Gastrointestinal Problems in Athletes

Several mechanisms combine to upset the gut during exercise, and no single one explains every symptom. Reduced gut blood flow, or splanchnic hypoperfusion, is particularly relevant during prolonged, intense exercise and in the heat, where reduced perfusion may increase epithelial injury and permeability. Mechanical factors contribute too, with the repetitive jolting of running and body position in cycling both playing a part. Nutritional factors matter, as high-fat, high-fibre or high-protein food and hypertonic drinks slow gastric emptying and provoke symptoms, and altered gut motility, including a fall in lower oesophageal sphincter tone, promotes reflux.

The symptoms divide usefully into upper and lower. Upper-gut symptoms include gastro-oesophageal reflux disease (GORD) with heartburn, nausea, vomiting and belching. Lower-gut symptoms include abdominal cramps, an urgent need to defecate, the classic runner's diarrhoea, and bloating. A more serious but less common presentation is ischaemic colitis, which is reserved for severe abdominal pain with bloody diarrhoea and needs urgent assessment. These complaints are common in endurance athletes and are a frequent reason for slowing down or dropping out of events, so they deserve to be taken seriously and worked up where the pattern warrants it, rather than dismissed as inevitable.

Most exercise gastrointestinal symptoms are managed with nutrition and preparation rather than drugs, and advice is symptom-specific rather than a blanket restriction for everyone. Where symptoms warrant, reducing high-fat, high-fibre and high-protein food and hypertonic drinks in the pre-event window improves gastric emptying, while staying well hydrated protects gut blood flow; lactose is restricted only where intolerance is suspected, and carbohydrate and overall energy intake are kept adequate. A short, targeted low FODMAP approach, reducing fermentable oligosaccharides, disaccharides, monosaccharides and polyols and ideally guided by a sports dietitian, can help, although the evidence is promising rather than definitive and prolonged restriction may reduce dietary variety, carbohydrate availability and energy intake, so it is time-limited. Gut training, gradually practising taking on carbohydrate during exercise, improves tolerance.

Medication has a supporting role. Non-essential use of non-steroidal anti-inflammatory drugs (NSAIDs), particularly around prolonged endurance exercise, is discouraged because it may increase gastrointestinal and renal risk. Exercise-related reflux is assessed like other reflux, checking usual gastro-oesophageal reflux features and any alarm symptoms, and managed with lifestyle measures and standard reflux treatment rather than automatically starting a proton pump inhibitor for every symptom. Throughout, other diagnoses are considered rather than assuming every symptom is exercise-induced: coeliac disease, irritable bowel syndrome (IBS), inflammatory bowel disease and lactose intolerance can all present in athletes, and persistent, atypical or alarm-feature symptoms prompt appropriate testing.

Gastrointestinal bleeding in athletes is often occult, meaning it is not visible, and may settle after the effort, with splanchnic hypoperfusion, mechanical trauma and non-steroidal anti-inflammatory drug use all contributing. Even when occult, repeated blood loss can cause or worsen iron-deficiency anaemia. The important point is that not every episode automatically warrants endoscopy, and equally that bleeding is not simply attributed to exercise. Visible or persistent bleeding is assessed clinically, with a full blood count and iron studies, and investigation or referral is guided by age, pattern, upper or lower gastrointestinal features and current cancer pathways. Severe abdominal pain with bloody diarrhoea after endurance exercise needs urgent assessment for ischaemic colitis.

Iron deficiency deserves a broad approach. Ferritin confirms low iron stores but does not identify the source, so unexplained iron deficiency in an endurance athlete prompts assessment of dietary intake, menstruation, coeliac disease and other gastrointestinal causes rather than assuming occult exercise-related bleeding. Once other causes have been considered and excluded, exercise-related symptoms are managed by discouraging non-essential non-steroidal anti-inflammatory drug use, optimising hydration and nutrition, managing reflux and training the gut, with iron replacement where there is genuine deficiency and its cause has been addressed.

• •Exercise gastrointestinal symptoms are multifactorial and split into upper (reflux, nausea, vomiting) and lower (cramps, urgency, diarrhoea); reduced gut blood flow may increase epithelial injury and permeability.
• •Reserve ischaemic colitis for severe abdominal pain with bloody diarrhoea, which needs urgent assessment.
• •Nutrition is symptom-specific and pre-event focused, keeping carbohydrate and energy adequate; a short, targeted low FODMAP trial with a sports dietitian can help, but evidence is limited.
• •Discourage non-essential non-steroidal anti-inflammatory drug use around endurance exercise; assess reflux features and alarm symptoms before starting acid suppression.
• •Do not automatically arrange endoscopy for every bleed, and do not simply attribute bleeding to exercise; assess with a full blood count and iron studies and refer by age, pattern and cancer pathways.
• •Ferritin confirms low stores but not the source, so unexplained iron deficiency prompts broad assessment including diet, menstruation and coeliac disease.