Athletes often underestimate the importance of the gastrointestinal (GI) tract. Supplying exogenous sources of fluids and carbohydrates can be fundamental to performance, especially during prolonged exercise (1). Furthermore, GI symptoms such as bloating, cramping, diarrhea, and vomiting are common in many sports, especially endurance sports (2).
Without a well-functioning GI system, nutrient delivery will be impaired, and a variety of GI symptoms can develop. Clearly, the intestinal tract is highly adaptable, and it has been suggested that specific training of the intestinal tract can improve nutrient delivery during exercise while alleviating some (or all) symptoms (3). This training, sometimes referred to as "gut training," has received relatively little attention in the literature, and, to my knowledge, there are no dedicated review articles on this topic.
What is it? Why?
Within the term Nutritional Training, we find different methods to optimize sports performance, such as Nutrient Periodization, previously discussed. Gut training is a process of adapting the Digestive System.
It aims to train and/or accustom the stomach and small intestine to tolerate larger volumes of fluids and amounts of food ingested during exercise, as well as to achieve adaptations in substrate transport and utilization.
It seeks to improve athletes' behavioral habits regarding intake during exercise, to optimize recovery, minimize the risk of injuries, internal and metabolic load, and improve anxiety, satiety, and control of extra-training intake.
The ultimate goal, therefore, is to minimize associated gastrointestinal problems and maximize, in each situation, the transport of different ingested nutrients, as well as their utilization and performance.
Why train the stomach and intestines?
High-intensity, long-duration exercise and its potential factors (mechanical impact, metabolic stress, dehydration, electrolyte imbalance, etc.) limit the proper functioning of the Digestive System.
Carbohydrate intake and hydration are key to optimizing medium and long-duration performance. An adequate intake pattern and habit during exercise improves performance, recovery, and the long-term health of athletes.
Minimizing exercise limitations and optimizing possibilities (ingesting more food and drink, always according to specific demands) is a fundamental part of athletic success, and this is where this training comes in.
Objective: To ingest the maximum amount of carbohydrates during exercise to achieve greater oxidation of these and allow the athlete to run the event at a higher intensity than planned.
How?
Phases and explanation:
Phase 1: With virtually no intervention during the activity, it consists of increasing the amount of Carbohydrates in meals prior to training, accompanied by a certain amount (500-1000ml) of liquid. We do this so that the athlete exercises with greater volumes of liquid and food in the stomach, which will help improve their tolerance and stomach comfort.
Phase 2: It consists of starting to ingest food during exercise (habit) to increase gastric volume throughout the training session. For this, "complex" digestion foods such as fruits with skin and certain fructose-rich foods (fruit sweets) are chosen, ensuring a minimum carbohydrate amount of 40-45g/h, along with a moderate amount of liquid: 300-400 ml/h of isotonic drink.
The first two phases are the most basic. Starting from a very primary point, i.e., an athlete who eats and drinks very little during sport, a person on a chronic low-carb diet, or an athlete with a given intestinal pathology (ulcerative colitis, for example).
Both phases can be combined (performed at the same time), always depending on the athlete's starting point. As a general rule, a period of 2 weeks is proposed, at a rate of 3 training sessions per week, to progress through the phases.
Phase 3: This involves starting the intake pattern during exercise with more suitable foods for this purpose. Therefore, more digestible foods composed of simple sugars (see table) are proposed, in an approximate amount of 45-60g CHO/h. In addition, a minimum amount of liquid must be ensured, with the adequate amount being up to 500ml/h with its respective electrolytes.
Phase 4: The next step is to optimize the amount of intake during exercise (reaching 90g CHO/h). To do this, we must combine sugars that are absorbed with different intestinal transporters (maltodextrin:fructose) in an adequate range of 2:1 or 1:0.8. Sports foods that meet this premise (gels), homemade foods based on sugar and fruits (quince paste, sweet potato paste, etc.) and/or drinks with this composition should be sought. With the same amount of fluid as in Phase 3, we ensure adequate hydration so that our digestive system can withstand this training.
Phase 5: We are now at a higher level. In this phase, it is assumed that if your body is already able to assimilate high amounts (like the previous ones) of carbohydrates, you are ready to seek the optimal quality and quantity (the maximum tolerable for each individual). This amount is around 120g CHO/h (although it should be personalized), as we have seen that such quantities can be tolerated without any problem and, at the same time, significantly improve performance and recovery (4).
