Ticking past 60 minutes on the treadmill, I’ve entered a new realm. While I’ve completed longer sessions on the turbo trainer, this is the longest I’ve spent running indoors.
With no virtual worlds to explore as in Zwift or Rouvy, I’ve had plenty of time to sit with my thoughts, venture down rabbit holes in my mind and come to the conclusion that it was a rookie error to have left my headphones at home.
Although I’m still only two-thirds of the way through, distractions to break up the monotony are on the way.
My third and final gel is scheduled for 15 minutes' time, and for the final 10 minutes, I get to don a Calibre metabolic face mask (think a breath-based metabolic testing apparatus that makes you look like Bane from Batman).
While the thought of 90 minutes on a treadmill might sound like a form of punishment, I’ve been invited to try ExoAnalytics’ at-home exogenous carbohydrate oxidation test (albeit in a swanky London gym).
The event has been organised by Puresport – an energy supplement company that partnered with ExoAnalytics to perfect the quantity of carbohydrates in its new energy gels – and although I’m conducting the test on a treadmill, it can also be completed on a turbo trainer in the comfort of your own home.
And it should be worth it too, with its results likely to influence my training and competing for years to come, and potentially unlock new cardio-based personal bests.
Rule of thumb
In the continual pursuit of performance gains, nutrition has become a big focus among athletes of all abilities in recent years.
At the elite end, professionals are pushing the envelope of how many exogenous carbohydrates (those ingested from external sources such as sports drinks or gels) they can consume, with WorldTour riders such as Tadej Pogacar reportedly taking on up to 120g per hour during stages of the Tour de France.
While not as extreme among amateurs, the idea of increasing carbohydrate intake during a ride has experienced a trickle-down effect. Gone are the days of getting round a sportive on jelly babies and bananas stuffed in your jersey pocket.
But with reports of professionals quaffing vast quantities and each brand of gel containing a different quantity of carbohydrates, what’s a good rule of thumb?
“60g per hour is a really good baseline and an appropriate amount for someone to start with,” explains Dr Sam Impey, co-founder of ExoAnalytics and former lead nutritionist with British Cycling.
He adds that this should be sufficient for “95% of the running community – people who aren’t elite, run regularly and are thinking about how to use gels”.
But like all aspects of performance science, there is a way to tailor your nutrition plan during training and racing to meet your body’s personal carbohydrate-oxidation rate – how much of the carbs you eat is used for energy versus how much stays in the gut or is lost.
The main benefit of an individualised approach is the improvements you will see in performance. By optimising your carb intake, you can maintain performance levels for longer – reducing the chances of underfuelling and tailing off towards the end of an event, or, at worst, bonking.
But the benefits aren’t limited to racing – they can play a key part in training adaptations, too.
“Most people, based on their diet [alone], can get through training sessions, and there are two things that can happen with that,” explains Dr Impey.
“There's an element of helping your energy balance. By accumulating periods of time where you're in energy deficit, you're potentially not fuelling your body enough for all the recovery processes to happen in the way that they should, and fuelling during exercise is a big contributor to that.
"Where that happens, you don't adapt as well to training, and you build up things like the risk of chronic fatigue.
“The flipside to that is, if you are fuelling okay but not brilliantly, you lose training quality – particularly in that last 25-30% of your training session.
"In the context of cycling, that might be you're not quite hitting the power numbers, or you may be getting sloppy with keeping your core tight and the mechanics of riding if you're going for a long base ride.”
A test can also reveal if you’re currently over-fuelling because of a lower physiological limit – symptoms include bowel discomfort or gut sensitivity.
“If you have an issue, you might have a naturally very low capacity to absorb and utilise carbohydrates, so that could be detected.”
This isn’t limited to those new to fuelling either, with Dr Impey saying he tested a competitive ultrarunner who had a capacity of only 30g per hour.
“It tends to trend in [a positive] direction, but just because someone is a good athlete doesn't always mean they've got a high intrinsic capacity to use carbohydrates from external sources.”
Action stations
As someone who has never suffered from gut issues, I don’t think I’m likely to be at the lower end of exogenous carbohydrate oxidation capacity.
