A Taxonomy of GI Distress

Upper-GI symptoms are felt mainly in the oesophagus and stomach:

• reflux or heartburn;
• belching;
• upper-abdominal pressure;
• fullness;
• bloating;
• nausea;
• retching or vomiting.

Reflux and heartburn

Reflux occurs when stomach contents move upward into the oesophagus. During exercise it may be influenced by large meals close to the start, high-fat foods, concentrated drinks, caffeine, body position and repeated bouncing. Running is especially provocative because of vertical impact and changes in gastrointestinal pressure gradients.¹

Fullness and bloating

Fullness describes a stomach that is not emptying quickly enough; bloating is expansion or pressure, sometimes with visible distension. Common contributors are large fuel or fluid boluses, highly concentrated carbohydrate, high-fat or high-fibre foods, carbonated drinks and swallowed air.

Nausea and vomiting

Nausea is among the most disruptive endurance symptoms because it quickly reduces both eating and drinking. Contributors can overlap: excessive intensity, heat stress, dehydration or overdrinking, low blood glucose, delayed gastric emptying, concentrated carbohydrate, caffeine, motion and running impact, stress, or underlying illness. Persistent vomiting is not simply “strong nausea” — it can indicate heat illness or hyponatraemia and demands attention.

Lower-GI symptoms are felt mainly in the intestines and colon:

• lower-abdominal cramping;
• intestinal bloating and gas;
• urgency and frequent bowel movements;
• diarrhoea;
• occasionally blood in the stool.

During hard exercise, blood is redirected toward working muscles and skin. Prolonged strenuous exercise can temporarily compromise gastrointestinal integrity and function, particularly when combined with heat and dehydration.¹

Pain that is severe, localised, persistent or accompanied by blood should not be treated as ordinary race discomfort.

Gas and lower bloating can be associated with large amounts of fermentable carbohydrate, fructose in excess of absorption capacity, sugar alcohols or high-FODMAP foods. This does not mean every athlete should follow a low-FODMAP diet — short-term reduction may help selected athletes with recurrent symptoms, but restrictive diets should be targeted, not automatic. Evidence does not support routine gluten avoidance without a diagnosed reason.⁷

Some symptoms are best understood not by location but by a mismatch between what enters the gut and what the gut can process at that moment: carbohydrate overload, excessive drink concentration, fluid–fuel mismatch, malabsorption, poor product tolerance.

Carbohydrate overload

Problems can occur when intake rises too quickly, the athlete has not trained with the target amount, servings are too large, one carbohydrate pathway is overloaded, or heat and intensity reduce tolerance. Glucose–fructose combinations can increase total absorption compared with glucose alone, but the athlete still needs to practise the exact product and dose.

Fluid–fuel mismatch

A single drink is often asked to deliver both carbohydrate and hydration. In heat, thirst can rise faster than carbohydrate needs — drinking more of a concentrated fuel overloads the gut, while avoiding an overly sweet drink reduces both fluid and fuel. Separating some of the fluid from the fuel gives more control.⁸

The untrained gut

The gastrointestinal system can adapt. Two-week gut-training protocols have reduced exercise-associated symptoms and carbohydrate malabsorption in runners.^{5 6}

Do not ask the gut to perform a race-day task it has never practised.

Heat deserves its own category because it can amplify nearly every other GI problem — increasing cardiovascular strain, redirecting more blood toward the skin, reducing gut tolerance, worsening dehydration, affecting intestinal integrity and blunting appetite for concentrated fuel. GI symptoms are commonly exacerbated in hot endurance conditions, especially at higher intensities.⁸

The digestive tract does not experience running the way it experiences cycling. Running creates repeated impact and vertical movement, which can contribute to reflux, urgency, abdominal discomfort, diarrhoea and difficulty tolerating solid food. Research comparing modes has found real differences in the magnitude of exercise-induced GI disturbance between running and cycling.²

A fuelling strategy that works on the bike will not automatically work during a trail race.

Not every fuelling failure begins in the stomach. Athletes may experience:

• dry mouth;
• thick or sticky saliva;
• accumulated sweetness;
• unpleasant coating after gels;
• taste fatigue;
• difficulty managing another concentrated product;
• reluctance to swallow.

These are oral and sensory symptoms — not GI disease. But they influence GI strategy because they decide whether the athlete keeps eating and drinking. An athlete may say, “My stomach cannot take another gel,” when the immediate barrier is actually the mouth.

Some serious conditions initially present as nausea, vomiting or abdominal discomfort.

The map below is not a diagnosis. It is a way to avoid treating every GI complaint as the same problem.

UNBTN does not treat gastrointestinal disease. It does not accelerate gastric emptying, prevent diarrhoea, correct malabsorption or remove the physiological effects of heat. Its role sits earlier in the fuelling pathway.

UNBTN is designed to stimulate saliva and support oral comfort between fuelling moments.

That may be relevant when the immediate problem is:

• dry mouth;
• sticky saliva;
• sweetness fatigue;
• gel residue;
• discomfort from prolonged mouth breathing;
• reluctance to take the next planned fuel because the mouth feels overloaded.

By stimulating natural saliva, UNBTN may support oral lubrication, a fresher mouthfeel, clearance of concentrated fuel residue, relief of dry-mouth sensations and a sensory break between sweet products — helping an athlete continue a fuelling plan that is otherwise still physiologically appropriate.

UNBTN should not be presented as a product that:

• prevents nausea;
• treats reflux;
• stops vomiting;
• prevents diarrhoea;
• heals intestinal permeability;
• improves carbohydrate absorption;
• prevents heat illness;
• guarantees continued fuelling.

An athlete has been running for five hours. The stomach is not painful. There is no severe bloating or urgency. But the mouth feels dry and coated, every gel tastes excessively sweet, and the athlete is beginning to avoid carbohydrate.

A practical sequence:

1. Take water as appropriate.
2. Reduce intensity briefly if needed.
3. Use UNBTN between fuel servings.
4. Allow saliva and mouthfeel to recover.
5. Continue the planned intake with a tolerable product.

UNBTN is not treating the gut. It is supporting the mouth so the fuelling strategy has a better chance of continuing.

When symptoms appear, athletes often make abrupt changes: stop all fuel, drink large amounts of water, take more salt, switch to unfamiliar food, add medication, force another large gel. A better response is systematic.

1. Identify the symptom. Upper gut, lower gut, oral, systemic — or a combination?
2. Reduce the immediate stress. Slow down, cool the body, avoid another large intake.
3. Simplify. Familiar products, smaller servings, water as appropriate.
4. Reassess. Improving, stable or worsening?
5. Respect red flags. Severe symptoms are not ordinary race discomfort.

GI distress is not one failure with one solution. It may originate from:

• the oesophagus;
• the stomach;
• the intestine;
• heat and circulation;
• movement and impact;
• carbohydrate delivery;
• the mouth;
• or a medical problem unrelated to fuelling.

The best strategy begins with classification.

Name the symptom. Locate the problem. Identify the load. Then respond.

UNBTN belongs to one specific part of that taxonomy — the dry, sticky, sweetness-saturated mouth that can make necessary fuel increasingly difficult to tolerate. It does not replace gut training, cooling, pacing or sound nutrition. It supports the first part of the fuelling pathway, before the gel ever reaches the stomach.³