The gut · about four minutes

Nine out of ten stomach failures land in the run.

The bike felt fine. That is not bad luck, and it is not something you ate at mile sixty. It is where the physiology changes.

Where it happens

90%of gastrointestinal problems in an Ironman strike during the marathon segment. At the hot World Championship, prevalence reaches 93%.

In ultrarunning the numbers are similar in spirit. Up to six in ten ultrarunners experience nausea during a race, far higher than road marathoners, and up to half of those cases are severe enough to hurt performance. Heat can quadruple the incidence against the same effort in mild conditions.

Why the run and not the bike

Two things change when you get off the bike. You are upright and being jostled, and you are usually deeper into both the effort and the heat.

Past roughly 70 to 80% of VO2max, your body starts diverting blood away from the digestive system to supply working muscle and skin.

Up to 80%of splanchnic blood flow can be redirected during maximal exercise. The gut is left ischemic, hypoxic and acidotic.

That is the accepted main mechanism behind exercise-induced GI distress. Gastric emptying slows. Whatever is in your stomach stops moving, and it is on its own until the effort comes down.

This is the finding that rules out the promise the entire category makes, ours included. Exercise GI distress is attributed mainly to splanchnic hypoperfusion, mechanical jostling, heat and dehydration. It is not attributed to what you drank. So we are not going to tell you a drink fixes your stomach, because the research says the cause is your blood leaving your gut to cool you down.

What actually helps, none of which we sell

Training the gut. Practising race-day intake in training is the most consistently supported intervention in the literature.
Managing heat, because heat multiplies the incidence rather than adding to it.
Pacing the first half honestly, since the mechanism is tied to intensity relative to your ceiling.
Not arriving at the run already behind on fluid, which is the one place the drink in your bottle is genuinely relevant.

Where a drink can and cannot reach

There is one honest thing to say about formulation here, and it is narrow. The forms a product uses affect how irritating the product itself is. Magnesium oxide that you do not absorb sits in the gut and behaves like a laxative. Potassium chloride is more irritating to the stomach than potassium citrate.

That is a claim about not making things worse. It is not a claim about fixing the underlying problem, which is circulatory, and which nothing in a bottle reaches.

Sources

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