March 30, 2023

"Restricting Carbs and its Impact on Physical Activity & Metabolism" with Prof Javier Gonzalez

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Episode 185 of the Institute of Performance Nutrition's "We Do Science" podcast! In this episode, I (Laurent Bannock) discuss "Restricting Carbs and its Impact on Physical Activity & Metabolism"with Professor Javier Gonzalez PhD (University of Bath, UK)

Discussion Topics Include:

  • Fuel use  and changes during activity and rest
  • Blood sugar and insulin responses to sugar and carbohydrate restriction
  • Sugar restriction and blood lactate concentrations
  • Fat oxidation rates after various types of meals
  • Changes in metabolism as a result of sugar and carbohydrate restriction and wether that actually results in physical activity and total energy intake differences
  • Pros and cons to sugar and carbohydrate restriction for health, body composition and performance

Key Paper(s) / Book(s) Referred to:

Related Podcast Episodes:

Check out our other podcasts, publications, events, and professional education programs for current and aspiring sports nutritionists at and follow our social media outputs via @TheIOPN






[00:00:00] LB:Welcome to Episode 185 of We Do Science, the Institute of Performance Nutrition’s podcast. I am the host, Dr. Laurent Bannock, and the Founder and Director of the Institute of Performance Nutrition. 


What did we talk about in today's podcast? Well, I brought back Professor Javier Gonzalez. Yes, he's been on our podcast before. I've not done any podcasts for a little while. So it's going to be busy catching up period here in March 2023 for those of you that are listening to this fresh episode. 


What did we talk about today? Well, we discussed something I think you're all going to be interested in which is this question of how does restricting dietary intake of sugars or all carbohydrates affect metabolism? How does that affect performance, training adaptations, body composition, et cetera, et cetera? It's a biggie, and we got deep into this topic. Professor Gonzalez is, of course, a world-renowned expert on this topic. So you're going to get a lot out of that. 


Before I unleash the episode on to you, please check out our website at, where you can learn about everything that we do. There's loads of new stuff on our website. Our new recently launched postgraduate level, master's level, diploma in sports nutrition, fully accredited. Our newly launched professional development program, which we call a PDP, it's a community, a range of resources, research reviews. You name it. Access to journal, all sorts of stuff. It's there for you to learn about on our website under PDP. 


You can find out about our sports nutrition software for sports nutrition professionals, nutrition coaches, that sort of thing, SENPRO. You can also see the freshly updated podcast page. It's all new and loads of other stuff. But here we have my conversation with Professor Javier Gonzalez, all about restricting dietary carbohydrates and sugars, and its impact on metabolism. Enjoy. 




[00:02:10] LB: Welcome back to the Institute of Performance Nutrition’s We Do Science podcast. I'm super happy. Well, for various reasons, I'm super happy. Although I'm probably not as super happy as my guest is for reasons that he may choose to reveal shortly. 


But it's been a while since I've been doing some podcasts, just because I've been busy. Like all of you guys, I'm sure. My guest today is no newbie to the podcast, although it's been a long time. For various reasons, it's been a long time. But welcome, Professor Javier Gonzalez.


[00:02:41] JG: Thanks, Laurent. Yes, it's great to be back. I’m really looking forward to this chat.


[00:02:44] LB: Well, isn't it weird what happens in time, personally and professionally? We were just talking offline there about – I mean, the first time I met you, it wasn't quite – well, it wasn't 10 years ago. But, I mean, it was like seven years ago or something. You were a burgeoning researcher back in the day. But as I was joking, everyone that's been on our program over the years, particularly in the days that we ran [inaudible 00:03:06], you've all become professors. What’s going on?


[00:03:08] JG: Yes. Yes, I know. We should maybe hire you as a talent scout for universities.


[00:03:12] LB: You know, that's a good idea. That's a good idea. I want a piece of the action. That's where I want to go with it. Anyway, firstly, congratulations on becoming professor. That's a serious achievement. We only have to delve into your work over the years, and we can see why. 


But you've not just been a researcher. You've been a practitioner, which is really perfect for the types of conversations that we have here. Bearing in mind, we're trying to sort of bridge this gap between science and practice. We'll come back to that in a minute. But also, personally, I believe you might have had an achievement. So welcome to fatherhood.


[00:03:48] JG: Yes, thank you. Yes, really nice to become a dad and now reading up on all of the neonatal nutrition. 


[00:03:54] LB: Yes, of course, you would. I know. It's funny, isn't it, as parents? Although I'm embarrassed at what my kids eat, I’ll admit. You will learn. You will see. It's a question of just keeping them alive.


[00:04:05] JG: Exactly. Yes. Yes. As long as they're getting something in. Yes.


[00:04:09] LB: The other thing is, is you don't have – well, firstly, you got hair. Well, so am I. But speaking of all those professors on our program, an awful lot of them, the males, have lost their hair. Maybe that's not a good link. But, yes, gray hairs. Look forward to those coming along. 


