Sports Dietitians, Leah Higl and Aidan Muir, explore the ongoing debate between low-carb and high-carb diets for endurance performance. They break down the science behind each approach, and dive deep into the research and nuances of each strategy.
(0:00) - Introduction
(0:56) - Mechanisms Behind High Carb or High Fat Diets for Performance
(2:50) - Summary of the Research & Our Interpretation
(13:26) - Low Carb Diets for Ultra Endurance Events
(14:50) - Anectodal Evidence & Other Considerations
(18:26) - Summary
WEBSITE: https://www.idealnutrition.com.au/
PODCAST: https://www.idealnutrition.com.au/podcast/
INSTAGRAM: https://www.instagram.com/idealnutrition__/?hl=en
Our dietitians 👇
Aidan Muir @aidan_the_dietitian
Leah Higl @plantstrong_dietitian
Tyler Brooks @lift_dietetics
Hanah Mills @hanahmills
Samantha Staines @nourished.by.sammy
Monica Cvoro @fuellingperformance
Josh Wernham @josh_does_health
Katelyn Bowden @katelyn_dietitian
(upbeat music) - Hello and welcome to the Ideal Nutrition podcast. I'm Leah Heigl and I'm here with my co-host, Aidan Muir. And today we will be discussing low carb diets for endurance athletes. Traditionally, higher carb diets are more preference for endurance athletes due to the high carb utilization in training and events. However, for several years, there has been quite a large interest in adopting a low carb, higher fat diet for these athletes. So today we're gonna go through why high carb diets are typically preference for these athletes, why low carb diets theoretically might make sense. We'll go over a summary of the research in this particular arena and then kind of get into the nitty gritty of our interpretation and current recommendations. The reason why higher carb approaches are typically recommended is because carbs are the most efficient fuel source for higher intensity activity. That's not really a debatable topic because carbs can be metabolized into ATP more easily than fat can. That's just a factual thing. Research comparing higher carb approaches versus normal intakes consistently has found improvements, particularly when looking at things like a carb loading and intro race carbs. So that's part of why the traditional model has often been higher carb. Like a lot of that stuff seems pretty simple, but let's go a bit more nuanced beyond that. - Yeah, so moving on to a brief summary of reasons why a lower carb, higher fat approach could make sense. So essentially, like when it comes to the storage of glycogen, there is a limited amount that we can store. So glycogen being the storage form of carbohydrates in our liver and muscle. So even with a full carb load, we are still likely to run out of glycogen at some stage in an event like a marathon or particularly longer events than that. On the other hand, eating fewer carbs improves our body's ability to use fat as a fuel source, including things like better mitochondrial efficiency and fat oxidation. Again, those things aren't debatable. So those are adaptations that do occur when someone reduces their carb intake and increases fat intake. We see that kind of move to a higher fat oxidation and being better at utilizing that as a fuel source. But there are a few other proposed benefits that are a bit more debatable in this space, including things like improved body composition, reduced inflammation, and also some people proposing that a lower intake of higher sugar products like sports gels might lead to a more nutritious diet in general and improve things like recovery. So those are all mechanisms. A lot of that's not debatable. Some of that is debatable. When we get deep into mechanisms, I love to just look at research on outcomes and be like, how much does this matter? 'Cause you can sometimes get caught in mechanisms. So looking at the research, stealing the punchline, making the golden content creation kind of mistake. The punchline basically is that in the research so far, on average low carb diets have not outperformed high carb diets for performance. The difference has not been big though, but on average they have under performed a little bit. At lower intensities, so say 60 to 65% of VO2 max, it doesn't seem to be a disadvantage at all, but at higher intensities it does. So a lot of race paced conditions. The biggest difference appears to be the metabolic costs that it's a higher metabolic cost to move at the same pace when you're doing these higher intensities. And a simple way of explaining that is fat metabolism requires more oxygen than carbohydrate metabolism to produce the same amount of ATP. At 80% of VO2 max, oxygen delivery to muscles is a bit of a limiting factor. The greater oxygen demand for fat oxidation can lead to a quicker onset of fatigue, which is part of why in a lot of this research that we are looking at things like rate of perceived effort can be higher in the lower carb conditions, even in scenarios where there hasn't been under performance 'cause they were just doing a set task or something like that. And both groups completed the task, but IP is higher in the low carb group in these examples. - While that broad summary does sound like a little bit of a nail in the coffin, there is way more that does need to be unpacked. So we're not going to stop there. We're going to keep going. - We didn't, we didn't just like give everything away. - This speed up is like no. But