ZOE Science & Nutrition - The TRUTH about intermittent fasting

Episode Date: April 11, 2024

What we learned from the world’s biggest intermittent fasting study. Did you know that intermittent fasting can have significant health benefits? By aligning your eating schedule with your body's natural rhythms, it can bolster heart health, enhance insulin sensitivity, and support weight loss. In today’s episode, Jonathan, Prof. Tim Spector, and Gin Stephens dive into the world of intermittent fasting, with a focus on time-restricted eating. Gin shares essential tips for beginners and explains what it takes to be successful. Tim explores the groundbreaking findings of The Big IF Study from 2022, the largest exploration of intermittent fasting to date. They also unpack controversies and describe who might want to avoid fasting. Gin Stephens is an intermittent fasting advocate, New York Times bestselling author, and podcast host. Gin has been living the intermittent fasting lifestyle since 2014. Tim Spector is a professor of genetic epidemiology at King’s College London, director of the Twins UK study, scientific co-founder of ZOE, and one of the world’s leading researchers.  🌱 Try our new plant based wholefood supplement - Daily 30 *Naturally high in copper which contributes to normal energy yielding metabolism and the normal function of the immune system Learn how your body responds to food 👉 zoe.com/podcast for 10% off Want ZOE Science & Nutrition’s top 10 tips for healthier living? Download our FREE guide. Timecodes: 01:25 Quick fire questions 03:25 What is intermittent fasting? 04:35 What are the most common types of fasting? 06:00 The circadian rhythm and fasting 08:22 The Big IF study explanation and results 13:41 Breakfast misconceptions explained 16:01 How do the Big IF study results compare to other research? 18:56 What are the health benefits of sticking to the Big IF study? 24:00 What is Tim’s intermittent fasting schedule like? 25:40 Jonathan's experience with the Big IF study 28:04 What is metabolic flexibility? 32:43 Practical advice for getting started. 35:40 Cephalic phase insulin response 40:30 Is there an ideal length for an eating window and time to start? 43:20 Can you eat whatever you want? 44:30 Can people over 70 years of age fast safely? 51:21 Summary Mentioned in today's episode: Flipping the metabolic switch: Understanding and applying the health benefits of fasting in Obesity The Big IF Study  Books by Gin Stephens: 28-day Fast Start: Day By Day Fast. Feast. Repeat.  Have feedback or a topic you'd like us to cover? Let us know here Episode transcripts are available here.

Welcome to Zoe, Science and Nutrition, where world-leading scientists explain how their research can improve your health. Today, I'm delighted to have Gin Stevens and Professor Tim Spector back on the show to talk about intermittent fasting. We also have a world exclusive. Since we last discussed this topic, we've completed the world's largest intermittent fasting study. Over 100,000 people participated in the Big If study. And today, Tim shares the initial results.

Jin tries to convince me to take up intermittent fasting and share some fantastic practical advice on how to get started. Follow the advice that she shares today for 28 days and you will be in a good place to make this a lifelong healthy habit. If you don't know Jin already, she's a leading voice in intermittent fasting with an amazing story. She's host of the popular Intermittent Fasting Stories podcast and a best-selling author. Her latest book is The 28-Day Fast, Start Day by Day. Tim is one of the world's top 100 most cited scientists, a professor of epidemiology, and my scientific co-founder here at Zoe.

Jin and Tim, thank you very much for joining me today. So we have a tradition, Jin, which you may remember, where we always start with a quick fire round of questions, which come from our listeners. Both ready to go for it? Yes. All right, I'll start with Tim. Does the latest science suggest that intermittent fasting can improve my health? Yes. Can intermittent fasting improve my energy levels and mood? Yes. If I fast for most of the day, can I eat whatever I want the rest of the time? No, sadly not. All right, Jen, do most people get used to intermittent fasting within 28 days? Yes. Am I the only one that finds intermittent fasting really hard?

No, but I can help you with that. We'll definitely get onto that. Okay. And then finally, and you can have a whole sentence, will I enjoy food less if I fast regularly? No, you will enjoy it more. Okay. We had a lot of questions about that. Yeah. Jen, it's fantastic to have you back on the show. And the last time we recorded, you were actually in the middle of a hurricane. Literally. So we're doing this remotely, and we were just waiting for the roof to come off and the internet to go. So I think hopefully, now that we're in person, we should be safe. But the other thing which is even more exciting than having you in person is we have a bit of a world exclusive.

So since our last recording, Tim and our incredible team of Zoe scientists led by Sarah Berry have completed what's actually the world's largest study of intermittent fasting. That's so exciting. With more than 100,000 participants. Wow. So like a really, really big study. And so I'm very excited to discuss some of our initial findings, which will be the first time we've talked about this, you know, in this sort of large scale public setting. And I'm also very much looking forward to discussing this because I took part in the study myself. And it's the first time that I tried intermittent fasting. And transparently, I absolutely hated it. And so I did follow. I did the required week and I could not wait.

The day I stopped, I was so happy. But before we get into that, I'd like to just start at the very beginning, actually, with you, Jen. Okay. What is intermittent fasting? The words sound so scary. The word fasting makes you think that you're going to, like, 40 days and 40 nights wandering in the desert. But intermittent fasting, the word intermittent is key. You are having periods of fasting and periods of eating, which every single person who is listening or watching already does that. It's just changing the balance of that.

