Training Your Gut — How to Build Tolerance to High-Carb Fuelling

Most gut issues during endurance events are not a product problem. They're a training problem.

The gastrointestinal system adapts to carbohydrate intake the same way muscles adapt to training load. Absorption capacity increases. Gastric emptying rates improve. Tolerance for higher volumes grows. The athletes who can consume 90g of carbohydrates per hour during a race without incident didn't arrive at that point naturally — they trained their gut to handle it, systematically, over time.

Athletes who skip that process and attempt race-day volumes without preparation should not be surprised when their stomach refuses to cooperate.

The Adaptation Mechanism

When carbohydrates arrive in the small intestine, they're transported into the bloodstream via specific protein channels — primarily SGLT1 for glucose and GLUT5 for fructose. The density and activity of these transporters is not fixed. Research shows that consistent carbohydrate exposure during exercise upregulates transporter expression — meaning the gut literally develops greater capacity to absorb carbohydrates when it's regularly trained to do so (Cox et al., 2010).

The stomach also adapts. Gastric emptying — the rate at which the stomach moves food into the small intestine — increases with training, allowing higher volumes to be processed more efficiently. Athletes who regularly consume liquid carbohydrates during exercise develop faster, more reliable gastric emptying than those who don't.

What "Training Your Gut" Actually Means

It means consuming carbohydrates during training at the volumes you intend to use in races, regularly, over a period of weeks.

Not occasionally. Not only on long rides. During training sessions of relevant intensity and duration, consistently enough that your gut accumulates the stimulus to adapt.

The practical approach: start at a volume that's comfortable — for most athletes, 40–50g of carbohydrates per hour is a manageable entry point — and increase progressively over four to six weeks until you reach your target intake level. Every week or two, add 10–15g per hour. Monitor how your gut responds. Adjust based on what you observe.

The goal is to reach your target race-day intake without distress during a normal training session. If that's possible, race day is not a new stimulus. Your gut already knows what to do.

Common Mistakes

Trying high volumes for the first time on race day is the most frequent and most damaging error. The conditions of a race — elevated intensity, race nerves, heat — already stress the gut. Asking it to process carbohydrate volumes it has never encountered in training compounds that stress significantly.

Using different products in training and racing is the second mistake. Each product has a different composition, concentration, and flavour profile. Gut response is specific to those variables. "Training your gut" with one product and then switching to something different on race day resets a meaningful part of the adaptation.

Training at lower intensities than race intensity is the third mistake. Gastric emptying slows at very high intensities, and the gut's response to carbohydrates changes under race-level stress. Some portion of gut training should happen at race-relevant intensities to reflect the actual conditions.

The Timeline

Meaningful gut adaptation takes four to eight weeks of consistent training. It's not a process that can be compressed significantly. Trying to rush it by jumping immediately to high volumes usually produces the distress athletes are trying to avoid.

For athletes preparing for a specific event, beginning gut training at least eight weeks out — and using the same product and target volumes throughout — gives the gastrointestinal system enough time to develop the capacity required.

The gut is trainable. The athletes who treat it that way find race-day fuelling to be a solved problem rather than a gamble.