I remember that many years ago a review paper was published with the title: Is the gut an athletic organ? It hinted to the fact that athletic performance is very dependent on fuel and fuel delivery is dependent on gut function. Could the gut therefore be a much underestimated organ for athletes? In professional cycling it is sometimes said that a stage race is as much an eating competition as it is a bike race. From marathons and triathlons in particular, we know that fuelling during a race can be problematic as it is not easy to eat and drink whilst pushing the pace in these events. It is very common that athletes report stomach problems in attempt to do so. So the options are fuel less and have a chance of running out of energy and become dehydrated or fuel more and have a chance of getting an upset stomach. The optimal approach is of course somewhere in the middle where you balance intake with stomach comfort. However, there is one other approach that may work. The gut is an extremely adaptable organ and can be “trained” in a similar way to the way we train the muscle.
In a recent review in Sports Medicine I discuss the evidence that the gut can be trained. A lot of this evidence comes from studies in animals. But the evidence is strong and the few human studies we have point in the same direction. Contestants in eating competitions are known to ‘‘train’’ their stomach to hold larger volumes of food with less discomfort and— through regular training—are able to eat volumes of food within a small time window that are unthinkable for the average and untrained person. The current all-time record is 69 hot dogs (with bun) in 10 min. To achieve this, competitive eaters train using a variety of methods: chewing large pieces of chewing gum for longer periods of time or stomach extension by drinking fluids or by eating the competition foods. Volumes are progressively increased, and it takes many weeks to reach a level where these eaters can be competitive.
This demonstrates the adaptability of the stomach. Conducting this ‘‘stomach training’’ has two main effects: (1) the stomach can extend and contain more food and (2) a full stomach is better tolerated and is not perceived as so full. Both aspects could be beneficial in an exercise situation.
Another example relates to intestinal absorption: carbohydrate absorption during exercise seems limited to about 60 grams per hour (at least when a single type of carbohydrate, for example glucose, is ingested). An intake much above 60 grams per hour will most likely result in accumulation of carbohydrate in the intestine. However, increasing daily carbohydrate intake, mostly by increasing intake during the activity, has been shown to increase the absorption and oxidation of ingested carbohydrate (2). Read also this blog on increasing intestinal absorption.
The duration of this adaptation is unknown but in animal studies significant changes can be observed after a change of diet for only 3 days. The human study referred to above (2) used 28 days. From a practical point of view, I have been recommending 5-10 weeks at least once a week (Please note that in the absence of concrete evidence this is a best guess based on information that is available). It may be best to pick one longer training session per week, or the training that is closest to the race the athlete is preparing for and use this to practice the race nutrition intake.
There are a number of methods available to prepare the gut for competition. These are summarised in the figure below: