Those decisive kilometers of a long run arrive, you open a gel and, a few minutes later, the problem appears: nausea, bloating, cramps, or that unpleasant feeling that the stomach has decided to stop cooperating. For some runners the conclusion is immediate: gels sit badly with me. However, often the problem isn’t the gel itself, but the amount consumed, the carbohydrate concentration, hydration, exercise intensity, or simply that the digestive system has never been trained to work under those conditions. Just as you wouldn’t debut new shoes on marathon day, you shouldn’t debut a nutritional strategy either. The intestine has adaptive capacity and can improve its tolerance to carbohydrate intake during exercise. That is what sports nutrition refers to as gastrointestinal training or, more simply, training the stomach. And it’s not a metaphorical expression. There are changes related to gastric emptying, tolerance to larger volumes, and intestinal absorption that can be practiced through repeated exposures.
Why gels can upset you when you run
During a prolonged race the digestive system works in an unfavourable setting. As intensity increases, a larger share of blood flow is directed toward the active muscles and the skin, especially when it’s hot. At the same time, the repetitive motion of running adds a significant mechanical component. If we suddenly introduce a large amount of carbohydrates into that scenario, the result can be uncomfortable. A review published in Sports Medicine explains that gastrointestinal problems are common among endurance athletes and that nutritional, physiological, and mechanical factors jointly contribute to their appearance. Not everyone reacts the same. Some runners tolerate perfectly three or four gels during a race, while others experience discomfort with much smaller quantities. Exactly for this reason, blindly copying another runner’s feeding strategy makes little sense.
The gut can be trained as well
One of the most interesting references comes from a study published in Applied Physiology, Nutrition and Metabolism. The researchers subjected endurance runners to two weeks of gastrointestinal training through repeated exposures to carbohydrates while running. After that period, the groups that had practiced carbohydrate ingestion showed a significant reduction in gastrointestinal symptoms compared with the placebo group. Improvements were also observed in some variables related to glucose availability and performance. That doesn’t mean fourteen days are enough to turn any problematic stomach invulnerable. Later works have shown more modest responses and there is considerable individual variability. But it does support a very useful idea for the runner: digestive tolerance can be trained. The review Training the Gut for Athletes, published in Sports Medicine, precisely notes that the gastrointestinal tract can adapt and that training the nutritional strategy can improve tolerance, gastric emptying, and absorption capacity.
Don’t start directly at 90 grams per hour
Here lies one of the most common current mistakes. High-carb-availability strategies have gained a lot of popularity in endurance tests, and it’s easy to interpret that the more fuel you take in, the better the performance. It doesn’t work that way. Classical recommendations generally place intake during prolonged exercise at around 30-60 grams of carbohydrates per hour. In endurance tests lasting more than roughly two and a half hours, amounts of up to about 90 grams per hour can be used, especially through combinations of different carbohydrates able to exploit different intestinal absorption pathways, according to studies published in the Journal of Sports Sciences. But 90 grams per hour is not mandatory, nor an appropriate target for every runner. Someone who currently takes nothing during training should not start tomorrow a long run aiming to ingest that amount. A more logical strategy is to start with a dose that is well tolerated and gradually increase the amount across several sessions.
How to train the stomach in practical terms
Let’s imagine a runner who is preparing a marathon and currently can only manage to take a gel containing about 25 grams of carbohydrates per hour. The goal should not be to immediately move to three or four gels. They can begin by practicing around 30-40 grams per hour on some long runs. When they find that they tolerate it, they can progress to 45-60 grams. If their race, expected duration, and strategy justify a higher intake, they can continue to advance. The important thing is repetition. The digestive system isn’t trained by taking six gels on a single Sunday, but by regularly exposing it to the strategy we want to use later. And it must be practiced while running. Being able to ingest a certain amount of carbohydrates while sitting at home does not prove you will tolerate it at kilometer 30 of a marathon.