You know, you go to bed, you sleep, you wake up in the fasted state. Probably everyone listening has had fasted blood work before. So our bodies are already fasted every single day. If you live an intermittent fasting lifestyle, the difference is you just extend that period. Instead of most people having this much for your feeding time and this much for fasting, we just switch it. And so you're fasting for a longer period of the day, intentionally. And I know that there've been a lot of different types of intermittent fasting and quite a lot of evolution, I think, about the science and view about what's best. We just explain high level what those are and then

talk about what is in fact, I think today seems to be the area that people are most interested in, both scientifically and also in terms of just practically being able to carry out and find that they have good experience with. All right. Well, the whole umbrella intermittent fasting is any kind of approach where you might have periods where you're intentionally fasting balanced with eating. And the most common or what most people think of when they think of intermittent fasting is also known as time-restricted eating. Or if you're like really doing the research in the lab with rats, you might call it time-restricted feeding. But those are the daily eating window approach where every day you might eat within an eight-hour period or a six-hour period or whatever eating window works for you, and it doesn't have to be the same day-to-day. There's a whole other branch of also intermittent fasting under that umbrella that is alternate

daily fasting. People in the UK probably heard of it back in the day under the 5-2 name back 2012. That was a big approach, 5-2, 4-3. But most people who do intermittent fasting as a lifestyle tend to gravitate towards time-restricted eating, the daily eating window approach. That's what I do. And the 5-2 wasn't really a proper fast either because it allowed you to have like 500 calories or something. So I think we're talking about something completely different here. And the 5-2 has really gone out of fashion anyway. People realized it wasn't as good as it was cut out to be. So I think people are gravitating towards this way of eating, which really, as we said, isn't so much fasting as just changing your eating times. Exactly. Yeah. We want to focus on that. If they called it intermittent

eating or something like that, it wouldn't be so scary. But like, oh, fasting. Oh, no, I'm out. It certainly scares Jonathan. So we need to convince him that it's not scary. We can do that. We've had a few episodes talking about circadian rhythm with a number of sort of really great scientists recently. And I think that's definitely part of what seems to have played into this. And this is obviously on a 24-hour cycle. So very much focused on this idea, there's a time of day when you should do things and a time of day when you shouldn't. And it seems like that's played in quite a lot into this idea of thinking about eating in particular parts of the day on a sort of regular 24-hour rhythm. Well, I would just like to point out that y'all know from your research with Zoe, bio-individuality is a big component of what you're doing. It's the whole thing that y'all

are doing. And I think that also applies to circadian rhythm. You know, I work with intermittent fasters from all around the world who live intermittent fasting as a lifestyle. You know, I've worked with hundreds of thousands of members of my communities over the years. And you figure out what works for you over time. There are people who tend to have an earlier eating window or a midday eating window. But most of us tend to gravitate towards afternoon, evening, just as a practical way of living our lives. But we're not all the same. That said, within that, there's that variation. But I think what you're getting at is the fact that all humans have a circadian rhythm, which means their body has to

do stuff, active stuff in a certain time, and then rest stuff in another time. And all our cells are geared up for that. So it's become clear that this eating needs to be done at the same time as your body is in sort of activity mode and not in total rest mode. And I think this is also being linked to the circadian rhythms of our gut microbes that are having to deal with the food you're eating if it's at weird times. And this also goes to this idea that shift workers and people who are doing stuff out of sync have poorer health because of it.

So I think it's more and more we're realizing we have to go back to our origins. And this means eating in times when we would normally be active rather than very late at night particularly. Exactly, yeah. But everybody figures that out over time. You find out where your body prefers, what just feels natural to you. Although advertisers would love us to be snacking all night in front of the TV. That's true.

You know, they can make profits. It's not for the benefit of our bodies and our health. So that's brilliant. So having just explained this idea that it's sort of like you're on this 24-hour rhythm and talking about sort of shrinking the time when you're doing this eating. Tim, I actually think it's a brilliant time to talk about this brand new study, you know, this world's largest study on intermittent fasting called the Big If Study. Could you tell us firstly about the study, what happened, and then what have we found? Yeah, the big if, it's not a question mark. It's IF, study of intermittent fasting.

And it came out of a big health study we were running in the UK called the Zoe Health Study. And there were 246,000 people that were sort of eligible for this. We told them what it entailed, which was just monitoring what you eat for a week so we actually could document what the normal eating windows were. And then the idea was to eat within a 10-hour eating window, 14 hours of overnight fasting

and 10 hours of eating, which isn't a very strict one, but it is one that has been shown across a range of animals and humans to be effective. 37,000 of them completed the study. And how long did they have to do the fasting for? They did for two weeks. 27,000 were highly engaged and continued it for many more weeks after that, some of them up to about 16 weeks, and a proportion was still going. So, you know know we got people

hooked on it at baseline they weren't too bad that their eating window was about 11 hours so for many people they only shifted it by about one hour the people that did complete it they lost on average about 1.1 kilogram that's just over a couple of pounds in weight. But importantly, we saw that people who reduced that eating window most changed most. And we got increases in energy levels of 22%, mood improved by 11%. And interesting, hunger, which most people thought would have gone up, actually slightly dropped. So hang on, you just want to make sure I got that right. You're saying that for the people who did this, even though they were eating in a shorter time than before,

they were actually less hungry than they were before they restricted their eating. That's what they were reporting, yes. Which sounds like... Counterintuitive. Very counterintuitive. When we looked at the people who weren't doing this consistently, so they were sort of having one day of doing it, another day not, we found that actually things reversed.

Those people were more hungry and they had less energy. So clearly you do need some sort of stability to this to get these benefits. And we also saw the people that benefited most had an eating window that was largest at the beginning. Those people that maybe were eating over a 14-hour window, they benefited the most from this than the people that were, you know, marginally near it when they started. So the other thing we found is that people actually had some gut symptoms that improved. So bloating went down by 11%, which was interesting because,

you know, there's been some links between people with gut problems. Maybe some doctors would say, well, eat little and often, that's good for you. But actually, we found the opposite. Snacking also was reduced in people. So nearly everybody said they were doing less snacking, which is pretty obvious in a way, but it's nice to actually document that. So it's one less snack on average per person. We also saw differences in the early time restricted eaters versus the late restricted eaters. And this is something Jim knows a lot about. All the studies suggest that metabolically early is slightly better for you if you, say, stop eating at 6 p.m. rather than at 9 p.m. And they ate less snacks accordingly.