Anyway, Javier, so welcome back. I'm looking forward to having this conversation with you. The reason why I wanted to get you back was primarily because you and your group had published a paper late last year, which I'm going to show the camera here, which was about restricting sugar or carbohydrate intake does not impact physical activity level or energy intake over 24 hours, despite changes in substrate use. That wasn't just sort of a review. It was a randomized crossover study in healthy men and women. 


For all sorts of reasons, I wanted to get into this topic, not because it's still just a topical conversation to talk about carbohydrates. Anyone that's followed this podcast, listened to previous episodes with yourself, Dylan Thompson and James Betts, your colleagues there at Bath, where we've talked, obviously, about the breakfast thing. We've talked about compensation mechanisms. We've talked about metabolic regulation, particularly with James in the last podcast I did with him not so long ago. 


But when we're talking about athletes, recreational athletes, we do like to get into things like macros. When we delve into macros, energy intake and so on, obviously, carbohydrates come up. It's still that thing that people talk about, “Oh, well. Let's not eat carbs. Carbs are king.” We've debated this on all sorts of podcasts. The thing that we haven't talked about so much is this idea of restricting types of carbohydrates and, in particular, sugars, for example. 


So anyway, I'm bringing the conversation back. But what I wanted to talk about today was this research that you've done and not just this particular paper because you and your colleagues have got years of knowledge and experience. But also, you yourself have worked, of course, with many different types of athletes, cyclists in particular. I'll let you talk about that in a second. 


But it is a topic that's still hotly debated, rightly or wrongly, with or without evidence or relevant evidence, and so on. So I'm looking forward to having this chat with you, Javier. But as I'm talking about this, of course, I know you. Many of the listeners know you. But not everyone knows you. So perhaps we can just quickly go back to who Professor Javier Gonzalez is, and then we'll delve back into this conversation.


[00:06:41] JG: Yes, sure. Well, this paper is a really nice example, actually, of my research interests in general because it combines carbohydrate and fat metabolism with physical activity and energy intake. They're really the areas of research interests that really kind of grasp me. 


I guess going back a little bit further, so I did a sport and exercise science undergraduate, followed by an exercise physiology masters. Really, at that point, when I started my master's, it was for the sole purpose of getting on to a PhD program. So I was really driven at that time. 


It took a while actually to get that driven academically. At school, I wasn't so driven. I was more interested in rugby and the third half of rugby. Then it was actually, yes, in my undergrad degree, I started to get really interested in understanding how the body works and how it responds to exercise and nutrition trends. Also seeing the benefits of hard work and how that can pay off. 


So that started a positive cycle. I really got into the studying then and got into a PhD program after that master’s. That was up at Northumbria Newcastle with Professor Emma Stevenson. Following that, I stayed in Newcastle for a postdoc, where we were doing work on carbohydrates and sugars and liver metabolism in cyclists. Then came down to Bath, and I've been at Bath now for – I think it's about eight years. Yes, just carrying on that line of research really and building up that evidence base.


[00:08:06] LB: I'm wondering if there's an interesting link between somebody who starts that process off, not just in your conversation but in reality, as I did, by the way, of being at school and not really – I mean, I wasn't remotely academic. It's questionable whether I am still. But either which way, is this an overcompensation that you've been going through? Because I'm pretty sure that I did.


[00:08:28] JG: Yes, yes. Quite possibly, yes. Yes, I agree.


[00:08:31] LB: I mean, what do you think if you were to talk to yourself when you were younger? I mean, now that you're a father, this is an interesting perspective to have. In that chat with yourself, do you feel, “Yes. Actually, I've found my calling. I’m going down a path.”? I mean, you've been remarkably successful, if you don't mind me saying. You're still quite young, Javier. You’re doing great things.


[00:08:55] JG: Thank you. Yes. I mean, I think there's probably a few things that kind of contribute to the path. I have always been interested in science without really knowing it. I've always been interested in how the body works. But it was just at school, the way you're taught isn't – it's more rote learning, which I'm not very good at. I'm not really good at remembering things just off face value. But when I understand the story and it makes sense, then I can remember it. 


I feel like physiology is like that. It's a story of what's going on. Once you understand that, then it makes sense. I also like, with university upwards, that there's that level of critical thinking and the logic and rational thought that really engages me as well. Then alongside that, I probably have always been quite competitive, even with others or just with myself, actually. So that came across in sport when I was younger, but it took a while to actually translate that over to studying hard.


[00:09:48] LB: Yes, that's the great thing, isn't it? When I was learning back in the black and white days, I guess. For those who are listening, by the way, for those of you that are still doing your undergraduate degrees, for example, you'll probably share some of this, where you're kind of taught stuff from the perspective of you're told, “Well, this is how it is. Just move on.” You're not encouraged necessarily to be curious and/or necessarily open-minded because that can be a bit of a shocker. Of course, as you transition to master’s, particularly to doctoral level, it's all completely different, isn't it?


[00:10:21] JG: Yes, definitely. I probably – going back to your question regarding now being a father and kind of imparting the wisdom, if I can use that term, on to others is, yes, I guess, looking back, I was a little bit concerned actually about doing a sport and exercise science degree because I thought, well, what is this going to lead to? 


