The Complete Guide to Fasting: How To Heal Your Body Through Intermittent, Alternate-Day, and Extended Fasting

I'm about to tell you everything that you need to know about fasting. This is fasting 101 and far beyond. It's an interview with the author of what I would consider to be the most complete guide to fasting that's out there in this episode of the Ben Greenfield Fitness Show.

If you are a hunter and a caveman and you don't eat for a couple of days and your body starts to shut down, you will never eat again. You have no energy to go out and hunt. You have no energy to catch those rabbits. What? So your body's simply not that stupid. What it does is it switches energy sources once you start to fast, where you just don't eat anything at all. You start to see little spikes of growth hormone all throughout the day. So your body's actually trying to build and care, and assuming that you eventually eat again and provide the nutrients that are needed, your body actually goes into that anabolic mode.

He's an expert in human performance and nutrition, voted America's top personal trainer and one of the globe's most influential people in health and fitness. His show provides you with everything you need to optimize physical and mental performance. He is Ben Greenfield.

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Hey, folks, it's Ben Greenfield. And I have to admit that probably thousands of books have been written about diets that help people lose weight or get better health or whatever, get into ketosis or become a vegan or learn how to barbecue, whatever. But one of the things that tends to be a prevailing theme from diet to diet in terms of actually moving the dial when it comes to health, is periods of time where you simply stop stuffing calories into your gaping maw. And that's what today's podcast is about. It's about something that you find spread across religion, something you find spread across diet, something you find spread across cultures where there's a lot of longevity. And that's this concept of fasting. And when fasting is done right, it's got an incredibly effective therapeutic effect on a lot of different diseases. It also, of course, has a fantastic effect on things like weight loss and type 2 diabetes and obesity. And my guest today, in addition to recently writing what I would consider to be the ultimate guide to fasting, it's actually called the Complete Guide to Fasting even though if it was me, I would have named it the Ultimate Guide to Fasting. His name is Dr. Jason Fung. He's a Toronto based.

How are you?

Hey. I'm good, man. I'm good. You're a nephrologist, which means you're a kidney expert in Toronto and you're still practicing as a nephrologist, right?

Oh, absolutely, yes.

Okay, cool. And then for those of you curious about Jason, he's also the chief of the Department of Medicine at Scarborough General Hospital. He's on the board of directors of the Low Carb Diabetes Association. He's also the scientific editor of the Journal of Insulin Resistance. And he knows what he's talking about. He's a smart cookie, and we're going to be able to dive into his brain and find out everything he knows about. About not eating. So, Jason, welcome to the show, man.

Thanks very much. Great to be here.

I guess the million dollar. Yeah, the million dollar question is, are you fasted right now?

Yes, I am. I don't eat breakfast a lot of the days of the week. So when I have something at lunchtime, then I'll often just kind of just keep working. And once you get used to it, it's really not very difficult because your body will simply take the calories it needs from your stores of energy, which is your body fat and the glycoge stored away. So it's a completely natural process. And when there's time, then I go and eat. And it makes it so much easier. You don't have to obsess all the time about, oh, I don't have time, what can I eat? And then you wind up going to the local donut store and then, oh, there's just muffins and you know that that's not good for you. But then you feel this pressure to eat. There's all this pressure on people, oh, you're not hungry, but you must eat. And it's like, well, eating when you're not hungry is not a great weight loss strategy.

Yeah. Now, now, what about, like, not to play devil's advocate right off the bat, man, but what about this concept of circadian rhythmicity and the fact that, you know, you'll see a lot of recommendations that to optimize circadian rhythms or to have a good body clock, that it helps upon waking, you know, within a few hours after waking to actually consume a meal? Are you concerned at all about circadian rhythm?

No. In fact, if you look at the. What happens when you wake up, it's Very interesting because there is a counter regulatory surge which happens just before you wake up. So somewhere around 4, 4:30am Your body releases certain hormones which are counter regulatory hormones, that is they run counter to insulin. So these are the sympathetic nervous system gets activated, growth hormone noradren, for example. So these all spike just before you wake up. And the whole point of it is that they are actually activating your body for the day ahead. And one of the things that these counter regulatory hormones do is they stimulate your liver to pump out some glucose. So in fact you don't have to eat to get ready for the day. Your body's already done that for you. If you start to eat well, that's fine, if you like to eat breakfast, it's fine. But you don't have to. There's no magic about.

Oh yeah, no, I wasn't saying that you have to, that you're gonna run out of energy. Because what you're saying makes sense, you know, in terms of like glycogenolysis by the liver and you know, production of glucose. What I was just curious about was, you know, some, some of these studies I've seen about habitual meal frequency and its effects on like a normalized circadian rhythm or better sleep later on in the day. Basically the relationship between nutrition and circadian rhythm in mammals I know, has been pretty well studied, but I was just curious if you'd thought about that much at all. I don't remember seeing that part in the book. Things about how regular eating might affect the circadian rhythm.

I don't think that it makes a big difference because lots of different cultures have done different, all kinds of different eating regimens throughout history and people have done well on all different ones. So some people eat early, some late. If you look at for example circadian rhythm of hunger, I actually think that's very interesting because if you take people and simply measure their hunger over 24 hours, there is a tendency to go in certain patterns and the rhythm is that your hunger is actually lowest at somewhere around 7:30, 8:00 clock in the morning. So again, and that's because of the counter regulatory surge and the fact is that the hormones is what determines your hunger because 8am is usually the longest period before you've eaten because you've often not eaten all night. So it may be 12, 14 hours since you've eaten. Yet your hunger is actually the lowest that it is through the day. So again, that's why a lot of children don't feel like eating breakfast and so on. And the only reason they start eating breakfast is that we force them and we tell them that you got to eat breakfast, you got to eat breakfast, got to eat breakfast. And it's always okay if you want to eat breakfast, but what you want to avoid is eating all this processed sort of highly refined foods that are so convenient and have taken over the breakfast aisle, so to speak, because they're so easy. We can grab them and go muffins and donuts and all these pop Tarts, all these sort of things that are super unhealthy for us because we don't want to cook bacon and eggs in the morning. It's like, who's got time for that? You got cook, clean, eat all this sort of stuff. So even if you look at the circadian rhythm, if you're again, if you're the least hungry at 8am why would you force yourself to eat? It doesn't make any sense. It's based on people who have said that, oh, it's gonna keep you full throughout the day, it's gonna both boost your metabolism. The data behind those claims is very, very sketchy.

