Dr Rupy: Have you ever wondered if some people simply have a fast metabolism? You know, the kind of people that always seem to be able to eat anything and still maintain a healthy weight? Or do you think your metabolism has slowed as you age? What about the effect of the menopause? And have you ever wondered about whether intermittent fasting or certain supplements are evidence-based ways to speed up your metabolism? If these are some of the questions you have, this is the episode for you. Hi, I'm Dr Rupy. I'm a medical doctor and nutritionist. And when I suffered a heart condition years ago, I was able to reverse it with diet and lifestyle. This opened up my eyes to the world of food as medicine to improve our health. On this podcast, I discuss ways in which you can use nutrition and lifestyle to improve your own wellbeing every day. I speak with expert guests and we lean into the science, but whilst making it as practical and as easy as possible so you can take steps to change your life today. Welcome to The Doctor's Kitchen podcast. If you've ever had questions about metabolism, whether you can speed it up, slow it down, whether it changes, what even is metabolism, these are all the questions that I put to Dr Adam Collins today. He's a researcher at University of Surrey where I completed my nutritional medicine masters at. He actually was one of my lecturers during the modules there. And Dr Collins in the field is a really well-known expert in metabolism and nutrition science. He's done a ton of research on energy expenditure, intermittent fasting and how different diets may impact metabolism. So today, it's going to be a bit of a deep dive episode, talking through what metabolism actually is, whether there are such things as fast and slow metabolism or is it an artefact of the way in which we measure metabolism? Can metabolism be affected by ageing and whether we can actually measure that as well? Is cold exposure good for metabolism? Does that mean we should all be jumping into cold plunges? Stress, what does that have an impact on? Does that impact our metabolism in a negative way? And we're also going to do a bit more of a dive into a relatively new concept called metabolic flexibility, which I'll briefly explain now is the efficiency and the effectiveness by which one can transfer their fuel source and utilise carbohydrates versus fats. So how readily can you burn fats when you need to and how quickly can you refer back to carbohydrates and how efficiently are you even burning carbohydrates? This concept of metabolic flexibility I think is going to become a lot more mainstream over the next couple of years. And we're also going to be talking about why metabolism matters outside of weight. What could the impact of metabolic inflexibility be or a poor metabolism be outside of just weight gain and weight loss. Like I said, this is going to be a bit of a technical episode. We use words like substrate and fuel to mean food. Sometimes we use the word carbohydrate and glucose, which is essentially the breakdown of carbohydrates into the simple molecules of glucose, also known as sugars, or that's a simple sugar. So just bear in mind, we do actually utilise those terms interchangeably. And if you can follow along, I really feel that this is going to give a lot of clarity to a subject that is quite nuanced and unfortunately gets brushed over quite a bit online. And that's why I wanted to do a deep dive where I'm hoping to guide Dr Adam to explain this complicated topic in a way that is approachable for you the listeners and the wider general public. I really do hope you find this useful. And if you have any extra questions about this, please do let us know on the YouTube comments.
Dr Rupy: Let's dive into what we mean by metabolism, because I think any answers to the question around or questions around metabolism have to be contextualised to we need to explain exactly what we mean when we say the word metabolism. So what is metabolism? How would you explain that to folks?
Dr Adam Collins: So, strictly speaking, metabolism is a term that describes the collection of sort of biochemical processes and chemical reactions that happen in your cells and then collectively in your tissues and then all of those different tissues and organs on a whole body level.
Dr Rupy: Okay.
Dr Adam Collins: So you could drill it down into what's happening at the cell or look at it in an organ or look at it across multiple organs.
Dr Rupy: Okay.
Dr Adam Collins: But it's how your using the energy substrates that you're eating. So that's predominantly your carbohydrate, fats, to some extent your amino acids, and how that is done using your oils of the machine, should we say, which is your vitamins and minerals which are acting as sort of co-factors, co-enzymes of of the metabolic machinery.
Dr Rupy: Okay. So when you say co-factors and co-enzymes?
Dr Adam Collins: So the things that are helping the reactions to happen. They're not necessarily broken down for any, they're not broken down or used for energy themselves, but they're making sure that the reactions can take place or that the metabolic processes can can happen.
Dr Rupy: Okay. So from my understanding, metabolism is the production of energy.
Dr Adam Collins: That's when we think of energy metabolism. And I suppose from a nutrition diet perspective, that's what we focus a lot of our energy, a lot of our focus on. But obviously there's more to metabolism than just taking energy or recycling energy because there's things related to detoxification and other metabolic processes that you might use, which do involve energy, which might be part of your immune response or other physiological processes.
Dr Rupy: Okay. So we're not just talking about the production of energy for all these different reactions around your body. We're talking about protein generation, immune complexes, hormone regulation, etc. So this is all lumped into this word that we band around called metabolism.
Dr Adam Collins: Yeah. Yeah. The other way to think about it is if you looked at the body and you looked at what you're feeding into the body, in terms of what's coming in from diet and what you're eating and drinking, and what you're excreting in terms of your urine and your faeces, and how much heat you're generating, how much work you're doing. So that's your sort of energy in, energy out, energy balance type model. And metabolism is essentially the black box that does all that. So what you're feeding in and what you're producing from what you're feeding in is basically the job of your black box, your metabolism.
Dr Rupy: Okay. So the sum of all these different reactions around the body that are happening at a cellular level, collectively in tissues and collectively even more so in different organs.
Dr Adam Collins: Yes.
Dr Rupy: So give us a ballpark figure of how many of these different reactions are actually occurring every second or every every every day, because I don't think people understand the magnitude of what we mean by metabolism.
Dr Adam Collins: Yeah. I suppose we're having to do these processes continually all the time. So we are recycling our unit of energy, several thousand times a day. And sometimes that can increase manyfold, maybe several thousandfold higher in terms of rate when we're doing physical activity or exercise. So because we can't hold enough energy or store enough energy in this energy currency form, we have to be able to recycle it. So we're recycling it by burning other things.
Dr Rupy: So when we talk about these reactions that are occurring all over the body, my understanding is that there are certain parts of the body that are more metabolically active. What are the most metabolically active cells in our body? Where would we find those?
Dr Adam Collins: So if I was thinking of my sort of metabolic rate, so if we go back to what do we mean by metabolism? I've gone straight in with my technical definition of what it is, because metabolism is the sum of all the parts that I described on a whole body level. And a lot of that is around energy metabolism or our ability to take the energy in what we're consuming and use it to recycle and and supply the energy that we need on a daily basis, which is constantly a demand and also we need to supply the things that are being turned over. So things like proteins, for example, are being broken down that we need to obviously replace and and restore. But another way in which people think about metabolism is going back to that whole black box idea of energy and energy out, is that people think of metabolism as just the product of metabolism, which is how many calories am I burning? Right. So how much is my, how fast or slow is my metabolic rate? And our and people talk about metabolic rate, which is technically just the amount of energy I'm burning lying at rest doing nothing. And and that is obviously a product of your metabolism. That's what you're breaking that energy down that you're supplying to fuel. But if I think of that metabolic rate, so let's say I'm burning just lying at rest doing nothing, 1500 calories a day, which could be around one calorie per kilogram per hour, let's say, roughly. So if I was looking at that, not, I'm not just one homogenous mass. I'm made up of different organs. And so what is it that's actually demanding that energy or using that energy in the body? So that's the question that you're really asking. And if I was to look at it, and there's been a certain really nice work by people that have tried to model metabolic rate and predict metabolic rate based on organ mass and almost assign metabolic rates for individual organs. And if you do that, then if I was looking at your metabolic rate, probably about half your metabolic rate is being drawn from just your liver and your brain. So and the rest of it is from everything else.