I'm so glad that my parents encouraged me to just study what I'm interested in because I'm not very good at doing things that are not enjoying. Whereas if it's something I enjoy and, I guess, I can't remember who told me this now, but I've heard people say, “Well, don't worry about necessarily finding a job straightaway. But find something you enjoy and then find out how to make money from it.” That's probably good advice. Yes. If you do it well. 


[00:11:02] LB: I'd agree. I’d agree. When you love something and you develop passion for it, that passion drives you. But it also – it’s infectious. That can translate in many different directions. But without it, it's just doing something for the sake of it, which for the likes of you and I is not something we can do. 


That’s important because, look, it's incredibly hard to achieve things like degrees and especially doctorates and so on. You've got to have an awful lot of patience and a thick skin, really, in many different ways. This paper we're discussing is – I don't know what number in your list of publications that have been published, daresay one or two might not have been published. It's a very long process to go from research to publication. 


So before I go there, I did mention not only have you spent all this time evolving from not being an academic school kid to achieving what you have in terms of your academic career and publications and so on. But you're not just a researcher, and there's nothing wrong with being just a researcher. But you have also managed to spend a fair amount of time working, as I call it, “in the real world.” Do you want to just quickly talk about that? Because I think that's an important perspective.


[00:12:19] JG: Yes. I probably wouldn't want to overplay this, compared to some of my colleagues who do a lot more of this than I do. But, yes, ever since I started research, I did do little bits of applied work in rugby and triathlon and cycling. But it wasn't a major part of what I did. Then I focused on the research career, and it was in 2020 – well, end of 2019, early 2020. I got the opportunity to really get involved in the applied world with [inaudible 00:12:45]cycling team as the performance nutritionist. 


So it was a very different entry to that world for many other people. I've kind of focused on research that was relevant to cycling, and then was able to learn about how that world works, and learned a lot from the couple of years I was working in that sport. I still do a bit of consultancy as well with various teams. But it's a really interesting sport as well to get involved in, not only from the direct impact that nutrition can play but also the logistics and everything else around it. 


I think the biggest difference between professional cycling and, say, most team sports is the fact that the athletes aren't in one base training every day. So learning how to build a relationship with them and communicate with them and educate them is a really key element to cycling that differs from some of the other sports where you can see those athletes day in, day out.


[00:13:39] LB: Yes. I remember talking to James Morton about this years ago now. The amount of knowledge that the cyclists can have is pretty shocking. I've worked a lot with triathletes but not so much cyclists, specifically, especially not at the level that you guys have worked at. But it's always a bit mind-blowing when they almost know more about nutritional values of foods than you do. 


I imagined jockeys would be the same if I recall what [inaudible 00:14:03]told me before. But how did you find that being as achieved as you have been academically? But talking to a population of people who are way above average in terms of, well, obsession if nothing else?


[00:14:16] JG: Yes. I really enjoyed it. It was a great challenge to get to know them and to choose the moments to engage in those conversations. So I was, obviously, very aware that they are professionals. They've been doing that sport for many years, and they've got learnings from the things they've tried and tested, which might disagree with scientific literature. Of course, neither of those pieces of evidence are the perfect piece of evidence. So it was a case of being humble on both sides in terms of how they map onto each other and coming up with a plan. 


I guess one of the useful pieces of information that a colleague gave to me, he’s actually my line manager there, he said that just remember that it's not you who does the performance. It's the athlete. That actually takes a bit of weight off your shoulders. He was like, “They are the professional. They're responsible for their performance, and you're there to advise and assist and help.” Yes, that certainly helped me reframe it a little bit.


[00:15:08] LB: Yes. No, that's great, I think, to be on various sides of that. You've engaged somewhat in sporting activities yourself, so you've also been a consumer of this stuff. I imagine it’s probably a dangerous thing to have as much knowledge as you have. It probably drives you bonkers when, no insult meant, but you may not be getting any Olympic medals yourself. It's probably a challenge. 


Anyway, Javier, to bring us back to this chat then. So it's interesting to ask the question, which is how does restricting dietary intake of sugars or of all carbohydrates actually affect metabolism? I think this is interesting for lots of different reasons, mainly because so many people already seem to think they know the answer to this, despite the lack of evidence. 


As I mentioned in my preamble at the beginning, which hopefully was recorded, it's also a very emotive topic here for one reason or another. But since we're dealing with performance and athletes and outcomes in particular, even if it's just body composition outcomes, we do actually need to know. Because as human beings, we have to sit there and make choices. 


But some of those choices are also based on, well, I'd like to have that because I like it. Or it's a convenience matter, like you were talking about your cyclists who did traveling everywhere on –when I've worked a lot in tournament fields, traveling is such an issue. So let's just go back then. So why did you feel the need to do this research?


[00:16:33] JG: Yes, I completely agree with what you've said there in terms of the misunderstandings around carbohydrates and sugars in particular. That was one reason for doing this study, and it has a two-pronged approach. So one is trying to understand carbohydrates as a whole and their role in energy balance and energy balance behaviors. The other is the role of sugars. 