But interestingly, when we looked at our data about who completed the study, more people chose to do the late time-restricted eating than the early one. Which means they continue to eat into the evening, but they start, just to understand this, so they start late, so they might start eating at 11 o'clock and go through to 9 p.m., for example, rather than start at 7 and go through to 5. In the UK, that was a more popular option than the other one, which might be finishing 8 in the morning until 6 p.m. at night. So it was a two-thirds to one-third split on that, which I think is really interesting because it suggests that what we're looking for here is how do we get people to change their habits in a long-term way that's sustainable. So they need to choose one that suits them. So there's no point telling everyone, you must do this in these particular hours, otherwise it's

worthless. Exactly. And getting people to choose their own regime seemed to be effective. And that's really what we saw. So we had 37,000 people who completed this out of 148,000 who signed up for it. So clearly, some people didn't like the idea of it once they were forced. A bit like you, Jonathan, if you'd had the choice, you'd say, well, maybe I'll skip that one. But what we are seeing is about a third of people who weren't picked for their nutrition interest can do this quite effectively and benefit from it. And I think that's the real message out of it is people listening to this, you know, there's a good chance that you'll be in that group and that, you know, we should all be experimenting ourselves and trying it. Yeah. I think what's really important there is

you allow them to choose when they wanted to do it, and they naturally chose what felt right to them. And that's what we find in our community is most people do end up having the window that shifts a little later. You wake up in the morning, you're already fasting. You know, think about the history of eating and food and what we call things, you know, an appetizer wakes up the appetite. You know, the first time you eat in the day, that wakes up your appetite. So if you just delay when you open your window, it's a lot easier to keep fasting than it is to stop for the day and close that window when you're used to snacking. You know, for me, I stop eating usually around 7 p.m. every day. That's when I've had enough. You know, I'm not tempted to eat because I'm not hungry anymore between 7 p.m. and bedtime. Now, if I were told I must have an early eating window and I must open at 8 in the morning and then stop, I would be hungry again before bedtime. And that would not be a very

enjoyable lifestyle. Whereas I'm sleeping through the part where I would be the hungriest. I wake up and I'm fine. It's been a bit of a shock to me because I was definitely brought up with this assumption that you have to have breakfast. It's really important before you go and do something else. I think also the other misconception we were brought up with is you've got to have food inside you before you undertake any activity. Otherwise, you know, whether it's walking to school or going for a jog or going on your bike, you've got to have carbs up there, you know, to be ready for it.

And now we know that's not true. Yeah, and we did a fascinating podcast looking at this, only for men in this stage, and we're looking forward to seeing the data also for women looking at actually fasted exercise or not. And actually, this amazing fact that actually doing some fasted exercise could actually be beneficial for your health is one of the things that's made me more and more intrigued by this idea that it's not just crazy to… I do all my exercise fasted. Yeah, so do I. And I think, you know, actually many people now will get out of bed and the best way to do exercise is to not think about it and just do it.

And then, you know, once you've done that, then you might start thinking about food. But it's very different to maybe even 10 years ago when, you know, science and doctors were telling us, you know, you must have reserves before you can do this activity, otherwise it's all going to fail. So I think everything's being turned on its head. And this is why, you know, we need to be looking at how these sort of lifestyle changes fit into our culture and what we're doing and our way of working. You sort of gave us this overview of the results. And I know that these are the preliminary results. What about this for you

is surprising or where something sort of new has been discovered? Well, looking at the literature, I mean, a lot of the stuff on this has been done in mice. And clearly, it works super well in mice, but humans are not mice. We don't eat like them. We don't sleep like them. We don't sleep like them. And, you know, if you combine most of the studies to date that have been done, small numbers of usually young men, 600-odd participants in total, it hasn't really been broad. So I think it was the first time we'd done a pragmatic study of people to see how many found it easy, how many found it hard. Because the key part of this is not only does it work if you do it, but how easy is it to do it long term as a lifestyle.

And I think this sort of community science study is doing something very different. It's both assessing the acute science for a few weeks, but also really gives us real insights into how easy it is to continue it. And I think the surprising other results that other people haven't looked at are things like energy, mood, and hunger. Because there's been a lot of fixation around this, around sort of weight loss and sort of cardiovascular, like heart health and things like that hasn't there because it's very hard to ask a mouse, I guess, what its mood is or how much energy it feels it has. So these things feel very different from that, right? To say you could actually feel different energy in a couple of weeks seems, you know, as just a regular person, that seems completely different from anything to do with like your heart health or your weight? So to me, just looking at these results crudely, I'm seeing minor changes on things like weight.

You see small changes in metabolism and insulin and things like this. But the only thing I got from the other literature really that I don't think people have highlighted is that you are getting consistent reduction in inflammation. Now, inflammation is this activation of the immune system. And that's the one thing that stood out of all the studies is, on average, you're getting reduced inflammation from giving your gut a rest. And you are improving your gut microbes. And this has this effect all across our body. And this is why it's important for energy and mood

and our immune system in general. And small amounts of weight loss are a bit of a side effect for some people. They're not the main reason for doing this modest time-restricted eating because it is very modest. You know, we're only shifting people, a lot of them just by one or two hours in a day. It's not a huge difference compared to these trials, which have

often combined this with calorie restriction. This study was quite short term, right? So it's two weeks. And I think one of the things we talk about all the time on Zoe is like, you know, the only thing that really matters is like a long-term sustained change to life. So this is clearly like a test of something, as scientists often do, right? This sort of short-term to then learn about something long-term. So if people had stuck to this shorter eating window, based upon all the other science that's out there, what health benefits might they expect? And you mentioned something about inflammation. Could you maybe help us understand that and elsewhere? What would that actually mean for some of us?

Listen, it's like I want to make this as a permanent change to my life. If we see this reduction in inflammation and we assume that's going to continue, no reason to think any otherwise, then you would see small changes across a wide area of health. So that means that your immune system is working better, that it's reacting better, that your gut microbes are in a better state, they're producing better chemicals.

It means your mood is going to be heightened, you'll get less depression, less anxiety, you will feel more general energy, less tiredness, and you will have some small changes in perhaps your response to sugars, minor changes in your fat levels, reducing your risk of heart disease, etc., etc. So, you won't suddenly change your risk in a few weeks from being, you know, high risk to low risk. It's not like the silver bullet that cures all ills and you're going to live for another decade healthy life just because of the time-restricted eating. Absolutely not. But if you multiply that small difference in inflammation over years. Right. That's when it adds up. Which is what we want people to do. There's no point doing this for three weeks and

then stop it. No. You've got to find something that suits you and then do it for a long time. Realize how important it is to give your gut and your body a rest. And if you can just prune that, you know, those inflammation levels down, that will have huge effects on your risk of virtually every disease because we know that aging, related diseases, cancers, all these other things are linked to this baseline level of inflammation that, you know, through our poor lifestyle and, you know, the Western diet has made it worse. So, I think it's seeing a small change over really long periods of time that will bring these benefits. Plus, maybe some people will notice, you know, these energy and mood levels more than others. So, every time we talk about study, we're talking about averages.

