Does when you eat affect your body weight? Do meal-timing based diets like intermittent fasting really work? How are your hormones involved and does this new diet fad stand up to scientific scrutiny?
Hello and welcome to Hormones: The Inside Story, the podcast from the Society for Endocrinology where we look at the tiny things inside us pulling the strings.
Back in series one we learned about how hormones hijack our hunger levels, leading to weight gain. But when it comes to losing or gaining weight, what if it’s not just about what you eat, but when you eat.
Over the past few years there’s been a huge rise in interest in diets that control meal timings, such as intermittent fasting, with proponents saying not only that it can help you with your weight, but also that it can improve your heart health, reduce likelihood of cancer, or even prevent Alzheimer's disease. But is there anything to it?
I’m Georgia Mills, and in this episode we’re setting the clock ticking to uncover the science behind mealtime manipulation, and what our hormones have to do with it. If you are what you eat, are you when you eat too?
Sadaf: As a population, we're getting heavier, but within that, some people are particularly much more likely to gain weight than other people.
Georgia: This is Professor Sadaf Farooqi, Professor of Metabolism and Medicine at the University of Cambridge.
Sadaf: We know that people who are struggling with their weight are much more likely to have other health problems. So they're much more likely to get type two diabetes, heart disease, even some forms of cancer are more common in people who struggle with their weight. So we really want to find new ways to help people who struggle with their weight.
Georgia: Bodyweight is down to a whole load of factors, and as you’ll remember from series 1, hormones are a big one.
Sadaf: Hormones are really important for lots of different things. And in the past, people didn't really think about hormones affecting weight. We tended to think it was really simple. It's just what you eat and how many calories you burn. But actually, the body has a really intricate system of hormones for controlling many, many things and weight is no different. So, we actually have hormones that are keeping the weight stable in most people over long periods of time. Now, we all know that sometimes you'll have days where you skip a meal, you don't eat very much, and then other times you'll have a day where you really splash out and you have quite a bit to eat. Now, your body has a way of adjusting itself to keep your weight stable. Otherwise you'd be going up and down all the time. And that job is done by our hormones.
Georgia: If you want a deeper dive into why hormones shoulder a lot of the blame for our size, check out the first episode of series 1. But one area we didn’t cover was how timing comes into play.
Sadaf: So this is a bit of a new and emerging area. So we know certain things very clearly that lots of hormones change during the day. So certain hormones are necessary for you to be alert for being awake. Certain hormones change with your meals. But there's a lot of interest now in whether the timing of your meals can influence how hard or easy it is to lose weight. And the way in which the timing of your meals might do that is because of the hormones that are released when you eat a meal. So, I would say that actually at the moment we don't really know the full details of this. We know there's probably a link because, across the board, people who tend to have a longer period of time without any food, sometimes like a short fast, generally seem to do a bit better in terms of their metabolism. But we don't really know for sure and we don't really know why.
Georgia: A good indicator that timing is important comes from when you look at night shift workers, who often end up eating at odd times of the day and night.
Sadaf: So there's a very clear link between shift work and higher rates of obesity and diabetes and metabolism not being as good. And it's quite likely that a part of that is the time at which people will eat because of their shift work. Now, some of it may be because they also may be less active because they're obviously sleeping off the effects of the shift. So it's a bit complicated, but I think there is pretty good evidence. The other way that there is good evidence is when people bring in people for research and they try to make up what happens in a shift, so they might deliberately keep people awake. So there are studies that are called sleep deprivation studies where you bring people into a research unit and you deliberately deprive them of sleep. It's quite cruel. But when you do that, you find that very quickly their blood tests change and you can detect a change in their hormones and in their metabolism.
Georgia: So why might timing make a difference? Well - we are a rhythmic animal.
Jonathan: Pretty much every tissue, in every cell within our body, has its own clock mechanism, driving local aspects of biology.
Georgia: This is Jonathan Johnston, a Professor at the University of Surrey, who is interested in our body’s internal rhythm section.
