What does science say about intermittent fasting?

Intermittent fasting. It seems like every day, someone new is talking about it. But what is it, exactly? 

There seem to be so many options — from the 5:2 diet, with 2 days of of extreme calorie restriction each week, to the warrior diet, which involves eating only raw fruit during the day and a mammoth feast at night.

Whatever the approach, intermittent fasting involves restricting the window of time when you eat.

Supporters can be evangelical about the benefits, promising weight loss, disease prevention, and even life extension. 

Currently, the scientific evidence is unclear, but it's an exciting area that may be full of potential. 

In today’s episode, Jonathan speaks with Gin Stephens, who has had a powerful experience of intermittent fasting transforming her health and weight. He also talks to Tim Spector, who will share what science can tell us about intermittent fasting today —  and, interestingly, how much it can’t, yet. Plus, an exciting announcement about how this is set to change.

  • Gin Stephens is a New York Times bestselling author and podcast host.

  • Tim Spector is a co-founder of ZOE and one of the world's top 100 most cited scientists.

If you want to uncover the right foods for your body, head to joinZOE.com/podcast and get 10% off your personalized nutrition program.

Episode transcripts are available here.

Join us for the world’s biggest intermittent fasting study at joinzoe.com/fasting.

Follow Gin, and get her books here.

Follow ZOE on Instagram.

This podcast was produced by Fascinate Productions.


[00:00:00] Jonathan Wolf: Welcome to ZOE Science and Nutrition, where world-leading scientists explain how their research can improve your health.

Intermittent fasting, it seems like every day, someone new is talking about it. But what is it exactly? There seem to be so many options: from the 5:2 suggesting two days each week of extreme calorie restriction to the Warrior Diet, which involves eating only raw fruit during the day before a mammoth feast at night. Whatever the approach it involves restricting the window of time when you're allowed to eat.

Supporters of intermittent fastening are evangelical about the benefits. Promising weight loss, disease prevention, and even life extension. Currently, the scientific evidence is unclear, but it's an exciting area that may be full of potential.

Today's guests certainly think so. Gin Stephens has her own powerful experience of how intermittent fasting transformed her health and her weight. She has written two books on the subject, Fast Feast, Repeat, and Clean(ish), and supports a global community of fasters. She's here to tell us her story.

We're also joined by Tim Spector, one of the world's Top 100 most-cited scientists and my scientific co-founder at ZOE. Tim will share what the science can tell us about intermittent fasting today, and interestingly, how much it can't tell us - yet. He has an exciting announcement about how this is set to change.

Gin Stephens and Tim Spector, thank you for joining me today and Gin, thank you for joining us despite being in the middle of a hurricane.

[00:02:02] Gin Stephens: Yes, we are in category 1, hurricane Ian. Thank goodness it's not gonna hit us like it hit Florida because I'm on the South Carolina coast, but it's still very windy, and lots of rain.

[00:02:14] Jonathan Wolf: But you are okay? 

[00:02:15] Gin Stephens: I'm fine. Yeah. I'm on the third floor, so whatever happens, I should be okay. 

[00:02:19] Jonathan Wolf: Alright this is a first for the ZOE Podcast and, Tim and I are sitting in London are thinking, Well, you know, maybe it rains a little bit today, but we're definitely not used to a hurricane, so we are very impressed that you've joined us.

[00:02:30] Gin Stephens: Thank you. And I'm not used to them either. We just moved to the coast in May, so this is our first time. But it helps me understand why people have hurricane parties because it's definitely nerve-wracking even as a category 1.

[00:02:41] Jonathan Wolf: You're making us a bit nervous just hearing about it and seeing you here.

So let's start before the hurricane cuts off the electricity with our quick-fire round of questions from our listeners and Gin, I have three questions for you to start with. So the first is, in your experience, can intermittent fasting have a big impact on people's health and weight? 

[00:03:01] Gin Stephens: Yes, it is literally the most powerful thing I've ever done for my weight and my health.

[00:03:07] Jonathan Wolf: All right. Will I feel weak and hungry if I start intermittent fasting? 

[00:03:13] Gin Stephens: Well, the answer is yes, and then no. 

[00:03:16] Jonathan Wolf: We'll talk a little bit more about that, I am sure. And then thirdly, in your experience does intermittent fasting lead to weight loss for everybody? 

[00:03:24] Gin Stephens: Weight loss is multifactorial. I love that word because our bodies are complicated.

So intermittent fasting is a great health strategy, but you might need to do some tweaking. For example, your gut health, what you're eating, other things, hormones, all of those play a role. Intermittent fasting has a lot of powerful things that it does in the body, but it doesn't fix every single problem you might be having, but you can tweak it till it's easy and find your magical weight loss solution.

Well, I don't wanna use the word magical, but you can find your weight loss solution. 

[00:03:55] Jonathan Wolf: And I told Gin she was allowed one sentence. I didn't realize she was really good at long sentences. So I'll title this up from the next episode. Tim, are the health benefits of intermittent fasting proven? 

[00:04:07] Tim Spector: Yes. Although we don't know what goes on long term, so definitely short term.

[00:04:14] Jonathan Wolf: Are there risks from intermittent fasting? 

[00:04:17] Tim Spector: There are some risks, but minimal if you are fairly healthy and it doesn't last very long.

[00:04:22] Jonathan Wolf: And finally, we had a lot of questions about whether intermittent fasting affects men and women differently, particularly due to female hormones and menopause. 

[00:04:30] Tim Spector: We simply don't know.

We don't really have enough data at the moment to answer that question. That's why we need bigger studies. 

[00:04:38] Jonathan Wolf: And I think that's actually a perfect way to introduce some very exciting news. Right, Tim? So, ZOE is actually about to launch the world's biggest-ever intermittent fasting study, and part of that is because we don't know most of the answers right to these scientific questions.

Tim, can you tell us what's going to happen?

[00:04:55] Tim Spector: Yeah, up to now most of these studies have been done with about 50 people, followed for a few months in very tight conditions, and no one's really looked at thousands or hundreds of thousands of people in real-life scenarios, in their real environments, day jobs, you know, looking after kids, going to work, et cetera.

So what we are doing with ZOE is getting this massive community, these citizen scientists who are already signed up for the ZOE Health Study, and others who might want to join to participate in this mass intervention.

For several weeks we ask people to shift the time they're eating, not to necessarily calorie restrict, but just eat their meals at different times so they are within a smaller time window and we're looking at 10 hours is around where we're aiming at, and we want this to be done as well as people can do it. And then just look at the real-life results and see how many people feel better. How many people feel energized? How does it change their lives?

And some people find it really difficult and work out why that might be. So it's a real-life study On a scale a thousand times bigger than has been done before. That is gonna tell us enormous amounts about how this might work as public policy for all kinds of people, whether they're young, old males, females, menopausal on HRT, all these kinds of questions we could answer.

And it's super exciting and I can't wait to get started.

[00:06:27] Jonathan Wolf: And Tim, it does sound super exciting. I'm excited as well. How big a study are you hoping to get here?

[00:06:32] Tim Spector: I'd love to get at least 50,000 people doing this. And our estimates are that there are plenty of people out there really keen to do this kind of study and we are gonna start in the UK and then open up to the US and then hopefully the rest of the world if it all takes off.

Because it'd be lovely to see how this works in different scenarios, in different food cultures, in different places where people eat meals at different times, et cetera. So I think it's gonna be, you know, one of the most exciting projects I've ever done in my career.

[00:07:04] Jonathan Wolf: Amazing. And if you want to participate, then we are gonna give you the website link right now, which is join ZOE.com/fasting.

That's F-A-S-T-I-N-G. And you can go there and you can get the instructions for how to sign up for if you're in one of the countries we're not yet supporting how to put your name down, and hopefully do that as we expand it.

[00:07:25] Tim Spector: And just to say it doesn't cost anything, so you know, there's no signup fee. You don't have to buy any special meal kits or anything, no special recipes. All you gotta do is really just say, I wanna find out if this makes me feel better or worse and help science. And that's all you gotta do. Just keenness is all we're after. 

[00:07:44] Jonathan Wolf: Absolutely, and I'm particularly interested because I think I should make this admission at the beginning, Gin: I'm a terrible snacker late into the evening. So generally my diet is pretty good, transformed over the last five years.

But I have to say that dark chocolate on the sofa at about 10:00 PM is probably one of my key vices. So I've always been too scared of being hungry to commit to intermittent fasting. So I think this conversation might be enough to give me the motivation to sign up and take part in this study and figure out how it works.

[00:08:15] Gin Stephens: Well, definitely don't be scared. There's a saying, I don't know who created it, but diets are easy in contemplation and hard in execution, whereas intermittent fasting is the opposite. It's hard in contemplation, but easy in execution. So I think you'll, you'll find that it isn't anything to be afraid of.

[00:08:34] Jonathan Wolf: Wonderful. And look Gin, why don't we just start actually today with your own personal experience? I thought that was a really interesting way to approach this whole topic. So what is intermittent fasting and how has intermittent fasting impacted your life? 

[00:08:48] Gin Stephens: Well, intermittent fasting is not a diet, and I want to get that right out there because your diet is what you eat.

You know, everyone has a diet that they're eating, whether it's the standard American diet or the Mediterranean diet, or whether you've gone through, you know, the ZOE Protocol and now you're eating according to the scores that you get on ZOE. But everyone eats the way that they eat. Intermittent fasting is about when you eat.

So however you're eating right this minute, whatever that is, you can add intermittent fasting to it. The most common form of intermittent fasting that most people end up as a long-term approach is one we call time-restricted eating. And that's actually what your study is going to be. It's a time-restricted eating study with about a 10-hour eating window.

In the intermittent fasting world, you know, there are a lot of different ways you can choose to structure your eating window. You know an eight-hour eating window became really popular, maybe 10 years ago, there was a book that came out called The 8-Hour Diet. There are other eating window links you might enjoy. I tend to be someone with around a four or five-hour daily eating window.

Now someone might be listening and thinking, Oh my gosh, that sounds so terrible, but it's actually over time, you know, I like to call it, you tweak it till it's easy and I've found a rhythm that feels really good for me. I don't eat until late afternoon every day, and I have great energy, and great mental clarity all throughout the day.

Then I open my eating window. Usually, I have a really hearty snack, and then later I have just an amazing dinner. I eat the foods that make me feel great, that are delicious, and that is satisfying. You know, I might have a little something sweet to close my window. So you see there. There's nothing wrong with having a little dark chocolate on the couch.

Then you close your eating window and then you know, I go to bed and I am able to sleep really well, and then wake up the next day and do it again. And it's just such a great way to live. Because I'm not counting macros, I'm not counting calories, I don't feel restricted.

And over time I've naturally gravitated towards eating the foods that work best for my body. It's very different than how I started. You know, I was eating the standard American diet, and eventually, my body let me know it was not how it felt best.

So intermittent fasting has been the most powerful thing that I ever have done in my entire life. I struggled with my weight and even was obese up to 2014, I weighed 210 pounds, which is a lot of weight for a five foot-five woman. And you know, my waist circumference was huge. And thanks to intermittent fasting, I was able to lose over 80 pounds, and I've been keeping it off since 2015, which is truly the most remarkable part. And so aside from the weight loss, you know, my health is also been transformed.

I'm 53 and I literally feel better at 53 than I did at 33. You know, thanks to the health benefits and the longevity benefits of intermittent fasting, I believe I'm gonna age well and I'm really looking forward to all that's coming my way.

[00:11:47] Jonathan Wolf: Fantastic. It's a brilliant story Gin, and I think a bunch of people listening will be saying, Well, I'm still a bit confused about what intermittent fasting is.

So you talk there about sort of this time-restricted eating, which I think is increasing what people talk about, but could you maybe just explain maybe some of the other things that people think about when they think about intermittent fasting and to what extent those are still, I guess, popularly done or whether this is really time-restricted eating and intermittent fasting are now sort of the same thing for most people?

[00:12:16] Gin Stephens: Well, intermittent fasting is a big umbrella word now, and it's just the way that the terminology has become to be used. Now. Some people are like, No, intermittent fasting is only if you're doing longer periods of fasting intermittently.

Right? But, really intermittent fasting is any time that you are purposefully having periods of time where you're fasting, and then periods of time where you're eating. It might look like alternate daily fasting where you have a day of fasting, and a day of eating, and you alternate that. Or in the UK 5:2 was really popular for a while and those were two days of fasting each week.

Actually, they were more calorie-restricted days, two days of that, and then five days of quote, "regular eating". But time-restricted eating tends to be the way that most people live intermittent fasting as a lifestyle because it's just such a great rhythm to get in from day to day, and you feel good while you're doing it.

And you know, your body knows what to expect. You know, you're flipping that metabolic switch to fat burning and getting into ketosis during your daily fast, and then you eat, and your body's ability to switch those fuel sources because it's learned how to do that after you've adapted.

But just think about it as if you're either fasting. Or you're feasting. Your eating window is open or it's closed. And so you know when your eating window is closed when you're fasting, you don't have any decisions to make. That's one of the best parts about it. You don't have to have all that willpower because you're like, Well my window is closed right now.

So you don't have to, you know, walk into the break room at work and think, Should I have that donut now? Should I have a... No! Your window's closed. You're like, Oh, a donut. I might save that for my eating window later. But you don't have to have that struggle of, you know, should I eat that or should I not? Because your window's closed. So when you're fasting, you wanna stick to black coffee, plain tea, plain water, and plain sparkling water, and those are perfectly satisfying during the fast. And then you open your window and you have what feels good to you during that time. Did that help?

[00:14:16] Jonathan Wolf: Yeah. You're an amazing proponent of this.

[00:14:18] Gin Stephens: Oh yeah!

[00:14:19] Jonathan Wolf: Tim, what does the science say about intermittent fasting?

[00:14:22] Tim Spector: Well, again, as Gin says, it's lots of different things and I think they often get mixed up. So some part of intermittent fasting is a way of overall getting less energy in, actually ingesting fewer calories, because you are sort of tricking, your body into not wanting it.

So the ultimate result, if you're trying to eat as Gin does in four hours a day, most people can't actually eat that many calories in four hours a day that someone who's eating over 18 hours could do. So that's one part of the equation. It's like away with calorie counting to actually reduce calories and to some extent fuels your body slightly more than just, you know, saying I'm gonna have low-calorie foods.

And so that seems to be one way that it works. Then, of course, you've got, while you are in this fasting period, your body is using these other fuels. It is using ketones if there are, you know, carbs that aren't floating around.

So it's processing things in a different way and then, of course, you've got also the role of the gut microbes that if you're in a proper fast, where we're not talking about the 5:2 diets, which I think largely fail because you were getting 500 calories a day and often messing up the fast period. So you were getting small amounts of calories and that was more focusing on the calorie, whereas this new way of eating is extending that non-eating window over as some of these alternate day fasts, you are, you know, going 24 hours without eating in some cases.

Well, the work all started in mice back in around 2003 when people started to look and see, well, the idea that calorie-restricting rodents increased their longevity.

And then some groups started to say, Well, why is that? And as well as calorie restriction, could you change the time of eating and get similar results? And so they did that and the whole field of not only reducing the calories but also tweaking when you gave the meals often had similar metabolic effects.

So for that first 10 years, richly everything was in mice, and it's only really the last five to eight years I think that we've got a decent amount of human data.

[00:16:40] Jonathan Wolf: And Tim, just on the mice, just to make sure we got that, are you saying that if you change when you feed mice, they suddenly live much longer? That sounds pretty crazy.

[00:16:49] Tim Spector: It does sound a bit wacky, but that's what they found. By manipulating the eating windows of mice, you could improve their metabolic state and get them to live shorter or longer depending on what you were doing.

[00:17:01] Gin Stephens: What's interesting is some of those early animal studies with the rats were just designed to be calorie restriction studies. They would feed the rats or the mice, their whole allotment of calories, however, in a short period of time, they would feed them the whole, like, Here's what you get. And they found that the rodents ate the entire allotment in four to six hours. So as a researcher, it's hard to nail down the variables.

They were attempting to just compare calorie restriction to, you know, the rats and the mice that were eating around the clock, their normal way. But it ended up being that they were time-restricted because of the way they just ate, all of them, those calories at one time. So even those early calorie restriction studies were actually accidentally intermittent fasting as well.

[00:17:49] Jonathan Wolf: So this is like when I empty the fridge by lunchtime and I'm too lazy to do anything else, so I'm not gonna eat any more food until nighttime. That's like my time restriction.

[00:17:57] Gin Stephens: Accidentally fasted!

[00:17:58] Jonathan Wolf: By the fact that I've just run out of food or for when I'm up here in my study and I've run out of the nuts and then it's like, well, I can't be bothered to get anymore, so I stop.

So you're saying it's sort of a byproduct of the limited amount of food.

[00:18:11] Tim Spector: Yeah. And, and the way, also the fast metabolism of mice, they're not the same as humans. They will eat fast and metabolize it very quickly. So that was the other problem, is it was one thing to show that this mechanism exists in rodents and could, you know, these metabolic benefits and could extend life.

It is quite another thing to say how much does that affect humans? I think that's why we are waiting for these studies. And the early studies then went on and did calorie restriction in humans showing that was beneficial in terms of metabolic effects.

[00:18:44] Jonathan Wolf: And could you explain a bit, are these short-term studies, long-term studies? Just help us tend to understand a little bit what you're saying beneficial because everyone's gonna jump off and start calorie restricting if it's beneficial.

[00:18:54] Tim Spector: Well, they were generally over months rather than years. And so most, the variety of them, none of them are big studies. They're all fairly small because these are really hard studies to do, to get people to stick to the program because humans, you know, like to eat and they suddenly don't like being treated like lab rats, really.

So this was a really tough thing for these people how to incentivize someone to carry on this study just so someone else could write a paper about it and get famous. So it was a challenge. So most of them would be three months, six months, this kind of time, really hard to get people to do much more than six months of this in a trial, understanding conditions.

So, in that, they showed all the parameters, your blood sugar levels, your insulin levels, and your blood fat levels. Everything that was showing things was healthy, was improving. And that was a key point there that yes, it showed that short term, these do work. Now it turned out to be really quite hard to get people to stay on these diets, and that was really the problem.

Because your body was screaming at you, you know, I want to eat now. I want to regain that weight. And that's where this whole idea started to come in about, well if we start changing the times of eating and, and start having, you know, one day of feeding and one day of fasting, does this have the same benefits and it is more likely to be sustainable.

And I think there've now been about 12 studies, which have compared the two where they randomized adults to either steady calorie restriction over time, or they did it with some intermittent fasting regime. And those studies, they're all quite small. They do vary a little bit, but if you take the summary of them, they show that both of these methods do work in terms of improving the metabolic health of the individual and losing weight.

And that's now pretty clear. Although the studies are all small, you know, we lack a lot of the details about men versus women, of different ages. Some only took obese people, some only took younger people. We haven't got a full picture of who it works best in and who it doesn't work in. But overall, the science is pretty clear that at least short term on these diets, you can lose weight and improve your metabolic status. And losing weight was the one that wasn't quite as good in these studies when you compare it with calorie restriction, but the metabolic health was. So I think that's the important message.

[00:21:31] Jonathan Wolf: And is it fair to say, Tim, this is definitely in that category of new and emerging scientific evidence?

So you are sounding pretty positive about it, but this is not yet at the point where this is as fully proven as maybe some of the other things that you might talk about.

[00:21:44] Tim Spector: It's all fairly new. So we've only recently moved from animal studies to humans. The size of the study so far is actually really small. You know, 50 people. It seems a big study.

And of course, this means that we can't generalize it to everybody. We don't know how everybody's going to do well, and as we've always talked about on these podcasts, everybody is different to some extent, and everyone's circumstances are different but I think what it's showing is it has enormous potential for everyone, even just by tweaking their meal times, just by 30 minutes, if they did that over 10 or 20 years, could have dramatic effects.

So I think it's really important we take it seriously. There doesn't seem to be much in the way of downsides and huge amounts of upsides. So yes, we're still accumulating evidence but it's something that I think everyone can self-experiment with themselves.

[00:22:40] Jonathan Wolf: Well, Tim, you know, giving up that dark chocolate at 10 o'clock is a big sacrifice from my side. So you've got to understand there are potentially a lot of emotional downsides. So I'm excited by the experiment, but I'm not yet sure that this is what I'm willing to commit to.

[00:22:53] Tim Spector: Well then if you give up your cornflakes in the morning, you can have all your chocolate in the evening!