yeah, going a little bit further, unpacking. There is a lot less research than you would anticipate on this topic and the sample sizes are always pretty small. So we're talking, you know, less than 20 participants. So like typically five to 16 participants is what we're seeing in this research. So we often talk about how small sample sizes can skew training studies like this because we have obviously differences in genetics and individual responses, as well as how good someone is just feeling on any particular day with such a small sample size. All those things are going to skew that data a little bit more than a large scale study. So again, something always to consider in this regard. - Talking about a very specific study, and we'll talk about something that is known as the supernova study from 2017. We've linked it in the show notes if anyone wanted to read it, but I'm using this as an example because it's one of the more well-known ones. I don't know how well-known it will be by people listening to this. I reckon a lot will. Some won't know this. If you spent any time on nutrition Twitter, which I don't think many people. (all laughing) - I know. - It was probably one of the most discussed studies around that time, and then also the follow-up supernova studies in the following years. But let's go just back to the original one. They had a high-carb group, a periodized group, and a low-carb group. The high-carb group, and it was race walkers, I should be specific. Anyway, the high-carb group improved their race times by 6.6%. The low-carb group had no change in performance. I don't know if I've said this already. It was a three-week, it was kind of like a three weeks in these interventions. Anyway, the low-carb group had no change in performance. Digging into the numbers though, and I think this is really crucial to listen to. Somebody in the low-carb group improved their performance by 5.3%. While somebody else in that group got slower by 8.5%. Huge individual variation there, even though there was small sample sizes, et cetera. And another thing that I don't have in the notes in front of me, but my understanding was that periodized kind of train-high, sleep-low group that they had, where they were basically all standing between high-carb, low-carb, where they were doing low-carb for like lower-intensity sessions, high-carb for higher-intensity sessions. From memory, they were a similar improvement to the high-carb all the time type of group as well, even though calories were matched and everything like that, which is just another little thing on this whole concept of like feel-for-the-work required/train-high-sleep-low kind of thing being like, it does add another layer of complexity, but I'm still yet to see evidence that that's outperforming as well, going a tiny step deeper on that as well. When we're talking, or when Leah was talking earlier about, some of those factual benefits of training in a low-carb state have increased mitochondria efficiency, better fat oxidization. The whole logic of this periodized approach is to kind of get best of both worlds, like get the advantages of this low-carb state. So it makes a lot of sense, but it's just I'm currently yet to see it outperforming. Another thing to consider for that particular study, so the supernova study, is the testing conditions in which it was done. So there were four testing days, two for each of the study groups, which was both like pre and post dietary intervention. And something to note here is there was quite a large discrepancy in wind and temperature between testing days. So the lowest temperature was 17 degrees Celsius, whilst the highest was all the way up at 28, with a mix of low mild and moderate wind conditions. The researchers did somewhat factor this into their results, but it is worthwhile being aware that one group having their pre-intervention be at 28 degrees when all the other testing days were more 17 to 20 degrees, has the potential to massively change the outcomes of this study. And that's why, or I guess part of why more research with large sample sizes has so much potential to help and for us to get a better understanding of whether this type of approach is something that could be beneficial. - Yeah, and this is because I'm so big on outcomes. I focus on outcomes so much more than mechanisms, even though I focus on both. But I just want to highlight that it's like, you'd be pretty dumb to see one study with something like this that could affect things so massively. It makes sense to pay attention to the larger body of research and stuff like that. - Hey everyone, I'm Hannah, one of the dietitians at ideal nutrition. If you like listening to the ideal nutrition podcast, then there's a good chance you're going to enjoy what about other podcasts, insights with ideal nutrition. That's the podcast where I interview different nutrition professionals on the latest research around different nutrition topics. Simply search up insights with ideal nutrition wherever you listen to your podcast and download for free today. - And because this kind of sample size issue, there is less research than we'd like. And that also leads into a very crucial question. How long does fat adaptation take? Researchers in this space have pretty typically used short study iterations. For example, that one that we just talked about only involved being low carb for three weeks. Is that long enough? Just as a genuine question, is that long enough? Critics of this research, back on 2017, nutrition Twitter when I was reading, just constantly pointed at that thing. And I'm like that, to be fair, that's a pretty valid criticism. I'll highlight the responses to that criticism and you can kind of use your own head to think about what you personally conclude from this. There are two major responses to that criticism. One is just around the logistics. They're trying to study elite athletes. 