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Coaching How to Break the 90g of Carbs Rule

BY Tom Epton

Understanding how a gut training protocol is used to find the limit of what an athlete can absorb for race duration can help push this limit over time, at least once per week.

In the 2018 Giro d’Italia, Kenyan-born British multiple grand tour-winning rider Chris Froome attacked around 80km from the finish line. He was a little bit down on the general classification at the time but won the stage, taking back a whopping 3:23 to a dangerous Tom Dumoulin. We were all left wondering, how on earth did he do that? If you go back and watch this stage now, Froome was constantly eating. Consuming carbohydrates from bottles and gels almost every time the camera was on him. This constant fueling left many viewers wondering — how much carbohydrate is possible to be absorbed over the course of an hour?

Carbohydrate absorption becomes a limiting factor in some people at 60g per hour and above — or roughly three gels, depending on the brand. However, we know that many athletes can absorb well more than 100 grams per hour. On top of this, we know there’s a correlation between carbohydrate intake and improved race performance (check out this TrainingPeaks blog if you’re not sold on carbs yet). Product selection is quite important for some athletes, which can vary across individuals. Only a little training is required for others with a less sensitive gut to increase carbohydrate tolerance.

The aim is to be able to consume as much carbohydrate as is tolerable for the duration of a race when racing for more than two hours. If the race duration is between one and two hours, then going into the race in a well-fueled state and consuming top-up carbs is usually optimal. This is because the intensity can limit absorption somewhat. If you’re racing for less than one hour, some water is probably all you need, if anything. If there’s a lull in the racing, taking a gel at the 20-minute mark may be optimal. The shorter the race, the more we can rely on our preexisting glycogen stores and the less we have to worry about topping them up.

But what if we are racing for four hours? Or eight? Or more? There are plenty of endurance events where this is the case. A half IRONMAN triathlon can be three and a half hours or more. Many road and gravel bike races are nearly as long or longer. There’s also the question of full-distance triathlon, which can take up to 17 hours. To tackle this, the best place to start is by improving our understanding of how carbohydrates are absorbed and how we can improve these absorption rates.

How Does Carb Absorption Work?

As our exercise intensity increases, our ability to utilize fat as fuel decreases — this means the proportion of energy we are burning from our fat stores goes down, and we are running out of carbohydrates. Having your athletes get tested for their substrate utilization during exercise can yield valuable insights — you’ll be able to see at which intensity this starts to occur. The problem is that we only have so much glycogen. This is why you often hear about athletes and coaches trying to become ‘fat adapted,’ which you can read more about here.

From the Packet to the Legs

The question of what happens when we eat is something all coaches should be broadly aware of — especially when we are eating carbohydrates. When we consume carbohydrates, a gel, for example, digestion begins in the mouth. Salivary amylase (an enzyme in our saliva) breaks the carbohydrate into smaller glucose chains — dextrin and maltose. Amylase doesn’t function in the stomach as the environment is too acidic. The next part of digestion occurs in the small intestine.

From the stomach to the small intestine, things get interesting for us. Once the mixture passes through the stomach, it’s expelled into the small intestine when our pancreas releases pancreatic amylase. Additional enzymes are also secreted by the small intestines (sucrase, maltase and lactase). Carbohydrates are then broken into singular sugars and transported to the intestinal cells.

The cells in the small intestine contain what are known as transport proteins. These allow our bodies to get the nutrients into the blood, where they can be distributed where they’re needed. The different means by which fructose and glucose are absorbed are the reasons for the 2:1 ratio we see so heavily advertised.

A proportion of the population does not appropriately produce some enzymes used to break down sugars. This can lead to GI distress, especially when consuming them during (and immediately before or the day before) a race when the GI system is under stress anyway. Having your athletes aware of minor intolerances and writing down episodes of GI distress and foods they ate may help avoid problems on race day by avoiding certain foods the day before or the day of the race. Lactose is the most common example of this.

How Does Gut Training Work

Over time, if we stress something, it adapts to that stress. Studies have shown that our gut is no different. The idea of gut training is to gradually increase the number of carbohydrates you can consume during training in the lead-up to your race. Gut training is something athletes should be thinking about a long way before the event; it’s not something that can be rushed. Gut training isn’t new. A 2017 article titled ‘Training the Gut for Athletes,’ concluded that “a substantial body of evidence suggests that the GI system is highly adaptable,” citing the ability to train gastric emptying and stomach comfort. Not only can we train our gut to absorb more carbohydrates, but we can also reduce the probability of GI issues by doing so.

Gut Training Test

Before we start gut training, like anything, we need to set a baseline. The process can be unpleasant, but the benefits of absorbing a large number of carbohydrates are clear and should be discussed with your athlete. A protocol for gut training should be done with duration in mind first. Let’s say we have an athlete who is completing a half IRONMAN but has never paid more attention to nutrition than having a gel when they feel like it. GI issues happen sometimes but not always. How would we test this athlete? 

Ride for race duration in zone one or two — not race distance — and attempt to consume 90g of carbs per hour for this ride, if possible. Then pick up carbohydrate consumption in the last hour and a half to 100g/hour. If your athlete is a triathlete, have them run off the bike for 30 to 60 minutes after the ride, consuming carbohydrates at the same rate. If your athlete has problems here, you’ve found the limit. If your athlete is fine, this is good. They have gut talent.

Gut Training Protocol

The aim is to find the limit of what you can absorb for race duration, then push this limit over time — maybe once a week, increasing the number of carbs per hour by five to 10% each week. Once you notice a plateau in the ability to absorb carbs, from personal experience, this happens between week six and week eight, it’s now time to start introducing race pace into the mix. Absorbing carbohydrates at easy aerobic intensity is more manageable, so consuming the same amount at higher paces may continue to force an adaptation.

Consuming more carbohydrates correlates with performance, and gut training is a straightforward gain for athletes. A huge improvement in capacity in efforts over two hours can be seen with no increase in ‘fitness’ needed. By testing where their gut is currently at, then following a gradual increase in carbohydrate consumption for the duration of race length and above training sessions, athletes will find their ability to consume and absorb more significant volumes of trouble-free carbohydrate increase. This adaptation will lead to the ability to race at greater intensities over the same duration, thus leading to better results. Fueling properly during a race is relatively simple — it’s a trainable capacity problem rather than an optimization problem, making ripe, low-hanging fruit for your athlete to make major gains.


Jeukendrup, A.E., McLaughlin, J. (2011) Exercise: Effects on Performance, Training Adaptations and Trainability of the Gut. Retrieved from https://www.karger.com/Article/Abstract/329268

Jeukendrup, A.E. (2017, March) Training the Gut for Athletes. Retrieved from https://link.springer.com/article/10.1007/s40279-017-0690-6

Costa, Ricardo, et al. (2017, January) Gut-training: the impact of two weeks repetitive gut-challenge during exercise on gastrointestinal status, glucose availability, fuel kinetics, and running performance. Retrieved from https://cdnsciencepub.com/doi/abs/10.1139/apnm-2016-0453

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About Tom Epton

Tom Epton is a writer and data scientist based in the South East of England. He is a founding member and principal data scientist at PyTri Ltd, a consultancy specializing in applying data science techniques to performance sports and healthcare. Tom has a first-class BSc in Physics and has worked at several well-known brands on big data and machine learning projects. Away from work, he is an elite triathlete racing a mixture of draft-legal short courses on the British Super Series to middle-distance non-drafting triathlons. Tom also offers coaching, physiological testing and endurance sport consultancy services. Email him for more information.

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