Dr Rupy: Half?
Dr Adam Collins: So about half your metabolic rate just lying at rest doing nothing is, you know, to meet the demand of your liver and your brain.
Dr Rupy: Gosh. And and just by mass, like your brain's a very small part of your of your body. So per kilogram of weight, it's doing a lot of stuff.
Dr Adam Collins: It's doing a lot. And I would say your liver is probably one of the most active metabolic organs. It's probably the metabolic organ because it's the one organ that can pretty much do most of most things you want to do from a metabolic perspective. Other tissues have limitations or specialisms in what they can do. Um, but another way, so yes, it might be your liver and your brain, but then if I look at what else is drawing energy at rest, if you look at your kidneys, your kidneys alone are probably drawing maybe about 20% of your metabolic rate, up to 20%. And think how small your kidneys are. It's just a an illustration of how metabolically active some tissues are relative to others.
Dr Rupy: That's incredible.
Dr Adam Collins: And obviously people think muscle is the thing. Oh, muscle is the thing that's going to be drawing demanding the energy, which it does, but not because per kilogram it's it's very active. It's just that you've got a lot more of it because obviously there's a lot more muscle. You've got several kilograms of muscle as opposed to maybe half a kilogram of kidneys and and so on. So it's a way of sort of getting people to understand what is actually happening. Obviously that's at rest. Okay. So when you're lying at rest and obviously your muscles are not doing anything. And it's not to say that they're sitting there inactive completely, but they're obviously not going to be demanding much energy. Of course, when you move, do any physical activity, that energy demand is going to go up. And obviously it's not because your liver and your kidneys and your brain necessarily are going to be demanding more energy. It's obviously your muscle. And muscle is is probably the most dynamic organ because it's the one organ that can go from not zero, but you know, very little energy use to like a thousandfold energy use when you go from rest to sort of exercise. For example. The other thing which we might come to later about muscle is not just about the variability in terms of the energy consumption, it's also the fact that your muscle is designed to basically burn anything for energy. It's like your main metabolic consumer. Not because it's the most metabolically active, but because it's the thing that you need to supply with energy in order to do any work. And it's the thing that is going to be the most variable in terms of energy consumption or fuel consumption. But it can burn anything. It can burn carbohydrate, obviously it can burn fat, it can burn ketones, it can burn amino acids. It's it's basically a consumer, the metabolic consumer. And one of the issues that you have is that if that metabolic consumer doesn't work or is compromised, then of course you're going to have all the consequences that you see with metabolic disease. And now part of that could be because you're completely sedentary and you're never actually demanding that energy from that muscle. So of course it's not serving you as a metabolic consumer as you think. But even aside that, there are things that we might come to that illustrate how your muscle is is impaired in its ability to serve as a metabolic consumer.
Dr Rupy: Okay. So going back to, this is great. I I I love sort of this idea of our brain and our liver taking up 50% of our energy needs at rest. And I think that's going to be really surprising for folks. Uh, our kidneys as well. I guess the other organ that comes to mind is our heart and my sites, the the the cells, the muscle cells within our heart because it's consistently pumping all day.
Dr Adam Collins: Yeah. And again, that's I suppose the thing that people are conscious of when they're so if you're lying at rest doing nothing, then of course your heart is still pumping and you're breathing. But those processes are actually very energy efficient. You don't really spend that much energy to because breathing is actually quite a very passive process. You know, you're not really doing much muscle contraction unless you're doing forced breathing. But if you're just doing quiet breathing, you know, the contraction of your diaphragm, which only has to go in one, just contracting and relaxing. It's not like you've got a whole combination of muscles. Very, very in terms of energy terms, very efficient. Same with the heart. Yes, it's a muscle and it's pumping, but relative to other things, it's not going to be demanding a lot of energy when you're lying at rest. Even though you're, that's the thing you're conscious of doing actively when you're lying at rest doing nothing.
Dr Rupy: So at rest, the as a percentage, the majority of my metabolism is going to be taken up by those organs collectively. If I was to get on a treadmill or a watt bike and just go for it, trying to do my hit training, every single muscle is being used, maybe I'm using dumbbells at the same time, you know, just run with me. I'm trying to utilise all my skeletal muscle as much as possible. As a percentage of my metabolism, how much can it shift towards skeletal muscles as a ballpark?
Dr Adam Collins: Well, it would, obviously your overall energy expenditure is going to go up and it could go up relative to rest 10 times what you have at rest.
Dr Rupy: Oh, wow.
Dr Adam Collins: Now, of course, that's not because your liver is burning more energy or your brain's necessarily burning more, obviously it's your skeletal muscle. That's what I was saying about how variable the energy consumption of your muscle is. And so you can in terms of energy expenditure, go from rest to exercise and that could be if you were walking, it might be three times your metabolic rate. If you were doing something more vigorous, it could be up to 10 times. If you're doing something like cross country skiing, could be in the teens, it could be 10, 15 times your metabolic rate. And so that just illustrates how variable your energy expenditure is because of course, we think of metabolic rate and we express metabolic rate in terms of calories per day. As if like you're going to spend the whole day lying at rest doing nothing. But of course, people don't. Even the most sedentary person is not going to have a energy expenditure or energy requirement, if you think of it in other way, because energy requirement would mean you're meeting your energy expenditure of the same as your metabolic rate. It probably be not twice as much, maybe not 50% more, but might be something like 20, 30% above your metabolic rate even in someone relatively sedentary.
Dr Rupy: Got you. Okay. So it can really flex up in terms of the percentage of your total.
Dr Adam Collins: Because you have to factor in that physical activity. And that's where a lot of interest is on metabolic rate because metabolic rate, if I was thinking of all the energy that I'm burning on a daily basis, for most people, metabolic rate is the biggest component. That explains probably about 70, 60, 70% of your total energy expenditure.
Dr Rupy: I want to double click on that in a second. So just to clarify, metabolic rate is an objective measure of our basal metabolism. So how many calories I burn at rest.
Dr Adam Collins: At rest.
Dr Rupy: Okay. So, you know, the combination of our liver, our brain, our kidneys, all those metabolically active organs, etc. And metabolism is more the sort of like definition of actually what's going on, all the different processes that are occurring in every cell in our body every second of every minute of every day. With this in mind, aside from the obvious difference in weight, if you had someone of the same weight, would you see variation in their basal metabolic rate? Are there some people that are slow versus some people that are fast?
Dr Adam Collins: In my experience, no.
Dr Rupy: Okay. Interesting.
Dr Adam Collins: I mean, I, I mean, this is going back many, many years, um, when I was doing my PhD and a lot of that at that time was me measuring metabolic rate in hundreds of people. And it's highly predictable. So you can have two people that are basically the same height, weight and age and they'll have very similar metabolic rates.
Dr Rupy: Okay.
Dr Adam Collins: So yes, it might vary, but the variation is relatively subtle. So you're not talking about, oh, that person's going to burn two or 300 calories a day more than that person. You're looking at like 50 calories, 100 calories.
Dr Rupy: Okay. So not a significant difference.
Dr Adam Collins: And it's probably something that is within almost within the realms of measurable difference as well, because there's obviously a certain error that's associated with measuring these things as well. So how sure can you be as to is that a true difference or not? Or is it just normal variation between different measurements that you're taking?
Dr Rupy: Can we, so I I take that. I think so from from the listener's perspective, very predictable. You could potentially use an online calculator to give you a rough estimate of what your basal metabolic rate is likely to be. If you have an extreme example where you have an 80 kilogram male, let's say, one of whom has got quite a lot more muscle mass than the other, but they are of the same weight, would that accentuate the difference between their metabolic rate?
