Instant Genius - How metabolism really affects your health, weight and wellbeing

Episode Date: February 23, 2026

Whether we’re working at a desk in an office, running a marathon or even asleep in bed, our bodies are host to thousands of chemical processes that are continually converting the food we eat into energy and repairing our bones, muscles and organs. Collectively these processes are known as the metabolism. But how are they affected by our genes, lifestyles and diet, and what can we do to make sure everything stays running smoothly? In this episode, we’re joined by Javier Gonzalez, a professor of nutrition and metabolism based at the University of Bath. He breaks down the links between our metabolisms and body types, tells us why our metabolisms aren’t necessarily fixed for life, and explains why it’s almost impossible to out exercise a poor diet if you’re looking to drop a few kilos. Learn more about your ad choices. Visit podcastchoices.com/adchoices

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I'm Jason Goodyear, commissioning editor at BBC Science Focus. Whether we're working at a desk in an office, running a marathon, or even asleep in bed, our bodies are host to thousands of chemical processes that are continually converting the food that we eat into energy and repairing our bones, muscles and organs. Collectively, these processes are known as the metabolism. But how are they affected by our genes, lifestyles and diet? And what can we do to make sure everything stays running smoothly? In this episode, we're joined by Javier Gonzalez,

a professor of nutrition and metabolism based at the University of Bath. He breaks down the links between our metabolisms and body types, tells us why our metabolisms aren't necessarily fixed for life, and explains why it's almost impossible to out-exercise poor diet if you're looking to drop a few kilos. So, Havayay, welcome to the show. Thanks so much for joining us. Thanks for having me. Oh, you're more than welcome. So today we're talking all about the metabolism, and hopefully we'll be busting many of the sort of persistent myths that seem to surround it.

So first off, as a scientist, what do you actually mean by the term? Yeah, that's a great question in itself. So metabolism generally refers to all of the chemical reactions that take place in our body. And mainly what I'm interested in is how those chemical reactions allow us to get energy from the food that we eat. And then we can use that energy to store it for later on, or we can burn it immediately to fuel things like the physical activity or exercise that we do throughout the day. And we can broadly categorize metabolic processes into two main things. One is building up reactions that we would call anabolic and others are breaking down reactions, which would be catabolic.

Then there's potentially a third which is where we're changing between things, which would be intermediary metabolism. Right. And so you often hear when talking about this, people call about your resting metabolic rate or your basal metabolic rate. So first off, you know, what do we mean by those terms? Yeah, I think that's a good place to start when we're thinking about NN. expenditure, the energy that we burn throughout the day.

So we've got our total energy expenditure, which would be over a 24-hour period, the total amount of energy we burn, and we can generally break that down into several different components. You've described one of those components as resting metabolic rate, sometimes referred to as basal metabolic rate or basal energy expenditure, and that's generally the lowest that you would measure over a 24-hour period, and it will happen normally when we're asleep actually, because when we're asleep, we're moving very little. We're not stimulated very much. And the third factor there is that we're normally fasted as well.

So when we're fasted, we're rested and we're asleep, that's when we have our lowest rate of energy expenditure, and that would be referred to as our basal energy expenditure. So you mentioned there that a sort of 24-hour period. So what sort of percentage of our kind of calorie burn is this responsible for? Yeah, so that will depend largely on the other things you do throughout the day. So for some people, their basal energy expenditure, could be the majority of their energy expenditure.

That would be for people who have fairly sedentary jobs and don't do very much physical activity or can't do very much physical activity. And it could be 60 or even 70% of their total energy expenditure. For other people, it could be the opposite. It could be a minor component of their energy expenditure. If you think about professional athletes, especially endurance athletes like Tour de France cyclists,

the majority of their energy expenditure could be from other components, such as exes. Oh, of course, yeah, got you got you. So is it our metabolism something that we can change sort of over the course of our lives? Because you often hear people say, oh, such and such a person's got a fast metabolism, such and such a person's got a slow one. Like it's a sort of stuck thing.