Jonathan: We all know that we go to bed at certain times usually, and we wake up and we know we feel tired during the night usually and usually we feel awake during the day. But what a lot of people don't realise is pretty much every major part of our biology is actually rhythmic across the day. And this is something that has been found in the vast majority of all biological species that have been studied. But we also have other rhythms, so some animals and plants have seasonal rhythms, so they go from summer to winter and show huge seasonal changes in their biology. But there are also other rhythms that occur much more frequently than every day, so a lot of hormones are released in what we call an ultradian rhythm, so they go up and down over the course of perhaps every hour or so. So how are these rhythms controlled? Well, there are different control mechanisms, depending upon what sort of rhythm you're looking at. The most studied and the best understood rhythms are our daily rhythms, which are sometimes called circadian rhythms.
Georgia: Thinking about these internal clocks, like, to be honest, I don't really have much idea about how any clock works. But how on earth does a tiny cell in our body know what time it is?
Jonathan: A number of groups around the world have been studying this now for the past few decades. And in fact, the Nobel Prize was awarded a few years ago to colleagues in the states who started off studying the molecular basis of the clock within fruit flies. And the basic mechanism is pretty similar in any species that you look at. Some animals have slightly different genes and proteins that regulate the clock, but the basic process is the same. And the way in which it works is a cell rhythmically produces or expresses genes and proteins, which then feed back onto the nucleus of that same cell a number of hours later and switch off the activation of their own coding genes. And so without going into the details of it, do you have a set of genes within each cell that are rhythmically producing messenger RNA and proteins? And these proteins feed back on themselves on a rhythmic basis and this entire loop takes 24 hours.
Georgia: So I know that there's a lot of focus on: You are what you eat, but should we be paying attention to when we eat?
Jonathan: The short answer is, is yes. There are lots of ways in which people have been studying the interaction between meal timing and our body clock system. And it's been known now for a number of decades that the way we process a meal depends on what time of day we eat it. So, for example, if we eat a meal over the few hours after that meal, we get a peak of things like sugar and fat within our blood. And then as those molecules get picked up by the metabolic parts of our body, they come back down to baseline levels after a meal. But if we eat a meal at night, we get a higher peak of sugar and fat within our blood. And it takes longer for those sugar and fat molecules to be stored within the body.
And although there's a little bit of discrepancy in the literature, the basic idea which has been taking hold is that if you tend to eat more of your calories in the morning and fewer in the evening, that's in at least, and certain people will actually tend to help you lose weight and possibly improve your your metabolic status.
Georgia: So, because of the changes in hormones levels through the day, at some times we are more “primed” to eat food than at others. So should we be planning our meals around our body clocks?
Jonathan: Generally, yes. For most of these interventions, the idea is that you should be consuming most of your energy, most of your food at the time of your internal rhythmic cycle when your body is best geared up to process that meal. So if you imagine the way our bodies have evolved, we're expecting to be eating when we are awake and we're expecting to be fasting when we are asleep. And so it kind of makes intuitive sense that our bodies are better able, better equipped to process a meal during the daytime than they are at night. And of course, this has potential consequences for, for example, night shift workers who are forcing themselves to eat during their biological nights. And it's been shown very clearly in multiple studies that night shift workers have a higher incidence of things like obesity and type two diabetes. So one of the groups that we hope to target and improve their health would be night shift workers.
Georgia: So does this explain the old adage that you should eat breakfast like a king, lunch like a prince, and dinner like a pauper?
Jonathan: Yeah, that's basically it. What we have now is some strong scientific evidence to support adages like that. But yes, basically that seems to hold true.
Georgia: And so I can understand why our bodies and hormones are set up to process food in the day when we're eating and be fasting at night. But presumably dinner is still when we're awake. So why is it better to eat in the morning?
Jonathan: A lot of this will depend upon what time of day you have your evening meal and how big that evening meal is. It's been known since the 1960s, 1970s, that our sensitivity to the hormone insulin will decrease as we go through the afternoon and into the evening. And indeed, there's a term, I think, in the late 60s called afternoon diabetes, which indicates simply that people have much less sensitivity to the insulin that bodies make in the evening compared to in the morning and the early afternoon. Why that is, I can't say exactly. I suspect it's just part of the way our natural rhythm has evolved that our bodies are not anticipating us to eat so much after the mid-to-late afternoon.