Yeah, it makes sense. And you know, where I'm at is I typically wait for about three hours or so after I get up to eat and if I finish dinner like around 8pm usually I'm eating breakfast around like 9:30am, 10am and so I'm kind of like halfway between. I eat but I eat late when I get up. Now my kids, they are not corn flakes kids, they're not pop tart kids, but they get up and make elaborate breakfast. I mean my, my twin nine year old boys will get up and they'll do like the full on. They gotta be on the bus by 7:30 and they're up at 6:30, like scrambling eggs, making waffles, like getting mom's nut butters and jams out of the pantry and they do a full on spread for breakfast, bless their little hearts. And I know that's probably because they're running around all day at school. But then the interesting thing is they will come home having barely eaten any of the lunches that we pack for them, like carrots and olives and avocado and pemmican and all these things we send them to school with. And then they'll play and play and play and play and typically eat dinner at about 7:38pm or so. So they kind of do like a two a meal type of, or two a day meal type of thing. But it's interesting to watch children and see what they might. Might naturally, naturally progress towards. Although I think their affinity for breakfast also comes from the fact that mom's a big breakfast person too, right?

She's like, oh, yeah. And there's nothing.

The homemade cinnamon rolls and ferment the waffles overnight, you know, she's very much into that.

Yeah. And as I said, there's nothing intrinsically wrong with it, but it gets kind of. People get browbeaten into, oh, you must eat, you must eat, you must eat. It's like, no, there's lots of different options. You can eat if you want and you can make it work for you. And you, if you're really not hungry at breakfast and you don't want to eat and you're way too. You want to rush out the door to get to work and get to school. Yes. You can just skip it and go to lunchtime. It's okay. What's interesting is the very word itself. Breakfast. It's the meal that breaks your fast. It's not the oh, you must eat as soon as you wake up, before your feet hit the ground sort of thing. You can break your fast at any point. You can break it at 8am, you could break it at 11am it makes no difference. And the other thing that's very interesting word is that what it implies is that you actually have to fast in order to be able to break your fast. So what this tells us is that it's part of the natural cycle of life. You have to eat, but you have to fast. Fasting is merely the absence of eating. So if you have a period of time, that's when you eat and you have a period of time where you should fast, you don't eat all the time. That's not a good weight loss strategy either. Yet we get told again and again, oh, before your feet hit the ground, you got to start shoving food in your mouth. And then, oh, you can't, you can't. You have to eat all the way up until you go to bed. You know, six or seven, eight, nine, ten meals in the day. Oh, that's gonna make you healthy. Where's your period of fat?

It'll make you swollen, though. I used to, you know, I used to bodybuild, and that's what I did. And, you know, I was actually fooled into thinking that it would also increase the metabolic rate, which, as you've just alluded to it, it doesn't. You don't need anything more than, I believe at this point, more than two meals a day. And I actually want to talk to you about this whole concept behind starvation mode and metabolic rate and how long you can go between meals. But I used to eat a ton of meals. I'd show up at work with my yogurt and my carrot sticks and my apple and my chicken and my broccoli and my rice and my three different protein shakes and just grays all day long. And it actually worked pretty well for putting on a ton of muscle and weight on my body, which is what I was trying to do at that point. But there's definitely some health implications, too, which I think this would be a perfect place for us to kind of delve into some of the health implications, or at least what happens from a physiological standpoint. You have in the book, I guess you describe it in five different phases, if I'm correct, or five different stages in the book of what happens when we stop eating. Can you go into exactly what's going on when we stop eating, from, like a phase one to phase five type of standpoint?

Yeah, so this was described by a lot of the physiologists, and many of these studies are very old. And it basically is a process of moving from storage of food energy to burning food energy. So what happens is that when you eat, insulin goes up. And insulin is a hormone. It's essentially a nutrient sensor. So a lot of things other than just carbohydrates, a lot of proteins, for example, will also stimulate insulin. When you eat, the insulin goes up, and it tells your body that you need to store food energy because you're eating and you've got more food energy than you actually need. So some of it goes to open up the cells to glucose, for example. And that's what we tend to think about for insulin, is that it lets the cells use the glucose. But the other thing it does is it lipolysis, which is the burning of fat, and glycogenolysis, which is the storage. So you store sugar in the form of glycogen in the liver. So glycogen is long chains of glucose. The liver packages them all together and stores it in the liver. And this is why you don't die when you go to sleep every single night, is because your body can store some of this food energy. So when you're not eating, it will bring it back out. Plants, for example, will link all these chains of glucose together as starches. So amylopectin, for example, there's amylopectin A, B, and C. So there's different forms of starches so, for example, white bread, white flour, uses amylopectin A. And this is the wheat that puts all this glucose together. So that's what plants do. They use starch and humans use glycogen. And lots of mammals use glycogen. But essentially you can think of it as stored sugar because that's what it is. And the body has two storage forms of energy. It can store glycogen, and when that's full, you can't store anymore because the liver simply can't hold it. So your body then turns to storing fat. And this is the process of de novo lipogenesis. So de novo is a word from Latin, means from new lipogenesis. Lipo means fat. Genesis means the creation of. So it's the creation of new fat, but it's the creation of new fat from carbohydrates. Dietary fat doesn't actually do this. It actually just gets absorbed directly and goes into your fat cells, the excess carbohydrates. If you take a lot of carbohydrates, it gets turned into glycogen, it gets stored and then body fat. So there are two complementary storage systems because they fulfill different roles in the body. The glycogen is easily accessible. That is, the body can kind of move in and out of glycogen very easily, but there's a limited amount. Body fat is much harder to get at, but it has unlimited storage. So the analogy I sometimes use is it's kind of like a refrigerator and a basement freezer, like a chest freezer. The refrigerator, it's right there. It's easy to put food in, it's easy to take food out, but once you're full, you're full. The freezer in the basement has unlimited capacity. That is, you can put several freezers downstairs, but it's harder to get to. And that's how the body stores energy as you stop to eat. So the process of fasting goes through several phases. So when you eat insulin goes up, you store food energy in those various forms. You'll notice, of course, that protein is not stored as energy. If you eat a lot of protein, then your body uses some of that protein for building, building protein. But the excess actually just gets turned into glucose because it can't store that excess protein. So people who are pounding back, it's very hard from a dietary standpoint to eat a high protein diet. It tends to be very unpalatable. But if you're pounding back, like whey shakes and protein bars, a lot of that excess protein and depends, man.

