How Metabolism Affects Your Health and Weight: Myths vs Facts

You can effectively cut 1,000 calories from your diet a day, but to burn an extra,000 calories takes a lot more time and effort. And putting on 5 kilos of muscle won't increase your energy expenditure by more than tens of calories. Hello and welcome to Instant Genius. I'm Jason Goodger, commissioning editor at BBC Science Focus. In today's episode, we're joined by Javier Gonzalez, a professor of nutrition and metabolism based at the University of Bath to talk about some of the myths surrounding the metabolism. So Javier, 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. [snorts] 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. Um, 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 meta metabolic processes into two main things. um 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 like your resting metabolic rate or or your basil 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 energy 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 basil metabolic rate or basil energy expenditure. And that's [clears throat] 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. Um we're not stimulated very much. And the third factor there is that we're normally fasted as well. So um 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 basil energy expenditure. So you mentioned there the 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 basil 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. Um 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 to France cyclists, the majority of their energy expenditure could be from other components such as exercise. Ah, of course. Yeah. Got you. So, is it our metabolism something that we can change sort of over the course of our lives? What is it? Because you've often hear people say, "Oh, such and such person's got a fast metabolism, such and such 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. Um so our resting metabolic rate is mostly based on the amount of what we call fat-free mass. So 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 diet and you restrict the calories that you're eating, yeah, that will lower your resting metabolic rate partly because you lose fat free mass, but also aside from that reduction

in fat free mass that and that's known as a phenomenon called adaptive thermogenesis. So a reduction in metabolic rate that is greater than we would predict based on just the muscle mass that's there. So you mentioned a term there metabolically active. So what exactly does that mean? Like 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. Um, a whole load of processes and that's why it's so metabolically active. So, another thing that you hear a lot is that um slimmer people have faster metabolisms and heavier people have slower ones. Is there any truth to that? Uh, not really when it's been looked at. So, um, it 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 um an overactive thyroid, for example, and that's producing a hormone that would stimulate metabolic rate. But for the vast majority of people um their rate of energy expenditure, the rate 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 rate if you know if somebody wants to find out? Yeah. Um so for resting metabolic rate that's normally done with uh a method called indirect calorimetry. So what you essentially do is measure the amount of oxygen that person is consuming and you the amount of carbon dioxide that

they're breathing out. And you can do that with a face mask and a you collect the air into a bag and measure those gases there. Um it's a little bit more tricky if you want to know how much energy they burn 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 um quite a sophisticated method known as doubly labeled water. So the person will drink this special water where both the hydrogen and the oxygen are labeled.

They're slightly heavier isotopes. Um 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 and on the breath. And so that means that 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 um 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 lot 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. Um sleep is linked to a whole range of um metabolic diseases. Um, we're still trying to understand exactly which of those diseases are caused by poor sleep rather than just poor sleep being a marker of other aspects of that lifestyle. It 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 um quite severe sleep restriction um can impair our glucose metabolism, our ability to metabolize the carbohydrates that we eat and might

even affect uh 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 uh thing that is often talked about in terms of health which we often feel um 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. Um one of the most profound effects of stress on metabolism when [snorts] we get that adrenaline spike um it will increase the release of fats from our fat tissue. So fatty acids are stored within the fat tissue largely under our skin.

Um and the adrenaline will stimulate the release of those. And from an evolutionary perspective, um that has a useful role in that it's releasing energy available for us to then run away or run a long distance. Um, 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, [snorts] um, then it's increasing the availability of those fats for organs and tissues that they probably shouldn't go to. Um, 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 often hear and that's that's aging. Mhm. So, um, a lot of people say, "Oh, you know, I'm sluggish now. I'm in my 40s or something in my 50s. Um, it's because my metabolism slowed down." Is there any truth to that? Again, a grain of truth, um, but sometimes overstated. So, it's true that metabolic rate declines as we age. Um, 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. Um, at least at the population level. Um, 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 um so that will exacerbate the age related muscle loss over time. um 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 um 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 um 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 there sort of burning calories etc. The first sort of port of call a lot of people will take when trying to lose a bit of weight is cutting calories. So obviously then we're consuming less energy that we can burn. So what effect does that have on our metabolism?

