Carbohydrate supplementation of ~30–90 g/h during prolonged exercise can delay fatigue and improve performance, with higher rates of intake and tolerance of intake being proportional to performance improvements. However, not all athletes are immediately able to tolerate high rates of carbohydrate intake during exercise due to a variety of factors including heat or heat stress, fat or fiber content of food intake, type of carbohydrate ingested, dehydration, following a low-carbohydrate diet, and more.
Therefore, it is generally recommended for athletes to repeatedly practice race nutrition strategies using the specific products and amounts that are planning to be used during the event. Additionally, compared to a moderate carbohydrate diet, several weeks of a high carbohydrate diet appears to be supportive of an athlete’s ability to handle higher rates of carbohydrate intake during exercise.
The concept of gut training involves a more structured approach that has been proposed to improve the tolerance and the absorption of carbohydrates during exercise. Gut training refers to repeated intentional exposure to fluids and foods during exercise for a period of ~ 2+ weeks that leads to the ability to increase nutrition intake, which in turn supports performance outcomes. Trained athletes who are accustomed to regular fuel intake during exercise may not see as many changes in gut-related outcomes compared to more recreational athletes or to those who do not regularly fuel training per the published recommendations.
Based on current research, a gut training protocol may look something like this:
For at least 2 weeks, complete an individualized protocol that includes ~3 days per week or more of gut training
During the sessions, find your limit of carb and fluid intake per hour using the actual products (food and drink) you would plan to use during the event. The limit would be when you start to experience upper or lower GI symptoms.
Consider some of the following factors that may influence your protocol: volume of fluid, concentration of fluid, frequency of intake, type of carbohydrate source, your regular diet (e.g. FODMAP content, amount of dietary carbohydrate), heat/humidity in environment, hydration status, fat/fiber content of your fueling plan etc.
Keep notes within your training journal or in TrainingPeaks so that you can keep track of what is working and what still needs improvement!
References: PMID 37061651, 35963615, 34557109