[00:22:57] Gin Stephens: That's what I was gonna say. You just shift your eating window, the direction where you like, Here's where I really wanna be able to eat. I really wanna have that. Maybe you could have it at 9:00 PM instead of 10, but you just nudge it this way. Nudge it that way. Boom, you're doing it.

[00:23:13] Jonathan Wolf: You know what? That's a brilliant transition. So, we've talked a bit about what it is and what science is. One of the things we really like to do on this podcast always talks about actionable advice. And it was one of the reasons I was really excited to have you join because you've helped so many people to actually understand how to do this in practice.

So, Would you maybe guide a listener who has never done this before, and is thinking of doing it? Hopefully, they are, you know, just about to sign up for our study, in fact, as part of this, and just tell us what it means. So imagine you're telling me I've never done this before. What is this thing? Help me to walk through how I can do it and be most likely to be successful.

[00:23:49] Gin Stephens: Yep. You really have to come into it with realistic expectations, and you're not gonna start doing intermittent fasting on day one and then lose a ton of weight quickly. It's not like that. This is something brand new that your body has to learn how to do. You're becoming metabolically adapted. Becoming fat adapted.

Learning how to tap into fat stores for fuel, your body is probably not very good at that. If you've been eating the traditional, you know, three meals a day, plus snacks you've been living, you know, as from meal to meal during the day. And instead, you've gotta teach your body to flip that metabolic switch and do something different.

So, in my book Fast Feast Repeat, I have a very important period of time called the 28-Day Fast Start. And it's actually kind of a funny story. When I was finishing it up and it was about to be, you know, there was thinking ahead to publication, my literary agent said, Now we need to get everything together, you know, for the publicity.

How much weight should we tell people they're gonna lose during the first 28 days? And I said, zero pounds. And they might even gain weight. And they're like, Well no, we can't really say that. I'm like, But that's the truth of it. You know, intermittent fasting is not a quick weight loss approach. And you know, for those first 28 days, six weeks, whatever it takes for your body to adjust.

You're just learning how to do something new and you're adapting to the clean fast. So you just want a fast, clean, tweak it till it's easy, meaning you're working to try to find a pattern that feels good to you. For me, I've ended up with probably a four to six-hour eating window most days. You know, I fast clean, meaning I stick to plain coffee, black, no flavors, nothing added.

[00:25:29] Jonathan Wolf: And just to make sure that's clear to me, because fasting, I think like I can't have anything. But are you saying I can have coffee? What am I allowed during this fasting period?

[00:25:39] Gin Stephens: Well, let's talk about it, plain water. Yes. Nothing added to it, no flavors. You know, plain sparkling water is also fine, but you know, people sometimes say, Well, how come I can have black coffee and plain tea? Because those have flavors. Well, they do have flavors, but they have a bitter flavor profile, and a bitter flavor profile is not associated with a cephalic phase insulin response.

So black coffee is actually stimulating autophagy. We have not used the word autophagy yet, but autophagy is our body's powerful, cellular housekeeping. It's like recycling and upcycling where our bodies during the fast can go in and clear up old junky proteins and really clean up things. Also, it's great for our immune systems.

They can really function best during the fasted state, and black coffee is likely to stimulate those processes. It even, you know, helps with fat burning and so black coffee is a great thing to add to your fast. Now, if. Find that black coffee makes you hungrier. If you don't wanna have coffee, you're not required to have coffee.

You can just stick to water if you want. But black coffee does tend to stimulate the things we want to have to go on during the fast.

[00:26:50] Jonathan Wolf: And Tim, any thoughts on that? I remember we had a lot of debate when we were doing our big, ZOE Predict studies about whether or not you can have tea and coffee during fast periods.

[00:27:02] Tim Spector: Yeah, I mean no one knows absolutely for sure because the tests haven't been done. So we are just getting an expert consensus on this, really. But most people do believe in the fasting world that yes, black teas, green teas, coffees, and water are perfectly fine.

Where people start to disagree is, can I have just a drop of, you know, a macchiato in my coffee? Just that tiniest little drop. And some people say, you know, if it's less than the equivalent of, I don't know, 10 or 20 calories, it's probably okay. Your body probably won't be able to sense that as a meal and therefore break its fast. Other people, I think as Gin would probably say, avoid that, that could be, you know, counterproductive and you actually lose all your benefits.

I don't think we quite know yet. It may be that Gin's actually tried it herself and seen anything different.

[00:27:54] Gin Stephens: Oh yeah. Well, I have a whole section in Fast.Feast.Repeat., where I talk about the clean fast and there is at the end of that, there are two chapters about the clean fast, and there's a section where I have, you know, anecdotal stories from intermittent fasters and you know, I've been in the intermittent fasting community since well before I ever wrote any books at all, or had podcasts, really in, you know, 2014, 2015 started with the support groups on Facebook and anecdotally, the difference between fast and clean and you know, putting a little bit of this, a little bit of that, the bulletproof coffee, a little bit of butter, you know, all the things that people might be, you know, putting in there.

The difference is night and day, and you have to really experience it for yourself. So anybody who's putting in a little sweetener or a little drop of cream or whatever, and you're like, It's fine, it works for me. I'm still losing weight. I feel okay. I would challenge you to try it with the clean, fast, Give yourself 30 days.