And that means that, you know, some people are going to see more, some people are going to see slightly less. I remember when you first talked about this, I was like, that sounds completely crazy. Like I'd never heard about this. And that was, you know, that was after the start of Zoe's. This was maybe like four or five years ago or something. Jen was already doing her time restricted eating, but that was the first time. And at that point, there was some of the first data, I know you said,

was starting to come out. I think really all our mice to start with. Or 20 students or something. Sitting here in 2024 now, and you described this big study we did with Zoe, but there's a whole bunch of other smaller scale, you know, different studies of different prime periods. Like how strong

is the evidence around that improvement in sort of information you're describing and more broadly, like, where do you feel quite confident? Where are you saying, hey, look, this is exciting, but the research is still early and we're still going to learn more to get more confidence? Well, we're definitely going to learn more. We definitely need long-term studies following people up to see what's happened to them and look at their bloods years after they've been starting this, which doesn't exist. So at the moment, we're joining lots of dots from mouse studies to spore human trials to big community studies like the Big If study. But if you take everything together, basically, you're seeing nothing harmful about this for the vast majority of people, with a few tiny rare

exceptions, and great potential for benefit that we can't quite articulate at the moment how big those benefits will be. But I would certainly say to anyone, my level of confidence is good that you'll get some benefit from it. Whether that's tiny or large, I think remains to be seen over time. And it'll probably depend, your starting point, just like we saw in this, if you're someone who is snacking all the time, as many government guidelines tell you to do, let's be honest, you're eating all the time, and you do this and you can get down to a 10 eating window or less, the chances are you're going to see pretty major changes. Yeah, this is your breakfast at 7am and then like a beer at 11pm before you go back to bed, as you're saying, like that's sort of your worst extreme because you've

been stretching this out at both ends, probably because you're like, oh, I really need a beer. There are a lot of people who follow that, that eat from when they first wake up until the very right before they go to bed. I'm about to move over to Jen and talk about how we do this really practically. But just before we do, Tim, what does that mean for you personally? I tend to start eating at 10.30 or 11 in the morning. If I'm doing any exercise or workouts or whatever, I do those in the morning. I'm not hungry when I wake up, on most days, unless I'm jet-lagged. So at the moment, all bells are off, I'm pretty jet-lagged. And then I would finish eating or drinking anything other than black tea or black coffee at nine o'clock at night.

And I do that for probably five or six days a week. I'm not absolutely rigid on it because I realize that I want to sustain this long term and I don't want to feel like a failure if I'm giving that because I've definitely seen you drink a glass of wine after nine o'clock with me. So I know that you are not, which is one of the things I love like you're always like you don't have to be perfect in order to get moderation and moderation so i think you've got to realize that you want to do this as a long-term goal and we know that if you can do things five days out of seven you're doing pretty well but occasionally you know i might might be in France and there's an incredible breakfast buffet and I'm saying, oh, really? Am I going to miss all that? I've paid for it. It's

free. And so sometimes I just do it. And, you know, life is too short not to take rare opportunities as well. We ask ourselves, is it window worthy? If it's window worthy, the window gets longer. That's what we say. That's brilliant. So that means this is window-worthy. Is this window-worthy? This is so appealing. Yes, it is. I am going to eat it. I love that.

That croissant looks so good. Oh, that's window-worthy. It's my one croissant of the year. I'm going to have it here. That's right. So look, I think that's a brilliant discussion about where we are. Gin, I'd like maybe to start almost a bit with me. Okay.

So I went through this study. I was like, I feel like, well, everybody's been asked to do this study. I should try it as well. Yeah. And you talked about how there's like an average, and then there's always like all the individual responses. So I did this. I followed it really properly, and I found it really hard.

And my mood got worse. It did not get better. Were you hangry? I was not even sure that hangry was quite like, I think genuinely it felt like my mood was worse. So different from being angry, but actually like I think I was, I felt like less happy.

Like it felt like a real struggle. Yeah, I get it. I'd love to talk about the practical advice in general, but also maybe talking through the idea that it's not always just super, because I've heard other people say, oh, it's so easy. I just started doing it tomorrow and it was fine. And my own experience of this has been, no, I found it quite easy to do like the fasted exercise because that's just like sort of ad hoc in just the way that you described

interesting, which I was terrified of. I was convinced I would fall over and drop something heavy, you know, on my other toe because I hadn't eaten food. And it turns out that you can, to me, really remarkable. I think for anyone who's never done this, it's like, it turns out that you can sort of do all this exercise, even if it was something hard, like something heavy you're lifting or going for a run or whatever. And you'd be like, oh, I did that without any any breakfast and it felt exactly the same so that has been sort of eye-opening but actually managing this sort of time restricted eating for long enough to start to come out yes truthfully i couldn't do and the day that it was over because

you described that many people in the study decided to keep going i was like so happy to not have to do but point out that you and I are quite different, right? So when we go out, you know, you're always thinking about your next meal, right? So you've got this, you say, where are my snacks? You know, we can't have a sit-down chat without something to nibble on. I'm feeling judged now, Tim. Well, you know, you appear to be naturally a grazer.

Correct. And I think I'm naturally, this is how I am. And I'm a gorger, right? I'll sit down and have a really big, and I might eat more at my main meals. So it could be the way he was brought up. It could be genetic. It could be any number of things. But there are clearly differences between people. And that's maybe why in the big if study, a lot of people dropped out or didn't like the idea of it who might have been a bit like Jonathan. The way I'm hearing this, you're just pretty much feeding,

grazing throughout the day. And when your blood sugar dips, you feed yourself something else, right? Is that kind of what? So I've definitely changed what I eat a great deal. So I would say that I eat a very good Zoe diet in terms of the food that I'm eating. But I'm naturally someone who continues

to snack between meals. I'm eating, but I'm naturally someone who continues to snack between meals. I'm eating things differently. And I absolutely love eating dark chocolate and on the sofa in front of the TV when I finally stop work, which can be quite late in the evening. And I think this is great. Let's talk with me practically. How would you get someone like me to get started in this? Because I think lots of people are listening to this saying, okay, that sounds interesting, but we've also described, yeah, how do you get someone like me to get started in this? Because I think lots of people are listening to this saying, okay, that sounds interesting. But we've also described, yeah, how do you get started? And also to explain more practically, what does that really mean?

