Instant Genius - The truth about fasting and time-restricted eating

Episode Date: September 3, 2023

Lose weight, gain more energy, boost your metabolism and even live for longer: these are some of the claims made by the adherents of fasting and time-restricted eating. But was does the science say? In this episode we catch up with Prof Javier Gonzalez, a physiologist based at the University of Bath’s Department of Health. He tells us all about the effects fasting has on our bodies and brains, breaks down some of the myths surrounding it and tells us the safest way we should go about it if we’re thinking about giving it a try. Learn more about your ad choices. Visit podcastchoices.com/adchoices

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delivering digital precision with analogue warmth. So you can experience exceptional sound at home. Music just as the artist intended. Visit name audio.com to learn more. And welcome to Instant Genius, a bite-sized masterclass in podcast form. I'm Jason Goodyear, commissioning editor at BBC Science Focus magazine. Lose weight, gain more energy, boost your metabolism, and even live for longer. These are some of the claims made by the adherence of fasting and time-restricted eating.

But what does the science say? In this episode, we catch up with Professor Javier Gonzalez, a physiologist based at the University of Bath's Department of Health. He tells us all about the effects fasting has on our bodies and brains, breaks down some of the myths surrounding it, and tells us the safest way we should go about it if we're thinking about giving it a try. So fasting has been a bit of a buzzword in health and nutrition for quite a while now, with all sorts of different types and approaches to fasting being discussed. But is there an exact scientific definition as to what it means? Yeah, so the definition of fasting is quite tricky. It could mean abstaining from any nutrients at all,

which could include water. And if that were the case, then death would occur in a matter of days. We actually need water quite frequently to survive. But most people probably mean the absence of energy-containing nutrients, so things like carbohydrates, fats and proteins. And so what people mostly mean by a fast is not eating those foods for a prolonged period of time. Now, that other element of time becomes an interesting part of the definition. So if you haven't had a meal for two hours, that probably isn't fasting because you're still digesting, metabolizing, and absorbing those nutrients from that previous meal. So normally we start to enter what we would call a fasting period at least six to eight hours after the last meal that we ate, assuming that was a meal of

a normal size. So you sort of touched on it there. We've, and I said earlier, we've got different approaches. So there's a sort of one that's called intermittent fasting, longer periods of fasting for a few days. And then also something that's known as calorie restriction, for example, the 5-2. So can we just break down the differences between those approaches, please? Yeah. So as you say, there are multiple types of fasting or time-restricted eating. So you might have heard of what some people call intermittent fasting. We've then got alternate day fasting. So that might be where you don't eat anything for a full day and then you eat on day two, and then you don't eat on day three, and then you eat on day four, for example. Or it could be

two days a week that you fast and the rest of the week you eat. And then we've also got time restricted eating, which is within each 24-hour period, you might limit your opportunity to eat within a certain window. So it might be. be a case of you're only going to eat between midday and 6pm and the rest of the time you're going to fast. And they will have different effects because the duration of fasting is different and also that pattern of fasting is different across a week. So coming off the back of that then, let's have a look at what happens in our bodies when we do enter the fasting state. You know, what happens? Yeah, so one of the main things that happens is a change in the

fuels that we're using. So if you imagine you've just woken up first thing in the morning, and the main fuels that are supplying our tissues with energy are a mixture of carbohydrates and fats, and the carbohydrates are a key fuel in that scenario. They're in the form of blood sugar. And some cells like the brain and our red blood cells can't really use fat as a fuel. And so running low on sugar can be a real issue for our brain in particular. But we've actually evolved an interesting way to circumvent that issue. So first of all, our liver provides sugar to the bloodstream, and that's via the breakdown of stored carbohydrates in the liver, and the liver can also convert other fuels into sugar for the brain to use. But if we continue to fast for about

a day or so, so after about 24 hours, then these stores of carbohydrates in the liver can become completely depleted. And so the total amount of sugar coming out of the liver for the brain and other tissues to use is now much lower. But luckily, we've got an alternative fuel that can kick in at this point. And our liver can start to produce these extra fuels called ketone bodies or sometimes just call ketones. And these ketones are neither carbohydrate nor fat, but they can be used by the brain and also our muscles actually as a fuel, an alternative fuel. So high levels of ketones in the blood can therefore prevent an energy crisis that would otherwise occur in our brain when carbohydrate availability is limited. So is that what people mean when they talk about ketones?