But is there any truth in that? There is an element of truth. So our resting metabolic rate is mostly based on the amount of what we call fat-free mass. the mass of our body that isn't fat, so that's largely muscle, but it's also the liver and the brain and other organs that are highly metabolically active. So the more of those tissues we have, if you think of a bodybuilder with a very large muscle mass, the higher their resting metabolic rate will be. So that's one factor that can change over the lifespan if we increase or decrease the amount of fat-free mass or muscle mass that we have. But there are some other smaller things as well

that might play a role. So for example, if you go on a lot, if you go on a lot, you go on a lot of fat-free mass, a diet and you restrict the calories that you're eating, that will lower your resting metabolic rate, partly because you lose fat-free mass, but also aside from that reduction in fat-free mass, and that's known as a phenomenon called adaptive thermogenesis. So a reduction in metabolic rate that is greater than we were predicts based on just the muscle mass that's there. So you mentioned a term they're metabolically active. So what exactly does that mean? A lot of people probably haven't heard of that. Yeah, and that essentially refers to the number and rate of these chemical reactions that happen within the tissues. So if you think of the liver is probably one of the most

metabolically active tissues in the body, and that's because it does a whole load of jobs for the body. It's converting things into other things. It's producing glucose, providing that to the bloodstream, so our brain has a fuel, a whole load of processes. And that's why it's so metabolically active. So another thing that you hear a lot is that Slimmer people have faster metabolisms and heavier people have slower ones. Is there any truth to that? Not really when it's been looked at.

So what's normally the case is that So I should add that there are a few caveats to this. So it's possible that someone could be very lean because they have an overactive thyroid, for example. And that's producing a hormone that would stimulate metabolic rate. but for the vast majority of people, their rate of energy expenditure, the race of resting metabolic rate,

is proportional to the amount of muscle mass they have. And so what that means is that it's the other side of the equation which probably accounts for the difference in their body size, and that is the amount of food that they eat. But the problem is we can't measure that very well. So we can measure metabolic rate quite well. We just can't measure energy intake or calorie intake very well. So how was I was going to say that next?

How do we measure our metabolic rates? If somebody wants to find out. Yeah. So for resting metabolic rate, that's normally done with a method called indirect calorimetry. So what you essentially do is measure the amount of oxygen that that person is consuming and the amount of carbon dioxide that they're breathing out. And you can do that with a face mask and you collect the air into a bag and measure those gases there. It's a little bit more tricky if you want to know how much energy they burn over.

over a 24-hour period. There are a few different ways you can do that. One of them, you basically get them to live in a big room, and you collect all of the gases going in and out of that room and do the same principle. Or you can use methods such as quite a sophisticated method known as doubly labelled water. So the person will drink this special water,

where both the hydrogen and the oxygen are labeled. They're slightly heavier isotopes. They behave chemically very similarly to normal water, but they just differ in the mass, and that's how we can distinguish them. And then over, say, a one or two-week period, those labels in the hydrogen and the oxygen will disappear from the body, but they'll disappear at different rates. So the oxygen will disappear on the carbon dioxide that's breathed out,

but also in any water losses, such as urine and sweat, whereas the hydrogen label is only lost in the water losses and not in the carbon dioxide on the breath. And so that means that the different, The difference in the elimination rate of those two labels gives us the average amount of carbon dioxide that someone produces over a one or two week period. And with some additional assumptions and equations, you can convert that into the average amount of energy they burn over the one or two week period. So they're the two main ways you can do energy expenditure measures. So let's have a look at some other things that might sort of play a role in this.

So earlier you mentioned sleep, like sleep when we're asleep. our bodies aren't doing nothing, they're still doing things, lots of things. So does, like, is it getting a good night's sleep? We all know that's important. Is that also important for our metabolic health? Yes, sleep is linked to a whole range of metabolic diseases. We're still trying to understand exactly which of those diseases are caused by poor sleep, sleep rather than just poor sleep being a marker of other aspects of that lifestyle. People with poor sleep tend to have quite erratic lifestyles, perhaps erratic shift patterns with their work and so on. So it's quite difficult to unpick exactly what's causing the

increase in disease risk. But there are some really tightly controlled studies where sleep is restricted and it does have some quite profound effects on metabolism. So for example, quite severe sleep restriction can impair our glucose metabolism, our ability to metabolize the carbohydrates that we eat, and might even affect our muscle protein metabolism. So in response to the protein that we eat, it might affect our ability for our muscle to regenerate and renew. So how about another sort of thing that is often talked about in terms of health, which we often feel everyone pretty much feels.