Dr Adam Collins: Yeah. Yeah, you could, you could definitely extend that. And that's again, going back to the sort of modelling. So for most, most of the time, a lot of the differences that you see between people or most of the differences you see between people is explained by simply height, weight and age. Yeah, the body size and the age and the gender. Um, now, if you had extremes of body composition where you had someone who had a lot of body fat and very little lean mass versus someone who was the opposite, you could see differences in in metabolic rate. But remember that 50% of your metabolic rate is the organs, isn't it? So you're not going to have a different, you're not going to have someone who's going to have bigger organs than another person.
Dr Rupy: Yeah, yeah.
Dr Adam Collins: Um, so again, it's when you look at the modelling, it ends up being relatively subtle, the difference.
Dr Rupy: Yeah, yeah. I think that's a really good explanation because I think a lot of people would have assumed the big muscly bloke is going to be a lot more metabolically active than the other one. But going back to what you said right at the start of this conversation, if the vast majority of your metabolism is going to be, your metabolic rate is going to be explained by your organs and what they do at rest, then it's not going to have a massive impact regardless of the differences in body composition.
Dr Adam Collins: Yeah. Yeah, I would say so. Um, now, I would maybe put a caveat to all of that I said. So if I was, like I said, going back to data that I've collected myself, if you look at cross-sectionally, just measuring loads of different people of all different shapes, size, ages, backgrounds, whatever, then it's pretty predictable. You could predict what someone's metabolic rate is going to be. But on an individual basis, you might see differences within that same individual. And that's the interesting thing about metabolic rate, which comes into this idea of metabolic rate being something that could be adjusted.
Dr Rupy: Yeah.
Dr Adam Collins: Right. So it's not the fact that that person's got a higher metabolic rate than that person, but circumstances in an individual might adjust their metabolic rate up or down relative to themselves.
Dr Rupy: Okay.
Dr Adam Collins: Does that make sense?
Dr Rupy: That makes sense. That does make sense because this brings me on to some more questions. So you already mentioned one about different stages of life. So you do see this gradual decline in metabolic rate. And that is taken to mean, you know, if you're eating the same amount of food, give or take, then you would expect an increase in weight. Am I correct in thinking that?
Dr Adam Collins: Potentially, yeah. But you would hope that as a system, your body's adjusting that over time because if you're thinking of energy balance, which is calories in, calories out, that is not just down to willpower and conscious control. Your body's obviously regulating that as part of any other homeostasis mechanism. So it would adjust, if you've got an adjustment in one side, it would also adjust the other side to compensate to try and keep you in balance.
Dr Rupy: Yeah. So that could be appetite, movement.
Dr Adam Collins: So appetite and intake could be adjusted to to levels of or changes in energy expenditure.
Dr Rupy: Okay.
Dr Adam Collins: Um, in theory. Now, of course, that is difficult to see experimentally because you can totally override all of that by being sedentary and choosing to overeat more than your appetite is driving you to. And maybe there's certain things that are easier to overeat than others. And there's lots of things that will drive either energy intake or energy expenditure that will override your homeostatic drivers of control.
Dr Rupy: Okay. And homeostatic drivers control basically everything keeping balance.
Dr Adam Collins: It's just keeping everything in balance. Yeah. And we know that there must be things that are keeping people in balance because we don't gain and lose weight freely all the time. We are relatively weight stable most of the time, despite the fact that we don't prescriptively eat, well, some people prescriptively eat the same thing all the time, same calories every day, expend the same calories every day. But there's so much variation in how much energy you're consuming on a daily basis as well as how much energy you're expending on a daily basis. But like I say, as a measurable entity, it's incredibly difficult to see because you can't really measure energy balance unless you're carefully controlling or observing it in a situation where you can accurately measure both sides.
Dr Rupy: Yeah, like in a experimental, you know, metabolic chamber or.
Dr Adam Collins: Yeah.
Dr Rupy: So I've got the listener in my ear right now. Dr Adam, I get your experiments and I understand people are, you know, relatively predictable, but I hit middle age and then I got middle-aged spread. You know, this uh suddenly increase in weight, largely concentrated around the midriff. And I eat the same and I exercise the same and it's affected. Hence, I feel like my metabolism has slowed. It's a phenomena that everyone is acutely aware of. How would we explain this?
Dr Adam Collins: Difficult from the point of view of metabolic rate. So it's not like you suddenly hit your 40th birthday and your metabolic rate drops by 200 calories a day.
Dr Rupy: Okay. I'm glad about that because I'm hitting 40 in a couple of weeks, so.
Dr Adam Collins: Um, but I, yes, there are going to be subtle changes in in metabolic rate that happen with ageing. Um, but I think, and this is going to sound totally anecdotal and stereotyping and I apologise in advance, but what tends to happen in middle age is you get life changes in middle age that people get married, they have a change in lifestyle and that could translate into not differences in metabolic rate, but differences in physical activity. So become less physically active. Equally, they might have um other things that are influencing their energy intake that weren't there before. So they might have more money to spend on nice food or eating out in restaurants or, you know, things, life or life stages can influence eating behaviour, which is obviously related to energy intake, and also physical activity behaviour. So I don't think it's something necessarily that you can blame from a from in terms of your metabolic rate.
Dr Rupy: Okay. What about menopause? Um, so does the shift in hormones, i.e. the drop in oestrogen and testosterone, have an impact that would explain the sudden um increase in weight?
Dr Adam Collins: Yeah. Interesting. That's an interesting sort of stage. Now, there's some suggestion or at least theoretical logic to say that, let's say in the menopause, um, and let's just scoop back a little bit to what I was saying about energy balance and energy homeostasis, the fact your your body's trying to keep everything in check, you know, to to match your calories into your calories out. Now, in order to do that, your your body has a bit of an adjustability to obviously appetite control, but also energy expenditure or metabolic rate. So it can sort of dial up and dial down metabolic rate. Um, and it can become more wasteful or more frugal. Um, and part of that's driven by things like your thyroid hormone and your sympathetic response. So there is a sort of central control over that. Um, and it's not to say it's the same mechanism, but a way to sort of prove that that capability exists is when you look at cold acclimatisation. Because if you stick someone in the cold, obviously they need to increase their body temperature and have to have to produce more heat to compensate. Now, what they do initially is they shiver. You have to shiver, so you do all these micro muscle contractions because that's a way of wasting energy because you you can't convert chemical energy into work very effectively. So a lot of that gets lost as heat. So that's a deliberate way of generating heat. But of course, that's debilitating. You can't shiver all the time. So, but you have got an ability to do what they call non-shivering thermogenesis or heat generation without having to shiver. Um, and we have an innate ability to do that all the time. But when you when you expose someone to the cold over a period of time, their ability to do that increases.
Dr Rupy: Okay.
Dr Adam Collins: So they now can maintain the same body temperature but without having to shiver. And when you measure their metabolic rate, someone who's in a cold acclimatised state would have a higher metabolic rate than someone who's in a temperate climate because they've they've basically wasted some of the energy that would have gone to other things purely for generating heat.
Dr Rupy: And how does one acclimatise to that colder temperature? What what kind of practices are that?
Dr Adam Collins: So that that um, well, so let's say what's doing the business there is is a certain form of your adipose tissue or your fat tissue, which is um, not pure brown. So you've got white adipose tissue and brown adipose tissue. And people talk about brown adipose tissue as a as something that's heat generating. But what happens is that your, we don't really have any recognisable stores of brown adipose tissue, pure brown adipose tissue, unless you're a neonate just, but but what we've got the ability to do is to brown our existing white adipose tissue.