Cetosis. Exactly, yeah. So when we're in ketosis, it means that we've got high levels of these ketones in our bloodstream that our liver's producing. And whilst that's probably evolved in these periods of fasting, we can enter something that's known as nutritional ketosis, which is where we're still eating food, but we basically restrict our carbohydrate intake. So it's a high fat, very low carbohydrate diet. And that's another way in which we can cause our body to start producing these ketones. So I think it's probably safe to say the primary reason, somebody will be thinking about fasting or going on a sort of fasting-based diet program, is that they want to lose weight. So is fasting a good way to lose a few extra pounds? Yeah, it can be one of multiple strategies in which are effective at losing weight. What's probably important to bear in mind here is based on the evidence we have so far, it doesn't seem to be the case that fasting does anything special over and above any other form of calorie restriction. So what we might normally consider as traditional calorie restriction is where we might limit our food intake by about 30% every single day.

We're eating slightly less than we normally would, and that's constant every single day. And that will lead to a certain amount of weight loss proportional to that reduction in the amount of energy that we're eating. and with these forms of fasting, if we take, for example, alternate day fasting where you don't eat for one day and then you eat the next day, it just so happens that when people have eaten nothing on day one, on day two, they don't eat 200%. In other words, they don't fully compensate for what they haven't eaten the day before. And so they spontaneously then eat slightly less and tend to lose weight. In studies that have controlled for the amount of energy that's been eaten, It doesn't seem to be the case that people lose more weight than would be expected.

So if I were to just summarise that very briefly, fasting can be one way to achieve a calorie deficit, but when you control for that calorie deficit, it doesn't seem to be any better than any other method of achieving a calorie deficit. So before we go on to some other questions that I've got then, I think it's worth talking about this notion of calories. So everyone's going to have heard the term calories. But what does it mean and are all calories equal? So a calorie is just a unit of measurement.

So a bit like we measure distance in metres. We measure energy actually in kilojoules, but for kind of layperson, we might refer to calories. It's an equivalent to these killer joules. And what it comes from is actually the energy that's required to heat water. So a killer calorie, which is what's commonly used as it terms as a calorie, is the amount of energy that's required to heat up a liter of water by one degree Celsius. So that's the unit of measurement.

And that's important for nutrition because when we gain weight as body fat, that's excess energy that's being stored. So if we're consuming more calories than we're burning, then we will eventually gain weight over time. And if we're consuming fewer calories than we're burning, then we'll lose weight over time. And the main way in which we store that excess energy in our body is as body fat. So is there any difference between a calorie of fat and a calorie of carbohydrates, say? The simple answer is yes and no. So in the sense, if we just measure the amount of energy in that,

if you overeat 100 calories of carbohydrate versus 100 calories of fat, then on the one hand, you might expect to gain equivalent amount of body fat. But there are some intricacies that kind of undermine that process. So one of them is in the digestion and absorption of those two foods. So probably proteins are better example here than the carbohydrate. In fact, because it's the most different. And basically, when we eat protein, if we eat 100 calories of protein, about 30 calories of those are actually already used in just the digestion and absorption. So it's an energy costly process to digest and absorb protein. If we compare that to fat, Fat is very efficient, so we don't lose as much of the energy when we're digesting and absorbing it.

So 100 calories of fat will mean that more of those calories are available for the body compared to 100 calories of protein. So they're not entirely equivalent. So just coming off the back of that again now, so I think a lot of people who are on these fasting diets will think, well, you know, I'm on a fasting diet that's healthy, even if their diet consists mainly of junk food. So are they getting that entirely wrong there? If you're fasting, you may be eliciting some physiological processes in the body that could be beneficial. As I said earlier, there's not really much evidence that they are beneficial over and above other forms of calorie restriction. And so if we take that out of the picture and focus on the junk food part of the diet, the nutrient quality of that diet probably is very important, even in the face of doing some kind of fasting.

So, yeah, they probably would benefit from improving their nutrient quality and eating less drunk food. Ambition comes in all shapes and sizes. At First Citizens Bank, we roll with your goals because we're built for what you're building. Fit for your ambition for Citizens Bank. It's peak pollination season and my business is scaling fast. To keep the nectar flowing, I need a phone plan with top priority data speed. That's why I chose GoogleFiwire. Wireless. My connection stays strong even when the hive is buzzing. Plus, unlimited plans start at $35 a month.