This is where it becomes a little bit tricky because the definition of sugars varies by the country you're in and the specific guidance that's given by the government. In the UK, we have something called free sugars. So there's a definition for sugars as a whole but also free sugars. It's the free sugars that we are being advised to restrict our intake of. 


Now, I'll give the complex definition. But we'll give some easy examples afterwards. So the official definition in the UK of free sugars is all monosaccharides and disaccharides. They’re added to foods by the manufacturer, the cook, or the consumer. Plus the sugars that are naturally present in honey, syrups, and unsweetened fruit juices. So what that really means is if you're adding table sugar, just your typical caster sugar into a cake or something that you're making, that's a free sugar. But also, if you're adding honey or syrups, or if you're having fruit juice, that is a free sugar. 


Now, if you have that same fruit but eat it as whole fruits, then it's not a free sugar. So a key distinction in the government guidance here is whether the food has been processed to the point where it's not in its original food matrix. That might be partly because it makes a difference as to what other components are in that. 


But also, once you've processed it and it's now a fruit juice, there's two further aspects there. One is the dose of sugar is normally much higher. So if you're drinking a glass of juice, there's going to be more sugar than if you just eat one single apple. But also, the rate at which you digest and absorb that sugar is going to be more rapid in the juice compared to the whole fruit. 


So there's a lot of complexities in there that we can delve into, and there are issues with those definitions. One issue, I think, is that actually it doesn't include maltodextrin. So maltodextrin is a very rapidly digested carbohydrate. If you measure the glycemic index, which is an index of how much it raises your blood sugar level, it's identical to pure glucose. So maltodextrin and glucose physiologically behave very similarly. 


Yet, based on this definition of free sugars, maltodextrin is not a free sugar. So this has implications because if you buy, say, Angel Delight off the shelf, your normal Angel Delight will have sugar in it. Your Angel Delight sugar-free will have maltodextrin as a replacement. So we need to understand the physiology of these different carbohydrates much more to understand whether that's an appropriate substitute and whether that should really be called a sugar-free substitute. 


[00:19:27] LB: You know, you just used a word that might become part of my new favorite repertoire of words, which includes, as everyone knows, context but also relevance, right? That is behave. Now, it's a word we use a lot as parents. But behave, how do these things behave? I love that concept actually. We make a lot of assumptions about the behavior of sugar and carbohydrates, and that's what leads people down to, or it's going to make me fat, or it's the root to all evil, and/or without it, you will not win your medals. 


I think that becomes interesting because the way in which these things behave is also not a static thing because you can train these things. Do you want to just quickly tell us about that? Because I guess that makes it – it’s not just complicated in terms of do we even know what we're eating because of labeling and definitions and various other things. But the behavior side of things is also something that is not a predetermined function.


[00:20:23] JG: Yes. So that relates to both. I guess the human behavior in response to these foods but also how these foods respond within our body both acutely but also if you adapt and show different responses. So I guess with this particular study, it was an acute study. So just the first 24 hours, if you switch on to these diets, what happens. 


The primary thing we were interested in was people's behavior. So the primary aim of this study was to understand the physical activity response to these three different diets. So a control diet or a diet where you restrict just the sugars or a diet where you restrict all of the carbohydrates from your diet. 


The reason we were interested in that is because a number of other studies had indicated that maybe the carbohydrate availability might be a stimulus to be more physically active. That comes from a couple of studies that we've done. One is a Bath Breakfast Project led by James Betts, where people who ate a high-carbohydrate breakfast were then spontaneously more physically active throughout the rest of the day, compared to when they skipped their breakfast and remained fasted. 


Then a second study was an intermittent fasting study. So day fasting. On the fasting days where people weren't consuming any calories, their physical activity was lower than when they were consuming calories. So we didn't know if it was carbohydrates specifically or energy. But there was clearly something going on there. 


So the primary aim of this study was to understand is it dietary carbohydrates that stimulate this. The mechanistic data that also supported that rationale was a couple. One is from a study that we ran where we found that the amount of glycogen in the liver that people use during exercise was associated with more energy intake later in the day, so influencing energy balance behaviors. 


Then the kind of really mechanistic data is found in rodent studies, where if you genetically increase or decrease the amount of glycogen they have in their liver, they're more or less physically active and also adjust their energy intake. So there was reasonable justification, I think, to run this study. So, yes, that was the primary aim. 


But a great part about it was that we also included a whole load of other measures of substrate metabolism. So we can also dispel a load of other myths there because things like the sugars cause a big spike and then a crash in your blood sugar level. That kind of thing we can probably delve into as well because we measured all of those things.


[00:22:39] LB: Yes. Again, you're using phrases like measure, for example, but also an acute study and so on. A lot of studies are done, and a lot of observations are made from those studies. Then that information is translated in whatever way and then applied into practice or translated over to consumers in some form or another. 


But, of course, there's a lot of assumptions made. A lot of correlations are made from this stuff. Why does it matter that you perhaps have gone to greater lengths in terms of how you've measured and not necessarily just observing over the course of an hour's – somebody comes in for an hour and that's it. Why does all of that matter?


