a lot to break down there, and we will. I want to get to some of those things, especially how intermittent fasting affects men and women differently. But talking about these, going back to these blood markers. One of the benefits is it’s supposed to help regulate insulin, regulate blood sugar. I mean, what’s the research say about that? What effect does it have on other hormones like testosterone because I think you’ve written about that. Can you dig a little deeper into that?

John Berardi: Yeah, and that was part of my, as you recall, my experimentation. It wasn’t just how do I feel in body comp. Again, I spent a ridiculously long time in university learning to be a scientist. We did photos, and we did measures, and we did strength measures and all that, but we also did a whole host of blood work analyses. We even did psychological and psychometric profiles because I wanted see, a lot of people talked about intermittent fasting being very good for attention and psychological focus.

What I found was, it was for a very specific type of task, but for other tasks, it made it worse. Again, that’s where the subtly of nuance comes in. Not all attention is the same.

But back to your question. The notion that something like intermittent fasting could help regulate blood sugar, it has a solid basis. The idea being that if you aren’t feeding yourself regularly, external food, breakfast, lunch, dinner, whatever, and you’re going periods of time without feeding yourself food, your body has to upregulate receptors, which are these little sort of docking stations on every cell that receive the chemical message of a hormone, for example, to tell the cell what to do. It upreglates some of those, it upregulates the production of certain hormones.

Most people heard of adrenaline or epinephrine or norepinephrine. Well, when we do an extended fast, epinephrine and norepinephrine are upregulated. These are what often called our fight or flight hormones, but what they also do is help to release body fat, stored carbohydrates from their storage depots so that they can actually make circulating nutrients and also fuel cells.

One of the ways that intermittent fasting may help with regulation of blood sugar is that you’re obviously not taking in external sugar, so your body finds a way, it trains itself, it becomes metabolically flexible to release only the amount of energy from storage that’s required at any point in time, and no additional, which means that it finds a way to regulate its own stuff pretty well, in most cases.

Now, this takes time, and again, it’s not a panacea. Generally, when people start intermittent fasting, and they’re not very metabolically flexible, the first two weeks are a disaster. They’re hungry and they’re irritable. Their ghrelin hasn’t adapted yet. “I can’t skip breakfast. I’m just a breakfast person.” Well, that’s not true. It just takes two weeks for you to realize whether you are or not, and generally, people aren’t breakfast or not breakfast people. They can be trained to be either.

Likewise, your ability to become metabolically flexible doesn’t change with one or two days. It takes a period of time. Again, regulating insulin. Yeah, it actually does a pretty good job of that, although, some of the markers have been slim in my own work, my own experiments, didn’t markedly change because I was already kind of healthy.

Again, the if-then statements come up. If you’re already healthy and you switch to intermittent fasting, you may see no changes in blood markers at all. If you are overweight and your blood markers are a mess, any intervention would probably improve that. Intermittent fasting is one of the ones that could. Testosterone and anabolic hormones. Well, they, there’s mixed data on those.

Again, it depends on the person, and it’s another set of if-then statements, but in some cases, you’ll see the anabolic hormones, the muscle-building hormones go down. Testosterone, free testosterone, insulin-like growth factor, growth hormone. Some of these things may chronically go down, but acutely increase, like growth hormone, for example, goes up pretty markedly during a fast, but chronically, in other words, measured with those daily increases and decreases are balanced out over time, there may be no effect to a total growth hormone load, although testosterone and IGF-1 might go down. A cortisol, which is a catabolic hormone tends to go up.

Really, if you’re not a physiologist or have expertise in endocrinology, it’s fine. Generally, the hormones can be lumped into catabolic and anabolic hormones. The catabolic ones are things like epinephrine, norepinephrine, and cortisol. The anabolic ones are like IGF-1 and testosterone.

With intermittent fasting, generally, the catabolic hormones, or the breakdown hormones, go up, which is what you want if you want to lose fat, to some extent. The anabolic ones go down. Now, it’s not the same for everyone, and again, there’s subtly and nuance here, but that’s what you can expect both logically and physiologically, in other words, when you look in the research.

Brett McKay: Let’s go back … I have a question. This is a personal one. Maybe it’ll help other people figure something out, too. Whenever I do intermittent fasting, I always think, well, I haven’t had any food, so my blood, like you, carbohydrates especially, my blood sugar should be like below 100, right?

John Berardi: Right.

Brett McKay: But whenever I fast, it’s always above a hundred. It’d be like 110, 111, like what is-

John Berardi: Do you just use the glucometer regularly to monitor that?

Brett McKay: Yeah.

John Berardi: That’s pretty hardcore, isn’t it? How’d you get into doing that?