Phase 6: In this last phase, we are at the top level. The goal is to train the race strategy in detail. To do this, we must plan and prepare said strategy and test it, in competition simulation training sessions, at least twice. In addition, we can personalize hydration to ensure an adequate pattern based on our needs. At this point, you should have the foods, the combination of different sugars, the drinks, the intake protocols, and other details, tested and measured down to the millimeter to optimize your sports performance to the maximum. That is, everything you are going to take to the competition.
Look at the images; the intensity graph is described in the easiest and simplest way possible for you to understand. The 3 main intensity zones correspond to blood lactate concentration. We could say that they represent the 3 metabolic zones, always bearing in mind that metabolic pathways do not function independently and squarely, but rather complement each other through the so-called "energetic continuum" (the image on the left).
In it, we wanted to represent, in red, the intensity zone above the supposed Ventilatory Threshold 2 (VT2), from which the use of glycolysis becomes more and more important in obtaining energy.
Furthermore, due to greater blood redistribution, a greater impact, and generally slight dehydration, gastrointestinal problems also increase beyond this intensity. We are then at a critical point where the need and expenditure of glucose increase exponentially, and yet the difficulties in digesting and absorbing sugars also increase.
Therefore, this "gut training" will help us avoid these problems at all costs and become increasingly efficient.
Two examples of possible training sessions where "gut training" would be appropriate:
Long series training on a bicycle: Typical training at an intensity around UL2 with 20-minute series. For example, 4 x 20 min on a mountain pass.
Medium series running training: Another classic, a "miles" training session on the athletics track. For example, 10 x 1000 meters at an intensity higher than your best 10 km time (higher than LT2 or VT2).
Training sessions that simulate a competition or race. This is especially important to know how the digestive system behaves during key and most demanding efforts, and also to test all the selected products for X day.
Physiological Effects
This training allows us to better tolerate food and fluid volumes in the stomach during sports activity. That is, it improves our tolerance and stomach comfort during exercise, especially at high intensities. Keep in mind that as exercise intensity increases, gastrointestinal problems tend to become more present.
It increases gastric emptying speed and volume. In other words, what we eat and drink passes through the stomach faster and reaches the absorption sites (small intestine) sooner.
With Gut Training, we increase the sugar transporters in the gut, which translates into a greater capacity for transport and oxidation of these in the corresponding muscle. In addition, we can combine this type of training with a high-fructose diet. This will increase the rate of oxidation by the GLUT 5 (fructose) transporter.
It also improves the athlete's glycolytic capacity (glucose burning) at high intensities. Why? Because glycolytic metabolism is stimulated to a greater extent, "converting" the athlete into a more efficient "burner." You can imagine a gasoline Ferrari compared to a diesel Renault Clio.
Benefits:
Fewer gastrointestinal problems during exercise, which ensures athletic success, as we minimize a significant limitation in sports.
Better performance during higher intensity training/competitions. If we manage to ingest more "fuel," without associated problems and more efficiently, we will also perform better at a higher intensity where the demand is almost exclusively for that "fuel" (glucose).
Better post-training or post-competition recovery, due to less depletion of muscle glycogen and therefore greater conservation, which allows the athlete to finish feeling more "whole."
Bibliography
- Jeukendrup AE. Nutrition and endurance sports: running, cycling, triathlon. J Sports Sci. 2011;29:S91–9.
- Oliveira EP, Burini RC, Jeukendrup A. Gastrointestinal complaints during exercise: prevalence, etiology, and nutritional recommendations. Sports Med. 2014;44:S79–85.
- Jeukendrup AE, McLaughlin J. Carbohydrate ingestion during exercise: effects on performance, training adaptations and trainability of the gut. Nestle Nutr Inst Workshop Ser. 2011;69:1–12 discussion 3–7.
- Jeukendrup A. Training the gut. Sports Med. 2016.
- Cox GR, Clark SA, Amanda J. Cox AJ, Halson SL, Hargreaves M, Hawley JA, Jeacocke N, Snow RJ, Yeo WK, Burke LM. Daily training with high carbohydrate availability increases exogenous carbohydrate oxidation during endurance cycling. Journal of Applied Physiology Published 1 July 2010 Vol. 109 no. 1, 126-134