[00:23:24] JG: That's a really important point because no single study will give us the answer for everything. This study is part of a larger program of study. So there'll be more papers that we’ll have coming out on this exact theme but over longer timeframe. So this study had its own aim. We wanted to understand what's the immediate 24-hour responses to these diets in a relatively controlled environment. 


So we can characterize how the body responds, both with metabolites that are circulating in the blood, the fuels that are circulating, but also the hormones that are circulating as well. We can also understand the immediate impact on people's behavior. Then with our other studies, when we combine it all together, we can hopefully understand. We can understand this immediate response very tightly. 


As we go to longer-term studies, the downside is that you lose a bit of control. But the upside is you gain more relevance to people's daily life, people's lives over longer timeframes. You want to understand the effect of these diets over those longer-term, two, three, six months, and so on. I think the strength of these types of projects when you combine different types of studies is each study has its own weakness, but each study has its strengths as well. So you triangulate the data. 


If they all point in the same direction as to this diet is healthier or will help you achieve your goals in a certain way, if they're all pointing in that same direction, you have a greater degree of confidence that that's probably likely to be true. 


[00:24:49] LB: What’s interesting to me, Javier, is this is an area as previously discussed that has been hotly debated for quite some time. Yet, we're having this conversation here at the beginning of March 2023, and you're still working on adding to that as yet still very light body of knowledge, where you're attempting to answer this question, but you're still not in a position to have a definitive answer. 


If we bring it back to why did you do this study, well, you sort of answered that. But why is this question of how does restricting dietary intake of sugars or of all carbohydrates affect metabolism? Why is this important, particularly, if we bear in mind the perspective of this podcast, of course, is finding information to influence the recommendations that we make as practitioners, as performance nutritionists? That's the main angle here. And/or helps the impact other researchers looking to impact the profession. Why is this question still so important for us to find an answer to?


[00:25:50] JG: I think it probably comes down to the ability to measure physical activity. I should say the government guidance on restricting sugars is mainly based on the potential effects on our teeth, so our dental health, and also on energy intake. So we do know that diets that are high in sugars can lead to higher energy intakes. But if we're interested in body composition or body weight of an athlete, it isn't just the energy intake that we're interested in. It’s energy balance. So we need to know how much energy they're expending as well. 


It’s only really been in the last couple of decades or so that we've been able to measure physical activity with sufficient precision directly. So you can measure physical activity indirectly with methods like doubly-labeled water. It's a very expensive method. You basically use a subtraction technique to try and estimate physical activity. 


You can also only measure physical activity over a longer timeframe. So you need about a week or two weeks, and it gives you the average physical activity over that timeframe. It also doesn't tell you the intensity of activity or when that activity was done. Whereas with some of the more modern methods which are based on the same principle as your smart watch, using heart rate and movement patterns but combining them with some clever algorithms, they can quite precisely tell us our physical activity, energy expenditure. So the amount of calories we're burning, specifically through physical activity. They do that directly, rather than by a subtraction method. They also tell us when we're doing that activity and the intensity. We get a lot more information from it. 


As I say, they've only been around with sufficient accuracy over the last kind of few years or so. I think that's probably a major reason why this type of study hasn't been done, where understanding the effects of these diets by measuring not only energy intake but also the components of energy expenditure, alongside all of the metabolism and hormones to really give us that full picture hasn't been done yet.


[00:27:46] LB: This makes me think of previous podcasts and lectures that, well, either yourself, but I'm thinking really particularly Dylan Thompson a few years back now and James, of course. But Dylan Thompson sort of blew my mind in particular when we started to learn about compensation effects.


It’s very easy for us to think about, well, I'm just going to cut some calories. Okay, let's go beyond that. Let's just cut out some sugar. But we assume that everything else is just going to function the same, but it doesn't, as it turns out. Do you want to just clearly remind us about that concept and which is why it is important for us to have the right measures, et cetera in place?


[00:28:22] JG: Yes. Yes, exactly. So compensation can happen on either side. On the energy expenditure side, that might be that if you start at – let's say, you don't really do much exercise at baseline, and you decide you're going to start running for half an hour every day. You would assume that your calories would increase directly from the number of calories that you burn during that run. So let's say you burn 500 calories on that run. Is that going to add 500 calories to your total daily energy expenditure? 


Well, there's two immediate things that could happen. One is substitution. So that is what were you doing anyway when you weren't doing that run. If you were just sitting on the sofa, then it is going to add, at least at that timeframe, those extra calories. But if you are doing some house chores, maybe cleaning or something like that, you are already burning some calories. So the effect isn't quite as big as what you would have imagined. 


Then the compensation part is later on in the day, maybe because you've done that run, you do less physical activity than you would have done otherwise. The same kind of thing can happen with energy intake. So as you mentioned that, if you specifically restrict sugars, it might have no effect on your other behaviors, but it might. So it might be a case of if you restrict your sugar intake, you've reduced your energy intake. But you might also spontaneously be less physically active as well. The net effect is then zero on energy balance. 


I think it's important to understand both energy intake and energy expenditure to really understand what's going on.