Brett McKay: I mean, I don’t know, I was just, I was interested in it, and so I decided to buy one because I was, well, there was a time when I was trying to ketosis. I was doing the keto strips, and then I was also just wanting to check my blood sugar on it as well, and yeah, so I started testing, but I’ve done that experiment on myself. Whenever I intermittent fast or even go low-carb or no carb, my blood sugar spikes, which you think, it’s so counterintuitive. I mean, it’s like, well, I haven’t had carbohydrates so it shouldn’t spike.

John Berardi: I know. This is the amazing beauty of the human body and physiology. We think we’re being so clever to outsmart our fat cells and our endocrinology, but there’s so many redundant systems to prevent us from doing anything too stupid. For example, regulated balanced blood sugar’s absolutely essential to life, regardless of what keto people will tell you or whatever, and they tell you, “Oh, yeah, blood glucose or glucose isn’t the best source of fuel. It’s ketone body, and the brain likes those best, and blah, blah, blah,” but, ahh, that’s very debatable.

In other words, let’s just look at what you’re talking about there. You don’t eat any carbohydrates, and your body finds a way to create enough blood glucose to keep glucose stable and keep you alive. Thank you, body.

No, we can outsmart you. In this case, it’s a perfect example of those catabolic hormones in action, what you’re talking about here. You are eating low to no carbohydrates, or you’re fasting so literally no food is coming in externally, but somehow your body is producing glucose from your liver, well, I mean that, or other sources. That is a product of what we talked about. Epinephrine goes up, norepinephrine goes up. Cortisol goes up. The catabolic hormones. Their function is to release stored energy, so your stored energy is being released and bam, goes into your bloodstream, and it goes to the cells as required.

It’s really, really a known physiological thing. Again, with a ton of biology or endocrinology training, it seems super weird. Incidentally, this is also the reason why people often say, let’s say with intermittent fasting, “Well, I feel more energetic. I feel like I have more energy,” and sometimes focus.

Well, the reason is, that’s what epinephrine does. Imagine the fight or flight scenario kicking in. What is supposed to happen if you’re being chased by a bear or started or scared? You’re supposed to get super hyper focused and energetic to get away from the threat. That is why. It has nothing to do with carbohydrates and insulin, blah, blah, blah. It has everything to do with the release of epinephrine, the fight or flight hormone that is released when you do an extended fast or when you don’t feed your body any carbohydrates and are in an energy deficit.

It’s the exact thing that’s keeping you alive or providing energy or helping you eat the meals that are on you that’s making you feel this particular kind of energy and also increasing your blood sugar.

Brett McKay: Okay, let’s go back to the body composition. You were able to lose a lot of body fat using intermittent fasting. What is it about intermittent fasting that allows you to do that. Is it a combination … Is there something special about intermittent fasting or is it, in the end, just sort of, when you intermittent fast, you end up eating fewer calories than you usually do?

John Berardi: Yeah, I mean, that’s a great debate that I don’t think is resolved, but it may not matter, really. The truth is, when you do intermittent fasting, you generally will eat fewer calories. The other thing … Not just because you are not eating one meal a day or if you’re fasting one day a week, an entire day of no eating, but also because, generally, people don’t make decisions like this in a vacuum. They don’t make it a science experiment.

They’re not like, “Oh, you know what I’m going to do? I’m going to try intermittent fasting, but I’m going to actually figure out how many calories I was eating on average before intermittent fasting, and then exactly match the number of calories while intermittent fasting as I was before intermittent fasting to see if it’s the fasting or the caloric load, and I’m going to keep caloric load constant.” No one’s doing that.

When you decide to try a health intervention, it makes a whole bunch of other decisions for you, too. You start doing intermittent fasting and also you start being more mindful about your food, and you start making better food choices, and that generally leads to a lower caloric intake. It’s not only the actual effect of skipping a meal or a whole day of meals, it’s the knock-on effect of making a whole bunch of other changes because you’re now mindful of this, and you’ve made it a project in your life.

That’s generally what’s happened. People are eating fewer calories than they otherwise would have. Now, is there some other magic physiology at place? Is it that growth hormone spike or that epinephrine, norepinephrine. Maybe. That may be part of it, or maybe 50/50 or maybe 75/25. We don’t know. Research, I don’t know if it will ever resolve this, like ever, ever. It may take 2,000 years, but again, it probably doesn’t matter. My gut feeling, as a scientist and as someone who’s practiced this and tried it in a lot of people, is that it’s a combination.

One, you tend to eat less because you’re not eating in certain windows. Two, you tend to eat better choices, which means fewer calories because you’re choosing better things. Three, there’s some hormonal and physiological things happening that are slightly different. It’s probably a combination of the three, but if I were to say the biggest factor, it’s probably the energy deficit and the mindfulness to your health and your nutrition that makes the biggest impact.