Dr Rupy: Okay.
Dr Adam Collins: So we can turn what would be just a way of temporarily storing fat to then turn it into something that is turning fat into heat.
Dr Rupy: Okay.
Dr Adam Collins: Um, and we can adjust, we can morph basically certain fat cells into these heat generating fat cells.
Dr Rupy: Okay.
Dr Adam Collins: Dynamically to to burn off heat.
Dr Rupy: How do we do that?
Dr Adam Collins: And that's something that's centrally controlled in exposure to the cold.
Dr Rupy: Okay.
Dr Adam Collins: So again, it's something that's mediated through a sympathetic response, through things like thyroid hormone. So people that have got an underactive thyroid, they they're not able to do that as effectively because they haven't got that mediator to to trigger that change.
Dr Rupy: And so do practices like cold exposure through ice baths or cryo or, you know, being in a colder climate, maybe taking off your jacket during the winter months and walking to the station instead of actually, you know, dressing up as warm as possible. Are these all practices that could potentially encourage that transformation?
Dr Adam Collins: Potentially, yeah. But I suppose you don't have to go to extremes. That's the interesting thing. So people obviously think about having like ice baths or having cold showers every morning or immersing themselves in sort of a glacial lake somewhere. But you don't have to go to that extreme because this cold acclimatisation or this trigger um can happen by just going down to say 15, 16 degrees centigrade. You don't have to go down to like minus 20.
Dr Rupy: Right. Yeah, yeah, yeah.
Dr Adam Collins: It's just an adjustment um down by a few degrees. So just turning your thermostat down could make a difference potentially. Um, but I mean, I I don't know whether that's going to make a massive difference, right? But you could follow the logic to say, and I'm not saying this is a single-handed reason why we're all, the rate of obesity is increased. But if you look at the rate of obesity that's increased since the 1980s to now, and you look at the average house temperature, that everybody's living in a temperate climate all the time because we've all got central heating, we've all got double glazing, we've all got nice duck down duvets. We're never in a situation where we're outside of like a thermoneutrality where we're we're never triggering any any cold induced thermogenesis.
Dr Rupy: That's so interesting. I haven't heard that as a reason as to why we're all obese.
Dr Adam Collins: It's a fanciful suggestion. I know, yeah. But anyway, I say all this because it's an illustration of the fact that we've got adjustability to our metabolic rate.
Dr Rupy: Yeah.
Dr Adam Collins: The fact that we can adjust it up and down. Okay. Um, I think what's interesting is that people have sort of fixated on brown adipose tissue or this sort of non-shivering thermogenesis way of wasting energy as heat as a great thing that we could use to to keep us from gaining weight or help us lose weight. Oh, if we can just burn off our fat as heat or be more energy wasteful, that'd be a great, great thing to do. But I think I I think from what the evidence has has shown is that if you were looking at changes in weight, yes, you get an adjustment as a course as a consequence of weight change, but it's probably not, it's a similar thing in the sense that you can adjust your metabolic rate by being even more wasteful or more frugal. But it's not mediated through brown adipose tissue. I think it's mediated by other things, other mechanisms in play that are doing the same thing. And I think it's it's likely candidate in terms of your organs, not adipose tissue, is muscle. That there's things in your muscle you become more energy wasting in muscle as a way of of um consuming more energy.
Dr Rupy: Okay. I want to get on to muscle a little bit later, but before, you know, in line with the number of people with central heating over the last couple of decades, stress has definitely gone up. Yeah. And mental health and awareness of mental health has definitely uh increased, which is obviously a good thing. Can stress affect one's metabolism? Some people might think so after they've just heard you talk about the brain as a massive consumer of of energy. If um someone is more stressed, can that disrupt some of these balancing homeostatic mechanisms that can lead to obesity?
Dr Adam Collins: Yeah. Yeah, I would say so. Maybe not so much in terms of metabolic rate. Okay. Um, but if you think of what stress is physiologically and how it manifests in the body, it's essentially your fight or flight response, which in a way is the same thing as exercise is, as another example of an an analogy to a fight or flight response. So you've got lots of catecholamines, adrenaline, noradrenaline being released because you're stressed or because you're about to exercise. Um, but if we take it in this scenario of stress, you're releasing loads of adrenaline, noradrenaline, and that's triggering your body to, right, we're in a situation of fight or flight, I've got to liberate fuel and and have it ready to chuck on the fire. So it starts to trigger your adipose tissue, your fat cells to release fatty acids. It starts to trigger your liver to start exporting more glucose out of the liver. So basically you're trying to liberate fuel so so you can fuel the fight or flight. Now, if it was a true fight or flight situation, of course you're going to expend energy to fight or flight. So that fuel is going to get used. But if you're stressed, you're not really using that energy. So you're just liberating these fuels that are now in the system and they've got nowhere to go because they're not going to be consumed. So they just end up either returning back to where they were or you've got a likelihood that they'll end up somewhere where they shouldn't be. And that starts to increase the the potential for for issues because it could be contributing to um disease risk further down the line.
Dr Rupy: Yeah, yeah.
Dr Adam Collins: Um, now if it was exercise or fight or flight, of course, it's beneficial because actually you can mobilise all this fuel and then you're actually increasing the consumption of these fuels to fight, flight or to go and do that run or go to do that spin class or whatever it is. Um, now it might not be that you can use all that fuel at the time of the exercise. You might use some of it during the exercise and then what you can't use during the exercise, you'll use after the exercise to pay back what you had to use during the exercise. But effectively, you've consumed or used that fuel that you've liberated.
Dr Rupy: Yeah.
Dr Adam Collins: So that I think it would be a one metabolic explanation for how stress could contribute to a potential issue because you're liberating things without using it.
Dr Rupy: Yeah. And I guess, you know, the the wider context of psychological stress leading to behaviours that could lead to that over consumption of calories. So there are sort of uh non-biological as well as biological explanations as to why it can lead to a greater propensity of certain diseases and metabolic diseases.
Dr Adam Collins: Yeah. Yeah, yeah, absolutely. Yeah.
Dr Rupy: On this subject of metabolism, metabolic flexibility is a term that is becoming increasingly popular. And I think it's been misused in a lot of ways as well because there is this idea that if you just shift your uh diet to having a bit more fat, it gives a little bit more metabolic flexibility. Why don't we define exactly what metabolic flexibility is taken to mean?
Dr Adam Collins: Okay. Yeah, I think this would lead us down a really interesting, maybe more in-depth path that we need to explore to fully appreciate it. But let's just tackle the term metabolic flexibility. Um, metabolic flexibility is a term that came about to try and illustrate in essence, the the flexibility or the the switching between burning carbohydrate versus burning fat.
Dr Rupy: Okay.
Dr Adam Collins: So there's periods of time where you're going to be burning carbohydrate and then there's periods of time when you need to be burning fat.
Dr Rupy: Okay.
Dr Adam Collins: Um, and to illustrate metabolic flexibility, we can look at certain scenarios where you would expect there to be metabolic or a switch of fuels. So one would be that difference between a fed and a fasted state.
Dr Rupy: Uh-huh.
Dr Adam Collins: So a fasted state, let's say you you haven't had anything to eat for 12 hours, you're in a truly fasted state. So you haven't had any new fuel coming into the system. So you are, yes, you might be break, you might have been releasing your carbohydrate stores, but you have certainly in earnest started to liberate your fat stores and release those fatty acids and you're using fatty acids where you can for energy. So you're not using 100% fat, but you've shifted towards more fat oxidation, more fat burning.
Dr Rupy: Okay.
