The decisive kilometers of a long run arrive, you open a gel and, a few minutes later, trouble appears: nausea, bloating, cramps, or that unpleasant sensation that the stomach has decided to stop cooperating. For some runners the conclusion is immediate: gels sit badly with me. However, very often the problem is not the gel itself, but the amount ingested, the concentration of carbohydrates, hydration, the intensity of exercise, or simply that the digestive system has never been trained to operate under those conditions. Just as you wouldn’t wear brand-new shoes on marathon day, you shouldn’t debut a nutritional strategy. The intestine possesses adaptive capacity and can improve its tolerance to carbohydrate intake during exercise. This is what sports nutrition refers to as gastrointestinal training or, more simply, training the stomach. And it is 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 May Upset Your Stomach While Running
During a prolonged race, the digestive system operates under less favorable conditions. As intensity rises, a greater proportion of blood flow is directed toward the active muscles and the skin, especially when it is hot. At the same time, the repetitive motion of running adds a significant mechanical component. If you 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 all contribute to their emergence. Not all of us react the same way. Some runners tolerate perfectly three or four gels during a race, while others experience discomfort with much smaller amounts. Precisely for this reason, blindly copying another runner’s feeding strategy makes little sense.
The Gut Can Also Be Trained
One of the more interesting references comes from a study published in Applied Physiology, Nutrition and Metabolism. Researchers subjected endurance runners to two weeks of gastrointestinal training through repeated exposures to carbohydrates while running. After that period, the groups that practiced carbohydrate intake 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. This does not mean fourteen days are enough to turn any problematic stomach invulnerable. Later studies 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 errors. High carbohydrate availability strategies have become very popular in endurance events, and it’s tempting to think that the more fuel you take in, the better your performance will be. It doesn’t work that way. Classic recommendations typically place carbohydrate intake during prolonged exercise at around 30-60 grams per hour. In endurance events lasting more than roughly two and a half hours, amounts up to about 90 grams per hour can be used, especially through combinations of different carbohydrates that can exploit different intestinal absorption pathways, according to works published in Journal of Sports Sciences. But 90 grams per hour is not an obligation, nor an appropriate target for every runner. Someone who currently does not take anything during training should not start tomorrow on a long run attempting to ingest that amount. A much more logical approach is to start with a tolerable dose and gradually increase the amount across several sessions.
How to Train Your Stomach Practically
Imagine a runner preparing for a marathon who currently can only take one gel of about 25 grams of carbohydrates per hour. The objective should not be to jump immediately to three or four gels. They can begin by practicing around 30-40 grams per hour on some long runs. When they verify that they tolerate it, they can move to 45-60 grams. If their race, expected duration, and strategy justify a higher intake, they can continue progressing. The important thing is to repeat. The digestive system isn’t trained by taking six gels on a single Sunday; it is exposed regularly to the strategy we plan to use later. Moreover, it must be practiced while running. Being able to ingest a certain amount of carbohydrates while sitting at home does not prove you’ll tolerate it at kilometer 30 of a marathon.
Also train the frequency, not just the amount. It matters not only how much you consume per hour, but also how you distribute that amount. For some people it is more tolerable to spread intake in small, frequent doses than to concentrate a large amount once an hour. A runner might discover, for example, that they tolerate better small sips every 20-30 minutes than a much larger load once per hour. This practice also helps reveal a fundamental question: how long it takes for any discomfort 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 construct your competition strategy. It is worth noting the product, carbohydrate amount, water consumed, temperature, intensity, and symptoms. Within a few weeks patterns often emerge that would otherwise go unnoticed.
Glucose and Fructose When Intake 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 the body can absorb and utilize. That is why many products designed for long tests 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 beginning of the article: any combination must be tested by training before using it in a race.
Water Matters More Than It Seems
Consuming very concentrated gels without enough liquid can encourage discomfort in certain runners. The stomach receives a highly concentrated solution while the body is simultaneously attempting to maintain exercise, regulate its temperature, and distribute blood flow. The exact strategy also depends on the type of gel. Some products are formulated to be taken with water, while others have a different texture and concentration. It is wise to follow the manufacturer’s instructions and reproduce during long runs exactly the combination that will be used on race day. It is not about drinking water without limit. Hydration should adapt to duration, temperature, sweating, and the availability of aid stations. And there is another reason to practice it: correctly taking a gel while running, opening it without stopping, and coordinating it with a water station 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 sports 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 involves testing the whole package: the previous dinner, breakfast, the time between meal and training, caffeine if used, gels, beverage, and water.
The Marathon Begins Nutritionally Long Before the First Aid Station
Do not experiment on race day. It is probably the simplest rule and the one most runners ignore. A runner’s fair is not the ideal place to test a new gel. Neither should you modify your strategy because another participant at the start says they take twice as many carbohydrates as you. The long runs in the weeks prior serve as a laboratory. Use the same breakfast, the same gels, roughly the same timing, and, when possible, similar hydration to what you will have on race day. You can even practice taking carbohydrates when you start to feel tired. Digestive problems do not always appear at the beginning; they can emerge after two or three hours of effort.
When Discomfort Merits More Than a Gel Change
Occasional gastrointestinal problems are relatively common in endurance sports. However, intense or persistent pain, repeated vomiting, blood in stools, recurrent diarrhea, unexplained weight loss, or discomfort that appears systematically even with small intakes justify professional assessment. Not everything should be attributed to training or to a supposed stomach weakness. In people with gastrointestinal pathologies, known intolerances, or relevant medical histories, a higher carbohydrate strategy should be individualized with a healthcare professional or a sports-dedicated dietitian-nutritionist.
The Marathon Strategy Also Needs Training
The ultimate goal is not to be able to endure the highest possible number of gels. It is to find the amount of carbohydrates you need and that your digestive system can tolerate while running at your planned pace. For some runners this may 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 close to or even above 90 grams under specific strategies. The highest figure is not automatically the best. If gels have upset you until now, before discarding them, observe how much you take, how frequently, with how much liquid, and at what intensity. Reduce the dose if necessary and progressively build tolerance. Because marathon day must train not only the legs, the heart, and the mind. The digestive system must also have rehearsed its work.