'Cause that's who we care about with this stuff. Like we're caring about like these one percentage and stuff like that when it comes to elite athletes. There's a quote from Louise Burke who did that study that I absolutely love. That was something along the lines of like, I don't have a bias here. I don't care whether high carb or low carb is better. I just wanna help athletes perform better. That's my goal. And I think that's super important when looking at all of this type of stuff. But they're trying to do this on elite athletes. How are you gonna get an elite athlete to do something like this for greater than 12 weeks? Like just to be clear, when they did this study, it was at the AIS, like they were in Canberra for this entire study duration. They were in a training camp together that they were doing to prepare for an upcoming event. So like these are athletes who were trying to get better for an upcoming event. And they had all their food provided for them by the AIS. You can do that for a three to six week training camp. How are you gonna get athletes together for 12 plus weeks? They've got to travel, they've got to compete, they've got to do all these things. And then there's also a little bit of logic about being like, what if halfway through that 12 week block, they came to the conclusion that the dietary approach that they were doing was making them a worse athlete? These people care about their performance. They're not gonna do something for 12 plus weeks if they think it's making them worse at what they're trying to get better on. So the logistics are a bit tough. To be fair, that doesn't like take away the validity of the question. Is this long enough? That's a very valid question. It's just like looking at the logistics of like, how do we even study this even if it is a valid question? And then the second response that they researchers will often have is, how do we measure fat adaptation? Those researchers have actually highlighted that they believe all of the measurable adaptations that occur actually occur within a matter of a few weeks. So based on their logic, three weeks is enough. But I also understand when other people say it probably takes longer, say two to three months or even longer than that. Even like little things, just me personally, thinking about it from the perspective of a pretty like amateur recreational athlete. Like imagine you were a runner or a race walk or whatever and you found yourself running at a certain pace and it felt good for you, that felt comfortable or whatever. And then you changed dietary approaches and going at that same pace felt different. Whether it's easier or harder, it probably takes a little bit of time to figure out the appropriate pacing strategy for a race. Like these things, even if the adaptations had occurred, it still takes time to like optimise performance. So those are a few things I wanted to point out. I, when I was prepping for this podcast, I didn't find this same thing that I feel like I found previously, I feel like I found a bit of a summary somewhere a while back looking at all of the research on ketogenic slash very low carb diets and endurance performance. And it looks like most of them came out slightly underperforming and one came out overperforming. I feel like I might have been talked about this previously on the podcast. And the one that came out overperforming had a longer duration than the other ones. But then there was some big body composition changes. I think like they lost substantially more body weight in that study. And it's like, well, that would explain improvement in endurance performance as well. But there's just gaps here still. Like small sample size is not long enough, all of these kind of things. So it's like, it's worthwhile paying attention to research and everything like that, but also acknowledging that we'd love more. It'd be good to have more. - Yeah, talking from a bit more of a theoretical point of view, looking at ultra endurance sports and ultra endurance athletes, the longer the event, the more likely that potentially a low carb approach might have benefits. Again, just from a theoretical perspective, we don't have research kind of backing this up. But for example, if you have a full carb load, you would get around 90 minutes of glycogen storage while running at a marathon pace. That covers a half marathon if performing at a high level. So it helps for a full marathon, but for, let's say, 100 kilometer events. So something that is substantially longer, you could start to question, okay, at that point, we're such a long event time. Does something like this actually make a difference to race times and outcomes? So events like ultra marathons typically involve a lot of times spent at around 65% of VO2 max where you are less likely to