Dr Adam Collins: Because you haven't had any new carbs coming in.
Dr Rupy: No carbs coming in, yeah.
Dr Adam Collins: So, so that's like a situation of of predominantly fat burning. And then you have a meal and let's say that meal contains carbohydrate. Then of course you've got new carbohydrate coming into the system. Your whole metabolic apparatus, mainly orchestrated through the liver, is now going to shift you towards utilising that carbohydrate. So replenishing stores of carbohydrate in your liver, taking up and using carbohydrate in your muscle to store to use. And the fact you've got carbohydrate means that no longer you're releasing fat from your adipose tissue, you're now storing fat in your adipose tissue. So you've gone from burning fat in a fasted state to now burning, utilising carbs in a fed state.
Dr Rupy: And why does in the presence of both fat and carbohydrates, why does my body prefer using carbs when actually I want it to be using fat as much as possible, particularly if I want to lose a bit of fat mass?
Dr Adam Collins: Um, that's an interesting question. I've never had anyone ask it in that exact way before. Um, does it prefer to use carbohydrates? That sort of almost implies it's got a conscious choice to choose to burn carbohydrate.
Dr Rupy: I mean, everyone likes carbs.
Dr Adam Collins: Um, I suppose if you go back to the metabolic perspective of it, if you were to do any biochemistry, is to recognise that the whole of your metabolic apparatus is orchestrated around carbohydrate availability.
Dr Rupy: Uh-huh.
Dr Adam Collins: Right. So everything is dictated by glucose availability effectively. So when you've got glucose, you use glucose, and when you don't use glucose, you spare glucose. So from a metabolic perspective, it's not to say your body's got a preference for it, but because carbohydrate is a preferred fuel, everything is going to be orchestrated around its its availability. And part of that's because you it certain things have to be fuelled by carbohydrate. For example, the brain. So we have to supply carbohydrate and we have to have a constant supply line of carbohydrate. Um, so and we can't maintain that supply line indefinitely without a new supply of carbohydrate coming into the system. So when we have carbohydrate coming into the system, we need to be able to really utilise that as as efficiently as possible.
Dr Rupy: Okay.
Dr Adam Collins: So that's why everything is is going to be switched on around carbohydrate availability.
Dr Rupy: Okay.
Dr Adam Collins: And that's mainly driven by insulin because insulin is the, it's not because you're going to have fluctuations in glucose in the blood, but your any change in circumstance is reflected in insulin. Because when you've got new insulin, new carbohydrate coming into the system, that triggers insulin release from your pancreas. And it's that insulin which does all the coordination that stops you manufacturing glucose from the liver and stops you exporting glucose out of the liver and enables you to store some carbohydrate in the liver instead. It enables your muscle to take up and utilise that carbohydrate, whether it's storing or utilising it for energy, enables your adipose tissue to take it up because you need that carbohydrate to store that fat. It's not just fat that ends up as fat. Um, and you might even use some of that carbohydrate to manufacture other things, maybe some amino acids, maybe some fatty acids, because you can afford to do it because you've got a new supply. And then when that supply dwindles and you haven't got a new supply coming in, you start to not immediately switch, it's not just like a on-off thing, but over time, things will start to gradually shift towards away from carbohydrate utilisation towards fat utilisation.
Dr Rupy: Okay. So if I was to take an example, right, and I I this will be an extreme example. Let's say I stop eating at 8:00 p.m. I've just had a pizza, let's say. I do sometimes eat pizza. Um, I fast for 12 hours. And in the morning, I I've I've started already burning fat. And when I break my fast, I'll start using carbohydrates or whatever I'm consuming, whether that's protein and fats, etc. If uh I'm efficient, I metabolically flexible, that should happen. Let's say I'm metabolically inflexible and I extend that fasting window. So I didn't break my fast until 12:00 p.m. the following day. So I've been fasting for a longer period of time and I still don't start burning fat. Is that a strong indicator that I am metabolically inflexible? I.e. I'm not switching to another fuel source utilisation to power my metabolism.
Dr Adam Collins: Yeah. Well, you're pretty much there. Yeah. So if you were going to test someone's metabolic flexibility, you would probably do that. You would feed them, you would measure them overnight, let's say. So you might put them in a metabolic chamber or you might measure them say after a meal at dinner and then measure them again in the morning after however many hours of fasting.
Dr Rupy: Got you.
Dr Adam Collins: To see how they've switched from carbohydrate to to fat. And if they're metabolically inflexible, then the extent by which they make that switch would be less pronounced than someone who's metabolically flexible.
Dr Rupy: Got you.
Dr Adam Collins: But it's not just about transitioning from carbohydrate to fat. It's also the other way, which is again, what people don't appreciate. People always think about it's all about fat burning. But it's about your ability to deal with the carbohydrates as well. So if you feed them the carbohydrate, if that person's metabolically flexible, they'll be able to take that carbohydrate, clear it from the blood, utilise it very effectively. So they'll be able to switch to carbohydrate utilisation much more effectively. And of course, that's coming into play in terms of things like um glucose intolerance, pre-diabetes, which effectively is another manifestation of metabolic inflexibility because you're not able to um switch, you're not able to use that carbohydrate. Um, now that also means that when you measure them several hours after a meal, they're not going to be switching to burning fat as effectively because they're still trying to manage the carbohydrate that you fed them in the meal that you gave them before.
Dr Rupy: Okay.
Dr Adam Collins: So it's not just about your ability to switch to burning fat or how good you are at burning fat, it's how good you are at dealing with the carbohydrate as well, which is important.
Dr Rupy: So if I'm metabolically inflexible, I want to talk a bit about how we become more flexible, but if I'm metabolically inflexible, what kind of conditions, you mentioned a couple already, does that predispose me to?
Dr Adam Collins: So that would be the conditions that we all suffer from pretty much in the Western civilization, which is cardiovascular disease and insulin resistance, diabetes. So insulin resistance, diabetes is probably a more obvious one that people can understand given if you think of metabolic flexibility because like I said, if you've got an impaired ability to clear and utilise that carbohydrate, that's an example of metabolic inflexibility. Um, but cardiovascular disease is also part of metabolic inflexibility because if you're not switching from carbohydrate to fat, then it's not because you're not able to burn your own stores of fat, but you're also impairing your ability to manage the flow of lipid or fat around the blood. And that's not just the fats coming that you're liberating from your fat stores, it's fat that you've um fed in whatever it is you're eating because people don't just eat pure carbohydrates. Um, they're eating a mixed meal which has got protein and fat and carbohydrate in it. So your ability to deal with that ingested, that new lipid coming into the system, but also the management of the lipid that you've already got in the system, the the fat that's circulating around the blood, you know, all your lipoproteins, these shopping bags full of fat that are floating around your blood. Um, which links to what people measure as your cholesterol. Where's your cholesterol? It's in these lipoproteins, these shopping bags floating around the blood. And the reason why it becomes an issue is because you're not able to clear those shopping bags anymore. It just they're just carrying on being kicked back out into the circulation all the time. And then they start to interact with each other and they become smaller and denser and then they're acting like bullets getting through endothelium. You might also get, and the endothelium being the lining of cells around the artery. You also might, if you've got a perfect storm of of say something that is also damaging the endothelium itself. And you've got things like high blood pressure, that's a big cardiovascular risk. You've got more likelihood of of that atherosclerosis. So all these things are linked together.
Dr Rupy: Yeah.
Dr Adam Collins: So I haven't answered your question yet.
Dr Rupy: Yeah, yeah. So how do we get metabolically flexible?
Dr Adam Collins: But this is all good context.