Georgia: Now I think this is the first time we’ve mentioned insulin on the podcast, which is maybe a disservice as insulin is very, very important.
Insulin comes from a place called the islets of langerhans, which sounds like a beautiful Mediterranean holiday destination - but is actually a very fancy name for a group of cells in the pancreas. We have their discoverer, Paul Langerhans, to thank for that. Insulin does a whole bunch of things in the body, one of its main jobs is to allow our cells to hoover up sugar from the blood to use for energy - if we can’t do this, things start to go very wrong - leading to coma and death. Insulin is usually released after a meal, which makes sense as it’s when you are likely to have lots of sugar in your bloodstream. This is why people with type 1 diabetes, who can’t make insulin themselves, need regular injections of the hormone after meals.
Even for people without diabetes, our sensitivity to insulin changes throughout the day, with its effects being most potent in the mornings. And, because sugar that isn’t taken up by cells and used for energy eventually gets converted into fat and stored away for later, could this mean a calorie in your breakfast is better for you than a calorie in your dinner?
Well, the answer is another science backed ‘maybe’ - you know you love them!
Sadaf: I think that's a little hard to see from right now. I think the timing question really needs a lot more research to figure that out. At the moment, we don't really have any good evidence that if you eat a certain type of food or calorie at a certain time, it'll be better for you.
We do know that if you eat later in the day or very late in the day, that is generally linked with poorer blood sugar control or poorer metabolism because your body does have a relatively slow metabolism later on in the day. So generally the one bit of advice that I think is reasonably backed up is to not necessarily have meals too late.
Georgia: We’ve taken a look at what the science says about when you should eat, but what about skipping meals altogether? Many cultures and religions around the world incorporate elements of fasting, where people don’t eat for certain periods of time.
And if you’ve been following the health trends of the past few years, you’ll have probably come across an eating plan known as intermittent fasting, which involves squishing your meals into a certain period during the day, such as just 8 hours, and leaving a big stretch of no eating either side.
Advocates of intermittent fasting are enthusiastic about its benefits for weight control and health, but does the science stack up?
Results from animal studies are particularly exciting - rats have been shown to live longer and appear smarter with restricted feeding times, while obese animals lost weight.
The idea of controlling meal timings isn’t new - the first paper I found was from the 1940s - but more that 70 years later we still don’t really know exactly how this works in people.
Jonathan: And it's still very much an open question. What is the mechanism that causes the benefit of this time restricted feeding? It may well be in part that it means that we are eating a higher proportion of our calories at the time of day when our body is geared up to receive that meal. But equally, it could simply be that we are having a longer fasting period every day. And there's some evidence out there that prolonged fasting is actually something that's good for us.
As long as it works, then it's something which should be healthy and safe to do for the vast majority of people. There are some people, for example, patients with Type one diabetes where their meals have to be timed, particularly relative to their insulin injections. But for the vast majority of people, time restricted feeding is something which should be relatively safe and straightforward to do and potentially doesn't require a large change in your dietary intake.
Georgia: Basically, if it works for you, great. But, as Sadaf Farooqi points out, it’s OK if it doesn't.
Sadaf: There's a lot of hype. There's a lot of promises and there's a lot of not great science being talked about. So, I think overall what we can say is that, clearly people who are struggling with their weight or worried about their weight need to be generally consuming less calories and keeping more active. Now, there are different ways to do that, and everybody's different in how they might approach that. There isn't really strong evidence for one type or style over another. In fact, when they're compared head to head, most of them are pretty much the same. Now, we all know some people who've lost loads of weight on a particular diet or by doing a particular regime of fasting or some other pattern. What it tends to mean is that they found that diet or that pattern is easier to stick to. So it's often about what you can stick to, and if you can stick to it, then you're probably going to see better results. As long as it's reasonably healthy and you're not doing things like skipping loads of nutrients that would otherwise harm your health.