see disadvantages from low carb approaches as well. So again, theoretically, the longer the event is, the more likely it is to be beneficial, although we are not at a point where we're like, yeah, this is definitely gonna be something that is beneficial and that works. It's just that technically from a theoretical point of view, there are less downsides and potentially more of a benefit possible there. - Now, let's go anecdotal. I feel like if anyone's like super, super pro low carb, they're probably not listening at this stage, but if somebody was, I feel like there would be, there's a bit of a campus like, no, look at this athlete who's competing well, look at this athlete who's competing well. And I say that logic, let's literally talk about it. Why are there people who are at an elite level who are performing relatively well or performing well on a low carb diet? 'Cause there are many who exist. So, going through it, one thing could be body composition changes. Let's say somebody was not as lean as they ideally would be for their sport or anything like that. And then they went low carb and they got leaner and that moved them into direction of what was required. Then I could see that helping. That has nothing to do about it being magical for getting leaner. We know that just being in a calorie deficit, that will do the job. But sometimes people have been doing a certain thing for a long period of time. They make a change, it changes their overall dietary habits. Maybe their calorie intake reduces or anything like that. Maybe that plays a role. So that's one factor. The next big one that I really think is so overlooked here is that the differences in performance are not actually huge. Like if we look at a bunch of those studies, like I know even that supernova one how it's like 6.6% versus like 0% or whatever, that's a decent one. But my overall broad interpretation of the research as a whole, if I'm adding my two cents onto it, but beyond just like the direct stuff that we've kind of quoted, is that over the course of a marathon distance, it's probably less than 3% difference on average, particularly once again, my own interpretation, particularly if somebody had been doing a low carb approach for a long period of time as well. That's the average. That's not even factoring an individual variation or anything like that. We can go even deeper on that topic. But if something is less than 3% on average, you then add in the next problem. Most people can and should just get better just by training. Just simply by training, putting in another training block, doing six more months of training or whatever. If somebody trained for six months in a low carb state, in many cases, they should get better. That is just what happens if you train hard, you don't get injured, all of those kinds of things. It's another food-to-thought thing being like, if somebody's just measuring their times like, this is what I was doing in a high carb state, and then they did another training block where they didn't get injured and it happened to be in a low carb state. It's not surprising if they did get better. Talked about individual variation a little bit. But then let's go deeper on that being like, some people struggle with things like gels and stuff like that. Like they can't have 90 grams of carbs per hour because they keep running into gastrointestinal issues. If they can't maximize the benefits of the higher carb approach, and same thing with like if they can't maximize carb loading or whatever, like maybe that causes gut upset. Like if they can't maximize those benefits, suddenly they're probably even more likely to fall into a camp of actually getting performance improvements by switching to a low carb diet as well. Once again, I do think in many cases, there are many steps that can be taken to get better at taking on gels and stuff like that. But it's just another example of where this could be beneficial. There's many, many anecdotal cases. Something I do find funny though is like, for what it's worth, if we wanted to go anecdotal, there are many more cases of people performing well on high carb diets as well. This is a silly example, particularly with a plant based co-host, but like the game changers documentary, how they like kept pointing at like individual athletes in these sports being like these people are performing better because they're at my base, just kind of like the thing that they point out. It's like, well, if we want to go down that route, like if we want to go down that route, like there's a lot of anecdotal cases opposite to this as well. - Well, you can point to most athletes who are not plant based at a little elite level. And like, well, they're still what high performing athletes. - Yeah, yeah, so like outliers who are doing this, but there's many reasons why these outliers will exist as well. - Yeah, so overall, the research at this stage is mostly in favor of high carb diets over low carb diets for endurance performance, but there are gaps in the research that is individual variation, and there are cases where low carb diets might have advantages. - This has been episode 169 of the Ardo Nutrition Podcast. As always, if you could please leave a rating and review if you have not already, that would be greatly appreciated. (upbeat music) (upbeat music) (upbeat music) (dramatic music)