What are you allowed to do outside? So maybe take us through that, Jim, but then maybe with me also in mind what I might be able to do better if I try this again. Well, the key is becoming metabolically flexible. And that is when you are able to wake up, delay, and you're fine, and your body just uses the fuel that's already on hand. You know, we've got glycogen stored in our liver and in our muscles. We have lots of stored fat on our bodies. Even if we're lean, you still have fat stored on your body. And the key is, is your body going to tap into that,

or is your body telling you, all right, go ahead and send something down? And you're used to feeding yourself whenever you're hungry, and there's nothing wrong with that. It works well for you. You're a healthy weight. You're eating high-quality foods. But if you want to extend that period of time since you get some of those lowered inflammation, things like that, that Tim was talking about, it's just a matter of just kind of nudging it in a little bit. You know, I would stop a little earlier at night, nudge that breakfast a little later. You know, my husband never needed to lose weight. He starts right around when you do. He has his first meal of the day around 11, and we're usually done by about 7 p.m. at night, like I said. So, he does all of his eating between

like 11 and 7. Never needed to lose weight. He just found, you know, well, you know, I'm sold on the health benefits. He'd watched me do well, you know, I'm sold on the health benefits. He'd watched me do it. You know, I lost 80 pounds with intermittent fasting. I've been doing it for about 10 years now, and I've kept off the weight. But he said, yeah, I'm sold on the health benefits. I would like to have some time when I'm fasting. So he just skips the breakfast and goes about his day. The thing is, is that there is that adjustment period. And you were just right in the trenches of that. You know, I first heard about intermittent fasting in around 2009. And I dabbled in it for years and continued to get heavier and heavier because I would do it for a

couple days, and then I would take a few days off. My body never became metabolically flexible. I lived in the hard part. But you really have to get past that hard part where your body learns to flip that metabolic switch. That's terminology I got from Dr. Mark Matheson. Have you read some of his work? He was a Johns Hopkins researcher, and he wrote a paper called Flipping the Metabolic Switch, where he really talked about what has to go on in your body to really become adapted to intermittent fasting. And I love that terminology because that's really, boom, that's what happens. You're not metabolically flexible when you begin, and the more you've been a grazer, the longer you've stretched your eating,

the less metabolically flexible you probably are. And when you're saying that, you're meaning that instead of relying on there being sort of energy that's coming from food that you've recently eaten, it's suddenly I can actually tap into all these energy stores in my body that haven't just literally come from something I've eaten in the last couple of hours. Think about how we're designed. If you go back thousands of years, there wasn't a snack on every corner. People had to be metabolically flexible. You had to be able to have sustained energy to go hunting or gathering. You had to be able to make it. Your body had to be able to say,

all right, no fuel's coming in. I can access the fuel I've already got. Can you imagine if people were like, well, I can't go hunting because I'm too tired. I can see you're in big trouble. It's like there's not enough food to go out and get any would be a big problem. I can see that. Yeah, you wouldn't live trouble. It's like there's not enough food to go out and get any would be a big problem. I can see that.

Yeah, you wouldn't live long. Tim's laughing here. I lived with this hunter-gatherers for a week, you know, 10 years ago. And, yeah, they don't eat anything before 11 o'clock in the morning. See, they just do it. Nobody told them to do that. No, and they don't really eat after 7 p.m. These are our ancestors, you know, from East

Africa. So I think it makes sense that our bodies were adapted for that and that your pattern of eating has adapted the fact we, you know, we're surrounded by food all the time and that it's not actually a necessity of your body. Right. So Jin, talk me through, imagine, start at the beginning, like day one, and I know that this is something you cover in your latest book. Right. So, Jin, talk me through. Imagine, start at the beginning, like day one, and I know that this is something you cover in your latest book. Right. So, someone's listening to this, and they're like, this sounds interesting, but I don't actually really have any understanding of what I'm supposed to do. What do you need to do? Well, the book is called 28 Day Fast Start Day by Day, and it's for people who really need, like, what do I do? It's day one. How do I start?

And you get to pick how to start. Everyone isn't starting at the same place. You start at the beginning. You take a quiz, and you figure out, am I a rip-off-the-Band-Aid person, or do I need to ease in, or am I somewhere in the middle? And then every day, I tell you exactly what your eating window is going to be for that day. You're starting off with a longer eating window at the beginning, week one, and then you gradually shorten that. Depending on which plan you follow. Everyone is shortening it by the end. With the goal of you're teaching your body

to become metabolically flexible, you're adapting, you're pushing through those hard parts. So there's a sort of step-by-step reduction you're describing. So you don't suddenly jump from like, I eat for 14 hours and suddenly jump to eating for eight hours the next day. No.

What does eating mean? Because I think we sort of referenced the side, hey, you know, if you go and have a beer, that actually counts as eating. But could you help people understand what is eating and what isn't? So when you're fasting, you are having black coffee, nothing added to it. We're having plain water, no flavors, water enhancers or anything like that. And we're having plain tea, regular tea, not an Earl Grey, because Earl Grey has got flavors added. It's got the bergamot, wakes up people's appetite. So, we're sticking to plain water, black coffee, plain actual tea during the fast. No herbal teas. Well, here's the thing about herbal tea. We want to stick to

something that has a bitter flavor profile. We're trying to prevent an insulin response, and the cephalic phase insulin response can happen if your brain perceives that something's coming in that's got a hint of sweetness. So, you know, if you're having like a raspberry or apple cinnamon. So, definitely no sweetener or anything like that. No, you definitely don't want anything like that. Even though it doesn't have calories in it. Right. You're saying that this is your practical experience. It's a practical experience for people that have struggled with intermittent fasting. And then they're like, well, you know, I saw a video that said I could put a little splash of almond milk and it didn't matter. Or I could have, you know, zero calorie sweeteners in there. Or I was drinking a lot of these herbal teas.

And they found fasting to be very, very difficult. Black coffee, nothing added. Plain tea, plain water. You know, nothing added, plain tea, plain water. You know, just that little bit of lemon in there, it's going to make it harder. So, when you were doing the fasting part of it, were you drinking just black coffee, just plain water? Earl Grey tea, probably. Earl Grey tea, Earl Grey tea, but nothing else. I thought I was doing very well. Earl Grey has the bergamot oil in there, and it's got that citrus flavor. You just made it harder for me now, though.

