We know that intermittent fasting is good for health, and now the latest free Zoe study can help you discover if it’s right for you and how to tailor it to your life

There have been many diets claiming to be ‘The One’ for health, mood and weight management, from the gut health-friendly Mediterranean diet to Dr Michael Mosley’s Fast 800 Keto diet and the Pioppi Diet, created by cardiologist Dr Aseem Malhotra to reap the secrets of one of the world’s longest-living populations. They all require you to change what you eat, and for some of us that can be the very thing that makes us shelve our healthy intentions for another day.

Now a new study from the researchers behind the Zoe Diet Programme, the Blue Poop challenge and Zoe Covid Health Study, wants to know whether simply changing the times at which we eat – a type of intermittent fasting called time-restricted eating or TRE ­­– can not only have a marked benefit on our health long-term but also have more immediate effects on the way we feel. And it’s asking hundreds of thousands of us to take part.

What is the Zoe Big IF intermittent fasting study?

From today you can sign up to the free Big IF programme via the Zoe Health Study app, the same app that thousands of people still use to report Covid symptoms. You can start any time. You’re asked to eat as you normally would for a week and log the effects on mood, energy levels, sleep, hunger etc. Then for another week (or longer if you’re enjoying it) to eat all your meals and snacks within a ten-hour window of your choosing and again log how you feel.

In return, you’ll receive data on how this way of eating benefits you and nutrition tips from the Zoe scientists. Dr Sarah Berry, a nutritional scientist at King’s College London, who is leading the research for Zoe says, “the aim is to be the world's largest ever intermittent fasting study. We're inviting people to be citizen scientists to answer some really important questions around time-restricted eating.”

The study will release results in real time as more people complete the two-week programme. “What's really exciting for people taking part is they will have their results at the end of every day, they'll have a little graph showing how things are changing over the two-week or three-week period. And for us as researchers, we hope that within a month or so we will have thousands of people's data. Every few months, we'll crunch the numbers have a look at the data and we'll be feeding this back to participants in the app.”

Currently, the largest study on time-restricted eating is on 50-100 people. "That doesn’t mean it’s not a good one, but it doesn’t take into account how fasting works en masse and in a real-life setting, which is what sets the Zoe Big IF apart," Dr Berry explains.

“There's some great research out there regarding the effectiveness of time-restricted eating, but this research is done in really tightly controlled clinical settings,” says Dr Berry. “What we don't know is how it translates to the noisy way in which we live our lives. Can people actually follow it? Which way works for different people? Do people want to eat earlier in the day or later in the day? Is it particularly effective for postmenopausal versus pre-menopausal women or for men versus women, or for older versus younger people? It's going to be a seminal project; firstly, it's going to be the first big citizen science project of its kind. But more importantly, it's the first study that's going to allow us to look at really interesting science [on intermittent fasting] in a real-life setting on a huge population scale.”

The results could potentially inform government dietary guidelines as well. “In order to be able to advise people, it's all very well having lots of knowledge and lots of data about how different interventions and diets affect people. But if people can't adhere to them and follow them within the way that they normally live their lives, it's pretty useless. This is a huge problem. We know that less than one per cent of people actually follow the UK dietary guidelines,” she says.

What is time-restricted eating?

“Time-restricted eating is a form of intermittent fasting. Intermittent fasting can take lots of different forms” says Dr Berry. “Some people might have heard of the 5:2 Diet, where for a number of days [five] you consume normal food and then you have a number of days [two] where you consume a really low calorie, almost fasted diet.

“The other type of intermittent fasting that's typically followed by people is called time-restricted eating. And this is where you restrict the time that you're consuming food to a smaller, what we call, eating window. So typically, it might be an eight or ten-hour eating window, where you're actually consuming foods, and then a 16-to-14-hour fast period.”

What are the benefits of time-restricted eating?