[00:29:49] LB: If we delve even deeper into the physiology of this, the metabolic machinery, et cetera, we can also see changes, if we use behavior again, in how the body uses different fuel types. Substrate utilization springs to mind. I find that area – we talked about this a lot with James in our metabolic regulation podcast. 


But given we're questioning, should we bother to not eat sugar, or should we be cutting carbs with the outcome of, say, fat loss or reduction in energy intake or whatever, the machinery itself, what are the changes that are occurring there? Are they directly because of the restrictions? And/or is it just, well, again, a compensation mechanism? What’s going on there?


[00:30:38] JG: Yes. I guess if we start with carbohydrates versus fats as a whole first and then go on to sugars, it might make logical sense. So with – if you go in a low-carbohydrate, high-fat diet, you might be expecting to burn more fat as a fuel and less carbohydrate. It might also affect your blood sugar control and fat in the blood and that kind of thing. That's been known for a number of years. We kind of confirmed that in this study. 


But I think the part that we add quite interestingly is that these changes happen quite quickly. For example, if your fat oxidation rate, burning fat, that changes very rapidly by a ketogenic diet. So this is a very low-carbohydrate diet. We know from other work that it's probably not because you're eating more fat, but it's rather the restriction of carbohydrates that results in the higher fat oxidation rate. 


With blood sugar control, again, that changes very quickly. You get the direct effect of when you eat carbohydrates in a meal. They're going to raise your blood sugar concentration, your blood glucose concentration, whereas the levels of fat in the blood, the triglycerides in the blood show the opposite effect. So the more fats in a meal, the higher your triglyceride responses. They kind of seem to make logical sense. 


One interesting thing in our study was we also took a blood sample the following morning in the fasted state. What happened there with the triglycerides, the fats in the blood, was the opposite. So the high-fat low-carbohydrate diet had a lower triglyceride concentration in the blood the following morning, compared to the two other diets. That’s perhaps reflecting the higher fat oxidation rate. So whilst these people are eating more fat, they're also burning more fat in that scenario, and that might reflect that response there.


[00:32:22] LB: For people who are listening who may be not as familiar with this angle, and face it, even if you're a well-educated sports scientist, it's pretty complicated still because substrate utilization is – it’s what’s going on at that time. It doesn't mean, for example, that increased fat oxidation means that you're reducing fat stores in the long-term because fat balance is still an issue, isn't it? Because that is a central argument. 


Of course, people go, “Oh, I'm on the keto diet, and I'm increasing fat oxidation. I'm just burning fat all day.” But is that actually getting rid of your body fat is another question, isn't it?


[00:32:58] JG: Yes, definitely. So they were clearly burning more fat, but they were also eating more fat as well. So it is ultimately about energy balance. The only way we can really store large amounts of energy long term is as fats, and so excess energy will be stored as fats. If we have insufficient energy intake relative to our energy expenditure in the long-term, we will ultimately lose fat. Yes. 


So the increase in fat oxidation is still important for other things. It probably plays a role in performance, for example. In certain sports, you might want to be burning more fat and carbohydrate. In other sports, you might want to be burning more carbohydrate than fat. Probably also has an impact on health as well. There are certain scenarios where you probably want to be burning one fuel rather than another. But for body composition and body weight in particular, it really is about energy balance.


[00:33:47] LB: Yes. Anybody who's listened to my podcast, I mean, we've had so many conversations with so many experts. But it's such a mind-blowing topic that it deserves all the attention it gets, as does this idea of, well, not so much going low carb. It depends on what you're trying to achieve, right? But specifically, going out of your way to restrict your dietary intake of one or other or both had some surprising outcomes, didn't it, that you found in your study? What sorts of things did you discover?


[00:34:18] JG: Yes. It's one of the interesting points which counts as a common myth is you might think that if you restrict just your sugar intake, compared to eating a diet high in sugars, that it might affect your blood sugar level. If we just look at the overall blood sugar response, at least that first meal of the day didn't really differ whether people ate high or low sugar. 


So the kind of common thinking is, well, if you eat a high-sugar breakfast, it’s going to cause a big spike in your glucose, and then you're going to crash later on. The peak glucose was slightly higher with the higher-sugar versus lower-sugar diet. But the overall response wasn't different, and it certainly didn't crash below baseline later on. 


Part of that might be some of the other measurements that people use like the continuous glucose monitors and measuring from a vein. They can sometimes lead to some spurious findings there because of some physiological effects with a forearm tissue. We were measuring arterial eyes blood here. So this is the blood that's being delivered to your brain, to your tissues. When you measure it there, you don't really see that dip below baseline and that sugar crash. This might be partly because of the misunderstanding of what sugars really are. 


Table sugar is a combination of two smaller sugars, so glucose and fructose. Fructose actually doesn't produce a big blood sugar response. It's a low glycemic index carbohydrate. So it's perhaps no surprise that if you compare that to, say, maltodextrin, which is a high glycemic index carbohydrate but not a sugar, the effects on blood sugar aren't as apparent or as clear as people might seem. So just restricting your sugar intake might not affect your blood sugar concentrations. 


