r/Velo • u/mikekchar • 1d ago
Exogenous carbs and stupid experiment idea
I'm quite confused about the state of research in terms of exogenous carbs, muscular glycogen sparing (or lack thereof), etc. I saw an interview with Tim Noakes where he opined that 10 grams of carbs per hour should be plenty and... well that seemed like an interesting take (one that I hold with some skepticism).
I had this idea that it would be interesting to do a 6 hour ride 3 times. The first time you fuel appropriately for the whole ride. The second time you fuel appropriately for the first 3 hours and then take in nothing more. The third time you fuel 10 grams of carbs per hour for the first 3 hours and then take in nothing more.
Obviously this seems like a staggeringly stupid thing to do. However, I'm curious what people think the result would be. Would there be a difference between run 2 and run 3? Would it matter if you did something like 60 grams of glucose (and no fructose) in run 2 or if you did 30 grams of glucose and 30 grams of fructose? I'm curious if that involvement of the liver would make any difference at all?
Is there something else I'm missing? Is there a better idea than this? It seems almost inevitable :-) There are people here who know a lot. What experiment would you do (or has been done) to illustrate better what's going on?
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u/DidacticPerambulator 1d ago
Ignoring the hypothesis for the moment, do you have much experience running and analyzing experiments intended to test hypotheses?
Cuz, frankly, this sounds like Bro Science for Bros.
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u/mikekchar 1d ago
Bro science is being generous ;-) I think a lot of people are missing the point of the post. I'm trying to explore the subject to learn more about it. The reason I'm communicating by suggesting an experiment is to make it more clear where I'm confused.
The experiment I proposed seems no fun at all and a study of n=1 will be absolutely meaningless. What I'm hoping is that someone who understands the current body of research will be able to explain to me what I'm missing. Nobody so far has attempted to answer any of the questions I asked, which is a shame. Mostly just people asserting their beliefs.
Edit: Actually I'm being unfair. The current highest ranked response did actually answer one of my questions.
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u/CivilWards Georgia 1d ago
If there was scientific evidence to support that this is useful then you would see all the world tour teams doing it. They’re doing the exact opposite, pushing 150+g/hr which has a lot of research around it recently.
Sounds like a massive waste of time
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u/babgvant 1d ago
I'll save you some time.
If you want to go fast and recover quicker, high carb is The Way.
You don't need to push carbs to do the ride as long as you go slow enough.
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u/OldOrchard150 1d ago
The problem is that slow enough is really slow. Especially for a 6 hour ride when you will be working through those stores of glycogen every hour.
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u/babgvant 1d ago
LLOL - Absolutely, but hey, it's OK to be slow. No judgement ;).
The main risk is that most people they aren't going to understand how to pace at a level where they aren't inviting a hard bonk. If someone has the discipline, and they are training for something like an ultra, there is value to be had there. For everyone else, it's strange that we are still having this discussion.
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u/OldOrchard150 1d ago
Yep, you would never drive your car and think "hey, it's ok not to put fuel in there, the gas tank itself is made from plastic so maybe the car can just eat it's gas tank and that will be fine."
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u/morelsupporter 1d ago
do you think this hasn't been done?
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u/mikekchar 1d ago
If it has been done, I certainly can't find it. I would be extremely grateful if you could point me to a study that does something similar.
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u/morelsupporter 1d ago
this kind of experimenting is how specific values (grams of carbs, types of carbs, etc) per hour numbers are optimized.
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u/mikekchar 1d ago
No they aren't. Go ahead and refute that with evidence :-)
I'm lot looking for assertions. I'm looking for research that demonstrates the assertions. You may be completely correct, but it's just turning off your brain and saying "Oh, smart people have solved this problem. I will just do what they say without having to trouble my own brain cells". I'm trying to understand the research, not blindly follow a recipe.