Dr Adam Collins: It's important to understand the the consequences of metabolic inflexibility because that's basically if you follow it all the way down those sequence of events, then that's where it starts to lead to disease.
Dr Rupy: Yeah.
Dr Adam Collins: Um, the good news is that of course, this is a sequence of events that I can reverse.
Dr Rupy: Yeah.
Dr Adam Collins: So I can go back in the reverse order of that sequence. Um, and if I was so say, let's say I I was looking at someone who has overweight or obesity, then we know that just losing 5% body weight has huge benefits in terms of glycemic control, cardiovascular risk, all these things, despite not because I've hit a certain weight or I've got a certain adiposity, but because I've scaled back. I've given a bit of breathing space to now roll back those sequence of events. So I've mitigated some of that risk. Um, the interesting thing about that and the reason why I've spent a long time describing that situation is that although I've illustrated it with the scenario of obesity, that situation can happen without obesity because that can be a consequence of diet and lifestyle generally. That you can get a situation where not necessarily because you've got excess body weight overall, but you are leading to this metabolic stress, these sequence of events are happening. Now, it could be due to stress, for example, because remember you've liberated all this fuel and it's got nowhere to go and it ends up where it shouldn't do. But if that's coupled to the fact you've got poor diet and lifestyle on top of that, the fact you're fuelling inappropriately, feeding the wrong things at the wrong time of day, or you're constantly over supplying things from the dietary perspective and you're you're relatively sedentary or you've got, you know, poor physical activity behaviour, then that's another perfect storm. It also explains how things like interventions of diet and lifestyle will have an impact too, irrespective of getting that person to be in an energy deficit. That if you did an exercise intervention or just modified the um composition, timing uh of what they're eating around physical and and obviously physical activity, exercise at the same time, you can solve the same, same issue.
Dr Rupy: So looking at all these different stages that lead to the end point of metabolic inflexibility, exercise that you just mentioned, I'm assuming a mixture of aerobic and strength training to improve the efficiency of the muscle as well as put one slightly in an energy deficit. I know it's not the main way of putting people into energy deficits, that's mainly diet based, but would that be one sort of core pillar of metabolic flexibility?
Dr Adam Collins: Yeah, so what you could think about is if we think of the muscle, let's focus on that and let's think of exercise. So what's exercise going to do to the muscle? So if I was doing cardio exercise or low um intensity, long duration exercise, obviously from an energy expenditure point of view, I can expend a lot of energy because I can do that for I can maintain that over a long period of time. Um, but it's not really about the energy so much. It's the fact that I'm I am able to utilise fuel over a long period of time. And I'm using my oxidative system. So I'm able to burn things efficiently down to carbon dioxide and water. Um, so I'm able to burn carbohydrate and I'm also able to burn fat during that exercise. Um, but whatever form of exercise that I'm doing, and maybe let's take endurance exercise as an example, you are creating a crisis in that muscle because of course you're demanding energy and that energy's got to be supplied. And you're going to be depleting energy stores in the muscle and the energy status of that muscle is going to be different to when it was at rest. And that trigger of of or that change in energy state or that energy crisis or fuel crisis, let's say, is what triggers adaptation in that muscle.
Dr Rupy: Okay.
Dr Adam Collins: So if you were going to do that exercise again or have that same stress again, your muscle would have adapted to cope with that stress better.
Dr Rupy: Okay.
Dr Adam Collins: That's the idea. And it's and I suppose that's the idea of metabolic flexibility or what I would say is better to think about is metabolic resilience. Because what you want is your body not to be in a constant state of of homeostasis all the time, but obviously it's going to be periods where it's in excess and periods where it's in deficit. But what you want to be is in a situation of of resilience. So you can cope with the excesses and the and the deficiencies that you've got. You can roll with the normal punches of of life because we don't live in a vacuum and we don't eat the same thing prescriptively all the time or exercise in the same way or. So we've constantly got the stresses and strains of living. So that's what we need to be able to be better to cope with. And that's the idea of metabolic resilience. Um, but how can you achieve that? One of the ways is through um creating a stress which triggers a response which improves your ability to deal with that stress next time. And exercise is a clear example of that.
Dr Rupy: Yeah.
Dr Adam Collins: It's that whole Nietzsche thing, whatever doesn't kill you makes you stronger.
Dr Rupy: Yeah. So you're increasing resilience and the adaptability of that muscle.
Dr Adam Collins: So that might be that you've improved your ability to take up glucose. You might improve your ability to oxidise fuel because you've created more mitochondria or you've improved your existing mitochondria's ability. Um, and that might require protein, for example, because you need protein to make these things or repair and build these tissues. Or you might create the blood supply to that muscle. So all these adaptations are important and improve your ability to deal with that crisis next time. Effectively, increase your ability to burn fat.
Dr Rupy: Yeah.
Dr Adam Collins: So you've got a better fat burning ability of your muscle. That's really what cardio or endurance type exercise is predominantly designed to do because that's your adaptive response to that type of exercise. So that you can carry on that for longer or you can exercise at a higher intensity for the same duration because you've got a better ability to oxidise fat.
Dr Rupy: Yeah.
Dr Adam Collins: Um, now that's an important thing from a metabolic flexibility point of view because there's no point you having the ability to release fat from your stores if you've got no ability to actually oxidise that fat in your muscle. Obviously you need to have the demand to oxidise that fat in your muscle, but you also have to have the machinery to be able to oxidise that fat in the muscle. And that's really what physical activity or exercise is helping you to do.
Dr Rupy: What about fasting as a as a method to to increase flexibility?
Dr Adam Collins: So that is one way in which you can, I know this is quite a vague term I've just used here. So maybe, you know, we can define it into, you know, intermittent fasting or time restricted feeding. So we're 16 hours of fasting versus eight hours of feeding window. Uh, or fasting as a mechanism of calorie restriction over a week, but as a way of sort of accelerating that ability of the of the muscle to respond to the stress and become more adaptable.
Dr Adam Collins: Yeah. Um, with if we think of intermittent fasting, the the interesting thing with intermittent fasting is exactly and this is why you did that segue into it from what I was talking about, is the metabolic effect of that fasting. Now, that is muddied because of course, when you do intermittent fasting, invariably you create weight loss. So particularly if you're doing intermittent energy restriction, where you're doing something like the 5:2 or the 4:3 or alternate day fasting, because invariably you're going to end up eating less. It's going to create a like on average across the week, maybe a 5 or 600 calorie a day deficit. So the argument then was, well, what's doing the the benefit? Is it the weight loss or the things that we talked about in terms of scaling back? Or is it something to do with the fact that you've extended the fast or given them a period of fasting more so than than before? Um, I would say, yes, the weight loss is going to have a big effect. But there is definitely metabolic changes that you would see on a fast day versus a feeding day.
Dr Rupy: Okay.
Dr Adam Collins: Because you would see that that person has shifted to oxidising fat on a fast day versus oxidising carbohydrate predominantly on a feeding day. And you'll see changes in insulin release. Obviously you're going to release more insulin on a feeding day than they would on a fasting day. Um, and if you look at it from a a whole body perspective, then that would translate into you taking up and storing carbohydrate and utilising carbohydrate on a feeding day generally versus you liberating and utilising fat on the fast day. Um, now does that then entrain better metabolic flexibility? I would say it probably does.
Dr Rupy: Okay.
Dr Adam Collins: It's just difficult to show it because you've got that confounder of weight loss.
Dr Rupy: Confounder, yeah.
Dr Adam Collins: Um, and that's the same for time restricted eating because you would you would guess that time restricted eating wouldn't have that confounder as much because prescriptively you're not getting people to eat any less.
