What breaks a fast? - The Big IF Dailies
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A black coffee probably won't. A white coffee might. Add two sugars and it definitely will. If you're thinking of fasting, giving up your morning coffee might be a deal breaker. The question is, does drinking it break your fast? And if not, what does? Is there room for a sweetener or even a splash of milk during your chosen fasting window? Or is it water all the way?

This is Zoe, Science and Nutrition. I'm Jonathan Wolff. Today we ask, what does it take to break a fast? This episode is part of a limited series to celebrate the launch of the Big If Study.

the world's biggest clinical study to discover how intermittent fasting really affects us. By inviting our amazing community of citizen scientists to take part in this experiment, we hope to understand whether intermittent fasting really can improve how we feel, from our hunger levels to our mood and energy. To take part for free and discover if intermittent fasting can work for you,

Simply go to joinzoe.com slash thebigif, that's T-H-E-B-I-G-I-F, or via the link in the show notes. The countdown is on. Three days until you can join. ♪

We've chosen to speak to Tim Spector today, not just because he's one of the top 100 most cited scientists in the world and has done scientific studies on the subject of fasting, but he's a regular faster himself. So I think I have a personal interest as well as a professional interest. Before Tim showers us with advice, which he will, we should probably explain what's happening when we fast.

your body has to switch from fuelling itself with food to using its own reserves. But that's not all. It seems your whole body changes metabolic state during a fast. In terms of precisely how much, there is no real consensus in this area. There's two schools of thought about what's called 'clean fasting' or 'slightly dirty fasting'. The group of clean fasters say you should only have water or black tea or black coffee.

Then there are others that say, well, you could add just a drop of milk or plant milk to your tea or coffee. And that doesn't give you enough calories to really trigger the body's full response. It has to change its mind and expect food. And we simply don't know enough about the signaling mechanisms in the brain and in the gut to know whether it can detect these tiny amounts of energy and then send out different signals back.

Despite the lack of evidence, many people think you can probably get away with having up to 50 calories before you're at a risk of breaking your fast. But while there are clean fasters and dirty fasters, there are also the purists, the hardcore elite who don't even entertain black coffee or tea. It's water all the way. It might seem extreme since neither black tea nor coffee contain calories, but maybe they're onto something?

Tea and coffee both do contain fiber. Some fiber, for example, will get down to the lower intestine, although it will take longer than usually an overnight fast to do that. And it's believed that that doesn't have a major impact on the brain signaling and other factors that we think are crucial to this fasting and eating periods and our circadian rhythms. Phew, I was getting worried there. Going down the purist route never sounds that much fun.

As Tim says, this is meant to be a sustainable way of eating. Tim Cooke: Many people get through long periods of fasting with a bit of amusement by having tea or coffee. It keeps the idea that you're not punishing yourself too much and takes your mind off it. And to be honest, people have been practicing this for several years now. And I think if it had failed completely with tea or coffee, we'd have seen it. So I think if there is a difference, it's pretty small.

Right, so it's likely you're safe having your black coffee. You can check that off your worry list. But a splash of milk? It's still a bit of a grey area. It's not definitely bad, but the research is foggy.

Among all this uncertainty, I think it's time to get some concrete answers. What's certain to break your fast? Certainly anything with sugar in it. So adding a spoonful of sugar to your coffee or tea definitely will break that fast. It will send signals to your brain and gut to wake up and expect food. And I think that's one of the critical elements here, that it goes from this repairing resting state to a sudden alertness state.

And we know that sugar and probably fats as well are the things that do do that.

Fantastic. And now we know exactly what to avoid, we never have to break a fast again. I mean, that's the theory, but we're only human. Sometimes we make mistakes. And Tim says that's okay, especially if you intend to make fasting a part of your life going forward. To do that, it's got to be pragmatic. So if you can't resist some cappuccino that someone offers you half an hour before the end of your fast,

You don't beat yourself up about it. We realized that you don't have to probably do this extended fasting every day for it to be effective. We simply don't know that. So I don't do this every single day because of social circumstances. So I think the importance is we try, all of us try and find what is that balance between practicality and perfection and

work out what suits us. And we all have different patterns of life, lifestyles, young children, work, etc. that make it different to do this absolutely every single day of our lives to the same extent. We're all allowed time off and I think our guts will be quite happy with that. So get it right as often as you can, but don't sweat it if you mess up.

The key things to remember are: you probably don't have to give up your morning tea or your coffee to stay in a fasted state. And if it helps you adhere to the plan, do add a splash of milk. Just keep it dark brown rather than pale white. Adding anything past that and you're probably going to break your fast.

The episode you just heard is part of a limited series to celebrate the launch of the Big If Study, the world's biggest clinical study to discover how intermittent fasting affects our mood, energy, and hunger. To take part for free and discover if intermittent fasting can work for you, simply go to joinzoe.com slash thebigif or via the link in the show notes.

This episode was produced by Fascinate Productions with support from Yellow Hewings Martin and Alex Jones at Zoe. Zoe Daly's come out each day between now and our next regular episode.