that's stress. So this is linked to our sort of hormonal system, which you sort of touched on earlier. So what effect does stress have on this? Yeah. One of the most profound effects of stress on metabolism, when we get that adrenaline spike, it will increase the release of fats from our fat tissue. So fatty acids are stored within the fat tissue largely under our skin, and the adrenaline will stimulate the release of those. And from an evolutionary perspective, that has a useful role in that it's releasing energy available for us to then run away or run a long distance. But if we're releasing those fats in a scenario where we're completely sedentary, so we've got the adrenaline spike, but we're not actually doing the physical activity to burn that energy, then it's increasing the availability of those fats for organs and tissues that they probably shouldn't go to. So if we do that over a very prolonged period of time, it might explain why stress is a bad thing for metabolism

because the fats are then going to other tissues like muscle and the liver when it should be stored in our fat tissue. So how about another thing that you're often here, and that's aging? So a lot of people say, oh, you know, I'm sluggish now I'm in my 40s or something, and my 50s. It's because my metabolism slowed down. Is there any truth to that? Again, a grain of truth, but sometimes overstated. So it's true that metabolic rate declines as we age, but it's almost entirely accounted for by the reduction in muscle mass that we have with age. So from about the age of 30 to 40 years old, we'll get a gradual decline in muscle mass, at least at the population level.

One of the new ideas is actually we see it looks gradual at a population level, but it's probably exacerbated by very short periods of, extreme disuse. So if you get ill and you have a week or two in hospital or just in bed at home, then you'll get drastic muscle loss for that week, which you might not fully regain, and so that will exacerbate the age-related muscle loss over time. And that has implications for our metabolic rate, because the muscle mass itself is an energy-consuming tissue. But it also has effects on other aspects of metabolic health. So muscle is very important for our glucose control. When we consume carbohydrate in a meal, much of that glucose will go to our liver and our muscle. And so the more muscle we have and the better quality that muscle, the better we can

keep our blood glucose within a healthy range and store it safely away. So I think a lot of people who will be researching the topic of metabolism will be doing so because they're thinking of dropping a few kilos. And so we've mentioned that sort of burning calories, etc. The first sort of port of call, a lot of people we'll take when trying to lose a bit of weight. It's cutting calories. So obviously then we're consuming less energy that we can burn. So what effect does that have on our metabolism? Yeah.

Yeah, it's, first of all, it's probably the best way to approach it. So whilst I'm a big advocate for exercise, and we can maybe come on to that a little bit later, if we're thinking about weight loss, exercise isn't particularly effective for that, and the most appropriate way is to cut calorie intake. But there are effects of that on metabolism. One of the initial ones is that we get a reduction in our resting metabolic rate.

Another is that we will lose fat mass, which we probably want to lose. But we'll also lose a bit of muscle mass as well. It's almost impossible to only lose fat mass. We'll always lose a bit of both. We can shift the proportion of those, and that's partly where exercise comes in, and also protein intake as well. Peak pollination season, and my business is scaling fast. To keep the nectar flowing, I need a phone plan with top priority data speeds.

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Alcohol definitely affects our metabolism. The age-related aspects exist to some extent. But in terms of the potency of alcohol, if we think about completely, um, completely, comparing it to other things we eat. So, for example, sugars are known to not be the healthiest when consumed in excess. And one of the effects they have is they stimulate this process known as de novo lipogenesis, which all that means is converting things that aren't fat into fat. Okay, so the sugar can be converted into fat. Now, if you compare sugar with alcohol, Alcohol does that much more potently than sugar does.

So one of the negative effects of alcohol intake is converting the sugars and other things in our body into fat. And the liver does that. And that probably accounts for the fatty liver that you can see with very high sugar intake and very high alcohol intake. So another thing that people are from here is they'll say, well, if I want to sort of give my metabolism a bit of a boost, a bit of a kick, I can have like a coffee or something or like a blazing hot. curry, is there any truth in that? There is, but it's a very, very small effect. So, yeah, for both caffeine and capsacin is the kind of active, spicy ingredient in a curry, in a chili, they have both been shown to increase our energy expenditure.

