What the gut can and cannot absorb while an athlete is working

The gut loses priority during exercise

During hard exercise the body redistributes blood towards working muscle and towards the skin for cooling. The digestive tract is one of the regions from which that blood is drawn, and its function is reduced accordingly. Reduced blood flow slows the movement of contents through the gut and impairs the absorption of what is already there.

The effect scales with intensity and with heat, which is why problems concentrate in hot events at high effort. This is the underlying reason that fuelling strategies which work in training can fail entirely under competitive conditions. The reduction is temporary and reverses once effort falls, which is why symptoms often ease as soon as an athlete slows down.

Transport is the bottleneck

Carbohydrate crosses the wall of the intestine through specific transport proteins, and those proteins have a limited capacity. Once that capacity is reached, additional carbohydrate remains in the gut, drawing water in and producing discomfort. Different sugars use different transport routes, which is why combinations can be absorbed at a higher total rate than any single type.

This is the mechanism behind the formulation of modern sports products rather than a marketing distinction. The concentration of a drink matters for the same reason, since a very concentrated solution draws water into the gut rather than out of it. None of this indicates what any particular athlete should consume, which remains a question for a qualified practitioner.

Symptoms are common and under-reported

Gastrointestinal symptoms are among the most frequently reported problems in endurance events and are a common reason for withdrawal. Athletes often regard them as inevitable, which means they are frequently not mentioned to medical or nutrition staff. Symptoms during exercise can also indicate conditions unrelated to fuelling, which is a further reason to report them.

Persistent or severe gastrointestinal problems in an athlete warrant medical assessment rather than adjustment of intake by trial and error. Blood in the stool or severe abdominal pain after exercise are specific reasons to seek medical attention promptly rather than to wait.

Absorption appears partly trainable

Repeated exposure to carbohydrate intake during training appears to increase the capacity of the transport machinery involved. This is why fuelling strategies are practised in training rather than introduced on the day of competition.

The adaptation takes weeks rather than days and requires consistent exposure rather than occasional attempts. How this should be structured for a specific athlete is a question for a sports dietitian familiar with their event.

Mechanical and psychological contributors

Running produces mechanical jostling of the abdominal contents that cycling and swimming largely do not, which explains part of the sport difference. Anxiety before competition affects gut motility directly, which is why symptoms often appear before an event has even begun.

Dehydration compounds the problem, since reduced blood volume further limits what can be spared for the digestive tract. These factors interact, so identifying a single cause in an individual athlete is rarely straightforward.

Why this changes how fuelling is planned

A plan that specifies intake without considering whether the gut can process it under race conditions is incomplete. The practical constraint is therefore tolerance rather than theoretical requirement, and tolerance is individual. This is why practitioners test strategies in conditions resembling competition rather than in comfortable training sessions.

It also explains why identical products suit some athletes and not others without either being inherently superior. The general lesson is that the digestive system is part of the athlete rather than a neutral pipe through which fuel simply passes.