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So first off, what is your metabolism and then how is it affected by fasting? Yeah, so our metabolism is the amount of energy that we burn basically in a day. And we can broadly split that up into three main categories. So we've got our resting metabolism. So that's normally our metabolic rate, the amount of energy we're burning. That's normally measured when we're actually asleep or we've just woken up in the morning. That's when it's at its lowest point. We're nice and relaxed.

We haven't had anything to eat. If we then have our breakfast, we get the next stage of our metabolism known as diet-induced thermogenesis, and that is the energy that we use to digest and absorb and metabolize the food that we eat. And for most people, that's about 10% of their total daily calorie burn. And then the part that's quite variable between people, but also day-to-day within a person, is their physical activity energy expenditure. And that's any energy that our muscles basically use to produce force, whether that's to move around. But even if you're sitting in a chair, your postural muscles that are supporting your spine will also be expending some energy. And so, of course,

the amount of movement that you do in a day will hugely vary and will influence that measurement, whereas your resting metabolic rate will be relatively stable day to day, and that's mainly determined by the amount of muscle mass that you have. So what sort of effect can fasting have on this process then? So fasting can actually lower your resting metabolic rate, partly because if you fast for a very long period of time, you'll actually lose some muscle mass. And in some studies, even a kind of intermittent fasting might lead to some loss of muscle mass. So that could be some thing to bear in mind if you're looking to maintain muscle mass, that fasting wouldn't necessarily be advocated. But it might also influence physical activity. So there are some studies suggest that

that particularly on the days that people fast, they seem to spontaneously reduce their amount of physical activity. So fasting seems to lower the energy expended, but that's probably offset by the calorie reduction as well. So what about, it's often claimed that there are certain effects on blood sugar levels. So is there any truth to that? So there's a short term and a longer term effect on blood sugar levels. So the immediate effect is that you would get a lowering of blood sugar slightly, and that's because you're not eating anything, including carbohydrates, and so your blood sugar will lower slightly. But after a period of fasting for a few days, if you then eat a meal that contains carbohydrate, because we've had this switch to burning fat as a fuel, our body isn't

quite as adapted to using carbohydrate as effectively. And we can actually get a rise in blood sugar that's more exaggerated than we would have otherwise. So there's a bit of a complex interplay there between short-term effects and longer-term effects, and it might also be mediated by your current status. So if you have pre-diabetes, for example, to begin with, there could be a beneficial effect, whereas if you didn't have any signs of pre-diabetes to begin with, it could actually be what might be seen as a harmful effect. It's really normal physiology. So how about any effects that we can see on blood pressure and cholesterol, for example? Yeah, so most of these kind of markers of cardiovascular health will show an improvement, even with relatively short-term fasting.

Some of it will be physiological adaptation and some of it might be a true kind of health benefit. So with blood pressure as one example, part of that reduction might be because you're not eating any sodium, so any salt in the diet. And that will reduce the amount of blood that's circulating. and so our blood pressure can reduce as a result of that. And similarly with cholesterol as to the carbohydrates and blood sugar, if we're not eating any fat, including saturated fat, then we start reducing the amount of cholesterol in our blood, partly because our liver's using up more of that cholesterol. So how about any associated sort of hormonal effects? Like a lot of people talk about testosterone and health at the moment, for example.

Yeah. So for both men and women, actually, with a certainly prolonged, fasting, it mimics what we might see with low body weight. And normally what happens in that scenario is we reduce the levels of these sex hormones. So things like estrogen and testosterone can be lower. That tends to go hand in hand actually with this reduction in metabolic rate as well. So when we see the reduction in resting metabolic rate, it seems to coincide with the reduction of some of these other hormones as well. And that can be harmful if it's seen in the longer term. So for women, for example, they can stop menstruating, stop going through their monthly period. And for men, it might be more subtle signs of things like tiredness and loss of strength and perhaps

libido as well. So they are signs to be aware of. So another topic that's very popular at the moment is the gut microbiome and gut health. Do we know anything about the effects fasting could have on that? We don't know very much from the kinds of studies that can help us establishes. cause and effect. But we do think that there's probably something going on, at least from what we know about the gut microbiome already. So one of the main things that our gut microbiome will respond to from a dietary perspective is the fibre in our diet. So if we eat a lot of fiber, that's providing essentially food for our gut microbiome to feed off. And when we eat a lot of