Brett McKay: When I’ve done intermittent fasting, I’ll see results pretty quickly, but then eventually, I don’t see them anymore. Are the effects of intermittent fasting acute or chronic?

John Berardi: Yeah, I mean, it’s really interesting. I think it’s highly individual, very much like whether a low-carb diet or a moderate-carb diet is better. Also, has a lot to do with how you exercise and what your training looks like.

It’s really interesting, Chris, the co-founder of Precision Nutrition, the guy who started the company with me … I had been doing intermittent fasting for a while for this experiment and for my own body change, and it was going really, really well. I mean, I lost about 20 pounds over 16 weeks. I tried a whole bunch of different protocols. I felt like I plateaued towards the end simply because I got too lean. If anyone’s interested, you can go look at the pictures. I mean, I did a ultrasound-type body fat testing technology, so it may not be perfect, but it’s pretty good and it was reading me at like 4% body fat. I didn’t have a lot more to lose.

Generally, when you tend to get that lean, a whole bunch of mechanisms kick in to keep you alive. They’re like, “Hey dummy. We’re getting in the danger zone.” Appetite gets dramatically upregulated, a whole bunch of neurotransmitters and hormones are released to make you less active. Even though I could show up at the gym the same number of days a week, or at the track, in between workouts, my body is like, “Hey, you should stay on this couch a little bit longer.”

The activities of daily life go down, and we see it over and over and over again in research. When your energy balance gets to negative or you get too lean, your body finds a way to slow you down and conserve energy. You can be like, “No, no, but I’m still working out the same.” That’s not what I’m talking about here. Your workouts generally only account for about 30% of your metabolic rate each day. The other 70 is your activities of daily life, and your body finds away to slow you down. It finds a way to consume, or sorry, expends less energy.

For me, that’s where the plateau happened. It happened at the absolute edge of leanness. My business partner, Phil, though, right around the time I was at my leanest, we were away together. He was getting ready to get married, so we were away in Italy for his bachelor party, his bachelor trip, and he was like, “Holy crap, dude. I’ve never seen you in this good of shape. What’s going on?” I told him about it, and he’s like, “Cool, I’m going to try that for my wedding.

I remember his wedding came up very shortly thereafter, and he got in grate shape doing this, just like you said. Short period of time, he lost like 15 pounds, looked great in his tux, whatever. Then, literally, I didn’t see him for a few weeks because he went on a honeymoon, and we work in a virtual company, so we only see each other basically from the neck up in virtual meetings, and then the next time I saw him was like a month later, and he had totally gained all the weight back. I was like, “Oh, hey, how’s the intermittent fasting going?” and he’s like, “Easy come, easy go, buddy. The minute I stopped intermittent fasting, it was like the weight just came right back on.”

I mean, it just points out one of the dangers of these what I consider very quick-fixy type of implementations of eating plans where you’re like, “Yeah, intermittent fasting rules, shred like 10 pounds really fast.” I mean, you just essentially cut out, if you eat three meals a day, you cut out one third of your intake. Yeah, of course. But will you sustain that for the rest of your life? If not, then the day you start eating breakfast again without any sort of commence where it changes, it’s going to come right back just as quickly. It’s a really important thing to consider.

Brett McKay: Let’s talk about, you referred to something earlier that intermittent fasting is very individual. There’s a lot of if-then conditionals going on there. How does, as you mention men and women, there’s a difference there, how do men and women respond differently to intermittent fasting?

John Berardi: Yeah, I mean, obviously, different individuals are different genetically, lifestyle, lives, et cetera, but generally, when we look at all women’s physiological and nutritional exercise research, women are exquisitely sensitive to low carbohydrates and energy imbalances. In other words, when they get into a negative energy balance. However it happens, they are burning more calories than they ingest in a day.

Whenever they go too low carb for too long, their whole sensing mechanism, hypothalamus, pituitary, these glands that sit at the base of your brain, which are sort of the master command centers for all hormones and physiological activity, they sense what’s happening, and then they send out messages to the rest of the cells to tell them what to do, and those messages are hormones. They’re exquisitely sensitive to energy imbalance and to low carbohydrate intakes.

Generally, I mean, the physiological or evolutionary explanation is to preserve body fat and reproductive health because men are very, they’re of very little importance to the reproductive equation. Women are fundamentally important because not only do they have to have a healthy reproductive system to get pregnant, but they also have to have a healthy hormonal system to sustain a pregnancy and deliver a healthy offspring into the world.

This is the evolutionary explanation for why this happens, but it doesn’t really matter. It just happens. When women do too much intermittent fasting, they see all kinds of hypothalamic amenorrhea, so they lose their menstrual cycling.