Dr Rupy: Sure. Yeah.
Dr Adam Collins: It's just you're reducing the eating window and extending the fast. But we know that people end up eating less. We just did a study that's finished a couple of weeks ago. And we just wanted to see what people's change in eating behaviour was on a time restricted eating. And they end up eating 500 calories a day less.
Dr Rupy: Yeah.
Dr Adam Collins: Because they end up skipping meals or skipping snacks, whether that's consciously or unconsciously. There's less opportunity to eat. So they end up eating less.
Dr Rupy: Yeah, yeah.
Dr Adam Collins: Um, so it's difficult to untangle that thing.
Dr Rupy: Yeah, I mean, that's why practically I'm a fan of uh time restricted feeding because the outcome is usually that, you know, you tend to eat less out of boredom, you tend to not snack after 8:00 p.m. You tend to, you know, be a little bit more aware of your eating habits and when your meals are and when you shouldn't be eating. So the knock on effect is generally a reduction in energy, which, as you said, will make you a little bit more metabolically flexible.
Dr Adam Collins: And they ended up eating healthier.
Dr Rupy: Yeah, yeah.
Dr Adam Collins: That's the other thing that happened in the study we just finished. Um, because like any diet, I suppose, as soon as you get someone to follow a rule, and that's what you're doing with time restricted eating, people become more mindful of what they're eating.
Dr Rupy: Yeah.
Dr Adam Collins: And that in a way ends up them eating better or improving their intake.
Dr Rupy: Are there easy ways in which I can tell that I'm in fat burning mode? Like any consumer available tools or things that I can look at?
Dr Adam Collins: I know there's the Lumen tool that's obviously got a lot of um traction is obviously been highly advertised on social media. Um, and we have had contact with the company and we've had ideas of of sort of testing that product against our conventional ways in which we measure it because it's based on the same principles that we do from a research point of view in terms of measuring substrate utilisation. Are you burning carbohydrate or are you burning fat? Um, I don't know, I haven't done enough evaluation, validation studies of my own to say how good it is as a tool to measure it specifically. But it is quite a good behaviour reinforcer because of course, it's it's a way of testing yourself to see whether you are following what you want to follow.
Dr Rupy: Right. Okay.
Dr Adam Collins: You know.
Dr Rupy: Yeah, yeah.
Dr Adam Collins: So,
Dr Rupy: And what's the research tool that you use? Is it a ketone breath test?
Dr Adam Collins: So we use indirect calorimetry it's called. So we essentially measure oxygen consumption and carbon dioxide production. So you measure either with a face mask um and you measure how much people are breathing and what the composition of their breath is to get those two things. Or we put people in metabolic chambers. Um, where or we tend to use because we are interested, well the research I tend to do is where we want to assess a metabolic impact by looking at something dynamically. So we tend to use meal challenge as our as our way of testing the impact of what we're what we're doing. So we take someone in a fasted state, we measure their metabolic rate, what they're burning in a fasted state, then we feed them, usually a mixed meal which has got both carbohydrate and fat and and a bit of protein in it. And then we measure how they respond to that meal um in terms of the energy consumption, yes, but the um shift in substrate utilisation, how they shift from a mixture of carbohydrate and fat to predominantly carbohydrate and then back to fat again over the six hours that you follow them after that meal.
Dr Rupy: Okay.
Dr Adam Collins: As well as obviously measuring the appearance of the carbohydrate and the fat in the blood and how easy it is to clear that fat and carbohydrate you fed them. So that metabolic handling really of that meal is a is a test.
Dr Rupy: Okay.
Dr Adam Collins: Um, and that's why I've probably got confidence that things like intermittent fasting, yeah, do have an impact on that metabolic handling because you are allowing them to have those situations of having feeding and fasting.
Dr Rupy: Yeah.
Dr Adam Collins: So you do get that acute effect because of course, if you fast someone, they have to start burning fat because you haven't got any carbohydrate. Now, that does temporarily make them intolerant to carbohydrate.
Dr Rupy: Yeah.
Dr Adam Collins: So if you just suddenly dump loads of carbohydrate in them, you do get a slight intolerance to it. But that's because you're still dealing with the fat. Now, if you did that on a repeated basis, then you would probably start to get a bit of a trade-off of the two. And that's the whole idea of doing intermittent fasting. Um, but you've still got the confounder of weight though because of course, doing that as an intervention, invariably changes how many calories people eat overall.
Dr Rupy: Yeah.
Dr Adam Collins: Um, and that's leads on to obviously the most recent study that we published, which is can we get that same metabolic effect independently of weight loss?
Dr Rupy: Okay.
Dr Adam Collins: Now we've sort of done that already, shown that already because we've done interventions where we had people on a continuous energy restriction versus an intermittent energy restriction. And we they we followed them until they both lost the same amount of weight and then we measured their metabolic handling. And you saw better metabolic handling or an improvement in metabolic handling in the intermittent group, not so much in the continuous group.
Dr Rupy: Ah, okay.
Dr Adam Collins: So that's sort of evidence that you've got an improvement through that mode of intermittent fasting. Um, but the more recent thing is, well, can I do that without creating an energy deficit at all? And when you think about it, it is the carbohydrate that really is doing the transitioning into that fed state. So if I can attenuate the carbohydrate or have periods where I'm restricting the carbohydrate, I could achieve the same result.
Dr Rupy: Yeah.
Dr Adam Collins: And that's what we showed in the more recent paper that we showed is that because we know the acute effects of fasting, that actually you get the same acute effects if you just carbohydrate restrict. There is a caveat to that, obviously, because if you're going to keep the calories the same, you're going to have to displace the carbohydrate with something else.
Dr Rupy: Yeah.
Dr Adam Collins: And what you invariably displace it with is fat. So it's not to say that all of a sudden you're going to start melting away your body fat because you'll be burning a lot of the fat that you're consuming, not the fat that's inside of you.
Dr Rupy: Yeah.
Dr Adam Collins: And that's the same argument against sort of low carbohydrate diets. Going back to one of your really early questions that carbohydrate, low carbohydrate diets are not going to just miraculously get you to lose weight if you're if you're in energy balance because you're just be consuming, you'll be burning more fat, absolutely. But you'll be burning the fat that you're feeding, not the fat that's inside of you.
Dr Rupy: Yeah.
Dr Adam Collins: So it's it's basically neutral. Um, and actually if you look at the Kevin Hall studies, the metabolic ward studies, if anything, it's the low fat diets that give you more body fat loss than the low carbohydrate diets when you correct everything for calories.
Dr Rupy: Ah, okay.
Dr Adam Collins: Um, and that's not just his studies, there's other isocaloric studies have shown the same. So it's not necessarily this metabolic advantage from a weight loss point of view.
Dr Rupy: Yeah, yeah.
Dr Adam Collins: Um, but certainly manipulating the carbohydrate is going to have clear metabolic effects.
Dr Rupy: Uh-huh.
Dr Adam Collins: Um, and that's why we we're interested in, can we advocate an intermittent carbohydrate restriction rather than an intermittent energy restriction?
Dr Rupy: Restriction. Yeah.
Dr Adam Collins: So is that going to be a low carb 5:2? This is what we've piloted in our more recent study. Um, but previously we've also dabbled with a low carb time restricted eating. So you can eat things outside of the window, provided it's not carbohydrate. So it's sort of it's like a a modern manifestation of no carbs after five.
Dr Rupy: Yeah, yeah, yeah.
Dr Adam Collins: Right. So I wouldn't necessarily say that's a straightforward thing, but but it's a regimented way, I suppose, to get people to not over fuel. I think that's the end end result is the problem we have today is that people are just over fuelling all the time.