I call it the clean fast challenge. Go to plain black coffee, plain tea, plain water, nothing flavored, nothing sweetened, nothing to lighten up your coffee. I've never had anybody try it for 30 days and then go back the other way. So, you know, it really, you just take that challenge and try it for yourself and see, Most people report that they can't believe the difference that it makes.

[00:29:14] Jonathan Wolf: This is another example where what you're saying is, you know, anecdotally seeing this in practice.

[00:29:19] Gin Stephens: Yeah.

[00:29:19] Jonathan Wolf: This is a model that works. 

[00:29:20] Gin Stephens: You know, whatever the mechanism is, you know, whatever's going on behind the scenes. You know, I can give you the theory as to what I think, why it's easier without all that, you know, based on what we do know here are the theories, but in practice, you'll just see you're not white-knuckling it. You're not hangry.

All of a sudden you're like, Oh, I really can fast till three o'clock and I feel great. Whereas before, when you were having that little bit of almond milk or a little bit of cream or the butter or the MCT oil or whatever, you know that you saw YouTube video that said it was okay, you leave that out and you're like, Wow.

The whole experience is different.

[00:29:54] Jonathan Wolf: This is all back to this is just very new and not very well studied, so Gin, just to play back to like somebody listening, trying to do this. So I need to go to a clean fast. When I'm fasting, you know, I need to only have like this water and tea and coffee. Do I immediately go to like some constrained period on day one and stick with it?

Help me to understand what else I do in these 28 days.

[00:30:15] Gin Stephens: That's a great question and in Fast.Feast.Repeat., in the 28-day fast start chapter, there are three different plans you can kind of choose from that are helping you adapt. You know, you might need to be someone who really eases in and starts slow. You know, I'm not suggesting anyone start with what I'm doing as like day one.

You know, you've really gotta build up to it. I like to compare it to Couch, to 5k, you know, if someone wants to go run a 5k, you don't get off the couch on day one and run a 5k. You have to build up to it. And so fasting is the same way. We're very much building up our fasting "muscle", right? It's not technically a muscle, but you know what I mean by that analogy.

So you're building up to it. And that's what the, you know, the 28 days is really for. You know, you're learning how to fast clean. There are gonna be days when you feel hangry and you have to open your window earlier than you expected. And that's not a failure, that's just part of the process. We're learning to listen to our bodies.

You know, we never wanna feel shaky like we're having a blood sugar crash. If you're ever a shake, you're nauseous, go ahead and eat. You know, forget about what the plan said to do that day. Go ahead and eat. And, you know, gradually as your body gets adapted, You'll find, you know what feels good to you.

Some people always feel better with a midday eating window, for example, they like to, you know, skip breakfast, and eat lunch. Have a little, maybe early kind of dinner, close their window. No couch snacking on chocolate for them because their window is closed, but they sleep better when they have that middle-of-the-day eating window.

I'm not one of those people. I actually sleep better when my window is closer to bedtime. I've tried it all in different ways. I wait till afternoon, open my window, and eat till I'm satisfied. Close my window, but only through experimentation have I learned that you're not gonna learn that in the first 28 days.

It's very much a process. And your goal is to think of intermittent fasting as a lifestyle. You know, I interviewed a longevity expert for intermittent fasting stories, Dr. Gil Blander, he has a company now that does some things with biomarkers, but he has studied longevity in general. And he said to me, this was a couple of years ago, but he said he believes that one of the most powerful things we can do to increase longevity is intermittent fasting.

You know, just have that piece in there. Understand, you know, why we're doing it. I don't want anybody to start intermittent fasting only because they might lose some weight. That's not what intermittent fasting really is all about. You know, of course, I came to it for weight loss. I like to say, you know, we come for weight loss, but we stick around for health benefits.

You just have to experience it to see what we're talking about.

[00:32:51] Jonathan Wolf: Amazing. And Tim, can you? Tell us a bit about like that value of that window length, because you could say, Hey, just lots of people are eating for 18 hours a day today because of the way that the world works, and what really matters is just to shrink that to 12, which is still like 9:00 AM to 9:00 PM, Right? That's pretty different from, you know, shrinking it too much shorter periods too, I think that I've heard you, Tim, talk a bit about some of those, perhaps the microbiome having at least part of the answer to that story?

[00:33:21] Tim Spector: Yes. I mean, microbes themselves have their own circadian rhythms as well, and they're driven just like humans are by food.

So when the food arrives, that sort of sets them off on their particular clocks and things. And we do know that your microbiome changes as you're fasting compared to when you are eating. So within the 24 hours, Remember these species can, you know, change and replicate within an hour. They've had new babies and new lives, et cetera.

And so what happens when you are fasting is some microbes that don't live off food, but they live off the debris and the lining of the gut mucosa, suddenly come to life. So when suddenly all that snacking's ended and Jonathan's finished his chocolate, thank God, move on, you know, and get all those other chocolate-eating microbes out the way and the cleaning staff comes out and there are some microbes like Akkermansia, that's well known.