That’s just, that’s the external system, but that’s really happening is a massive depression of reproductive hormones, a massive change in neurotransmitter output, and all kinds of problems down the line, everything from reduced bone mass to thyroid hormone health to the ability to lose body fat or have a healthy balance of muscle to fat. All these things go really haywire in women, and it happens much more easily more men, and it happens much more readily when they do longer term low-carb diets and even intermittent fasting.

Now, there’s loads of women who may be listening to this. They post on our forums, and they send us emails all the time where they’re like, “You’re wrong, Dr. Berardi, I’ve been doing this, and I’m fine.” Obviously, there’s always going to be outliers. We hear from them, but also, we have to look at the time course. I’m not saying it happens in a week or even three weeks or even three months, but over time, this can happen. I’ve seen it happen over a year or two years.

People who are saying, “No, it’s not true. That’s not me, and I’ve done it. I have personal experience.” They may not just be far along yet, or far enough along to actually see the downstream consequences.

That’s the male-female difference. Generally, I found it really fascinating when I was following the intermittent fasting conversation because most of the very vocal advocates of intermittent fasting on the Internet, because you don’t find a lot of them in the research world or whatever because it’s not settled yet there, so you find them on the Internet telling stories, writing articles, being on forums and on Facebook.

Most of them, most vocal advocates are young, single dudes. Totally makes sense. They don’t have female physiology, and young, single dudes tend not to have partners and kids and demanding careers yet, and so generally, their overall stress load is much lower, and that is a very good set of circumstances for intermittent fasting. Overall stress, low, because intermittent fasting is, itself, a stressor, it’s probably one of the reasons it’s beneficial because it is a stressor, but thrown on to a very stressful lifestyle. This is one of the reasons why I stopped doing it.

My wife and I, we have four children now. I have this company, Precision Nutrition with a hundred team members. Intermittent fasting was just another stressor I did not need, and after a while, it became very difficult to sustain. Like I said, it helped with a certain kind of concentration but made other things much worse like interacting with my team and interacting with our children. I find that when you’re intermittent fasting, your focus on single, solo writing tasks and things like that is very good, single-minded, tunnel vision kind of tasks, but if you have to collaborate, solve problems with a team, I don’t know, parent, teach, patiently, your children, really, really difficult.

Again, there’s a couple of the if-thens. If you’re a woman, you have to be really careful with this. Probably don’t do it on your own. If you’re going to do it, have a coach. Make sure that coach knows what they’re doing, and make sure that you guys have an escape plan, like a safe word for when it’s time to stop the intermittent fasting because things are going wrong.

If you’re a man, consider your other lifestyle stressors and what’s important in your life. Young, single dude? Yeah, give it a try. You’re probably going to be fine. Middle-aged dude with a lot of work and family responsibilities? This may not be so good for you because the reasons I said. Those are some of the conditionals.

Brett McKay: Gotcha. Well, let’s add another conditional. If you’re young, single, give it a go. If you’re middle-aged, got family, kids, a stressful job, maybe not. But what about the type of training you do? Does intermittent fasting work better for different types of training. Let’s … I know intermittent fasting’s really popular amongst long distance runners, particularly of the low-carb stuff because they’re all about you’re aerobic, you want to burn fat instead of carbs, and intermittent fasting is a tool for that. Is there any credence to that idea?

John Berardi: I think this is one of the heated debates because the entire history of sport nutrition is really founded on endurance exercise. I often joke, the field of sport science was founded when there was a little extra space in a real physiology lab, and someone stuck a treadmill in it, and they’re like, “We can do muscle biopsies and measure glycogen, and people can run on that thing until they get exhausted.”

That’s actually, and that was like the first 20 years of sport science and sports nutrition in particular. Measure the amount of glycogen in the muscle by chopping a chunk of muscle out. Do it in runners who can run on that treadmill because treadmill doesn’t take up a lot of space in the lab.

But the history of sports nutrition actually bears out that, well, I don’t know, this whole modern idea of athletes doing best on low-carb diets, you’re just hearing the outliers. Now, I’m not saying it’s false. What I’m saying is there’s nuance here, too. For example, if you are the type of person who could train on a lower carb diet to get a whole host of physiological adaptations that are beneficial, but then leading up to an event can load your glycogen high, you’re probably in the minority, but it’s a cool minority to be in because you might see some interesting benefits, but not everyone’s in that group.

I think we have to be really careful. There’s a lot of loud voices on the Internet that take up too much of the mindshare around this. Most of the elite endurance athletes do not follow a low-carbohydrate diet, but with that said, we’ll get to the broader point you were trying to make, which is exercise. Does exercise influence this? The answer’s yes.

I mean, there’s obviously people who don’t exercise at all. The question is, is intermittent fasting for them? I think the answer is whether they exercise, if they don’t exercise, there’s another criteria that’s more important, which is, how much nutritional experience od they have? When we coach people … Again, there’s a lot of them in every year, and we see all different levels. We work with 20 different professional sports teams right now all the way to non-exercisers who are interested in just losing weight, maybe three, four hundred pounds.