But having swallowed a gel every 20 minutes during recent marathon attempts (working out at 66g of carbohydrates per hour), am I fuelling right? Or could I process even greater quantities of carbohydrates, and unlock additional performance gains?
The test started two days before, with instructions to restrict certain foodstuffs in my diet (notably corn-based products), as well as avoid caffeine, alcohol and processed foods high in sugar.
“Those foods we asked you to remove are naturally high in carbon-13. Every day, you top up your body's stores of glycogen, but you did it in a way to make sure that you had a low naturally occurring amount of carbon-13,” explains Dr Impey. A carbon isotope commonly used as a tracer during studies, its presence would be crucial to calculating exogenous carbohydrate oxidation.
Before getting onto the treadmill, I also had to breathe into two test tubes, which would provide my baseline, and I’d breathe into another two on completing my 90 minutes.
“As your body metabolises carbohydrate, one of the things that's produced in that chemical equation is carbon dioxide, which you breathe out, so we measure the carbon dioxide for the amount of carbon-12 and carbon-13 that's in your breath.”
During the 90-minute test, I had to run at an intensity lower than lactate threshold (LT) 1 (“Sunday long run-type pace, for most people, is comfortably below LT 1”) throughout.
I was also given three Puresport energy gels at 30-minute intervals, starting after 15 minutes. The gels each contain 30g of carbs and are high in carbon-13.
Dr Impey explains: “During the session, your body digested it, you absorbed it and a percentage of it was taken to the muscles and burnt as a fuel source.
"When your muscles burn that fuel source, they convert some of it into carbon dioxide that's breathed out.”
Post-test, my breath should contain a higher amount of carbon-13 than at the start, and the difference between the two would be used, alongside a measure of total body carbohydrate oxidation taken with the Calibre metabolic face mask, to calculate how much of the total carbohydrate I was burning came from the gels ingested during the test.
Take-away teachings
After a two-week wait, my results were in. My exogenous oxidation rate was 56g/hr and I had an oxidative efficiency of 93%.
So is the 60g benchmark right for me? “If you do the test and your efficiency is between 80-90%, that's your optimal range for fuelling,” says Dr Impey.
“If you're 60-80%, you're not far, and there's probably a bit of optimisation you can either do with your diet or how you fuel during training if you want to increase it, but probably having a slight decrease might be better for you.
"If you're above 90%, particularly with something like 56g, there's a good chance that your body has a higher natural ceiling, but we weren't able to detect that because we didn't give you more carbs in the first place.”
He adds: “It also shows that [60g per hour] is still an appropriate amount for you, but your body potentially has the capacity to go slightly higher – probably into the mid-70g as a way to reach that 80-90% efficiency rate.”
These recommendations aren’t limited to LT 1 work, either, with exogenous oxidation rates staying relatively flat before decreasing as intensity increases.
“If someone was going out at half-marathon pace, which can be up closer to LT 2, you shouldn't really see a shift at all.
"If you go really high, like VO2 max work, you might even get a decrease, because blood flow is moved away from the gut because it's needed more in the other muscles.”
Dr Impey also notes that these results are sport-specific, with your body less efficient at oxidising carbohydrates when running than cycling, meaning my carbohydrate ceiling could be even higher on a bike.
“Your body's doing more things [when running], and it's a bit more challenging to extract carbohydrates from your gut and circulate it,” he says.
So is it possible to increase my processing potential and unlock further performance gains? Dr Impey says the ‘how’ of training your gut to oxidise greater volumes of carbs is still a grey area, but suggests that training with carbohydrates above your maximum in key sessions is thought to work.
However, while you can train your gut to a certain degree, your body will have a natural ceiling, and if you go above this, you will either start to get symptoms such as gut issues or you won’t be able to physically process it.
“There's a big difference between being asymptomatic and being able to oxidise. I can ingest 120g per hour when I run, but I can only burn 69g, but I don't get any symptoms from it,” he says.
So while I’m not going to ramp up to Pogacar’s 120g/h fuelling strategies just yet, my results certainly provide food for thought, with a separate turbo trainer-based test and an increase in carbohydrates firmly on the menu.