I can't even have that. No, it's going to be so much easier. Here's just an example from my life, and it's truly just an anecdotal, but this is just an example of that. So I was at a restaurant with my son and about to eat, but I hadn't eaten yet. So I was about to open my eating window, and I'm just fine. When I'm fasting, I'm just fine. So, I order a club soda with lime, start drinking my club soda with lime. Suddenly, I could, like, have eaten my arm off. I was like,

I have got to eat now. I am starving. That little bit of lime in that club soda, you know, I know what it feels like to fast clean because I've been doing it for so many years. But just that little thing that you might think, how could that possibly make a difference? It absolutely does. And is it just in your mouth or do you have to swallow it? The cephalic phase insulin response, these were rodent studies that they did, but it was fascinating. They painted the oral cavity of these little rodents with artificial sweeteners. So it's like their mouth. They painted their mouth with it. They didn't even swallow it. They just tasted it. And they had a cephalic phase insulin response. That means their little rat pancreas released some insulin in response to that.

So then in part two of the study, which was not as humane, they snipped the nerves from the tongue to the brain, repeated it. The rats could no longer taste it because those nerves were severed. They did not have the cephalic phase insulin response. So it was the taste that made the difference. And what does insulin do? Insulin lowers your blood sugar. Why is your body releasing insulin in response to these sweet flavors or these food flavors? Because our brains know that sweetness is coming from honey or fruit or whatever.

Our body doesn't understand. So they think you're going to eat a raspberry, even if it's just raspberry tea that doesn't have any. So your brain says, we know fruit. Fruit has a sugar load coming in. We're going to need a little insulin to handle that. And so then your pancreas releases the insulin, only you've had like zero calories. So then it actually could lower your blood sugar and make you shaky and you feel worse than if you hadn't had it. So that's the cephalopathy. And your body might think you're hungry then. Then you're starving because your blood sugar has crashed. And if you're wearing a CGM, you could actually see it as a blood sugar crash after having that cephalic phase insulin response.

Coming back to thinking about this day by day, this really clean fast outside of the hours I'm deciding to eat, what else do you need to know in order to make this transition? Well, that's it. You just fast clean and you just work your way in to what in order to make this transition? Well, that's it. You just fast clean and you just work your way into what feels good to you. We don't expect weight loss for the first month because your body is learning how to do something new. Sometimes people find, because you're not metabolically flexible yet, you're not fat adapted, we call it. You're not really tapping into your fat stores very well because your body hadn't had to do that for a long time if you've been eating

like most people eat. You actually might overeat in your eating window at first because you're not well-fueled during the fast. Your body can't really tap into your stored fat. So, you open your window and you're like, I'm starving. And then you just overeat. That goes away once your body flips that metabolic switch. So, don't get anxious about that if you feel like you're overeating. Right. You're like, this is never going to work. I open my window and I'm eating too much. Eventually, that settles down. Yeah, I definitely feel like, I don't know anxious about that if you feel like you're overeating. You're like, this is never going to work. I open my window and I'm eating too much. Eventually that settles down. I definitely feel like, I don't know if that's common,

I feel like I had some anxiety about Tim's laughing, but I had some anxiety as it's coming towards the end of the window. Right. That I might not have eaten enough food. You might not survive the night. Not that I might not survive, but that I might be hungry and it's going to affect how I feel. Well, here's the funny thing about that.

Because normally I don't worry about that. If I still want to eat more food, I can get more food, right? You don't have to eat for future hunger. That's the beauty of it. You close your window. You're not eating for future hunger. You go to bed. Actually, eating later into the night is going to make you hungrier earlier than if you had stopped sooner.

So, like, I better eat a lot because tomorrow morning I'm going to be fasting. Actually earlier than if you had stopped sooner. So like, I better eat a lot because tomorrow morning I'm going to be fasting. Actually, we'll make it harder for you tomorrow morning than if you hadn't. Quite counterintuitive. Yes. And how long would it take you to get Jonathan into shape then? Well, if he's going to fast clean now and not have the Earl Grey, I think you're going to be amazed. So how long do you think it would take me to switch from saying this is really hard to saying I'm starting to get all those benefits of mood and energy. Two weeks. Yes.

Like, I'm having hot water in a mug right now. Just plain hot water. Just wake up and have that. Or you're used to a little caffeine in the morning. You could have some black tea. I stick to my black coffee. I bet you could do it right away. Well, I think you're challenging me.

I am. I am going. I'm willing to give it two weeks. I can't do a whole month. I'm willing to give it another two weeks. Give it two weeks. Well, I think you're challenging me. I am going, I'm willing to give it two weeks a whole month. I'm willing to give it another two weeks with the clean fast. Just try to wait till 11. See if you can wait till 11 and imagine, you know, your body is doing all these great things. So that really helps instead of thinking, can I eat yet? Can I eat yet? You know, checking your watch, think I am giving my body some time to have some digestive rest, my gut microbes are having some downtime. I've always found when I did more extended fasts that the busier I was, the easier it is. Absolutely.

So I would never suggest to anyone to start at a weekend. Right. Stay busy. Because that's the worst time. Yeah, you're staying busy. Is that your experience? Oh, absolutely. Right. If you have time to sit around and mope about it, that's when you're like, well, I've got nothing to do. I sure would like to have a muffin, right? But if you're busy, you don't even think about it. Yeah. And what about the length of time, Tim? What's the evidence there? Because I know this is an area that's still up for a lot of discussion. But for example, I'm probably eating 14 hours a day. Not every day, but there's definitely a whole bunch of

days when I'm eating 14 hours. I'm understanding from quite a few scientists I've been talking to recently, that's probably not ideal. But on the other hand, I think you're definitely not sitting here saying people should eat all their food in four hours. We don't have big enough studies in a wide enough group of people and not enough, particularly women in our studies. Although, you know, we were lucky in the majority were women in the big if study. But it looks like the sweet spot for when the results start sort of turning significant is around at this 10-hour eating window. So if you can get to 10 hours, that's