Yeah. Um, 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. Um, but there are effects of that on metabolism. One of the initial ones is that we get a reduction in our resting metabolic rate. Um, another is that we will lose fat mass, which we probably want to lose. Um, 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. Ah, okay. Interesting. So, how about So, let's have a look at some other diet things though. Um, so a lot sticking with the aging thing. A lot of people say, "Oh, they can't drink as much alcohol as they get older because they can't metabolize it." So, is there any truth in that? You know, does alcohol affect our metabolism at all? Alcohol definitely affects our metabolism. Um, the age related um aspects exist to some extent. Um but in terms of the potency of alcohol, if we think about um comparing it to other things we eat. So for example, sugars uh are known to um not be the healthiest when consumed in

excess. Um and one of the effects they have is they stimulate this process known as denovo 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. Um 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 often hear 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." Mhm. Is there any truth in that? There is, but it's a very small effect. Um, so yeah, for both caffeine and capsain is the kind of active spicy ingredient in a curry, in a chili. Um, they have both been shown to increase our energy expenditure. Um, but it 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 and

a half to 3,000 calories a day. So, how about another thing that you often hear is um people 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. Mhm. That concept is sometimes referred to as timerestricted eating or um where yeah people will restrict foods at certain times of the day. Um 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 um 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. Um

and it's probably the one that is the healthiest or on the healthier side. Whereas lunch is probably in the same 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 um 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 eaten. You're

you're not consuming as much alcohol and the snack type foods after your evening meal. So how about another sort of popular uh form of timerestricted eating and that's the sort of intermittent fasting diets surely they have some sort of effect on this. They do. Um so if for example you go without food for 24 hours and 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 libitum period. So sometimes referred to as alternate day fasting. So you fast for one day and not for the next. Um, and in the fasting day, one of

the metabolic changes that you see and you would 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 um then we need we can't 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 um 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. Um then when we go into that feeding state we switch back to um a normal postprandial metabolism. We 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, right? 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 ask 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 a day. So it works largely because it naturally results in an energy deficit. So you mentioned exercise there. So let's let's let's talk about exercise. So um we know we 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 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 metabolisms? Yeah. So um card cardio or kind of endurance or aerobic type exercise is where we might go running or walk brisk walking um 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 um 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 are on insulin therapy that can be a really useful strategy um if timed appropriately. Um after exercise then the muscle becomes sensitized to insulin. So that's a second benefit. Um 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 um

people call it the like the afterburn effect whereas you know if I do some exercise then that kicks up my metabolism for I mean some people even claim for a couple of couple of days. Yep that'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. Um, 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.

Um, it's seen with resistance exercising like lifting weights, but is also seen with the kind of cardio endurance type training. Um, 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 2 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 um used beneficially rather than just um oxidized and burned as fuel.

Yes. So I've got a few questions about strength training, but you've just brought something up there that I just thought, oh, is does doing exercise actually stimulate our appetites more? Yes, that's a very common question. Um, 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 with especially with vigorous intensity exercise is actually a reduction in appetite immediately after exercise. However, over the longer term, um, exercise would increase appetite. Um, we've never or 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 that 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 5 kilos of muscle so that's going to have a pretty big impact on my resting metabolic rate right um it will have an impact um probably not as large as you would hope um 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 um 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 5 kilos of muscle won't increase your energy expenditure by more than tens of calories. Okay.

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 5 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 5 kilos of muscle probably has more of a role in terms of your glucose control. Um your um functional capacity during daily activities just able to lift your shopping bags um 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 um to for example break our hip and then we get in a vicious cycle of a hospital stay and muscle disseuse um and so on. So Javier, thanks so much for joining us. Fascinating stuff. Thank you for the invitation. Thank you for watching 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 saw then please do consider liking and subscribing to the channel.