There’s already good science to show that consuming your food within a time-restricted eating window has positive health benefits, as we allow our body a period of digestive rest without blood sugar spikes and to get on with other important health-supporting processes. There are also fewer hours in the day in which to snack, so TRE can have a natural weight management effect too.

“We do know that for most people taking part in time-restricted eating, there's an unintentional calorie deficit of about 300 calories,” says Dr Berry. Evidence shows that TRE, without any other factors changing, can result in “about three to five per cent weight loss,” she says.

But there’s a bigger health gain to be had. “What's really interesting is that even when people don't lose weight when they're practising time-restricted eating, there are massive metabolic benefits. We see improvements in sleep, in inflammation, blood pressure, insulin sensitivity, and blood sugar control and so many other factors that we know are really important for our long-term health”

She adds: “What’s so great is that it's promoting health from the inside out – and a by-product is that you will most likely lose weight as well. But primarily you're going to feel better and function better.”

Why the ten-hour eating window?

The most widely-known form of TRE is 16:8 – eating within an eight-hour window and fasting (drinking only water, black coffee and black or herbal tea) – during the other waking hours. The Zoe Big IF intermittent fasting study has picked ten hours as a more realistic eating time frame that people in large numbers could stick to.

What is the best intermittent fasting schedule?

Dr Berry says it’s really up to you. If you’re not naturally a breakfast person, then an eating window of, say, 10.30am to 8.30pm could work well. But what does the existing science say?

“There's lots of research looking at early time-restricted eating versus late time-restricted eating. The evidence seems to suggest that for people who eat early in the day and finish early in the day, it has greater health benefits than for those people practising late time-restricted eating.”

Late-night eating is less favourable to our circadian clock, she points out. “Every cell in our body has a clock that is guided by two main things: the time that we eat our food and the dark-light cycle. If we're eating our food outside of the natural body clock that we have for all of the cells, then we know we don't quite function or process [foods] in the best way. We know from our own studies and other published research that late-night snacking is associated with all sorts of unfavourable health effects. But my advice to people would be to do time-restricted eating however it works for you. Many people find it difficult to stop eating at three in the afternoon. We know earlier in the day might be more effective. But if you want to do it later in the day, that's fine, but just try later to avoid those late-night snacks or drinks.”

Dr Berry cautions that eating should still be a pleasure. “We don't become overly fixated on the food that we're eating; we need to enjoy our lives. Anyone doing this, if you want to have a few glasses of wine in the evening, have a few glasses of wine. Please make sure that you still enjoy what you're doing. My mantra really is if a food is too healthy to be enjoyed, it's not healthy at all. Food is so much part of our social setting, our culture and our emotions. And it's really important that we continue to allow food to be a pleasure.”

So yes, you can still go out for dinner if you’re on the study. You simply shift your window on that day to suit your life. However, TRE it’s not suitable for everyone. If you have an eating disorder or are pregnant then this isn’t for you, and if you have a medical condition, please check with your doctor first.

When intermittent fasting what can you drink?

“The general consensus is black tea, black coffee, or any unsweetened herbal tea are OK to enjoy in the fast period,” says Dr Berry. Sparkling water is OK too. But once you start adding in anything that needs to be metabolised, such as milk in your tea, or that early morning lemon in your hot water or your apple cider vinegar drink if you’re Victoria Beckham, “that will affect things like your glucose fat inflammation, then that's not going to allow you that fast period.”

Will I feel worse before I feel better?

With any change to your schedule, you may feel a bit different initially. It’s not uncommon to experience fatigue and some irritability when you start time-restricted eating. “For most people, this goes away quickly,” says the Zoe website. But based on the IF research out there, Zoe says they anticipate “that, overall, you may actually experience improvements in your energy levels and mood.”

Dr Berry adds that you may feel so good that you want to continue. For many people, IF becomes a way of life because they feel so much better on it. And as a bonus, we’re benefitting our long-term health too and with the study potentially helping to shape better health advice for the future.

Find out more about the Zoe Big IF Study and sign up for free via the Zoe Health Study App.