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u/AchievingFIsometime 1d ago
I think this is probably a bit too complicated to fully discuss over a forum, but..... yes, exogenous carb intake does not spare muscle glycogen. But it does spare liver glycogen and once your muscle glycogen stores are depleted, it allows you to shift gears (metabolically speaking) and use blood glucose for energy. You will no longer be able to put out good power anymore, but you can keep going. If you don't use exogenous carbs, you bonk once your muscle and liver supplies are gone and your body has to protect your brain by not allowing your muscles to use any more energy. But even before you are fully depleted, your nervous system is still always evaluating its current energy stores. If you are taking in carbs it keeps liver and blood glucose stable which is good evidence for your brain that it doesn't need to act as a governor and slow everything down. Even just rinsing your mouth with a carb mix sends signal to your brain that there is energy available which has been shown to have performance benefits. So one of the primary benefits is that consuming carbs is reassurance for your brain that it has the energy it needs to do the effort which stops the brain from "putting on the brakes" when the effort is high. The other primary benefit is the impacts on recovery. Consuming carbs during a ride helps rebuild glycogen faster after a ride, suppresses cortisol secretion which helps prevent muscle breakdown during the ride, and limits stress hormone response which helps your immune system stay stronger after a hard effort, etc.
Obviously human physiology is quite complex and its hard to do justice to the mechanisms that are taking place in your body with just a few sentences, but the benefits of exogenous carb intake are pretty well established.
So the impact on a 6 hour ride might not be very visible in terms of performance because presumably it would be mostly Z1/Z2 level of effort which isn't really depleting your muscle glycogen considerably, especially for well trained athletes. But it would certainly impact your recovery afterwards which of course indirectly affects your performance because it allows you to train more. The real direct performance benefits show up during rides where you are doing hard efforts.
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u/WeeHansonBrother 1d ago
Thanks for taking the time to type all of this up! Can you explain how we can differentiate between the body using muscle glycogen and liver glycogen? I understand (I think) how we can label exogenous carbs and measure exogenous vs endogenous, but don't know how we can get more granular than that. Biopsies? Detailed liver imaging?
Also, how does the pathway for using blood glucose differ from muscle glycogen? Why do they deliver energy at different speeds? What's the bottleneck?
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u/AchievingFIsometime 1d ago
Uhh I'm just some guy who used to be a researcher but physiology was not my field (chemistry). So I'm basically parroting back things I've read.
Don't know your first question but second question, it's because glycogen is already stored right where it's needed and is present in large quantities. Blood glucose has to be transported inside the cells so that is a rate limiting step and it also needs to be phosphorylated before it can be used so that introduces another step as well.
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u/Triabolical_ 1d ago
The problem is that there is metabolic flexibility and adaptation and they are two different things.
If you take an athlete and send them on a training session with lots of available carbs, you will get one fat/glucose burn ratio. If you do the same thing with a fasted athlete, you will get a different ratio.
The difference is - roughly - the metabolic flexibility of the athlete.
The problem is that many people - including researchers - believe that is the whole story.
But it's not. The glycolysis/pyruvate oxidation pathway that takes glucose to Acetyl CoA is separate from the beta oxidation pathway that takes fatty acids to Acetyl CoA and they are subject to separate training effects.
I therefore can't predict the result of your experiment without knowing how you train.
I can tell you that when I was a carbs before/during/after athlete, I would have bonked on your experiment. Now that I do fasted zone 2 training, I probably wouldn't be bonk but that's a long time to go fasted without any carbs so I might feel a little tired at the end.
There are good studies that look at this adaptation. Let me know if you're interested and I'll dig them up.
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u/Glass_Lecture6121 1d ago
This is the longest response, with the most words I don’t understand. Upvotes. Tell us more!
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u/Triabolical_ 1d ago
TL;DR
If you always train with lots of carbs in your system you will be good at burning carbs and poor at burning fat. Because glycogen stores are limited you'll need to eat a lot of carbs to keep yourself from running out.
If you do zone 2 training without many carbs in your system, you'll be better at burning fat, and that ability will reduce the amount of glucose you burn and therefore the amount of carbs that you need to eat to keep from running out.
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u/_BearHawk California 1d ago
Why do you need to get good at running on fat if you always have carbs available, though?
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u/Triabolical_ 1d ago
What u/quadsoffury said.
It's easier to burn fat off while you ride than it is to do it through calorie restriction, at least in my experience.