I don't know. Rib eye steaks are pretty tasty.

Yeah. Natural foods, of course, are good, but it tends to. When you're just eating, there's a lot of fat in there as well. But if you just have those shakes and stuff, which is kind of pure protein, it's, it's, it's not that. It's not that effective. It's not as effective as you think it would be.

So the first phase is good for the biceps, though.

Yeah, A lot of bodybuilders use those sort of protein supplements. But you got to realize that the bodybuilders are actually breaking down a lot of muscle all the time because they're working out, they're breaking down the protein, so they. They may require more protein. So the first phase. The first phase is the eating phase. Then you start to go into the fasting phase. So from about zero to sort of 24 hours, what happens is that your insulin starts to fall. So the food that you eat is gone. And now you need to start pulling out energy. So you need to start pulling energy back out from the body in order to keep working. So the first place it goes is the glycogen. So what you see is that your body continues to burn glucose up to about 24 hours. At about 24 hours, the glycogen stores run out and there's a period of gluconeogenesis. So gluconeogenesis is again from the word glucose, meaning glucose neo, which is new genesis, creation of. So it's the creation of new glucose. It gets created from protein. So you will break down some protein and create glucose in order to feed the cells. You're ramping up fat oxidation at the same time. But that takes a little bit of time because as I said, the fat stores are very large or can be very large, but they're difficult to get at. So it takes a bit of time. So in that 24 to 36 hours, you get this period where you're actually breaking down some protein. You're still living off the rest of your glycogen, and then that's it. After 36 hours, then you get the kind of fat oxidation starts to take over. So protein burning really drops to a minimal sort of turnover, and then the breakdown. And then you start to see the fat oxidation. So fatty acid levels go up in the blood, and you see fat in the blood, and basically the body lives off of stored fat from then on.

And that's. That's basically ketosis. Right. When you get to that point, 36 hours.

Yeah.

So although you don't, you're not saying you have to, to get these effects, you would have to say, go 36 hours with zero calories at all. But what you're saying is if you were to, this is what would happen. So whereas some people might take a couple weeks of low calorie intake, high fat intake, restriction of carbohydrates to really begin to efficiently enter ketosis, if you just go three days without eating, period, you get there a lot faster.

Exactly. So this is the pure. This is just physiologically what happens when you stop eating. There are ways to kind of hack the system, so to speak. So ketogenic diets, for example, will use very, very high fat diets in order to reduce serum insulin, because insulin is one of the sort of master hormones that controls where your energy is coming or going. So you have to understand that insulin is essentially a hormone that tells our body to store energy. When insulin is low, your body wants to burn energy, but you can't do both. And this is something that's very interesting is that. And it makes sense because if you're burning energy, you don't want to store it and vice versa. So if insulin is high or it's low, the effects are completely opposite. And there's something called the Randall cycle in physiology as well, which is that when your body is burning glucose, it's not burning fat, and when it's burning fat, it doesn't burn glucose. So they actually, and this is a system that is designed because you want to kind of use one fuel or the other and not both at the same time. It's not very efficient. So that's why there's this kind of sequential move from burning glucose at first and then fat. And that's how you kind of hack the system, because if you have super, super, super low carbohydrate intake, you essentially are just providing fat to the body. Obviously there's some protein there, but mostly fast fat. So ketogenic diets, for example, are very high fat diets. And because it's mostly fat, your body turns into burning fat, which is why you get ketone production. So ketones, actually the body does. Most of the body doesn't use ketones. Most of the body directly uses fatty acids. So the triglycerides. So fat body fat is triglycerides. It's a glycerol backbone with three fatty acid chains. As you break those fatty acid chains off, muscles will use fat directly. The glycerol gets turned into glucose because Certain tissues need glucose. The brain is one, but the brain uses way too much glucose. So in order to get the fuel into the brain, your body produces ketones, which can cross the blood brain barrier and feed the brain. And that reduces the reliance on glucose. So that's where ketosis is. That's where ketogenic diets are. And that's how people kind of, kind of game the system to get where they right.

But, but no matter what kind of diet you're eating, whether it's a standard American diet or whether it's the McDonald's diet, or whether whatever it is, if you stop eating for three days, you'll be in ketosis.

Basically there is a good chance now there's something called the glucose ketone ratios. So different people actually are quite different. It's interesting that as your glucose falls, you expect your ketones to go up. But that's not what happens in everybody. So some people actually don't bring up their ketones and that's why some people get this sort of keto flu or they don't feel so good when they're not adapted to that because their ketones don't respond normally. Most people will go back to it very quickly. But yeah, if you don't eat, that's a very fast way to get into ketosis. If you want for thefor the large majority of people got it.

Okay, cool. And then finally, so that would be the fourth stage. You've gotyou've got your feeding stage where you actually have glucose that you talked about getting stored as glycogen in the liver or getting converted to fat. And then your what you call the post absorptive phase, which is like 6 to 24 hours after you start fasting, where your liver starts to break down its own glycogen and the glycogen stores last until like that 24, 36 hour mark. Then once glycogen runs out, then you begin to manufacture the glucose from amino acids and proteins. Then you go into ketosis and then you have this final stage you call the protein conservation stage.

Yeah, and that's where you're essentially just burning fat. So fat oxidation goes up when you measure where people are getting their energy from. It's from the fat. And that's great because that's really what we want to do. We want to burn the fat. So this kind of, and this is very important because the period from kind of 24 to 36 hours roughly where people are burning protein, that's where people get the misconception that, oh, you're going to lose muscle, you're going to burn muscle, you're going to burn muscle. And that's. If you're just to look at that, you might think that makes sense. But what they miss is that when you start refeeding, when you start eating again, growth hormone levels have gone way up. So again, remember that growth hormone is one of the counter regulatory hormones. So norepinephrine, growth hormone, the sympathetic nervous system, all go up as your insulin goes down. So growth hormone is going up. So when you actually start to eat again, you are starting to rebuild all that lost protein. But it's actually better because. Because what you've done is when you break down those protein, your body actually identifies the kind of old junky sort of proteins that it doesn't need, the cells that it doesn't need, and gets rid of them and then rebuilds new ones. So it's actually a complete renovation cycle as opposed to leaving that old protein where it is. Because if you continually eat all the time, you don't have that breakdown. That breakdown is very important in medicine. For example, for new bone, you don't simply have the bone just sticking around. There's actually a continual cycle of renewal, of breakdown and regeneration, breakdown and regeneration. And this is what the fasting does. It gives you that period of breakdown and regeneration. So if you look at studies of alternate daily fasting, for example, they've done studies where they've compared calorie restriction with alternate daily fasting over a period of 32 weeks, they've measured lean mass at the end of it. In fact, the calorie restriction people, their lean mass percentage went up by 0.5% because they had lost some weight. But the alternate daily fasting group went up by 2.2%. So in other words, the fasting group was four times better at preserving lean mass, which is directly contrary. Absolutely. And it's directly contrary.