It also didn't really have a massive effect on many other things like insulin or fatty acids in the blood or the fat oxidation. So by restricting your sugar intake, it's not going to affect how much fat you're burning, at least in the short-term. The only difference we really saw with sugar restriction was a decrease in lactate concentrations. 


Again, that probably reflects the fructose component. When we eat fructose, sometimes called fruit sugar, that can't really be used by the tissues in the body directly. It needs to be converted. The liver does most of the conversion of fructose, and it converts it into glucose and lactate a lot of the time. It can also convert it into fat, and that's one of the kind of health concerns of fructose. People get worried that the fructose is going to be converted into fat. 


That does happen with very high intakes. But with these typical intakes, the amount of fructose that's converted into fat is very, very low. When we measured in our study the levels of fat in the blood, it didn't really change whether people restricted their sugar intake or not.


[00:37:00] LB: You're the expert on this. I'm going to say presumably, when we're thinking about the behavior, the fate of these things, and the impact that they have on the body is also influenced by whether or not, for example, someone's active or not, the types of activities they have, their age, and whether or not they've got other issues, which, of course, athletes may be all of those things, except that they may also have diabetes or other issues. But what's relevant there?


[00:37:24] JG: Absolutely. It's a really important point. I should probably emphasize that this study was in relatively young, healthy people and lean. It was in males and females. We did do as an exploratory part some analysis of potential sex differences. But we know from other work that our response to sugar intake probably does depend on various aspects of our physiology and our lifestyle. 


People who are overweight or obese tend to respond more poorly to a high sugar intake. So any potential effects on the levels of fat in the blood and so on are kind of exaggerated in that scenario. The same thing happens with sedentary conditions. So even when you take into account energy balance, if you do a bit of physical activity every day, it seems to protect against some of these potential negative effects of sugar intake. 


For particularly endurance athletes, the metabolic effects of sugars on the liver and on levels of fat in the blood probably aren't a concern. It’s probably because the way in which the body handles that sugar when you're highly physically active is very different to if you're just sitting there at rest.


[00:38:30] LB: Yes. It's amazing, isn't it? I mean, there's a lot to think about, of course, with all of this. I think, though, that that's a key word is thinking. You need to think about this stuff. What is it you're trying to achieve? What is your own personal set of circumstances, your unique needs and preferences as a human being, as an athlete? 


But if you're asking yourself, does it matter whether I restrict sugars or carbs, the answer, therefore, might be, well, not so much then maybe. It just depends on what you're trying to achieve. Like we talked about at the beginning, there are some practical considerations because, of course, particularly in Elite Pro Sport, the consumption of sugars is a strategy for replenishment of glycogen. Whether you're a cyclist with your gels or a football player at halftime, or whatever, these things are certainly necessary. 


But broadly, your answer to that question of how does restricting dietary intake of sugars or carbohydrates affect metabolism, if you had to come up with just a broad basic answer to that, what is it?


[00:39:32] JG: Yes. I guess there's a take home of overarching the whole study. It would be to drastically change metabolism, you need to really restrict total carbohydrates because just restricting sugars doesn't really affect very much. But whichever one of those you do, it doesn't really affect physical activity in the short-term. 


At least in the timeframes we've measured, the type of diet people are consuming does affects metabolism if you’re changing carbohydrate. But it doesn't seem to affect their energy balance behaviors.


[00:40:00] LB: Yes, yes. So I'm thinking as a nutritionist, and I'm not going to go down public health advice and so on because that is largely aimed at sedentary people, and we're dealing with active people one way or the other. But, of course, sugars, in order to arrive at producing a sugar, there's a lot of processing involved. The victim of that is all the other stuff from fiber to vitamins and minerals. 


Maybe perhaps when we're thinking about energy balance and particularly getting athletes or your clients, weight loss people in particular, into a slight energy deficit to bring about that change in in body composition, the quality of those foods maybe does have a bigger influence on what your choices should be, right?


[00:40:47] JG: Yes. That's probably an indirect effect there of sugars where you're absolutely right, If you've got an athlete who is perhaps making weight where they're on an energy budget, so they've got to be selective about the foods they have because they've got to work within that budget, then foods that tend to be low in free sugars are probably going to be higher in some of those other nutrients that we need. 


You can more easily get away with a higher sugar diet if you've got a higher energy budget, and you're a cyclist training six hours a day. When you are on that energy budget, then, yes, it makes sense for kind of other reasons outside of this study to eat a lower free-sugar diet, probably.


[00:41:26] LB: Yes. That's where I can bring my new favorite word of is it relevant then. Yes, you can. You can do all these things. But if you think, if you're a bit more curious, then perhaps it's easier to answer your own questions on this regard, isn't it? But quite obviously, if you spend, which you should never do, too much time on social media thinking about what everyone else does, that perspective is actually lacking in evidence, obviously. It depends, as always. 


[00:41:53] JG: Yes, exactly. 


[00:41:54] LB: Yes. So where are we going next with all this, Javier? I mean, this is – as you say, it's just a piece of the puzzle. It's a bit of what you're doing amongst all your other colleagues in the research sphere, adding to this body of knowledge. Where are you guys going next with this? 