diverse fibers and fruits and vegetables, then we tend to have a diverse microbiome. So lots of different species in various abundances. And so if we're fasting, then at least theoretically, we wouldn't be consuming any fibre. And so that could have implications for our microbiome. It could also have implications in other ways, maybe changes in blood flow to the gut, for example, but that's really just speculation at this point. So another thing I've also read is that fasting could possibly have benefits for the health of our brains. Is there any truth to that? So similar to the gut microbiome, there are some theoretical ways in which it might. So there are some of the changes, for example, in these increasing ketone bodies providing an alternative fuel for the brain. And the ketone bodies

might have additional effects as signaling molecules as well as just fuels. And so people might speculate that that could have implications for brain health. Right now, we don't really have solid evidence on whether that's true or not, but it's an area of active exploration. Another one which I think might be along the same lines is it's been linked to effects on a sleeping pattern and circadian rhythm. Yeah, so similarly, no solid evidence, but certainly some of the kind of general sleep hygiene rules about not eating a large meal before bed would then make sense that if you had been following that pattern before and you're now a fasting, you might get a benefit. It could equally go to the opposite extreme if you're feeling very, very hungry before bed,

then your sleep might actually be worse as a result. So it probably depends on where your baseline status begins with for that one. So I mentioned earlier about most people probably would go on a fasting diet plan if they wanted to lose weight. But there's another, I know a lot of or some studies in mice have shown that perhaps it has the potential to extend. our lives. I mean, what do we know about that? Yeah, so there's, at least in mice, there are many studies that have shown that any form of calorie restriction, including intermittent fasting, seems to extend lifespan. The most commonly studied form there is actually the 30% calorie restriction. So they basically eat 30% fewer calories than they would choose to. And that's just

extended over their lifetime and they tend to live longer. We obviously can't ethically randomize people or even practically randomise people to do that for their lifetime. But there are some studies that have been done in non-human primates. There's only been two studies in that species that are very close to humans. Unfortunately, the studies seem to conflict. So one of them seems to show it extends lifespan. The other one suggests it doesn't. So it's not easy to interpret, but some people argue that some of the studies in mice confounded by other factors. So for example, they're living in an environment that's germ-free, so they don't need necessarily an immune system. And some would argue that one of the trade-offs you get when you calorie restricts is you might

get a reduction of, say, heart disease, but you could increase your risk of exposure to pathogens and viruses because your immune system could be suppressed. And so some people would argue that it might not translate to humans where we live in the world that has a lot of germs and viruses. And so we're trading off the cardiovascular benefits that you might get with the immune system benefits that that might suffer. So obviously this is a really complicated subject. Are there any sort of like big myths that surround fasting that you'd like to bust? Yeah, probably the biggest myth is that fasting produces health and weight loss effects independent from the energy deficit that's produced. to date, there isn't really any evidence that, robust evidence that you get additional health

benefits over and above the calorie restriction that you could achieve with other means. Now, that doesn't mean to say that for some people actually adhering to the intermittent fasting is an easier way to induce that energy deficit compared to other forms. So I do think it's a valid option, but probably the overstating of some of the health benefits is a myth that I would try to bust. So we've talked a lot about the benefits. I mean, can fasting have any negative effects? I mean, for example, is there anyone who should absolutely not try it at all? I mean, there are potential downside. So one is the potential for

additional muscle loss. There's also potential for people who are already at a lower body weight, that it could tip them over the edge to get these low hormone concentrations that we discussed a little bit earlier on. And in particular for women, that could be harmful for their menstrual cycle, and therefore their fertility. So there are certain populations like that that should certainly be cautious and really question whether they need to try any forms of this fasting. So sort of by way of summing up then, what advice would you give to listeners who are perhaps thinking about trying out fasting for themselves? Yeah, I would say if you are thinking about it, then consider all of the options available first. And if fasting seems like the most suitable to

you, then give it a go, but start small and you can scale it up if needed. So you you don't need to go into a three-day fast straight away. You could just try the time-restricted eating, for example. So maybe skipping one meal in the day and seeing how you go with that. It's also worth being aware of potential nutrients that you might be missing out on and make sure that you're still staying hydrated and getting a good quality diet in and around that. Thank you for listening to this episode of Instant Genius, brought to you from the team behind BBC Science Focus magazine. That was physiologist Professor Javier Gonzalez.

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