And that's because you get high levels of growth hormone that maintain muscle mass and lean tissue when you're in that fasted state. And that's related to this protein conservation phase you talk about.

Absolutely. And this is what's very interesting because you have studies of people who are growth hormone deficient, for example, older men, some people, when they measure growth hormone and they're deficient, then when they replace them, they see this increase in lean mass, increase in bone mass, and all these sort of really beneficial things. So I think that that's what people miss when they just look at that period of gluconeogenesis when it's breaking down, that they miss that, that period where you're rebuilding. Just like if you were to do a renovation, you have to tear stuff down first. You have to take down all the old cabinets before you put up new cabinets. So if you just look at the tear down, you say, oh, that's terrible. But you haven't looked at the full cycle of feast and fast, which is much more powerful. Powerful.

It's kind of interesting because usually you would think that not eating would actually fly in the face of anabolism, that not eating would potentially suppress something like growth hormone. But what you're saying is over a fasting period, I think you even have some research that you go over in the book, you actually see a pretty significant growth hormone increase. Like you talk about the religious frame 40 day fast where baseline growth hormone levels went from 0.73. It's like nanograms per milliliter up to like almost 10, like a 12, 50% increase in growth hormone without any drugs or injections. Just from. From not eating.

Yeah. It's fascinating because you would think that it's the opposite, that hey, if you're eating a lot, your body's going to say growth hormone but produce growth hormone. But it's actually opposite, very interestingly enough, because growth hormone is a counter regular hormone. So as you eat, insulin goes up, so growth hormone goes down. Nothing shuts down growth hormone secretion like eating. It's fascinating when they do these long fasts, five days and so on, the growth hormone levels just go way, way up. When you see the pattern of growth hormone secretion get this big spike in the beginning of the day, that during that period of longer fasting that most people have that 12 to 14 hours where you don't eat. Once you start to fast, where you just don't eat anything at all, you start to see little spikes of growth hormone all throughout the day. So your body's actually trying to build and repair and so on. Obviously, if you fast forever, you'll die. But assuming that you eventually eat again and provide the nutrients that are needed, your body actually goes into that anode anabolic mode. So it is fascinating. And I wouldn't have thought of it logically either, but this is, this is physiology, this is what happens. We know that nothing turns off growth hormone like eating.

Yeah. And basically, in terms of conservation of lean muscle mass, if you're fasting, what you're saying is, based on what you were talking about with this Randall cycle, carbohydrate oxidation is Going to go down towards zero. Fat oxidation is gonna increase. And when fat oxidation increases, the normal rate at which you'd break down protein, even if you were like eating a, you know, like normal meals, goes way down. Like your body actually naturally kicks into protein conservation and growth hormone increase in the absence of food.

Yeah, that's just the normal, that's just the normal way of things. So fasting provides sort of a lot of different hormonal changes. There's changes in obviously insulin and norepinephrine and growth hormone, sympathetic nervous activation. This is one of the other things that people don't necessarily understand is that if you think about what these hormones do, they actually activate the body. So sympathetic nervous system is the so called fight or flight response. So the body's actually getting activated during this period of not eating. Norepinephrine or adrenaline, which is noradrenaline, is pumping up your body, it's not shutting it down. And this is what they say, oh, you don't eat, you're going to go into starvation mode. Your basal metabolic rate will go down. It's the complete opposite. When they say you'll go into starvation mode, they're actually not just wrong, they're like 100% wrong because it actually activates. So people come back from fast and they go, whoa, I cannot believe how much energy I had. Yeah, because your body is pumping you full of energy. And if you think about it from an evolutionary standpoint, for example, it makes total sense if you are a hunter and a caveman and you don't eat for a couple of days and your body starts to shut down, you will never eat again. You have no energy to go out and hunt, you have no energy to catch those rabbits, whatever, right? So your body's simply not that stupid. What it does is it switches energy sources. So it says, okay, there's no more sugar. I can't burn any sugar. I've run through my glycogen, I need to switch to body fat. Not only am I going to switch to body fat, but I need to pump you up so that you can go out and hunt some woolly mammoth. Otherwise I'm going to die. So that's what it does. So it actually burns fat, pumps you up, gives you more energy, and it's like, wow, that's exactly what you want to do. Because the alternative is not viable. But these are physiologic responses. It's not just kind of hocus pocus. You can measure these hormones. And you know what noradrenaline does it pumps you up.

So you're saying that your basal metabolic rate is not going to decrease at all from, from fasting.

In fact, it's the exact opposite. When you measure people after prolonged fasting, their basal metabolic rate goes up. So, for example, a study I cited which measured metabolic rate after four days of fasting, shows that when you measure the resting energy expenditure at the beginning and the end of the Fast, it's about 10% higher at the end of those four days. So your body's not shutting down, it's actually being pumped up. So all this stuff about, oh, you should never skip this meal because you're going to go into starvation mode, they're completely wrong because your body is pumping up, not shutting down. In fact, if you do calorie restriction, we know from study after study, all those studies on the Biggest Loser, for example, that if you simply try to reduce a few calories per day, so if you say, oh, I'm going to go from 2000 calories to 1600 calories per day, we know that your energy expenditure is going to go from 2000 down to 1600. You will for sure go into so called starvation mode. But it's the calorie restriction that does that. Why? Because your body, remember, has two sources of energy. It has food and it has stored food or body fat. So if you keep eating, insulin is high. Your body says, I need to store energy, but you only have 1600 calories. Your body wants to burn 2000, but it's only getting 1600. Your insulin is high, so you can't store energy and burn energy. So you can't. So insulin is high, so your fat stores are locked away. You can't access those because you're still eating all the time.