[00:42:09] JG: Yes. Well, we've got the longer-term part of this project to come out. So we're still doing some analysis on that, where we've got four weeks and 12-week outcomes. We've also got a line of work trying to understand the role of milk sugars on health. So in that definition of free sugars that I mentioned earlier, milk sugars, as the present in milk, don't contribute to that definition, even though they're not really in a food matrix. 


We don't really know very much about the physiology of milk sugars. So we've got a study, which we’re just wrapping up on that. So, yes, certainly the kind of carbohydrate sugar area I'm still interested in. I think there's still plenty of questions to answer with regards to whether free sugars do play a role in health in certain contexts, whether they play a role in energy balance, and then unpicking their effects on health in energy balance and in energy surplus and energy deficit. So, yes, certainly plenty to do.


[00:43:00] LB: Yes. You're not going to be bored, are you? I mean, there's still a lot for you guys to do. This isn't the only area of research that you're working in, is it? What are some of the areas that you're currently involved in out of interest?


[00:43:13] JG: I guess I broadly split it into kind of sport performance work and health work. In the performance area, we're doing some work on the role of prebiotics in nutrient availability in athletes. So does certain prebiotics – so you eat your normal diet. But by supplementing with a prebiotic, do you extract more nutrients from that diet, so potentially increasing health and performance that way? 


That's led by a PhD student, Adam Collins, who's started with us just this last year. Then another PhD student, Louise Bradshaw, is doing work on the health side. So she's studying the effects of exercise in a fasted versus fed state, which is a theme we've been having ongoing for a number of years. 


To date, we've mainly focused on males in that particular topic. So as this project, we're trying to understand male or female responses to that intervention.


[00:44:01] LB: Yes. There are some challenges. I can see there are definitely some challenges. Although just to go back to the prebiotic issue, I'm particularly interested in microbiome and probiotics. I contributed to a few publications in that area. That if we link that to what this conversation was about, there are two angles. 


Number one is, obviously, the lack of fiber, which is a result of maybe prioritizing sugar over whole grain carbohydrates, for example, has a pretty negative impact on the microbiome and microbial diversity and all that. The prebiotic impact, I don't know. I mean, I'm looking forward to seeing what you guys find out about that. But the consumption of sugar itself has fairly interesting impacts, doesn't it, on gut diversity, which is not necessarily good.


[00:44:45] JG: Yes, yes. That’s where you kind of hit the nail on the head there with trying to understand this because there will be some effects that are direct effects of sugar. Then there are other effects that are indirect effects of a high-sugar diet. So a higher-sugar diet is probably more energy-dense. It’s probably lower in fiber and certain vitamins and minerals. 


The way we tackle it, I think, is with multiple types of studies, some of which tackle the direct effects of sugars, and some of them tackle the other components. When you piece that all together, you can understand. Is it the direct effect of sugar? Or is it the higher sugar diet in general?


If you have that level of understanding, you can then – if you're a manufacturer of foodstuffs or menus and things, you can design them in very precise ways to still achieve certain goals for athletes or the general population without the negative consequences.


[00:45:34] LB: It's good point we end on really. I think as with all research, you have to focus things down so that you can – well, you got to control all these variables, and you've got to try and understand things. But the reality, of course, is that these things, they all get thrown into the pot, into the human being at the same time, and in free living conditions. 


That was actually something I wanted to quickly chat about. I know you and your group have done a lot of work using gadgets like Actihearts and various other things. Assessing individuals, whether it's for research or doing an assessment on an athlete in a very convenient, controlled environment in the lab is not what happens in a free living environment. I'd be interested to know what you think about that.


[00:46:16] JG: Yes, definitely. Again, I think what we should really aim for is both types of studies. Then you can understand things with greater detail. One issue that I’m not sure we can get over yet, or you probably could with a clever study design is still the observation effect. So you put a device on someone. I guess you could tell them you're not measuring physical activity, and there's a level of deception there. 


But when people know they're being observed, they quite often change their behavior. So they might be in a free living environment, but it might not be their habitual behavior. To really understand habitual behavior, we also need to think about, even by observing, are we changing their behavior. So that's a tricky one to overcome for kind of all measurements of humans, really.


[00:46:58] LB: I mean, this is a whole another topic, isn't it? It's like white coat syndrome and people checking blood pressure and/or were they late for their appointment. Therefore, they ran there. Of course, their blood pressure's up and so on. I mean, it's one of those things, isn't it? 


But listen, look, we could talk for hours about this stuff. We probably will at some point. But I just wanted to say thank you for your time. I know you're a busy professional, a busy parent now. But that was a really interesting conversation. I benefited greatly from it. Yes. Good luck with everything that you're doing. I look forward to getting you back on at some point in the future, and we'll carry on this conversation. 


[00:47:31] JG: Yes, thanks. Likewise. Yes, I really enjoyed the chat. 


[00:47:33] LB: Yes. Thank you so much, Javier. Thank you so much. 


[00:47:36] JG: Thanks.