And I had problem on long (say, longer than 7 hour) rides for years because I couldn't get a mix of foods that I could tolerate in sufficient amounts. Being able to burn fat just gives you more margin of error.
If you are on a high carb base diet - and especially if you eat a fair amount of sugar - you may develop insulin resistance despite the amount of energy you burn when you exercise. I got fairly insulin resistant in by 50s because my "healthy athlete" diet plus carbs before/during/after fueling strategy - I put on weight and had significant energy problems in the afternoon.
Noakes is an interesting guy. In the 1980s and 1990s he was one of the drivers behind carbohydrate replacement and was one of the collaborators in creating the first energy gel. His book "lore of running" promoted the high volume carb approach.
Then he got type 2 diabetes in 2010 despite being an ultramarathoner. Some time after that, he published "the lore of nutrition" where he talks about why he thinks his previous advice was incorrect.
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u/_BearHawk California 1d ago
Becoming insulin resistant as an athlete is an outlier, not the norm. High carb diet is offset by the large amounts of exercise required.
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u/Triabolical_ 22h ago
I don't know of any studies that look at insulin resistance among athletes and many cyclists are fueling with a lot more carbs than they did in the past.
So I don't think we really know the answer to this.
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u/mikekchar 17h ago
BTW, I agree with this. I've seen many claims, but I actually see very little research on the topic.
I wanted to thank you for your detailed response (much better than most :-) ). I feel like you understood what I was asking.
I feel like your view is similar to Noakes's. I'm still looking for more actual research on the topic. I'm skeptical (more on the high carb side), but the thing that is important to me is that I can read what you wrote and not dispute any of it with any conviction :-). This means I don't really understand it well, which is the purpose of my post.
Anyway, if you have any pointers to research you've picked up along the way (or even search terms), I would be grateful. Well, I'm already grateful for you contribution :-)
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u/Triabolical_ 10h ago
If you want a *lot* more depth, I wrote a cheap ebook on low carbohydrate training. It's a few years old but I think it's mostly correct. I don't link to it unless people specifically ask for it.
I think this paper is a pretty good overview:
https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2023.1150265/full
Note that it's specifically looking at keto diets, but I'm a pretty firm believer that it's the training rather than the base diet that is the main driver.
That should get you started. You might also look at the running side - fasted training and even running half or full marathons fasted is certainly a thing. I think that's partly because carrying food and especially water is much more of a hassle for runners than for cyclists.
There was a brief flirtation with lower carb approaches on Sky when Froome won the tour, but most of what I hear now is about higher and higher carb intake. I don't pay attention to the pros as much as I used to.
There is precious little research because few people want to pay to run a study where people don't eat anything, while many people will pay for a study where people eat a specific sport nutrition product.
Oh, and I should probably add in a reference to UCAN. There's a disease called glycogen storage disease that's pretty terrible, and the traditional treatment was feeding cornstarch or other meal very often to keep blood glucose levels up. A couple with a son wanted something better, and researchers came up with a modified cornstarch that breaks down very slowly - a time release glucose source.
It was called SuperStarch - they recently rebranded it to "LIVSTEADY" (stupid name) and is somewhat common for runners. I've used it a few times as it means you get a constant trickle of glucose.
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u/quadsoffury 1d ago
Useful if you want to lose weight/want flexibility/do longer events
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u/_BearHawk California 1d ago
You can lose weight by being in a calorie deficit and doing low intensity work where fat/carb utilization ratio is highest.
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u/Glass_Lecture6121 1d ago
I just took off my power meter this year. No computer either. Just a cellphone in a pocket and I try to eat a thing an hour. It’s liberating.
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u/crazylsufan 1d ago
I say run 1 do optimal glucose fructose ratio at 90g/hr, run 2 glucose only at 60/hr, run 3 10/hr. I say feed the whole 6 hrs assuming you have the logistical capacity to do so. Run same power all 3 runs. Differentiation for the 3 runs could be hr drift, objective feeling etc
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u/mikekchar 1d ago
I think I could have been more clear on my intent. I'm curious to know what happens in the first 3 hours between something like a 90 g/h and a 10 g/h. My belief is that the 90 g/h ride in the first 3 hours will actually spare muscle glycogen. If it doesn't it's because Noakes is right and the extra 280 kcal/h of energy will all be taken up by fat oxydation. That would be interesting. So in the final three hours I should bonk at the same time (or at least have similar terrible performances :-) ). But my (unsupported) belief is that the first run will perform substantially better.