Right.

And what it does is Simply, I'm getting 1600 calories in, I'm gonna reduce my expenditure to 1600. So you've gone down the difference when you go down to zero, when you're, it' sit's about moving that insulin way down, is that all of a sudden you've unlocked all those stores of fat.

Okay, so basically it sounds to me like what you're saying is if you go on a restricted calorie diet, like if you drop from 3000 calories and you go down to 1500 calories a day for, let's say, you know, four weeks for losing weight, your insulin levels are still going to be elevated from the eating that you are doing. And the combination of high insulin and reduced calories would actually slow, not speed up your Metabolism like that would be a case where calorie restriction would actually slow the metabolism.

Yeah, that's what the studies show, is that almost all these attempts to simply restrict your calories. Now, insulin will go down because insulin goes down whenever you eat anything. Assuming your diet doesn't change much, but you eat a little less, your insulin will go down slightly, but it's not going down to like zero, where it's.

Not going down as low as it'd go if you were fasting.

Exactly. And low carbohydrate ketogenic diets do a pretty good job also of bringing that insulin level down.

Okay, so basically, if you were gonna restrict calories and you didn't want to get the drop in metabolic rate associated with calorie restriction, you would need to make sure that you were either measuring insulin or else eating foods like whatever, coconut oil and avocados and olive oil, that weren't actually gonna spike the insulin levels but still allowed you to consume a lower number of calories.

Yeah, it's really about the insulin. And actually you can lower the insulin levels even with a high carbohydrate diet. It's just that it's not sugar and refined grains. So there's lots of ways. Beans, for example.

Yeah, like. Like lower, lower glycemic index foods.

Exactly. Because the glycemic index is a good for carbohydrates, not for the other foods, but is a good reflection of what the insulin response is. There's a huge difference between beans, for example, and white bread. So they may be the same number of grams of carbohydrate, but the insulin effect, the glucose effect, is hugely different. And you see that on the glycemic index. So therefore, if you look at some traditional societies, for example, they ate a very high carbohydrate diet, but their insulin levels were not bad. The key, of course, is that they're not eating sugar, they're not eating refined grains, and they're not eating all the time. So when you do that, you can tolerate these carbohydrate foods. So natural carbohydrate containing foods, unprocessed sort of things, don't have nearly the insulin effect as our sort of, of refined, highly processed foods that we kind of eat in modern day North America.

Now, what about, though, to play devil's advocate again here, there's this story of the Biggest Loser contestants and how a whole bunch of them were. They weren't eating that much food, they were exercising a ton. But I believe in the study that showed how they all, or many of them like had this shocking increase in weight, like this yo, yo, weight gain effect after the show. Didn't they show that they actually had a drop in their metabolic rate like during the show or after the show or something like that?

Absolutely. So their metabolic rate was started dropping immediately. So the Biggest Loser diet is really the same standard sort of eat less, move more advice that physicians and dietitians have given, but it's sort of that eat less, move more on steroids. It's eat a lot less and move a lot more. So their calorie restriction goes down to about say 1200 calories, something like that. They don't want to be accused of starving their patients, which I think is ironic because I think if they went to zero, they'd have done a lot better. And their exercise goes way up because on the show you see them exercising and throwing up and all this kind of stuff. So it's essentially the same advice that people give, eat less, move more, but kind of jumbo sized. And what they show is that their metabolic rate just plummets. So they go from as they lose weight, which they do in the short term, their metabolic rate also drops. And because their metabolic rate drops, their feelings more tired, more cold, more hungry. And the worst part is that because they're burning less calories, even as they're eating less calories, their weight loss slows, eventually plateaus. So now your weight is not dropping, but you're burning 1200 calories instead of 2000. So your liver doesn't get enough energy or your heart gets less energy, you're not, you're not generating body heat, you feel really crappy and your weight is still down. So as you relax that you start eating say 1400 calories, but you're only burning 1200, so that weight comes right back. So you see on that, on that six year study that some people wound up like 50 pounds heavier than when they started.

But what you're saying is like that might not have even happened if they would have just for example, either A, fasted and done like some low intensity exercise that wasn't incredibly catabolic in that fasted state, or B, if they for their caloric restriction would have chosen a form of caloric restriction that doesn't increase insulin levels like a, like a ketogenic diet or like a slow carb diet or something that allowed them to not get the metabolic decrease accompanied with what they were doing, which was just like a standard low calorie diet that didn't control insulin. Levels combined with massive amounts of exercise and puke fests.

Absolutely. I think it's about controlling insulin, not about controlling calories. So you can do it many different ways. There's paleo diets and so on. Intermittent fasting just happens to be one of the better studied ways to maintain your basal metabolic rate. So we know from our studies, from physiology studies that this is what happens. So why don't we use our knowledge to tweak it? I mean, I don't say that everybody needs to fast or should fast. It's not easy, it's not fun. But why don't we let people use it as an option? If you don't eat, you're gonna lose weight. That's not that hard to understand. What you have to understand, too, is that this is completely natural. And as physicians, when I talk to physicians, physicians always understand this point because I say to them, well, we tell people that they should never, ever miss a meal. If you miss a meal, you're gonna die. But we tell people to miss meals all the time. When they have surgery, you have to fast. When you do a colonoscopy, you have to fast. When you do fasting blood work, you have to fast. When you do an ultrasound, you have to fast. So there's all these situations where we actually tell people to fast. And guess what? Nothing bad happens. Everybody's normal. They might be hungry, they may be a little cranky, but that's about it. Nothing bad happens. So why can't we simply use that as a therapeutic tool to make people lose weight? Because these people have type 2 diabetes. They're obese, they're having heart attacks, they're having strokes. This is a serious problem. To leave them at that level. The standard advice to eat less and move more doesn't work. We all know it, and the studies all show that it doesn't work. So why shouldn't we use this tool that we have, which is honestly the most powerful weight loss tool, because you cannot go lower than zero calories. You can't go negative calories. So therefore, this is the most powerful tool and we've decided to not use it. Like, are you kidding me? That's gotta be the craziest thing I've ever heard.

