Recap: What the science says about intermittent fasting | Gin Stephens

Summary

ZOE Science & Nutrition Podcast Summary

Episode: Recap: What the Science Says About Intermittent Fasting | Gin Stephens
Release Date: February 18, 2025
Host: Jonathan Wolf
Guests: Tim Spector, Jim Stevens, Jen

---

Introduction to Intermittent Fasting

In this episode of ZOE Science & Nutrition, host Jonathan Wolf delves into the topic of intermittent fasting (IF) with experts Tim Spector, Jim Stevens, and Jen. Intermittent fasting, often perceived as a modern dietary trend, involves alternating periods of eating and fasting. Jen clarifies the concept:

“Intermittent fasting… 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.”
— Jen [00:42]

Types of Intermittent Fasting

The discussion highlights various IF approaches:

• Time-Restricted Eating (TRE): Involves eating within a specific window each day, such as 8 or 10 hours.
• Alternate Day Fasting (ADF): Includes patterns like the 5:2 diet, where individuals eat normally for five days and restrict calories on two non-consecutive days.

Tim Spector distinguishes recent trends from older methods:

“The 5:2 wasn’t really a proper fast either because it allowed you to have like 500 calories or something… people are gravitating towards this way of eating, which really… isn't so much fasting as just changing your eating times.”
— Tim Spector [02:42]

The Big IF Study

Jim Stevens brings attention to the Big IF Study, the world's largest investigation into intermittent fasting, conducted under the ZOE Health Study umbrella.

“…246,000 people were eligible… 37,000 completed the study.”
— Tim Spector [03:19]

Study Parameters:

• Duration: Initial two-week monitoring, with many participants continuing up to 16 weeks.
• Eating Window: Shifted from an average of 11 hours to a 10-hour window, promoting a 14-hour overnight fast.

Key Findings:

• Weight Loss: Average loss of 1.1 kg (2.4 lbs).
• Energy Levels: Increased by 22%.
• Mood Improvement: Enhanced by 11%.
• Hunger Levels: Surprisingly decreased slightly.

“People who reduced that eating window most changed most… they lost weight, energy levels went up, mood improved, and hunger actually slightly dropped.”
— Tim Spector [04:09]

Impact on Hunger and Energy

Contrary to common beliefs, consistent intermittent fasting participants reported reduced hunger and increased energy.

“Even though they were eating in a shorter time than before, they were actually less hungry… they restricted their eating.”
— Jim Stevens [05:02]

Tim elaborates that inconsistent fasting (eating and not fasting intermittently) led to increased hunger and decreased energy, emphasizing the importance of stability:

“When we looked at the people who weren't doing this consistently… things reversed. Those people were more hungry and they had less energy.”
— Tim Spector [05:12]

Timing Matters: Early vs. Late Eating Windows

The study also explored early vs. late time-restricted eating:

• Early TRE: Concluding eating by 6 PM.
• Late TRE: Extending eating until 9 PM.

“Studies suggest that metabolically early is slightly better… but more people chose to do the late time restricted eating.”
— Tim Spector [06:53]

Jen adds that allowing individuals to choose their preferred eating window enhances sustainability:

“Allow them to choose when they wanted to do it, and they naturally chose what felt right to them.”
— Jen [08:15]

Sustainability and Personalization

A significant takeaway is the importance of personalization in IF practices. Tim emphasizes that adaptability to individual lifestyles is crucial for long-term adherence:

“There’s no point telling everyone, you must do this in these particular hours, otherwise it’s worthless.”
— Tim Spector [07:04]

Health Benefits Beyond Weight Loss

Beyond weight management, intermittent fasting offers other health benefits:

• Reduced Inflammation: Tim notes a consistent reduction in inflammation, which impacts overall immune function and mood.

  “On average, you’re getting reduced inflammation from giving your gut a rest and you are improving your gut microbes.”
  — Tim Spector [12:21]

• Improved Gut Health: Participants reported a decrease in bloating (down by 11%) and reduced snacking frequency.

  “Nearly everybody said they were doing less snacking, which is pretty obvious in a way, but it’s nice to actually document that.”
  — Tim Spector [05:12]

Fasted Exercise

The conversation also touches on the evolving perspective towards fasted exercise, challenging the old notion that one must eat before physical activity.

“You’ve got to have reserves before you can do this activity, otherwise… you must have food inside you before you undertake any activity… Now we know that’s not true.”
— Tim Spector [09:19]

Jim shares his intrigue with fasted exercise, supported by emerging data suggesting potential health benefits.

Conclusion and Recommendations

The episode concludes with an encouragement to experiment with intermittent fasting to determine personal suitability and benefits. Jen underscores the community's success in adopting flexible eating windows, while Tim advocates for self-experimentation to find what works best for each individual.