So the best evidence is from animals. I think finding out how it works in real people is the hard bit.
Georgia: Still, if we find out why manipulating meal timing is having this effect, and who it might be most beneficial for, it might pave the way for not only behavioural but pharmaceutical interventions
Sadaf: The way those changes in the meal, the timing of the meal, the size they are going to work is by changing the hormones so the meals will change the hormones and the hormones will then change what happens in the brain, the rest of the body. So if we could figure out that middle step, then in theory you could tap into that knowledge and you could maybe find a medicine or even a dietary supplement or something that does the same thing.
Georgia: Of course, most people’s ears prick up when we start talking about potential weight loss wonder drugs that mimic the effect of changing meal timings. But these treatments could also have important health benefits for people whose eating patterns are disrupted.
Jonathan: We’re trying to understand the basic biology in healthy volunteers. One of the big things we have to do now is to try and take that food to particular groups such as shiftwork. It's a very difficult question for many reasons, one of which is that there is a lot of different types of shiftwork out there. And so what might be right for someone doing long term night shifts might not be ideal for someone who's doing occasional night shifts or someone who is doing rotating shifts. So it's a very difficult question and probably the whole new podcast just to try to just to try and address it. I will say that people are looking into that now. I can't really give you a single, concise answer at this stage, but over the next five or ten years, I anticipate that there will be a lot more advice coming out as to how you can best adapt, particularly your your feeding to different shift work schedules to try and improve health for those particular people.
Georgia: There’s another good reason to find out exactly how fasting affects our hormones, because millions and millions of people do it every single year.
Shazia: So Ramadan is the ninth month of the Muslim holy calendar. Each fast will normally start at sunrise and end at sunset, and that will go on for 30 days.
Georgia: This is Shazia Hussain, a consultant in general medicine, cancer, diabetes and technology in London at the Hampton University Hospital.
Shazia: Fasting in general, be it a religious fasting or non-religious fasting, has a number of implications on hormonal secretion in general. Your cortisol surge, the high surge happens on waking, first thing in the morning. And sunrise can be as early as 3:00, 4:00 in the morning. So if individuals are waking up and starting their day earlier, then they will have a change in their circadian rhythm and release of cortisol secretion. Similarly, particularly during the month of Ramadan, some individuals will find that they're praying quite late into the night. Typically you have a fall in cortisol secretions.
Georgia: While fasting during Ramadan is safe for the vast majority of people - there is a lack of research on the health impacts of this kind of disruption to meals, especially for people with hormonal issues and other conditions.
Shazia: I don't think it is discussed enough. And I think a lot of individuals probably participate in fasting and may well be putting themselves at risk because they don't realise the importance of adherence to the medications and the fact that there is medical help and alternatives available to them. For example, in patients with adrenal insufficiency who require steroid replacement, if they don't take this steroid hormone replacement because they are taking part in fasting, then they obviously put themselves at life threatening risks through an adrenal crisis. And so if they take part in fasting, they will be missing medications, which isn't something advisable, particularly in adrenal insufficiency with a life threatening complications of emission of steroids.
Georgia: So while fasting - why ever you do it- is clearly safe for the vast majority of people, it might not be a good idea for people with some underlying health conditions, so you should chat with your GP if that's something you’re concerned about.
More generally, while there seems to be solid evidence that eating very late in the day is a bad idea, we still need more data from human studies about the health benefits of manipulating mealtimes with things like intermittent fasting before we can make the perfect caloric schedule.
Georgia: Is it bad this interview is making me really hungry and it’s about 5 in the evening?
Jonathan: I’m the same. I kind of go into phases like this where I remind myself actually that I've got something in the cupboard that might be quite nice to snack on. I think that's probably perhaps more psychological than circadian, but it all counts and interacts.
Thanks to our guests, Professor Sadaf Farooqi, Professor Jonathan Johnston and Dr Shazia Hussain.
This is a First Create The Media production for the Society for Endocrinology.