Endurance athletes such as marathon runners, triathletes, and long-distance cyclists typically utilize food during training and events to improve their performance.
The body can only store a limited amount of glycogen, the storage form of carbohydrates, in the liver and muscles. So once these storages have been exhausted or significantly reduced, the body has to turn to fat as a major fuel source.
The issue with this is that using fat as a fuel source is less efficient than carbohydrates and will limit the intensity that the athlete can continue to perform at. This is often what is occurring when an athlete says they have ‘hit a wall’.
So in events lasting longer than 1-2 hours, it is typically recommended that an athlete consume carbohydrate-rich snacks periodically to maintain good carbohydrate availability.
In long events lasting 4 or more hours, some athletes consume up to 110g of carbohydrates per hour. Although 40-90g per hour is more common in most endurance events.
Outside of carbohydrate intake, athletes also have to think about fluid intake. To avoid an impact on performance, athletes should aim to lose no more than 2% of their body weight in fluid losses. In an endurance event, particularly when sweat rates are high, this could mean a significant hourly fluid intake alongside carbohydrate intake.
So athletes are generally trying to manage hydration, carbohydrate availability and gastrointestinal discomfort all simultaneously.
Training The Gut & Gastrointestinal Distress
The gastrointestinal system is not naturally well adapted to functioning on all cylinders during exercise.
You may have heard of the sympathetic and parasympathetic nervous systems, also known as “fight or flight” and “rest & digest”.
During exercise your body is more so in fight or flight mode and is not prioritizing blood flow to the gastrointestinal system. So it definitely isn’t resting and digesting.
This is why gut problems are very common amongst athletes, especially those undertaking long distance events and trying to consume food and fluids during the event.
The specific physiological mechanisms that result in gastrointestinal symptoms are still up for debate as there are likely many factors that contribute. Although researchers in this space typically point to two possible factors that can actually be manipulated by ‘training the gut’. These are a high food and fluid intake leading to an uncomfortable bloated feeling and reduced gastric emptying.
This can lead to general discomfort, cramping, nausea, vomiting and diarrhea.
But it is thought that in training the gut, a tolerance for food and fluid intake can be built over time. Thus allowing athletes to consume as much food and fluid needed to maintain high performance whilst minimizing the risk of gut symptoms.
What Does ‘Training The Gut’ Mean?
The intestinal tract is highly adaptable. Targeted training of the gut can improve its ability to tolerate food and fluid during exercise as well as improve the delivery of nutrients.
A great example of this is seen in competitive eaters. These people literally train their stomach for their events so that they are able to tolerate more food volume with less discomfort.
Serious contestants are able to eat large volumes of food within a small timeframe that would be unthinkable for the average, untrained joe.
One of the all time records had a person eat 69 hot dogs (with the bun) in just 10 minutes. This was likely still an amount that was uncomfortable for this person but it does show how much the gut is able to adapt.
To achieve this ability, competitive eaters will eat huge volumes of things like salad and fluids to increase their stomach’s ability to extend. Volumes are progressively increased over weeks and potentially months prior to a competition.
Transferring this same concept to endurance athletes who definitely don’t need to eat anywhere near that much would surely be beneficial.
One study observed that trained runners were able to comfortably tolerate a carbohydrate–electrolyte solution at a rate approximately equal to their sweat rate during 90 minutes of running. The stomach comfort significantly improved over time by practicing these high intakes regularly.
Interestingly, in this study no change in the rate of gastric emptying was observed. So it is hypothesised that the improvement in tolerability came from the stomach adapting by extending the stomach walls to allow greater space for fluid. This would likely reduce feelings of stomach discomfort and reduce the stimulus for faster gastric emptying.
Other studies have demonstrated that gastric emptying of carbohydrates can be accelerated by increasing intake. Cunningham et al, found that after supplementing its volunteers with an additional 400g of glucose per day for 3 days and then giving them a high glucose meal that the gastric emptying time was significantly faster.
This was not done during exercise or in an athletic population but the concept can likely be applied. For example, many triathletes will have a 4-6 hours training session on the weekend in which they could consume ~400g of carbohydrates in training alone. If this large intake was done frequently enough, they could be able to reduce gastric emptying time, allowing for a greater food intake and/or tolerability. Nonethless, it is likely that the carbohydrate load that many athletes do in the few days leading into competition also assists with more efficient gastric emptying.
Unfortunately very few studies have specifically looked at training the gut to improve tolerance and gastric emptying during exercise. But from what we do know about the guts rapid ability to adapt, training the gut leading into an event makes a lot of sense for endurance athletes.
How To Train The Gut
Training the gut should occur in training, leading up to an event. Food and fluids in training should start at a comfortable level for that athlete and be incrementally increased session to session.
This allows the gut to slowly adapt to the new regime and will limit the risk of gastrointestinal distress in training.
There is no exact recommended protocol for how to go about this as it will be different athlete to athlete.
Typically when I am working with athletes, we will start by pinpointing the amount of food and fluid that the athlete is comfortable consuming during a long session. We will then compare that to where that athlete should theoretically be in order to perform as well as possible.
For most endurance athletes, 60-90g of carbohydrates per hour would be optimal, although this is highly variable depending on the athlete and their sport.
Optimal fluid intake can be slightly harder to pinpoint but the easiest way to assess it, is to weigh yourself pre and post training/event and aim for <2% loss of bodyweight. If it is frequently >2%, you will need to consume more fluids than you are.
Once you have an idea of ideal fluid and carbohydrate intakes per hour, you can start working up to tolerating these amounts in training.
Balancing Training The Gut With Fat Loss Goals
Managing fat loss goals with training the gut can be a difficult balance for endurance athletes. This is because fat loss requires a lower calorie intake, whilst training the gut usually means increasing your calorie intake.
The best way to get around this is to focus on either one or the other. There are definitely ways where you can achieve both simultaneously but without a really good handle on nutrition, it would be quite a difficult task.
Unless you are competing very frequently, there should be times in your year where you don’t need to be primed and ready for optimal race day nutrition. During these times you may just moderately fuel your training whilst being in a calorie deficit for fat loss. And in the weeks or months leading into an important event, pivot to focus on training your gut in order to tolerate the amount of food and fluid required to perform at your best.
Other Tips For Minimizing Gastrointestinal Distress
Even if you have trained your gut well leading into an event, race day conditions can come with their own issues.
Having a nervous gut on the day of competition is common so you may also want to:
- Avoid eating your main meal within 2-3 hours before the competition
- Avoid high fat or fibre foods which will take longer to digest
- Go for low FODMAP foods
- Start the event well hydrated
- Don’t try any new foods on competition day
- Practise mindfulness and breathing exercises to calm your gut-brain axis
Despite limited research in the area, training the gut seems to be standard practice amongst endurance athletes and for good reason.
Typically athletes won’t be able to tolerate the amount of food and fluid that is recommended for optimal performance. So training the gut should be done in training, starting at a level where the athlete has no GI distress, then slowly stepping up the amount ingested.
The gut adapts quite quickly, so if there are no events on the horizon, training the gut can take a backseat to other goals such as fat loss. Although, it is best to start training the gut weeks or sometimes a couple of months out from the event so that you are primed to tolerate the amount of food and fluid required on the day.