For people without much nutritional experience, we never, never start them at intermittent fasting. It’s probably a recipe for a whole host of weird food beliefs and possibly even disordered eating.

Sometimes you just need reps or practice at making healthy eating choices at breakfast, lunch, and dinner, so that’s where we begin. If someone is not really mindful about what they eat, the first thing get mindful about what you eat and practice, just like if you’re learning to play the guitar or language, you gotta put in reps every day or else you never get good at it. It’s breakfast, lunch, and dinner. Let’s get the reps rather than let’s just chop out a whole meal and teach you a whole different skillset that probably isn’t that beneficial to you in the long term.

If someone doesn’t exercise, we rarely ever start them at intermittent fasting unless they have a lot of nutritional experience and they make good food choices already, then it’s an option, but there’s probably 10 others we can go with.

If someone does exercise, it depends on the volume and intensity. Elite athletes probably shouldn’t do intermittent fasting as a regular practice throughout the entire train year. They could do it during low volume periods of training when body composition is really important, like they have to lose body fat, but as a way to enhance performance, I would say like 9.5 times out of 10, no, it won’t enhance performance.

The only reason I would ever give it to elite athlete is if they have body composition management to do. I would never do it during their heavy training periods of the year because just more stress on top of more stress, plus heavy training periods are for physiological adaptation to make you either stronger, faster, or have a better aerobic system, and this isn’t going to support that.

That’s kind of how I break it down. Now we’ve done two ends of the spectrum. We’ve done doesn’t exercise and elite athlete. Now let’s go into the middle, someone who goes to the gym three, four times a week, does strength training, you and I were talking, if you get really into barbell training or something like that. The way that … I think there’s a particular type of intermittent fasting that may be effective for that if you want to try it, and that’s the 16:8 style where on your non-training days, when you do actually eat, you just eat protein, vegetables, very moderate or even low carbohydrate, and then on your training days after you train, very high carbohydrate.

That, I found to be very, very effective if someone that’s going to do this. Again, recreational exerciser, on training days, a lot of calories during your eating window and a lot of carbs, and on your non-training days, keep the carbs really low and moderate your calories. That, I think, could be very effective because generally, recreational exercisers want to see some performance benefits, but generally, it’s about this fine balance between doing well in the gym and getting a little bit better, and also looking good naked. That’s what intermittent fasting can actually help with quite a bit, looking good naked.

Again, though, there’s a whole bunch of other ways to look good naked, through more mindful choices, through very specific manipulations in macronutrients and calories. It’s one choice. Start to see I’m very system-based thinker. A flow chart comes out of this. You’re like, if doesn’t exercise, no nutrition experience, probably not intermittent fasting. If doesn’t exercise, some nutrition experience, maybe intermittent fasting. If elite athlete, no under all conditions except has to lose body fat and in a low-volume training period. If recreational exerciser wants to look good naked and have small increases in strength over time, yes, it’s one of a few options for doing that.

Brett McKay: Right. I love that. I used to intermittent fast regularly a few years ago, and this was … I had a eating window from 12:00 p.m. until 7:00 p.m., but then I stopped because I got really into barbell training, and I just noticed that my progress, I just, I plateaued. I tried changing the programming. That didn’t do anything, so I was like, okay, it’s the diet. I was also doing low carb at the time, which I found out that’s not good if you want to get stronger because strength, it’s very intense, it’s very anaerobic, so you need carbohydrates.

Also, I got my genetic, did my genetic testing, and what I found out was I actually do better on a lower fat, higher carbohydrate diet.

John Berardi: That’s right.

Brett McKay: Which I, that’s another conditional put in there, like everyone, these guys that really promote high fat, low carb, intermittent fasting, it probably works for them because their body is geared towards that, and then they universalize that and generalize and say, “This is for everybody,” and that might not be the case.

John Berardi: It’s true, and there’s also this other factor. We often say it’s what works for you for now under the conditions that your life is in now, because the notion that somehow, as a young person, like we talked about, let’s say young dude with few stressors and responsibilities, that a particular diet that works for you now will be the same diet that works for you in 30 years if you get married, have children, have an aggressive career, you have to take into account that that may be false.

There is the genetics, there’s the lifestyle piece, and it all plays together. Usually the most vocal proponents are people pretty new in their journey. I mean, this isn’t just with nutrition, it’s with every other evangelism there is. You’re like, “I found this thing. It’s amazing. I’m feeling good on it,” whatever it is, belief in a particular theory, a religion, or a nutrition system, and it’s people very early in their journey who are the loudest about it, and they seem so convincing, but that ignores the for now. It worked for you, for now. It may not work for you for later.