Fun. You get hungry, man. Actually, I wanted to ask you about that because you get hungry. But what I've noticed, because I do throw in a 24 hour fast I've been doing about two to four times a month. I'll go from Saturday night until Sunday night. Then I usually have my Big rib eye steak and sweet potato fry. Chow down on Sunday night. Anyways though, for the first few hours of the morning, when I wake up on Sunday, I'm kind of hungry. Once I get to the point where I'd normally eat breakfast, like around 10am, I start to get hungry. And from like 10 until 1 on a Sunday, right? Like if I haven't eaten on a Saturday, like if I stop eating Saturday dinner around 8pm I get up Sunday, go about my normal routine. I get to the point where I normally eat breakfast and I start to get hungry. And then like around noon one, I get super hungry. And then like an hour later it goes away. What's going on there? Like why do you get hungry and then it gradually goes away once you kind of get to a certain point in the fast.

Yeah, this is thethe very interesting thing because everybody assumes that you can't do it because hunger will just build and build and build and then it'll be intolerable. But that's not actually what happens. So you can look at the so called hunger hormone ghrelin, and if you take somebody and you fast them, you can measure their ghrelin levels and see what happens. So in normal people you get this three sort of spikes at breakfast, lunch and dinner. So clearly there's some element of learning involved because we expect to eat, it's 12 o', clock, we expect to eat. So our body kind of preempts us and expects it and puts out ghrelin. But what happens when you don't eat, when you skip that meal, is that the ghrelin does not continue to go off, it just falls back towards baseline. So that's really interesting because again, when I speak to physicians, they all know this because we all, during our residency medical student days, we miss meals all the time. We're just way too busy. And you don't have 45 minutes to sit down at the cafeteria, so you skip lunch and that's it. So you're a bit hungry during that hour and then once it's faded, so at 12 o' clock to 1 o' clock, you're a little bit hungry, but you're so busy you just keep working. By 2 or 3 o' clock you've forgotten that you didn't eat. And then sometimes you go, huh, I wonder if I ate today. And you didn't. Because the ghrelin has gone completely back to baseline. So over the day this hunger actually comes in waves. And when people know that, they can plan for that. So I say, well, you Know it's going to come, prepare for it, keep yourself busy, have some tea, have some coffee, drink something and let that wave just ride over you. Let it pass, and then by two or three, it's done. And basically what you've done is you've let your body eat lunch or dinner or whatever of your own body fat. And that's perfect. That's exactly what we wanted to do. And when you look at multiple days of fasting, it's even more interesting because after about day one, day two, the hunger, actually the ghrelin, which is the hunger hormone, peaks and then starts to go down. So this is exactly what we see in our clinic. So when we have people doing longer fasts, 5 days, 7 days, 14 days, everybody says, wow, how can you do that? And the secret is that after day two, the hunger slowly fades. On day two, you're like, wow, I will never get through this seven day fast. On day five, you're like, oh, I can go on forever. The hunger has completely disappeared.

And it's because the ghrelin hormone basically just gets suppressed, goes away. And at that point you don't want to eat anymore.

Exactly. Because now your body is just feeding you body fat constantly. The levels of body fat, the fatty acid levels are high. So your body's like, why do I need to eat? I have tons of fuel here. Because you've shifted fuel sources. So again, people always get the wrong idea. They think it's a one compartment problem where all the calories goes in one box and all comes out of one box. It's a two compartment problem that there's the food and there's the stored food. And what you've done is you've shifted your fuel to come out of the body fat. And now there's just so much of it that. And it's interesting because by day five, day six, and not everybody but some people go, wow, I cannot believe how good I feel. It's like their energy level is way up because the noradrenaline, the sympathetic nervous system, I have no hunger at all. And a lot of people also note the sort of mental clarity which has been remarked over and over again, how they think that they're much sharper, more clear. And it's like, wow, I feel amazing. It's like, well, you know, not everybody's going to be like that. But if people have this natural physiologic response and they do well, why don't we let them do it? And then they'll lose weight. Because if you don't Eat for seven days, you will lose weight. One of the things that I would warn people about though is with about weight loss is that there's a lot of water loss. This is what people will tell you. The amount of weight loss, the amount of fat loss on a fasting is only average as half a pound of fat per day because a pound of fat is roughly 3500 calories. So if you're eating normally 18002000 calories, it'll take two days to burn one pound of fat. So on a seven day fast, which sounds really extreme, you can expect to lose three and a half pounds of fat. That's it. So if you have 100 pounds to lose and you say, wow, I'm expecting the seven day fast to do a lot, you will, you know, you have £100 to lose and you lost three and a half, that's it, you will lose more because the rest of that is water. So you'll probably lose seven pounds, ten pounds even. But if you lose ten pounds, six and a half of that will come back so that you're only three and a half down at the end of the seven days. And that's where people say, oh, you failed because look, you lost all that weight, but seven pounds came back. Well, you should have expected that, that seven pounds was going to come back. So you should never have expected that loss of water as it comes back. It's not a failure. It's basically to what to expect and having, having the experience. When people understand this and expect it, then they don't feel so bad when this weight starts to go back up. Because you know that that's what's supposed to happen, happen, Right? So it's a very interesting thing.

Yeah, it's like when I used to do bodybuilding and you'd kind of like have to have to do everything from sauna to extreme carbohydrate restriction to salt restriction before you'd step on stage that you'd make weight. As soon as you made weight, you'd step off and go do like a pancake feed. Like I could easily gain about 10 to 12 pounds within a few hours when I was doing that. And it was kind of cool because your muscles would be all swollen, you'd flex and you'd feel like your biceps are gonna pop through your shirt if you think that kind of stuff is cool. Anyways, though, I did wanna ask you about these longer fasts. Cause you even get into like extended fasting in the book. Have you ever done one of these where you where you. Cause I've thought about it before just to kind of like push the reboot button on my body and get some cellular autophagy and some of these other health effects that you talk about and that you also talk about in the book. You know, not eating for like two weeks or three weeks. Doing one of these water fasts. Have you ever done one of those?