fine. There's some evidence that maybe 11, you know, could be okay. And there's probably differences between people. So we're talking averages here, which might vary at different ages and between men and women. We don't really know yet those differences. So at the moment, I think it's do something that's sustainable. So if you can have a 10 hour eating window, that appears to be sustainable for years, because it's what our ancestors did. That's pretty easy. But for some people, they might want, you know, to get it down to eight hours. Others might be okay at 11. And to flip it on the other side, my sense from you and Sarah and Barry and others is like, over 12, you seem to be saying like, I'm really skeptical that that is a good thing for you. Correct. Correct. I think that that's right. All the data suggests that there is a linear relationship

between how long you can do it for and in terms of the biological benefits. But at the same time, it gets harder to do it for most people. So it's this trade-off of where these two lines go. Does that mean you're pushing that if somebody could eat all their food in one hour, that would be better? No, we're not going to, we don't recommend that day to day. No, I mean, we know that people who only have one meal a day struggle to get enough calories or vitamins or nutrients. And so for the vast majority of people, that is harmful. And so many athletes and things have tried that, and it fails miserably. We do not want to have a really short, short, short, tight eating window.

Now, how you define really short and tight depends on the person. You know, I'm someone, I've been doing this for a lot of years. I'm about a five-hour eating window. That is my sweet spot. That works really well for me. I'm able to nourish my body well. I'm able to maintain my weight. So, I think really the key is what are someone's goals? You are not attempting to lose weight. You do not need to lose weight. It would not be good for you to lose weight. But if someone had a great deal of fat to lose, they have to give their body time to burn more fat.

So that's when you might find shorter eating window. But still, no, one-hour eating window every day is definitely not something we recommend. So a 10-hour eating window. And then the question is, should you do that early or late? And the evidence says, well, if you find it easy, do it early. But if, like me, you know, it doesn't fit in with your life, et cetera, and you go out a lot in the evenings and you socialize, then it's better not to try and force it.

On average, it might be better to have it start early, but actually in reality, if it's the thing that's most important, it's a bit like we say with everything else, it's like what's sustainable is what's healthy. And that's why two-thirds of people in the Big If study picked the late eating window as, you know, easier for them to get on with their life. So I think there's always this difference between the theory,

what happens you can do to mice or to paid students in a laboratory, and what happens in the real world. And this is what we're discovering. What are people really going to do? Because that's what matters. If you can't do it and you don't enjoy doing it, you're not going to do it. Exactly. And if you went on holiday to Italy or Spain, you wouldn't be able to eat in the evenings.

You wouldn't find a restaurant. So I think there has to be some flexibility built in related to culture and work practices and where you are in the world, your family and all these other considerations. But I think what we're giving people is a blueprint here that they can adapt this science to their own lifestyle. Right. Can I come back to one of the questions at the very beginning? Because we had this from a number of listeners, which basically said, you know, if I follow this really strictly, can I eat what I want? Can I eat whatever I want? The answer to that is yes, but also no, right? And it's really on

the emphasis. You know, if you think about eat whatever you want, that implies that you're going to go crazy and you're just eating everything. That's not going to work very well for health. It's not going to work very well if you want to lose weight. But you can eat whatever you want, meaning if you're someone who does better on, you know, more whole food, plant-based approach, you can absolutely do that with intermittent fasting. If you're someone who is right now eating the standard American diet and you haven't changed your diet yet, you can implement intermittent fasting, continuing to eat what you're eating right now, knowing that you're making positive benefits.

But then a funny thing is going to happen. Most people who start off eating whatever they want, standard American diet, over time lose their taste for that. They find that, hmm, what I want is different now. Mine certainly did. When I started in 2014, I was eating the standard American diet. I was obese. I was 80 pounds heavier than I am now. Mine certainly did. When I started in 2014, I was eating the standard American diet. I was obese. I was 80 pounds heavier than I am now. I liked fast food. I liked to take out. Now, I don't eat that way. And it's happened naturally. My body has directed me towards more nutritious food. And so, you feel that the long periods of not eating have helped you, as it were,

to sort of fight back against some of maybe this ultra-processed food and these other things? Absolutely. Is that what you're sort of suggesting? It's a very common thing we see in our intermittent fasting community. People suddenly are like, huh, I tried my favorite coffee creamer that I used to enjoy. And now, you know, they're drinking their coffee black during the fast, but they had it during their eating window. And they're like, and it tasted like poison. I no longer like it. You know, we lose our taste for things that we used to enjoy. It's pretty much something that happens almost universally over time. Yeah. I think the question goes to, well, do I not have to worry about calories, I think is the sort of thing,

because if you're doing this intermittent fasting, does that mean you're off the hook and you can have unlimited calories? No. No is the answer. You know, donuts are not the answer. Right. Or the reward for this. But it just means that you still have to eat the same way. It's a healthy, plant-based way. The Zoe way, absolutely don't count calories because we know long term that's not a good solution. But still worry about food quality.

It's a compliment to quality, but it doesn't suddenly overrule that. Absolutely not. And as June's saying, by giving yourself a bit of a break, you perhaps are going to feel more the quality of your food than you've done before. By having black teas and black coffees, you are perhaps introducing more bitterness into your flavor systems in your brain, and you will naturally just slightly be lowering the need for sugar and things like this. You're exactly right.

I really think that having the black coffee, the plain tea, I mean, our taste buds change over time. They regenerate. You got new taste buds in a few weeks. And so if you train your palate to enjoy the black coffee, enjoy the black tea, suddenly you're like, oh, a Brussels sprout, that's delicious. Because you tolerate those bitter flavor profiles from the vegetables more. One of our previous guests that we had

recently, Professor Karen Esser, who's focused on circadian rhythm, particularly within muscles, was concerned that for people over 70, they might not be able to eat enough or get enough protein within a time-restricted period, despite actually being quite positive about this sort of more broadly. I was really interested in your thought and then also, Jin, your sort of practical experience across. But for this group, you might be saying, hey, this is a general rule, but actually here are categories where you would think differently. I think it's something you think about, but for the majority of people, it's not a problem. I think if you are both trying to lose weight and you are exercising a lot, then having too dramatic a change could be problematic in that small group.