We did a great piece on a physician who’s a good friend of mine who was a big low-carb advocate for years. We called it carb confessions. It’s the story of a low-carb convert. For years, he’s a big proponent of low-carb eating, and then someone convinced him to do a little experiment. Very much like you, he’s very much into strength training, and he just added some carbs and all these things improved. His blood work, his strength, all this, and he was just like, it was a very difficult realization for him because he was like, “Wow. How could I have been so locked into this one way of thinking for so long?” It’s very easy to explain but it’s very difficult when you’re living it.

That’s really it. Our whole philosophy at Precision Nutrition is being nutritionally agnostic. In other words, being open to every and all nutritional possibilities that could help a person at a different stage in their lives.

There’s a particular kind of person who, over the course of their life, might need to do a paleo style diet for a while and a low carb for a while and intermittent fasting for a while and then a high carb for a while and then maybe a vegan diet for a while. All those possibilities are okay. It’s our belief that there are principles that live at a higher level than the macronutrient split of a diet because you and I both know vegans who are very healthy, lift weights, strong, have a great blood profile, and also people who eat lots of meat and very little vegetables in the same exact situation. Healthy, strong, lean, whatever.

Neither of those camps can be totally right. If vegans are like, “No, the meat eaters are unhealthy and fat, whatever,” and the paleo people say the same about vegans, the evidence of just experience has to tell you that’s not true. There are people who are healthy in both camps, so it must not be the macronutrient breakdown or the absence or presence of meat that is the differentiator. Both camps must share something in common that transcends what they’re fighting over, that helps them both be healthy, lean, and strong, and that’s what we set out to find and help people achieve in whatever way fits into their lives. I think that’s really a key take-home message for the nutrition dialogue nowadays.

Brett McKay: I love that. Yeah, people can get very ideological when it comes to food. My wife and I joke, like food’s the new religion.

John Berardi: Totally.

Brett McKay: People are less religious, but we are religious about our food or our workouts.

John Berardi: That analogy extends. You could come up with 50 different examples of that. I believe it. It’s true. It is treated very much that way, and people organize into tribes, and they fight over belief systems, and the prevailing pressure is to come up with one true nutrition belief. One true God, whatever it is. Yeah, I very much agree. It’s become that way, at least in the subcultures that think a lot about this.

Brett McKay: We’ve been talking a lot about the nuance benefits of fasting and the downsides of fasting because there some. We haven’t talked about just fasting protocols. You mentioned one, which was, I think it was 16:8 is what you said, so it’s 16 hours of fasting, 8 hours feeding window. What would that look like for somebody in a typical day?

John Berardi: The one thing I tried in my experimentation that we logged and recorded, one was just a once a week fast, where every Sunday, I just wouldn’t eat any food. I’d eat my last meal Saturday evening around 10:00 p.m., and then I would either fast until Monday morning, so it was like a day and a half, or I’d have a meal at 10:00 p.m. on Sunday, so it’s just 24 hours without eating.

I know some listeners are probably shuttering to think about that, like, “Oh my god, won’t you die?” The answer is no. The first few times, it is very challenging, though. I’ll give you that because a whole host of expectations our body has about feeding, but that’s what I started with.

For people who are interested in, let’s say, the health benefits of fasting, it’s the one I actually recommend they start with because it’s very controlled. If you do this 16:8 thing, which we’ll talk about in a second, you have to commit to doing it often. This, you only have to commit to it, let’s say, once a month or once every three months or … I eventually got to once a week, but that was one of the protocols.

The most researched protocol, and I think this is the worst idea for active people, is called every other day fasting. Literally, you will only eat every other day. This is the one that’s been shown to have the most, let’s say, longevity benefits, possible health benefits, but I think that’s a function of very much like calorie restriction, just radically reducing your calorie intake, and that’s okay if you’re inactive. I’m not sure it’s the best way to get healthy, but if you’re active, it’s a nightmare. It’s terrible for performance in the long run.

Brett McKay: I also think there’s a social downside, too, because most of our socializing revolves around food.

John Berardi: Absolutely. I mean, there was a point, when I did my experiments, we had only two of our four children where I was literally not eating meals with my family. You know what I mean? “Well, daddy’s on a different plan.” That was just with my family. Then when you talk about outside-the-house social experiences, yeah, I mean, I might as well wore a t-shirt that said, “I have weird nutrition beliefs,” you know what I mean? There is a social downside.

The 16:8 protocol is generally, it can be constructed in a host of different ways, but the way most people talk about it is, like you did, from about noon to 7:00 or 8:00 p.m. is your eating window, and then the rest of the time, you just don’t eat. Functionally, what that looks like is you have lunch and dinner.

