8-week bowel training program

Gut training means gradually teaching your stomach and intestines to tolerate increasing amounts of carbohydrates during exercise. It is one of the most research-backed performance-enhancing strategies in endurance sports, and one of the most often overlooked. Here is the scientific background, along with the practical eight-week program that I use myself.

Why Gut Training Works

The “doors” in the intestinal wall that absorb sugar during exertion—one for glucose-based carbohydrates and a separate one for fructose—respond to repeated exposure. Regular, high carbohydrate intake during training increases the number and activity of these absorption sites, which raises the practical ceiling on how much fuel you can take in per hour.

Gut training also speeds up gastric emptying, meaning your stomach moves fuel into your intestines faster, and you feel lighter during exertion. It reduces the discomfort and bloating that most athletes experience during the first few times intake is increased. And it builds routine—the simple discipline of fueling on schedule, every twenty minutes, regardless of how you feel.

A study by Costa and colleagues showed that just two weeks of repeated gut loading during exercise significantly reduced gastrointestinal symptoms and improved glucose availability afterward. Over eight weeks, the adaptation is significantly greater.

In a discussion with Alex Larson—a registered dietitian and endurance nutrition coach with fifteen years of experience working with runners, cyclists, and triathletes—Alex highlighted that the body strongly resists sudden changes and that nutritional changes that truly stick are gradual. This is precisely why gut training is built on a slow, weekly increase in carbohydrate intake rather than a sudden leap on race week.

PODCAST LINK: Episode with Alex Larson — YouTube

Who Benefits from This

Any athlete aiming for more than 60 grams of carbohydrates per hour in a race will benefit. This applies to runners doing 100km or longer distances, runners who have experienced stomach problems with gels in previous races, runners who currently take less than 40 grams per hour on long runs, and anyone for whom high-intensity fueling is new.

Athletes who already tolerate 60-90 grams per hour on long runs without issues are partially trained and can start the program further along.

Weeks 1 and 2: Build the Routine

Goal: 40 grams of carbohydrates per hour on all runs longer than one hour. Method: 20 milliliters of fuel every twenty minutes. Focus on consistency, not quantity. Set an alarm on your watch. Take a dose even if you feel good. Some athletes notice mild bloating in the first week—this is normal and will subside as the gut begins to adapt.

Weeks 3 and 4: Increase the Quantity

Goal: 50–55 grams per hour on all runs longer than one hour. Method: 22–25 milliliters every twenty minutes. Focus on staying on schedule during harder training days and in warm conditions, both of which increase gut stress. Symptoms from the early weeks should begin to subside. Gastric emptying will start to speed up.

Weeks 5 and 6: Approach Race Intensity

Goal: 60–70 grams per hour on all runs longer than one hour. Method: 25–28 milliliters every twenty minutes. Focus on practicing fueling at race pace on long runs. The gut adapts precisely to the intensity at which it is trained, so this is the most important period of the entire eight weeks.

Weeks 7 and 8: Race Simulation

Goal: 70–90 grams per hour on long runs. Method: 30 milliliters every twenty minutes. Focus on fully simulating race day during your longest training runs—the same sip volume, the same timing, the same method of carrying fuel as in the race. By the end of the eighth week, most athletes tolerate 70–80 grams per hour with very few or no gastrointestinal issues.

Important Principles

Train your gut at the intensity you plan to race at. The gut adapts to the exact conditions in which it is trained. Fueling during easy runs will not fully prepare it for race-pace fueling.

Do not skip sessions. Eight weeks of consistency produces adaptation. Sporadic effort yields sporadic results.

Train with the exact fuel you plan to race with. Gut adaptation is partly product-specific, so train with the same carbohydrate profile, the same concentration, and the same packaging you intend to use on race day. The goal is that on race morning, nothing about fueling feels new.

If symptoms persist after the third week, reduce the amount by 20 percent and build up more slowly. Some athletes need twelve weeks instead of eight.

Summary

Gut training is not optional for athletes aiming for high carbohydrate intake in long ultra-events. Eight weeks of gradual, consistent loading produces measurable improvements in absorption capacity, tolerance of volume and intensity, and the simple discipline of fueling on schedule. Start eight weeks before your target race and treat training as an absolute part of your preparation.

References

Costa RJS, Miall A, Khoo A, Rauch C, Snipe R, Camões-Costa V, Gibson P. (2017). Gut-training: the impact of two weeks repetitive gut-challenge during exercise on gastrointestinal status, glucose availability, fuel kinetics, and running performance. Applied Physiology, Nutrition, and Metabolism. 42(5), 547–557.

Jeukendrup AE. (2017). Training the gut for athletes. Sports Medicine. 47(Suppl 1), 101–110.

Lambert GP. (2008). Intestinal barrier dysfunction, endotoxemia, and gastrointestinal symptoms: the canary in the coal mine during exercise in the heat? Medicine and Sport Science. 53, 61–73.