I've never done a very extended fast because it simply doesn't fit into my schedule very easily. I have dinner with my family most of the time, so I do a lot of shorter fasts. The 24 hour fasts. I have done a few. I'll tell you the reason I did them was that I went on a cruise. I gained a lot of weight because I wasn't watching what I was eating and I made a decision that I wasn't going to watch what I eat. So I ate all kinds of crap like ice cream and desserts and all that and everybody knows. So I gained a ton of weight. My rings weren't fitting and I was like, okay, this is not good. So then I went on a kind of longer fast about. I think it was about three days. And I'll tell you, at the end of the. I did three days, then I went kind of alternate daily fast and by the next week my weight was down to where it normally was. But I had a great time for that week. I ate a lot of food that I don't normally eat and I think that was probably the longest I did because. Because I was starting to get a little worried with, you know, when the rings weren't fitting and my pants weren't fitting properly and so on. It was all the carbohydrates. Right. I'm not used to that high a carbohydrate intake, but again, that's as long as I've gone. How long you have to go is unknown. So there isn't very much research into it. If you talk to some of the people who have done these longer ones, they'll often say five days, seven days, I'm not sure. I really don't know. You can get a lot of the autophagy cellular kind of what you want to do is get into that kind of 24, 36 hours where you're breaking down protein because that's where you're kind of getting that renewal cycle, the breakdown of the protein and then the regrowth.

Yeah.

And you can get a lot of those benefits by 24, 36 hours. So I tend to do quite a.

Few of Those, the interesting thing though with the, with the diabetics that you talk about in the study though is that they were like completely, I think it was like the two week mark with diabetes. You see people able to get completely off insulin, like just by obviously not eating for two weeks. Seems kinda sucky for some folks, but I mean if you could reverse a frickin disease by just not eating for two weeks, to me that' sthat's pretty profound in terms of being able to be completely off of insulin.

Oh absolutely. But they won't maintain that. You still have to maintain them, get them on the alternate day and monitor them. So we do a lot of that for people.

So you could, you could fast for like two weeks and then after you've done that to maintain the health effects, you could do something like one of the other scenarios that you talk about in the book, like 16 hour fasts each day or alternate day fasting or any of these other fasting scenarios.

Oh yeah, and absolutely, we see this every day. Almost every day I come in and I see somebody who's completely off of their diabetic medications and so on. One of the reasons we push some of the longer fasts is that these people are very sick. That is, we can do this in a controlled manner. That is, I can monitor them as a doctor, we have blood work, we have somebody that they can call. If they have problems, we tell them to stop. So it's a controlled situation. But the thing is that these are very, very high risk people. If you don't do anything, they will develop a lot of different diseases. Heart attacks, strokes, kidney disease, eye disease, amputations, all kinds of things. So the rewards are much higher. So we will push the envelope a little bit more for these patients. Now you can also do very well with shorter intermittent fasting, but it takes longer. It's simply not as powerful as doing an extended fast. But again, I would warn people that it is something that you have to adjust the medications because you can't take the same dose of insulin, for example, and not eat, your blood sugar will plummet and you're gonna get hypoglycemic. Yeah, exactly. So you have to do it in a safe manner, which we try to do. But yes, you have to understand that type 2 diabetes, which is sort of very near and dear to my heart, because that's the majority of patients that I treat are type 2 diabetes. And what we see is that when you put people on very low carbohydrate diets, when you use intermittent fasting you can actually reverse the disease because their blood sugars will come down, you'll take them off all their medications. And that's completely opposite to the message that comes out from the American Diabetes association, most doctors, which is that, oh, hey, type 2 diabetes is chronic and progressive. You've got it. You got it for life, buddy. That's the message that comes out. But it's not true, because we all know that as you lose weight, that diabetes goes away. So you have to be able to make them lose weight. But instead of that, the doctors, such as what I used to do, instead of really working hard to make them lose weight so that their disease gets better, we would simply pump them full of medications. And I was guilty of that as much as anybody else. But I understood eventually that these people were not getting healthier. They were getting heart attacks, they're getting strokes or getting kidney disease, which is where I come in. And they weren't getting better. Now I see their diabetes kind of melting away again. It's not everybody, not everybody's going to do well with a fasting diet. We're all individuals. Some people do very well with low carb diets. Some people sort of wild diet or paleo diets. You got to find out what works for you. But what my main message is that these are options for people. Don't take away those options of fasting, intermittent fasting, extended fasting, because some people will do extremely well on those other people. They'll do terribly and they'll hate it. And I say, well, if you. If it's not working and you hate it, don't do it. Do something else. But for those people who do very well, hey, this is beautiful.

Now, what about when you stop this long fast, though? This is something you go into in the book I hadn't actually heard of before. I guess I have felt like that effect of what my wife says is stomach shrinkage after you've gone for a while without eating, and you eat and your stomach kind of feels weird, or you get full really fast, or you even get nauseous. You call this refeeding syndrome in the book, and there's actually a reason for it. Can you go into what refeeding syndrome actually is?

Yeah, refeeding syndrome is actually something slightly different. So during extended fasts, what you can sometimes happen, and this was described more in sort of extreme prisoners of war In World War II, for example, when you start to eat, insulin goes up. But because you're very malnourished at the beginning, phosphorus levels in the blood actually plummet. So there are all the phosphorus is going back into the cell due to the high insulin load of the food. And it gets so low that you can get heart arrhythmias. And people have died, for example. So if you look at the risk factors for refeeding syndrome, it's sort of more extended fasting, five days and beyond, but also a baseline level of malnutrition, that is people who are truly starving, Holocaust victims, prisoners of war, that sort of thing. When you don't have that.

So it's like a mineral issue, it's a phosphorus issue. Or when your insulin levels go way up, once you start eating again and you get a bunch of synthesis of glycogen and fat and protein, all that new synthesis exhausts your mineral stores of things like phosphorus and magnesium.

Exactly.

So you could. So if you're fasting, you should use electrolytes, but when you stop fasting, you should definitely do something like. Could you use just like trace minerals or sea salt and things like that?