Now, for really performance or let’s say strength training oriented people, what they would try to do is have a strength training session at the end of their fast. Do that at 11:00, 11:30, or 12:30, so if, at like, let’s say, I work from home, so I have the luxury of working out when I want, but if you don’t, you might workout over your lunch break, and then you have a big lunch, and then you have a dinner, and then you just don’t eat again until the next day after your workout.

It’s pretty straightforward, and the idea is to be still mindful about your choices, eat lots of veggies, eat a required amount of protein. Again, if … I don’t believe that the average strength trainer who isn’t over-fat should be on a low-carb diet all the time, so get lots of carbs in after strength training on those days. Then, maybe like I said, on the non-strength training days, cut the carbs back a little. Not to zero, but a little bit.

That, pretty effectively, is the protocol. It’s a little bit of intermittent fasting and a little bit of carbohydrate cycling and a little bit of meal timing, so this one is sort of, has a little bit more complexity to it. It’s the one I saw the best results with, personally. But again, I mean, I’m not doing it anymore, so there’s a certain testimony to that there.

I often say when people ask me for nutritional supplement advice, “What supplement should I take?” Well, one of the first things you want to know is, what does the person that you’re asking take because your goals may be different, your physiology may be different, but you want to know, at least, that the person you’re asking eats their own dog food to begin with.

I thought intermittent fasting was cool, it was a cool thing to experiment with, but I didn’t want to do it in the long run because there were other options for eating well and being healthy and being fit that didn’t involve skipping meals every day, having this long period of time without meals, and also, the side effects of that, which, for me was I would become pretty irritable towards the end of the fast. I wasn’t able to share meals with my family regularly. Some of the costs were too great.

Generally those are the three big ones. The, maybe once a week or once a month, do a fast. That one is very easy to discount. Like, oh yeah, you just skip meals on a Sunday once a month? What could that possibly do? But there’s some data suggesting that it could actually help improve metabolic flexibility pretty effectively, just that one fast. Training your body one time a month to eat its own meals may actually last for the whole month until the next one. I wouldn’t discount that one. Then there’s doing that a bit more regularly.

I also tried fasting two times a week versus every other day or just once, and that really put a dent in my performance. Then again, there was the 16:8. Some people even take it further. There’s a protocol some people have called The Warrior Diet, which is a bit more nuance, but generally, it’s one meal a day. You just have one big meal to account for all of your calorie needs, and then the rest of the your day as a fast. Some people swear by it. I think it’s probably too difficult to get enough nutrition in that one meal if you’re highly active, but again, these are protocols that at least have a lot of attention right now.

There’s probably not too much else. I mean, we’re just talking about a particular theme here. Do you eat one meal a day, two meals a day, or three meals a day? The longer you extend the fast, the more of the fasting benefits you might get, although there eventually comes a bit of a trade-off with being able to get enough energy and actually feeling good about your life and your day.

Brett McKay: Yeah, I mean, is there a time frame, like you have to hit this time frame if you want to get the benefits of a fast. Could you fast for eight hours and get benefits or do you have to … Is there a minimum effective dose is what I’m asking.

John Berardi: Yeah, no one really knows. I mean, some people use the growth hormone response as a marker. Growth hormone is a hormone that’s released in times of bit of a negative energy balance or even stress. We’ll use the growth hormone sort of cortisol kind of release as a marker. They say, well, after eight hours, growth hormone and cortisol are still normal, or at least within a physiologically normally expected range. At 12 hours, they start to jump. At 16 hours, they’re pretty high. At 24 to 36 hours, it’s like you’re actually doing performance enhancing drugs.

If you look at those markers, generally, 12 to 16 hours shows a good spike. I mean, you could use that as a surrogate. It’s very crude, but yeah, I mean, if you want to get the benefits of fasting, somewhere in the 12 to 16 hours mark is probably good.

I had a friend who did a different protocol. He would only breakfast and dinners, so every day, he would get about two 12-hour fasts. He would get his overnight fast from 8:00 p.m. dinner until 8:00 in the morning breakfast, and then the 12 hours from breakfast to dinner. He seemed to enjoy that and feel good, and it fit his lifestyle well because he worked in a high-stress workplace and didn’t like to stop for lunch, so he had a big breakfast and a big dinner, and that was okay for him.

I think it’s really tempting to look for the magic formula. You’re like, it has to be a certain number of hours, it has to be this way. I mean, I get emails from people all the time. They’re like, “I’m doing the fast, and I have coffee in the morning. Does that break the fast?” “Well, no. There’s no calories in it.” “What if I put one teaspoon of almond milk in? Does that break the fast?” They start to get at these weird clarifications that probably are losing sight of the forest for the trees. “No, you’re not fasting if you have some calories during that time.” But have the benefits gone from high to zero? Absolutely not.