Because it has a name, it says Akkermansia muciniphila, means I love mucus. So it loves the sugary lining of your gut. So it's going around tidying up the gut lining that you haven't rested properly, and if you don't give it a rest, you don't have enough time for these cleaning microbes really to come outta the woodwork and tidy up your gut and help it regenerate.

What's interesting is these same microbes that have this job also seem to be crucial in preventing diabetes and obesity. So Akkermansia is one of these microbes that is stimulated when you go on a fast and is now a very trendy novel probiotic for helping your metabolic health and helping you lose weight.

So I think it's, you know, we are just starting to understand which microbes fit into these categories, but realizing that you're getting a whole new team comes out if you give them enough time to come out of the woodwork, tidy up your gut, do the repair work, and really you're in much better shape for the next time you know, that chocolate bar comes down.

[00:35:32] Jonathan Wolf: It's a brilliant analogy. So it's sort of like you've put the trash out overnight and early in the morning, you know —

[00:35:37] Tim Spector: - It's clean, it's the overnight cleaners in an office that come in and make everything shiny again. You know, it's the offense team and the American football versus the defense team.

You know, it's like giving them time to come out so that you got the right team ready there to deal with your body and what it needs to do. And if you put it out of sync by eating the way we weren't intended to be eating over 18 hours, it just simply doesn't have enough time to do its job. And I think what we're doing in these fasting is really extending the repairs side of the body.

And that's probably the general idea about why fasting is so good and why it has this huge potential for longevity.

[00:36:14] Jonathan Wolf: And so outside of the microbiome, do we have the data that supports that today, Tim? Or where are we on that?

[00:36:20] Tim Spector: Certainly there's really good data in all these animal models where it's easy to study these sorts of things and there are biomarkers in humans that suggest the same thing.

So I think there are these multiple mechanisms going on that are complicated that are all pointing the same way. That this is really essential for the body's repair process in the cells and in the gut.

[00:36:45] Jonathan Wolf: It's starting to get noisy outside Gin's window.

[00:36:47] Gin Stephens: Can you hear that?

[00:36:48] Jonathan Wolf: So I think the hurricane is really starting, to rattle. So — 

[00:36:51] Gin Stephens: - I was like, I can't believe if they can't hear that. 

[00:36:55] Jonathan Wolf: We heard that, for sure, Gin. So we appreciate you hanging in there. We've had a few episodes where people have talked about circadian rhythm, Tim, and just sort of how central that is. And that's obviously on this sort of 24-hour cycle and very related to the fact that you know, it used to be dark probably for 12 hours a day, and so I guess probably we weren't doing a great deal of eating. Do you think that's related to this sort of, you know, night work that it was, that you were talking about in the microbiome?

[00:37:26] Tim Spector: Yes. I think, you know, we've evolved to go in, you know, 12-hour cycles of light and dark, and our eating times were meant to be when we are active and our body needs to process it in those times.

And so by us shifting our eating windows outside that, you know, we are not processing the food as well as we should be. And therefore, what we're trying to do with these, certainly this time-restricted eating goes back to that hunter-gatherer type of time period for eating, which coincides with when our body is best able to deal with it.

And that's the resting period. So that's true. Now that's quite different from these periods of fasting where actually you are causing a bit of a disruption to the normal circadian clocks. It doesn't have the same clues.

So if you are going for a day without food, suddenly it's shaking up the body a little bit, and I think it's a different concept to the time-restricted eating because your body would expect and it's often switched on once you, you know, get food and you get light, they're an exercise.

They're the things that sort of get your body going and suddenly one of them is stopped. Your body's gonna be thinking, oh, what's going on here? And this part, some of the feelings that people get through fasting are because your body is being reset in a way, and gin might have some views on why that shaking up the body, you know, might be helpful.

[00:38:54] Jonathan Wolf: And so Gin, does that mean - because we had a lot of questions about this actually - is the consistency of the timing important? In other words, I'm gonna start eating at midday and I'm gonna finish at nine and I do that every day. Is that very important to this being sustainable and easy or can I just do it, sometimes?

[00:39:13] Gin Stephens: The consistency means that you're doing something every day, right? It's consistent with the fact that you maintain a fasting protocol. That doesn't mean it has to be the exact same time every day. It's just a matter of like, we know we don't like, quote, "take days off". You know we don't, you know, have cheat days.

But I mean, that doesn't mean though, that you can't decide today I'm going out to brunch and I'm gonna eat at 10 o'clock. And you know what, what worked for me really well was the idea of keeping my eating window to, you know, like five hours and shifting that around. So if I wanted to shift it to earlier in the day, I could do that.

Just slide it to a different part of the day. And then, you know, one day my fast was a little shorter because I opened my window earlier, but then I closed it earlier so the next day my fast, you know, was a little longer because I opened at the time I normally did.

So we don't want it to feel regimented and lots and lots of rules that you must follow, but you just wanna be consistent enough that your body maintains that metabolic flexibility. You know, if you go on a two-week vacation and don't fast at all, you're gonna have to come back and, you know, get back in the groove again. You know, it has to do with the amount of glycogen stored in your liver and getting through that. 

[00:40:29] Jonathan Wolf: Got it. So you're saying it's not fixed? It's not like I have to do it at the same time every day. You're actually relatively flexible, but the duration of the window, I'm sort of keeping constant. Even if I change it from day to day.