Dr Rupy: Yeah.
Dr Adam Collins: Uh, and that's partly down to society now because whereas maybe our parents, grandparents, depending on how old you are, um, where they used to have breakfast, lunch, dinner, and maybe even supper before they went to bed. Um, now we don't have distinct meals. We're like grazing all the time. We might have an energy load in the evening, but we've got a long eating window, but also we're constantly eating and drinking or consuming calories throughout the day. And that's driven by the fact that almost everywhere we go is giving us this idea that we can't go for more than two hours without having something to eat. We've got to constantly go and have a coffee here or have a cake there or go to that vending machine or when I'm filling up the car for petrol or nowadays, obviously even more so because I'm going to have to wait for the car to charge. I'm going to go and have something to eat.
Dr Rupy: Yeah, yeah.
Dr Adam Collins: So and everything is catered for that.
Dr Rupy: Yeah.
Dr Adam Collins: It's catered for people to to eat at every opportunity or drink at every opportunity.
Dr Rupy: Yeah, yeah.
Dr Adam Collins: Um, so the likelihood of people over fuelling is is rife.
Dr Rupy: And it seems like that's the main driver of what we've been talking about here today.
Dr Adam Collins: Exactly. Yeah. Because if you're constantly in that fed state all the time or you're fuelling more than you're using, then all that surplus fuel is going to end up accumulating where it shouldn't be.
Dr Rupy: Yeah.
Dr Adam Collins: Now that might lead to weight gain, of course, but it might also lead to metabolic consequences, increased visceral fat, increased cardiovascular risk, pre-diabetes in people that are lean.
Dr Rupy: Yeah.
Dr Adam Collins: And I mean, we did some work with Hammersmith not too far from here, um, where looking at MRI across a whole spectrum of people, if you look at lean people, people that have a normal BMI, about one in 10 of them have high levels of visceral fat, which you wouldn't normally think about because they would think, oh, they're they're normal weight. They're absolutely fine. In fact, they've got normal body fat levels, but they've got high liver fat, high visceral fat. And that's a consequence of poor diet and lifestyle, even though they're normal and lean. And that's a worry because of course, you're not going to be able to identify those normally.
Dr Rupy: No.
Dr Adam Collins: Someone who's overweight or obese, that's easy. Oh, you can spot them. Or you can screen for them or triage them. Not so much other people.
Dr Rupy: Would that make you a fan of people doing some of these DEXA scans to assess visceral fat?
Dr Adam Collins: Well, I wouldn't do a DEXA scan because they're it's not really a good measure of visceral fat. It's a bit of a fudge the way in which they do it. But, um, I suppose it's not so bad in lean people, but with people are large, it's difficult to use DEXA. Um, yeah, I don't want to say you have to go to that nth degree. But I think it might be a catalyst for people to have a look at their own diet and lifestyle and to think, is my current diet and lifestyle conducive to me running a risk of having this issue?
Dr Rupy: Yeah.
Dr Adam Collins: So, yeah, you can go and measure it to confirm or deny it, but actually, you could probably guess from the way in which you're you're living your life in a way.
Dr Rupy: Yeah.
Dr Adam Collins: Um, and that's not to say you've got to live like a monk.
Dr Rupy: Yeah.
Dr Adam Collins: Um, or you've got to suddenly change to another extreme. But just being conscious of what you're eating, when you're eating it, in relation to everything else that you're doing.
Dr Rupy: Yeah.
Dr Adam Collins: Um, and you can become more nuanced about things when you start to look at how that interplay between diet and exercise comes into play. So it's not just whether you're doing a certain amount of exercise or following a certain amount of diet, but how you're timing your meals and what the composition of those meals are around the exercise that you're doing becomes important as well. Um, so it can be, there's lots of facets to it that you can look at. Um, and that means that you can personalise it a bit better to yourself. Not because of a certain glucose response to a muffin or whatever it is that you're trying to use as a metric, but just having an appreciation, which is what true personalised nutrition is all about. What's your current diet and lifestyle? What is and what could we do to optimise that?
Dr Rupy: Yeah.
Dr Adam Collins: Um, in terms of a change.
Dr Rupy: So we've talked about metabolism. I think we've defined the difference and what we mean by metabolic rate, how much of that is actually contributed to by organs that people perhaps don't really think about. And we've defined metabolic flexibility and inflexibility and ways in which to improve one's flexibility through exercise, um energy restriction, but also ways in which you can have energy restriction as a byproduct of the uh the guide, like, you know, uh time restricted feeding or uh intermittent fasting. Um, is there anything else that might be a bit of a hack or a strategy outside of the obvious things that could enhance people's metabolic flexibility?
Dr Adam Collins: Um, I wouldn't say go down the route of saying it's a hack, but I think it's having that understanding of what impact things are having on you. I think that's an important thing for people to get a handle on.
Dr Rupy: Okay.
Dr Adam Collins: So if you say, let's say feeding carbohydrate versus feeding fat, or you know, this whole idea that I can't have carbohydrate at all. I think you can't think in in black and white terms like that. But if you're constantly having carbohydrate all the time, like every two hours, and you're totally sedentary, then obviously that's not going to be an issue. That's going to be an issue for you. But it can be more nuanced than that. So where you time your carbohydrate. So people talk about things like training low. So maybe deliberately not having breakfast before you do some sort of cardio exercise and then having your carbohydrate afterwards. How you might time your protein around the exercise that you're doing. Um, how you might meal structure things. You know, what might, what the meals where you have when, what the composition of those meals, what's the sort of energy um content of those meals. Um, there's lots of things that you can do. And you might also even go down to to strategies like periodic feeding. So you might be where you're doing periods of fasting, maybe, or it might be periods of uh where you're deliberately not having carbohydrate, whether that's for a whole day or for a meal versus other times where you are having carbohydrates. Um, so it is it's sort of there's lots of interplay there.
Dr Rupy: Yeah.
Dr Adam Collins: Um, but a lot of it is around a message that everybody has been trying to say for years, which is there's no bad or good. Everything in moderation. It's like horses for courses. It's fuel for for the work or the lifestyle that you've got. And if you're fuelling appropriately, you're not going to have any issues. Um, and that is including not just carbohydrate and fat, but also protein. I know um you've obviously got an interest in in protein, but protein is now everywhere. Everything is got labelled protein in it. And is that because we don't eat enough protein? Not necessarily to do with that, but we know that protein has a distinct role that we need to be mindful of. And particularly in people that are of an older generation, um that that protein intake and the timing of that protein becomes important. It's not a sin-free macronutrient, necessarily, but it performs a particular function. Um, and particularly if you're looking at maintaining muscle function or muscle mass or building muscle mass or um maybe attenuating the loss of muscle mass with age, then protein is is an important thing. It's also something that could be incorporated into other strategies. Like we said, if you're going to periodically restrict carbohydrate or do time restricted eating, you could have protein outside of that window. You know, you there's certain things that you can have at certain times versus others. So, so again, it's just that whole nuanced way of thinking about things.
Dr Rupy: Yeah, absolutely. Um, this is brilliant.
Dr Adam Collins: Yeah.
Dr Rupy: Thank you. Thank you a lot.
Dr Adam Collins: No, it was great. There's a lot there to unpack for a lot of people, but I think we've um, we've done a good job of explaining metabolism, metabolic flexibility. I think it's a really interesting subject. I think uh a lot more people are going to understand this quite difficult subject to wrap your head around, but it has huge knock on effect on the diseases of our time. So, um, no, I appreciate your, I appreciate your thoughts on this.