Also train the frequency, not just the quantity. It isn’t only about how much you consume per hour. It also matters how you distribute it. For some people it is more tolerable to spread ingestion into small, frequent portions than to concentrate a large dose once per hour. A runner may discover, for example, that they tolerate small sips every 20-30 minutes better than a much larger load once per hour. This approach also helps you learn something fundamental: how long any discomfort takes to appear. If nausea always starts after the third intake, if it only appears when you increase the pace, or if it disappears when you accompany the gel with enough water, you are obtaining useful information to build your race strategy. It’s worth noting the product, the amount of carbohydrates, water consumed, temperature, intensity, and symptoms. In a few weeks patterns tend to emerge that would otherwise go unnoticed.
Glucose and fructose when the amount increases
When carbohydrate intake is high, the type of carbohydrate used becomes important. The intestinal absorption of glucose has a limited capacity. Combining glucose or maltodextrin with fructose allows the use of different intestinal transporters and increases the amount of carbohydrates that the body can absorb and utilize. That’s why many products aimed at long-duration events currently rely on combinations of these carbohydrates. However, glucose plus fructose does not automatically guarantee perfect digestion. Fructose tolerance varies considerably, and we would return to the start of the article: any combination must be trained before using it in a race.
Water matters more than you think
Drinking highly concentrated gels without enough liquid can promote discomfort for certain runners. The stomach receives a very concentrated solution while the body is simultaneously trying to maintain exercise, regulate temperature, and distribute blood flow. The exact strategy also depends on the type of gel. Some products are formulated to be consumed with water and others present different textures and concentrations. It is wise to respect the manufacturer’s instructions and to reproduce during long runs exactly the combination we will use on race day. It isn’t about drinking water without limit either. Hydration should adapt to duration, temperature, sweating, and the availability of aid stations. And there is another reason to practice it: correctly ingesting a gel while running, opening it without stopping, and coordinating it with a water point is also part of the preparation.
Be careful with what you eat before running
Not all problems that appear after taking a gel are caused by the gel itself. A heavy meal, too close to training, or especially rich in fiber, fat, or protein can slow gastric emptying and increase the risk of discomfort during prolonged efforts. The endurance nutrition literature has warned for years that a high carbohydrate concentration, certain foods, and a substantial intake of fiber, fat, or protein before or during effort can promote gastrointestinal symptoms in susceptible runners. Therefore training the stomach means testing the whole: previous dinner, breakfast, time between meal and training, caffeine if used, gels, drink, and water.
The marathon starts nutritionally long before the first aid station.
Don’t experiment on race day. It’s probably the simplest rule and the one most runners fail to follow. A race expo isn’t the ideal place to be tempted to test a new gel. Nor should you modify your strategy because another participant at the start tells you they take twice the carbohydrates you do. The long runs in the previous weeks serve as a laboratory. Use the same breakfast, the same gels, roughly the same timings and, when possible, hydration similar to what you expect on race day. You can even practice taking carbohydrates when you’re starting to feel tired. Digestive problems don’t always appear at the beginning; they can show up after two or three hours of effort.
When discomfort warrants more than just changing gels
Occasional gastrointestinal problems are relatively common in endurance sports. However, intense or persistent pain, repeated vomiting, blood in the stools, recurrent diarrhea, unexplained weight loss, or discomfort that appears systematically even with small intakes justify professional evaluation. Not everything should be attributed to training or a supposed stomach weakness. In people with gastrointestinal pathologies, known intolerances, or relevant medical history, an elevated carbohydrate strategy should be individualized with a healthcare professional or a sports dietitian-nutritionist.
The Marathon Strategy Also Needs Training
The ultimate goal isn’t to be able to tolerate the largest possible number of gels. It’s to find the amount of carbohydrates you need and what your digestive system can tolerate while running at the planned pace. For some runners that will be 40 or 50 grams per hour. For others it will be 60. And well-trained athletes, subjected to very prolonged efforts and accustomed to large intakes, may use amounts near or even above 90 grams under specific strategies. The highest figure is not automatically the best. If gels have upset you so far, before discarding them, observe how much you take, how often, how much liquid, and at what intensity. Reduce the dose if necessary and build tolerance gradually. Because on marathon day not only must your legs, your heart, and your head be trained. The digestive system also needs to have practiced its work.