[00:40:45] Tim Spector: There are plenty of people I know that just, you know, will do this for two or three days a week, and they still feel better generally when they do it, but they're not so rigidly fixed on it.

And this is one of the things we're going to find out in our massive study because we'll find that some people are only able to do it two or three days a week and others will be doing it all the time. It'll be really interesting to compare them.

[00:41:04] Jonathan Wolf: You're hoping Tim to figure out whether you can get some benefit, even if you're doing it some of the time?

[00:41:09] Tim Spector: Exactly. Yeah.

[00:41:10] Jonathan Wolf: And can I ask one final question? Because we had a lot of questions around this, which was really about female hormones and I think we both had a lot of questions around, if you're in perimenopause or menopause, is this all going to be too stressful for my body. And also people asking, you know, because of changes of hormones during my cycle before menopause, is that going to mean that it only works some of the time?

Do you have any sort of anecdotal view on this, Gin?

[00:41:35] Gin Stephens: I have very strong feelings about this and it's just so interesting how, you know, people always start asking this about women. Well, yeah, but isn't it bad for women? And you know, what is bad for women? Is over restriction. Being overly restrictive with our bodies.

And when I think back to the way that I used to "diet", you know, throughout my twenties and thirties, the very low-calorie diets I was doing, that was actually a lot more restrictive for my body than the way I eat with intermittent fasting. So we don't want to do intermittent fasting in a way that's overly restrictive for our bodies, whether we're men or women.

But women we definitely do need to be careful about not over-restricting. And not over-exercising. Like I wouldn't do, you know, a one-hour eating window and train for an Iron Man and you know, do all those things at the same time. You have to find what feels good. But our bodies really have great feedback mechanisms in place that let us know what feels good is usually good for your body.

So don't think of intermittent fasting as overly restrictive. It really shouldn't be. And I am going to tell you that, you know, I started intermittent fasting when I was perimenopausal and went through the menopausal transition. I started intermittent fasting in 2014. Went through the menopausal transition around 2019 ish.

Now I'm on the other side. I'm menopausal. Just started hormone replacement therapy. Thank goodness. That's made a big change already. But I really think intermittent fasting helped me go through the menopausal transition with... Yeah, it wasn't a terrible thing, you know, and I didn't, you know, put on a ton of weight, as most women do. I really think intermittent fasting was a great adjunct to making the menopausal transition.

[00:43:21] Jonathan Wolf: Amazing. Gin, Tim, I have so many more questions, but I can see I'm hitting time. So I'm going to try and do a quick summary of what we covered and keep me honest if I got it wrong.

So first I think we explain that intermittent fasting is a lot of things, but increasingly it's really focused on this idea of time-restricted 18 on a regular sort of pattern every day, rather than having full days of fasting. The clinical evidence is still relatively early in human beings.

And then I think we talked, Gin, about this great idea about like, what do you need to do in order to do this? And I think you say, you know, 28 days to sort of adjusting your body. Clean fast. So I'm allowed black tea, I'm allowed black coffee, I'm allowed water, but no milk, no sweetness.

Listen to your body. So if you're shaky or you're nauseous or anything's not working, then stop. Think of it as a lifestyle. So this isn't something that you're going to do for a short period of time. This is, this is going to be like everything else to do with like lifestyle. It's either something you do always or it doesn't matter, but you don't have to do it every day.

I think the final thing we said is that there's this amazing new study, which we hope will be the world's biggest intermittent fasting study. And Gin, I hope you'll be joining it with everybody else.

[00:44:32] Gin Stephens: Well, I would love to! 

[00:44:34] Jonathan Wolf: Wonderful. Thank you both very much. And Gin, I hope you'll join us when we report back on the results.

[00:44:40] Gin Stephens: Fabulous. I can't wait to hear what you found out. And can you believe we made it through with no power outage?

[00:44:45] Jonathan Wolf: I wanted to, I was gonna say definitely time to stop before that happened.

[00:44:49] Tim Spector: You survived. We survived. 

[00:44:51] Gin Stephens: I did so far.

[00:44:56] Jonathan Wolf: Thank you to Gin and Tim for joining me on ZOE Science and Nutrition today. We hope you enjoyed today's episode. If you want to participate in what we hope will be the world's largest study of intermittent fasting, then go to joinZOE.com/fasting. And once we have the results from this study, we of course hope to be able to give you personalized advice about whether intermittent fasting is right for you.

In the meantime, if you want to understand what to eat when you aren't fasting, then you may want to try ZOE's personalized nutrition program, which will identify the right foods for your body. Each member starts with an at-home test, comparing them with participants in the world's largest nutrition science study.

We then use the results to create a program to improve your health and help you manage your weight. If you're interested in learning more about ZOE, you can head to joinZOE.com/podcast and get 10% off your personalized nutrition program. If you enjoyed today's episode, please be sure to subscribe and leave us a review.

We do love reading your feedback and if this episode left you with any questions, please send them in on Instagram or Facebook, and we will try to answer them in a future episode. As always, I'm your host, Jonathan Wolf. ZOE Science Nutrition is produced by Fascinate Productions with support from Sharon Feder, Dr. Yella Hewings-Martin, and Alex Jones here at ZOE. See you next time and hopefully with a full voice recovered situation. Bye bye.