Brett McKay: Gotcha. I mean, in my experience, I used to do the 8:16 thing. I no longer do that. I only fast, I fast on Sundays. Most Sundays, I’ll do a fast from Saturday night, my dinner, to 12:00 on Sunday, and then once a month, I’ll do a full 24-hour fast. That’s my thing, now, is what I do. During the rest of the week, I eat from breakfast until dinner.

John Berardi: I still do that in the same that you’re talking about periodically. I do it randomly, but periodically. For me, the one thing I found that’s interesting, and if you’re interested in body comp and health, then you’ll be like, “Ah, whatever. What is this guy talking about now,” but the one thing that I found that’s interesting about it is while there may be a small physiological benefit in terms of retaining metabolic flexibility from doing that every once in a while, I find some other things happening in my mind and personally for me.

One is, it’s kind of cool to not eat all day on that day, like to not even think about my meals, to have to cook meals myself or clean up for myself. I still have to do it for my family. I’m not fasting my three year old. But that’s kind of cool.

Two is, it’s a little bit of a test of my, I don’t know, discipline and strength because at points during that day, as I suspect you feel as well, you get hungry. It’s very comfortable to just go have a snack, but to actually sustain yourself during that uncomfortable feeling and not give into it feels like a test of my discipline and strength, and I like to do that periodically.

The third thing is, it is a very strong reminder for me, and this may not be important to other people, about what a privilege food and eating is for me. There are lots of people who are food-insecure in our own countries, US and Canada and elsewhere around the world who don’t have the choice whether they eat or not. They don’t get to make this decision. It kind of reminds me of that, and I find that helpful just as a human being.

Brett McKay: Yeah. I mean, that’s why I fast, not so much for the health benefits, more of just an exercise and discipline and willpower.

John Berardi: Yeah, I mean, it kind of reminds me of the cold shower thing. I don’t know if you ever got into doing that.

Brett McKay: Yeah. No, yeah. I do cold showers regularly.

John Berardi: Absolutely. It’s, that for me … One of the guys who works are Precision Nutrition, who’s a good friend of mine, he’s a former special operator in the Navy, he and I’ve talked about this a lot, and where we both concluded is, are there physiological benefits? Maybe. Are they overstated? Absolutely. What’s the biggest benefit I get out of doing it once in a while? It’s a practice of being comfortable with discomfort.

If people give business advice all the time, you have to lean into your discomfort. If you’re uncomfortable, you gotta do it. But how often … But that’s just people wagging their finger and proselytizing over how you should live your life. When do you get practice doing uncomfortable things? Well, there’s psychologically uncomfortable things. There’s work things related to that. But cold showers are the most concrete example of that.

It sucks to stand in freezing, cold water. It never stops sucking, but if you can find a way to step into freezing, cold water, not tense your body up, to breathe through it, to actually have an inner dialogue that says something like, “This is what cold water feels like,” not, “I have to get out of this. This is an emergency,” … I bring this up because it relates to one of the interesting things with intermittent fasting, obviously.

It’s that hunger, and we started the call with this, hunger is not an emergency, generally. It’s just the thing you’ve trained yourself to think is an emergency, very much like when you jump into cold water. All of your instincts tell you, “Jump out. This is horrible,” but you can actually change a narrative and say, “This isn’t horrible. It’s what cold water feels like. Oh, okay. If I can sustain this, I can probably do other challenging things in my life.”

I think both of these things actually speak to that in some way, and I think a lot of people try and glorify the physiological benefits when a lot of benefits actually come from this.

Brett McKay: That’s awesome. Yeah. Well, John, this has been a great conversation. Where can people go to learn more about your work?

John Berardi: Yeah, I mean, just, if they pop over to, that’s where we do all of our stuff. Like I said, very much like your site, which was a early model for ours, we’ve just been publishing free, very well-researched, comprehensive nutrition, fitness, health articles for a very long time. I think we have over a thousand free articles on the site. You don’t have to give your name and email address or anything. They’re just there for people to learn more from.

For everyone listening in, I thank you for spending all this time with us today. Hopefully you learned something, and if you’re interested or passionate about health and fitness, come check us out at

Brett McKay: Awesome. John Berardi, thank you so much for your time. It’s been a pleasure.

John Berardi: Thanks, Brett.

Brett McKay: Like I said, he was John Berardi. He is the co-owner of Precision Nutrition, it’s an online nutrition coaching company. You can find out more information about his work at If you’re looking specifically for that ebook on intermittent fasting go to Could download it. It is free.

Also, check out our show notes at where you can find links to resources, where you can delve deeper into this topic.

Well, that wraps up another edition of The Art of Manliness podcast. For more manly tips and advice, make sure to check out The Art of Manliness website at If you enjoy this show, have got something out of it, I’d appreciate it you just take a minute to give us a review on iTunes or Stitcher. That helps us out a lot. As always, thank you for your continued support, and until next time, this is Brett McKay, telling you to stay manly.

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