But if you work it out in terms of calories per day, we're talking around 10 or so calories, so not very meaningful when we think our total energy expenditures are more like 2.5 to 3,000 calories a day. So how about another thing that you often hear is people will say, well, you shouldn't eat sort of later in the evening as this can have an effect on weight gain. And some people say on your metabolism. That concept is sometimes referred to as time restricted eating or where people will restrict foods at certain times of the day. There may be subtle metabolic effects of timing that are independent of the foods that are actually eaten, but they're quite difficult to detect.

What is probably the biggest factor there is the types of foods we eat. So in the morning when we have our breakfast, that's probably the most regimented meal we have of the day for most people. And it's probably the one that is the healthiest or on the healthier side. Whereas lunch is probably in the similar range to that. And then our dinner, especially after dinner snacks and alcoholic drinks, that's where most of those consumption, the consumption of those mostly takes place. And so a large part of the benefit of having a time cut in which when you eat your

foods relates to the types of foods we eat and the total amount that we eat. So you've now said, I'm not going to eat anymore today. Your breakfast is then normally a fixed portion. And so you're back onto a cycle where you're A, keeping calorie intake controlled and B, the types of foods that are eating. and you're not consuming as much alcohol and the snack-type foods after your evening meal. So how about another sort of popular form of time-restricted eating?

And that's the sort of intermittent fasting diets. Surely they have some sort of effect on this. They do. So if, for example, you go without food for 24 hours, I should start by explaining that one of the types of intermittent fasting would be to essentially avoid almost all calorie-containing foods and drinks for 24 hours and then to eat for another 24 hours. And you can then eat as much as you like in that ad libidum period.

So sometimes refer to as alternate day fasting. So you fast for one day and not for the next. And in the fasting day, one of the metabolic changes that you see and you'd rarely see in normal life is this process known as ketosis. And that's where the liver starts converting fats into these other metabolites known as ketone bodies. And these ketone bodies can be used by the brain and by the muscle as an alternative fuel. And they've evolved in these fasting scenarios because the brain normally relies on carbohydrates as a fuel, on glucose. And so if we're fasting and we haven't got

carbohydrates coming into the circulation, then we need, the brain can't use fats. And so it needs an alternative source. And these ketone bodies can provide that fuel. And if we fast for 24 hours or so, especially if we do some exercise, then we'll get an increase in the levels of these ketone bodies in the blood. So normally they're almost undetectable, but if we fast, we can get this increase. And they have a wide range of signaling effects on the body, and they are a fuel, as I've already mentioned. Then when we go into that feeding state, we switch back to a normal post-pranial metabolism. We've got the carbohydrates circulating again. So again, there are metabolic changes with this alternate day fasting that you wouldn't see in normal daily life.

But in terms of weight loss and most of the health benefits of these types of diets, it's almost entirely due to the calorie deficit that's achieved. And that's because the amount of calories you skip on your fasting day are rarely fully compensated for on your feed day. And we did run one study on this where we wanted to understand the effects independent of the energy intake. So we asked people to consume zero calories on one day. but then 200% of what they normally eat, double what they normally eat on the second day,

and repeat that over a few weeks. And people find it really challenging to actually get that amount of calories in in a day. So it works largely because it naturally results in an energy deficit. So you mentioned exercise there. So let's talk about exercise. So we know exercise is just good for you in general. But how about its effects on the metabolism? because there's a lot of conflicting information out there on this.

So I've read some people will say, well, the best way to keep a healthy metabolism, just do a whole bunch of cardio. So what does doing cardio do to our metabolism? Yeah, so cardio or kind of endurance or aerobic type exercise is where we might go running or brisk walking, cycling, that kind of thing. And that will result in an increase in our energy expenditure. We'll start burning mainly carbohydrates and fats. as fuels. That has immediate benefits during exercise in terms of glucose control. So for people

with diabetes, for example, the muscle will start taking up glucose. And one of the real benefits is that it can do that without the need for insulin. So when we normally eat a meal at rest, we will produce insulin and that insulin will allow glucose to enter the muscle. During exercise, the muscle can take up glucose without needing insulin. And so for people with diabetes, who on insulin therapy, that can be a really useful strategy if timed appropriately. After exercise then, the muscle becomes sensitized to insulin, so that's a second benefit. So for the same amount of insulin, the muscle will take up more glucose. And then if we do regular exercise training, then the muscle will remodel and become

inherently then more sensitive to insulin and other stimuli that are important. So kind of related to that then. I've heard of this sort of people call it the afterburn effect. Whereas, you know, if I do some exercise, then that kicks up my metabolism. I mean, some people even claim for a couple of days. Yep, it's true. And it depends on what we're talking about with metabolism there. So it will increase energy expenditure, and that will stay elevated for a few hours afterwards.