But I think what we're talking about here, these rather modest changes, changing your eating window from, say, 12 hours to 10 hours, is not really going to affect anybody. I think only if you're going down dramatically would that have an effect because you wouldn't be able to get enough calories or protein during the day in that eating time. And that's perhaps what she was referring to is these more extreme, extremes of these fasting periods. But for what we're talking about here, which is very modest

changes that would last years, people would be getting all the nutrition, in my opinion, that they need. And one thing we find over and over again in the intermittent fasting community is that we become so much better at listening to our bodies. You know, we have built-in hunger and satiety signals that drive our eating. And protein is an example. You know, I eat a lot of plants these days. But every now and then, I'm like, I really need some meat today. My body lets me know. If I eat too much meat, I start to feel sluggish. So I don't worry about, well, am I getting enough protein? Because I really feel like my body lets me know. You know, people are not really satisfied until they've had enough.

And your body will tell you. I think that's absolutely right. As long as we're sensible about it, as long as we have a whole variety of foods, that we're not excluding large areas of foods, and we're not trying to go into a tiny window and at the same time lose a lot of weight, then I don't see this as a problem. We'll have plenty of protein in plants and other things as long as we get that variety. And we're not so limited that we haven't got enough time to eat all we need. And I wanted to add, you know, working your muscle is important as you get older. So I've become a lot more intentional when it comes to working my muscles because use it or lose it, right? Eat protein, eat sufficiently within your window,

but you also have to be doing activity. I'm an elderly person now. Are you elderly? Very elderly. Definitely does not look elderly. But, you know, in my 60s and, you know, I've practiced this for years and certainly only seen benefits. So I'm not seeing my muscles fading away. So it sounds like you're saying be pragmatic about particular cases where people may be trying to lose weight or you see that they're losing weight.

Or they're sick or they can't go to that. Yeah, obviously there's common sense. But in general, again, because you're not talking about very narrow windows where it's sort of impossible to eat. You nourish your body well. My book that I wrote in 2020 is called Fast Feast Repeat. It's not fast, eat a little diet meal, repeat. You know, we want to nourish our body well. I think that's a beautiful place to stop this time.

Let me try and summarize, and I know you're both going to keep me honest. We started by saying sort of what is intermittent fasting? And today in 2024, that's mainly this idea with this other term of sort of time-restricted eating. It's saying, I'm only going to eat for a certain number of hours each day. And in general, it's going to be on sort of the same cycle each day. And then there's discussion about how long, which could be, you know, as long as maybe 12 hours, in some cases down to, you know, six to eight or something at the other end. And Tim, I think you said, like, there's really nothing harmful for most people and great potential for benefits. So it's something that

we shouldn't be scared of. And equally well, it's not going to solve everything either. But it's something that is quite exciting. You then share the results from this brand new study, the Big If study that we did at Zoe with 148,000 people who participated, which was a 10-hour eating window, which from Jin's perspective is still a relatively wide window, which means like 14 hours of not eating. It was for two weeks. And what you saw was there was a small amount of weight loss, which is like two pounds, 1.1 kilogram, which you said is a minor change. But the thing that was sort of more exciting is you saw these improvements in energy and mood and a fall in hunger in just two weeks on this very large sample of sort of

real life people. And that interestingly, it worked particularly well when they were consistent in their patterns. People were more inconsistent, actually ended up more hungry or less energy. And then there was people like me who just were like, this is miserable. And that practically speaking, again, you saw that people who got this to work and stick tended actually to be having their eating later in the day, which is a bit at odds with some of the guidance about health. The reason why we believe this might be working is we think that this is lowering inflammation in the body because it's sort of not being forced to sort of work at times when sort of we're evolved, you know, not to have to deal with all this stress from food and that

this small change in inflammation over time can really add up. So it can really affect your health from your immune system to then affecting like every part of your body. So it really could add up and improve, you know, how long you have quality of life. Jin, I think you talked about then practically, what does it mean? Critically, if you think about it, like you need to accept there's a period of transition. So you can't just switch tomorrow and find it easy. So I was probably expecting something too easy because you need to move to what you described as metabolically flexible, which means being able to just do your normal stuff off your reserves rather than off food,

that in general people find it easier to continue their fast in the morning. So if you're trying this transition, it's a lot easier to say, instead of eating breakfast at 7.30, I'm just going to shift it later. Secondly, you need to be very strict about what you're doing outside of the eating window. So clean fast is what you said. So I can have black coffee, plain tea, but I'm not even allowed like my Earl Grey or my herbal tea or any of these sorts of things. Keep all that in your eating window. You could have hot water, you can have coffee, but that's basically it. Not to worry too much if you're overeating during the fasting period at first.

It will balance out. And then also to recognize, though, it's not like this magic bullet. You can't like eat unlimited calories during the eating window and expect to magically not put on weight. You have to continue to think about the quality of food, right? Like ultimately, this is not going to outweigh the fact that you need to eat food that is really healthy for you rather than, oh, it's fine. I can go and eat my donuts because I'm only eating for four hours a day.

And then maybe to finish, we talked about sort of the eating window. And I think my takeaway, and Tim, correct me if I go this wrong, is like over 12 hours is not good, which is actually similar to the advice that Sachin Panda gave on the podcast recently. Really short is also not good.

So you're not – neither of you are believers on like one meal a day is the secret to health. Now, can I pop in there about the idea of one meal a day? Yeah. I loosely think of myself as one meal a day but over five hours. Think about it as a multi-course meal spread out over like, I have an appetizer and maybe a salad course. It's a feast. It's a feast, right? But we're not talking about one hour. No, no. Got it. And therefore, I think, Tim, you said this, your sense at the moment is a sweet spot from like both health and sustainability is about 10 hours, but still the data is quite early.

And so, and it might vary between people. So, this is just very much average and we don't yet know for different ages and men and women and people with different conditions, but that's the current state of the evidence we have. And I can just tell you anecdotally, it absolutely varies from person to person. We are each a study of one when it comes to figuring out what works best for you. There is no like, here's how everyone should do it for intermittent fasting. Yeah, so that's why, you know, don't beat yourself up if you can't get exactly that,

if you need an extra hour and that works for you, that's absolutely fine. I think it's all about listening to your body. And this is really what we're trying to get people to do is not, don't listen to dogma, listen to your body. Love it. I think that's a wonderful place to stop. Thank you both very, very much.

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