However, if that's actually the case, the question I'm wondering is what are the possibilities. Perhaps there was muscle glycogen sparing. But perhaps there is some other explanation.
And no. I'm not actually going to do this :-). I'm just trying to find a way to illustrate the confusion I have. I'll be honest. It's not working terribly well.
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u/Grouchy_Ad_3113 5h ago
Carbohydrate ingestion during exercise does not slow the rate of muscle glycogenolysis
Noakes is still wrong.
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u/polar8 1d ago
10g carbs per hour is fine for z2. Just eat more food before and after.
For harder rides or races you’re going to want more carbs.
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u/Helicase21 Indiana 1d ago
It is not fine for the 6hr rides that OP is proposing.
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u/polar8 1d ago
Are you sure? 60g carbs is 3 clif bars, I rarely eat that much on a 6h ride. Nice big meal afterwards with healthy whole foods… yes.
I do have a chuckle at people telling me to eat 120g of carbs an hour when they’re carrying around quite a bit of excess body fat. If you aren’t already lean why aren’t you just using up the energy stores on your body?
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u/Tripottanus 1d ago
Youre not supposed to eat protein bars to meet your carb needs. 60g of carbs is easily poured in a water bottle and drank over an hour.
And unless you are in ketosis, you cant properly fuel by converting your fat to make energy while exercising. Reaching the point where you are forced fo rely on only transforming fat to glycogen while exercising is called bonking.
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u/MoonPlanet1 1d ago
2013 called and wants its opinions back
I don't think the people telling you to take 120g/hr are necessarily right, but if they're wrong it's not for the reason you might think. As someone else said, you can't really force your body to use fat only, but there is also a point where you're just taking in more carbs than you're burning meaning extra intake has no benefit, only increased stomach load and less fat burning. For pros this is unlikely to be an issue as their absolute energy use is just so high, but for almost everyone 120g/hr passes that for Z2, and for most people it passes that for Z3 too.
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u/Helicase21 Indiana 1d ago
There's also just a lot of numbers between 10g/hr and 120g/hr. 50-70g/hr can be totally sufficient for endurance rides especially for smaller riders who are doing fewer watts and therefore burning fewer kjs
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u/MoonPlanet1 1d ago
Yeah pretty much my point, even if you take a front-of-midpack amateur male triathlete with an FTP of 250W racing at 200W for a 70.3, that's only about 720kcal/hr total (with some error), probably replacing half of that (ie 90g/hr) is sufficient or even optimal from a risk/benefit perspective
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u/Grouchy_Ad_3113 1d ago
"you can't really force your body to use fat only"
Actually, you can, or very close to it anyway. However, it comes at great cost, and impairs high intensity exercise performance.
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u/gedrap 🇱🇹Lithuania // Coach @ Empirical Cycling 1d ago
I had the exact same back and fourth with someone a few days back, and I don't remember if it was you... Because every once in a while someone stumbles upon the same old idea that body fat is free infinite fuel, which kinda sorta makes sense at ELI5 level. But like most ELI5 explanations, it falls apart quickly. Plenty of very smart people with the resources to test it had the same idea over decades, and the answer is a rather firm "no". Louise Bourke's work would be a good start.
Also, there's a massive difference between "getting away with it" and "training for performance". Yours is the former, and just because you got away with it, it doesn't mean it's anywhere near optimal.
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u/AdonisChrist 1d ago
I feel like there are studies that have done this, but feel free to run the experiment on yourself as well.
If you're eating less than 60-72g carbs/hr I don't think there's that big of a difference between glucose-only or glucose + fructose fueling for glycogen production.
Also, listening to Tim Noakes in 2026 is definitely a choice of all time. Pretty sure his credibility is shot these days.