But what's more potent is that effect on insulin sensitivity. And that's what can last for up to three days. another really potent effect of exercise is on how we handle the protein that we eat. It's seen with resistance exercising like lifting weights but is also seen with the kind of cardio endurance type training. And that's where if we haven't done the exercise and we have a protein containing meal, the proteins in that meal are not only the building blocks for our muscle, but they also signal and stimulate the process of regenerating and building new muscle. And what exercise does is it sensitizes the muscle to that protein stimulus.

So for the same amount of protein, if you've done exercise in the last two days, 48 hours or so, you will make more of the protein that you eat. And more of it will be converted into muscle and used beneficially rather than just oxidized and burned as fuel. Yes, I've got a few questions about strength training. But you've just brought something up there. I just thought, oh, does doing exercise actually stimulate our appetites more? Yes, that's a very common question. And it partly, the answer will depend partly on the timing that we're talking about

and potentially even the type of exercise. Because one of the really interesting phenomena that you see, especially with vigorous intensity exercise, is actually a reduction in appetite immediately after exercise. However, over the longer term, exercise would increase appetite. We've never, well, it's very challenging to directly measure that, but we largely deduce it because if you get people to increase their exercise training, they naturally seem to not lose as much body weight as you would anticipate.

And when you do all of the measurements of energy expenditure, the gap there is that it must be that they're compensating with their energy intake and gradually increasing that over time. So the direct effect of exercise immediately is actually to suppress appetite, but over the longer term, it seems to then increase appetite to maintain our body weight within a certain range. Yeah, so let's have a look at some strength training facts then and myths. So you mentioned the key to this whole idea of our resting metabolic rate

is the amount of muscle mass that we have. So surely, like say if I start lifting weights going down the gym and I put on, let's say, I don't know, five kilos of muscle. So that's going to have a pretty big impact on my resting metabolic rate, right? It will have an impact. Probably not as large as you would hope. And that's where it comes back to why I said earlier on, if you're looking to lose weight, the most effective way is to cut calories.

Because you can easily cut, well, I say easily, easier for some than others, but you can effectively cut 1,000 calories from your diet a day. but to burn an extra thousand calories takes a lot more time and effort. And putting on five kilos of muscle won't increase your energy expenditure by more than tens of calories. Oh, right. So what I mean by muscle mass being a main predictor of resting metabolic rate, I'm talking about people who differ in their muscle mass by kind of twofold.

And it explains the difference in metabolic rate between someone who's 50 kilos and someone who's 100 kilos. but for you to gain five kilos has a much more subtle effect and disappointing, unfortunately. But obviously there's still lots of other benefits of doing that. Of course, yeah. So that extra five kilos of muscle probably has more of a role in terms of your glucose control, your functional capacity during daily activities, just able to lift your shopping bags,

which is important throughout all of our life, but especially as we get older. So one of the main benefits of strength training is to maintain our independent living as we get older. And the stronger we are, also the less likely we are to have a fall to, for example, break our hip, and then we get in a vicious cycle of a hospital stay and muscle disuse and so on. Thank you for listening to this episode of Instant Genius, brought to you from the team behind BBC Science Focus. That was Professor Javier Gonzalez. If you liked what you just heard, then please do consider subscribing to Instant Genius on your preferred podcast platform. If you'd like to see our guests and hosts in person, then why not check out our YouTube channel at ScienceFocus.

The current issue of BBC Science Focus magazine is out now. Pick up a copy wherever you buy your favourite magazines or download us on your app store of choice. You can also find us on Apple News or online at sciencefocus.com. This podcast is sponsored by name, audio and focal. The texture and emotional depth of music can be lost through digital. digital sources or poor signal. Name Audio believes you can have digital precision with analog

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