“We should all be experimenting ourselves and trying it.”
— Tim Spector [07:35]

---

Final Thoughts:
Intermittent fasting, particularly time-restricted eating, offers tangible benefits beyond weight loss, including improved energy, mood, and gut health. The Big IF Study underscores the importance of consistency and personalization in adopting IF practices, making it a sustainable lifestyle choice for many.

---

For more insights and personalized nutrition plans, consider exploring ZOE's membership offerings.

Transcript

[Zoe]

Hello and welcome to Zoe Recap, where each week we find the best bits from one of our podcast episodes to help you improve your health. Today we're talking about intermittent fasting. This form of time restricted eating has been gaining significant popularity with many people reporting positive effects after incorporating it into their daily routines. However, here at Zoe, we want to know what the science says about intermittent fasting. Is it the real deal or just another fad? I'm joined by Tim Spector and Jim Stevens to break down what intermittent fasting is, how it affects our bodies, and whether scientific evidence supports it.

[Jim Stevens]

I'd like to just start at the very beginning actually with you, Jen.

[Jen]

Okay.

[Jim Stevens]

What is intermittent fasting?

[Jen]

The words sound so scary. The word fasting makes you think that you're gonna like 40 days and 40 nights wandering in the desert. But intermittent fasting, the word intermittent is key. You are having periods of fast, 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.

[Jim Stevens]

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 explained 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.

[Jen]

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, you know, 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.

[Tim Spector]

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 five twos were only 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.

[Jim Stevens]

Tim 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?

[Tim Spector]

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 range of animals and humans to be effective. 37,000 of them completed the study.

[Jim Stevens]

And how long did they have to do the fasting for?

[Tim Spector]

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 that, you know, we got people hooked on it. At baseline they weren't too bad. Their eating window was about 11 hours. So for many people then he shifted it by about one hour. The people that did complete it, they lost on average about 1.1kg. That's just over a couple of pounds in weight. But importantly we saw that people who reduced that eating window most change 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.

[Jim Stevens]

So hang on, just want to make.

[Tim Spector]

Sure I got that right.

[Jim Stevens]

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.

[Tim Spector]

That's what they were reporting, which sounds 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 do need some sort of stability to this to get these benefits. 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. And 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 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 there'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 Gin knows a lot about. All the studies suggest that metabolically early is slightly better for you if you say stop eating at 6pm rather than at 9pm 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 ones.

[Jim Stevens]

So 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 11 o'clock, 11 o'clock and go through to 9, 9pm in that example, rather than start at 7 and go through.

[Tim Spector]

In the UK, that was a more popular option than the other one, which, you know, might be finishing 8 in the morning till 6pm at night. So it was a two thirds to one third split on that, which I think is really interesting because it, it suggests that what we're looking for here is how do we get people to, 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.

[Jen]

Exactly.

[Tim Spector]

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 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, it's a good chance that you'll be in that group and that, you know, we should all be experimenting ourselves and trying it.

[Jen]

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, and 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 7pm every day. That's when I've had enough. You know, I'm not tempted to eat because I'm not hungry anymore between 7pm 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.

[Jim Stevens]

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 things.

[Tim Spector]

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.

[Jim Stevens]

Yeah. And we did a fascinating podcast looking at this only, 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, you know, more and more intrigued by this idea that it's not just crazy to I.

[Jen]

Do all my exercise fasted.

[Tim Spector]

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.

[Jen]

Yeah.

[Tim Spector]

And then, you know, once you've done that, then you might start thinking about food. But so it's very different to maybe even 10 years ago when, you know, science was. 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, 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.

[Jim Stevens]

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?

[Tim Spector]

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. 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 find 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 is, it's both assessing the acute science for a few weeks, but also really give 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.

[Jim Stevens]

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 like how much energy it feels it has. So these things feel very different from that.

[Tim Spector]

Right?

[Jim Stevens]

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, or your weight.

[Tim Spector]

So to me, just looking at these results crudely, I'm seeing, you know, minor changes on things like weight. You see small changes in metabolism and insulin and things like this. But the only, 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. 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, 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.

[Zoe]

That's it for this week's recap. If you're hearing this, you're already on your way to making smarter food choices. With world leading science. Speaking of making smarter food choices, Zoe membership helps you to do just that with scientific solutions that let you listen to your gut. Listening to your gut starts with an at home test kit that helps you understand your body's response responses to food and the good and bad microbes in your gut. Then Zoe's app helps you build life changing nutrition habits step by step. Become a Zoe member today and give your gut the love it deserves. The first step is easy. Go to Zoe.com and use the code podcast to get 10% off. As a Zoe member, you'll get a comprehensive at home test kit and personalized nutrition program. So go to Zoe.com and get 10% off with code podcast.