Oh, yeah, you could. But the key is to that people who are not in that sort of malnourished state are at fairly low risk of that. So again, these are people who would be, for example, 6ft tall and 90 pounds, like, they're so skinny, these prisoners of war. When you apply it to people on what I apply to their 200, 200, 300 pounds, the risk is very, very low. But you have to be aware that that is a potential risk and it's out there. The key is to and from when they've treated them in the past, as I said, I've treated over a thousand patients. I've never seen it. But then I'm not treating 90 pound people, I'm treating 390 pound people. So the risk is highly different. But you want to break the fast very slowly so that insulin doesn't go up. You want to try to avoid refined grains and sugar when you're refeeding, you want to go with more things such as proteins and so on, stuff with phosphorus, so meat and so on, and you want to take it slowly so that it doesn't just shoot up. And in terms of the phenomenon where people say their stomach shrank, like it's hard to know what that is. Maybe it's the ghrelin which has gone down, but we've all done this where we've gone through a sort of extended fast and go, oh, I'm gonna eat a lot. And then you eat a lot and you have this big stomach Ache. And everybody's gotten that. Everybody who does fasting is like, we've all gotten that. And quickly teaches you not to do it because your body is actually saying, no, you don't need this. I've been feeding on my own fat. Slow down, don't take so much because I don't want to stop, switch back so quickly to all this stuff, right? So it's very interesting because a lot of people will notice, like, yeah, I'm so hungry after my 36 hour fast. But then I eat a little bit, I'm completely full.

I'm like, yeah, I mean like, I don't like it's because that drop and growing that I think you're talking about. But you know, I mentioned that I do that, you know, like the ribeye steak feed on Sunday nights. But I got to do like a little lemon juice and some bitters and sometimes like I'll have a little vodka and kombucha or something like that. And by the way, you get just plowed off like one drink when you're fasted. For any of you who wonder about that. And then some of those digestifs get my appetite going. I think I create some digestive juices and then I'm actually hungry to eat and food tastes really good. And in the absence of anything else, weed is great for breaking a fast because then you definitely get that spark of appetite then and everything tastes great. But yeah, you're right, it's kind of weird. Like you don't get that hungry. You'd think that you get hungry. But I mean for anybody listening in, I mean just try, just get Jason's book and try a fricking like 24 hour fast. You'd be surprised at how easy it gets once you get past that craving you experience during the time that you'd normally have your first meal. And then you're just like, you're good to go. Hey Jason, I wanted to mention something in your book, I guess kind of in closing here, if you don't mind. I'm hoping you don't sue me if I read part of your book to folks. But it says in the 1970s, a 27 year old Scottish man started fasting at a weight of 456 pounds. Over the next 382 days, he subsisted on only non calorie fluids, a daily multivitamin and various supplements, which I think were mostly like minerals. From what I understand, he set the world record for the longest fast. His body weight decreased from 456 pounds to 180 pounds. And then, unlike these Biggest Loser guys, even five years after his fast, he remained at 196 pounds. His blood sugar levels went down, but remained within the normal range. He had no episodes of hypoglycemia. Boom. So it can be done for a very long time.

Absolutely. And this is the key. When you look at these people who do hunger strikes and all this sort of stuff, everybody kind of says, wow, how can you do that? And again, remember, once you get past day two, every day after that gets easier and easier because you're actually just feeding yourself body fat. If you look at 382 days, half a pound per day, half a pound of body fat per day is somewhere just less than 200 pounds, 190 pounds, which is approximately what he lost. So if you're looking to lose 180 pounds, and unfortunately there are people who need to lose that much. That's the magnitude of the sort of caloric restriction that we're talking about. You could go down to zero for that amount of time. So it's funny because that's the world record. It was monitored by physicians, they wrote it up and so on. And it's like, from, okay, people can survive for 382 days and we won't even let people go 24 hours without eating before somebody starts brow beating them into stuffing a muffin into their mouth. It's hilarious. It's like, are you kidding me? One other thing I would just say in terms of when you're getting into it, there is a period of adaptation where about two weeks, three weeks, when you start fasting, where there are lots of issues that come up, up. It's kind of like exercise. If you have never run in your life and then you ran and then your muscles are all sore, you'd be like, oh, running is the stupidest thing I've ever done. But you have to give it time. You have to get your body used to it. So your body's not used to the fasting, it's not used to shifting over to fat oxidation. So it's kind of rusty and it's not getting there. So you may get this kind of keto flu where you're just kind of, you're not quite yourself. You get a bit of brain fog, you get a bit of this, get a bit of that. And what you have to understand is that that happens to a lot of people. And the key is just to kind of keep doing it, keep letting your body get used to it. And then after about two weeks, three weeks, four weeks. You'll find that it's like nothing at all then. Now, for example, I can fast 24 or 30 hours without noticing much at all. Half the time when I'm really busy, like writing the book, for example, I'm like, oh, I do tons and tons of fasting. Not because I, I really want to lose weight, but because I just want that extra half an hour to do my work. So then there's all these different benefits that come into play when you're fasting, such as having extra time and so on. And that's terrific. It's such a good option for people. It's not a cure all. It's not a, you know, one of these miracle things. But at the same time, you got to realize it's not like the latest and greatest fad. Oldest dietary intervention in the book, there's no intervention. More like more ancient. Don't eat for a while. That's what, that's what dogs do. They get sick, they stop eating. It's. It's kind of this natural cleansing period that people talk about.

Yeah.

So it's, it's not funny.

That's. My wife does too. She'll be like, I have a stomachache. She's like, won't eat for three days. My jaw drops because I'm like hungry water watching her. But it works, so it's interesting. And plus, you've got a whole chapter in the book about how these influential men in the history of the world, like Jesus Christ and Buddha and the prophet Muhammad and Greek Orthodox Christian and monks and Hindus and all these people who have these pretty cool religious practices, they all have some element of caloric restriction or fasting. And then of course, if Jesus Christ, Buddha and Muhammad aren't enough for you, then you also get into some of these historical figures like Benjamin Franklin and of course one of my favorites, Mark Twain, who says, and this might be a good point to end on a little starvation can really do more for the average sick man than can the best medicines and the best doctors. I love that.

Yeah, that's such a great quote.

I'm going to link to the book. For those of you who want to grab this book, just go to BenGreenfieldFitness.comFasting that's BenGreenfieldFitness.comfasting you can grab Jason's book, the show notes for today's episode. You can leave your comments and your questions. All the goodness is over there. Jason, thanks for coming on the show and sharing all this stuff. With us, man. You make me really, really want to not eat, which is weird. But good.

Yeah, absolutely. Thanks for having me on your show. That's terrific.

All right, man. Well, folks, until next time, I'm Ben Greenfield along with Dr. Jason Fung, the author of the Complete Guide to Fasting. Signing out from BenGreenfieldFitness.com have a healthy week.

You've been listening to the Ben Greenfield Fitness podcast. Go to BenGreenfieldFitness.com for even more cutting edge fitness and performance advice.

Sam.