From the Guidelines
Gastrointestinal (GI) upset following a marathon is most likely caused by a combination of factors, including dehydration, altered blood flow to the digestive system, and nutritional factors such as high fiber, fat, protein, and fructose intake, as well as high osmolality beverages. The exact cause of GI upset after a marathon can be complex and multifactorial, involving various physiological and nutritional factors. Some key factors that contribute to GI upset include:
- Dehydration, which can exacerbate symptoms 1
- Altered blood flow to the digestive system during prolonged exercise 1
- Nutritional factors, such as:
- High fiber, fat, protein, and fructose intake, which can delay gastric emptying and cause a shift of fluids into the intestinal lumen 1
- High osmolality beverages, which can increase the incidence of symptoms 1 To prevent or alleviate GI upset after a marathon, athletes can take several steps:
- Hydrate properly by drinking clear fluids like water or electrolyte solutions, aiming for 16-24 oz per hour for the first few hours post-race
- Eat easily digestible foods, starting with small portions of bland, low-fiber foods like bananas, rice, toast, or crackers, and gradually introducing more complex foods as the stomach settles 1
- Avoid high-fiber foods, aspirin, and NSAIDs like ibuprofen, which can increase intestinal permeability and worsen symptoms 1
- Practise new nutrition strategies and experiment with the pre-race and race-day nutrition plan to reduce the chances of getting gastrointestinal symptoms 1. It is essential to note that while these measures can help alleviate symptoms, if GI upset persists beyond 48 hours or worsens, athletes should consult a healthcare professional for further evaluation and guidance.
From the Research
Gastrointestinal Upset Causes
- Dehydration and carbohydrate depletion are common causes of fatigue in endurance events, and gastrointestinal (GI) problems can be potentially health-threatening 2
- GI disturbances are a prevalent cause of marathon-related complaints, and can be related to intestinal cell injury, as indicated by increased intestinal fatty acid binding protein (I-FABP) levels 3
- The intake of highly concentrated carbohydrate solutions, hyperosmotic drinks, fibre, fat, and protein can contribute to GI problems 2
- Endotoxaemia has been suggested as an explanation for some GI problems, but recent research has not confirmed this 2
- Dehydration above a certain limit can increase the frequency of GI disorders, possibly due to reduced blood flow to the GI region and disrupted normal secretion/absorption of the digestive tract 4
Risk Factors
- Inadequate sleep can exacerbate GI disturbances in healthy, trained endurance runners 3
- Recent illness, medications, and other factors can promote GI disturbances 3
- High levels of dehydration, as indicated by body weight losses greater than 4% BW, can increase the frequency of GI problems 4
- Running-related musculoskeletal injuries can also contribute to GI symptoms 5
Physiological Responses
- Marathon running can cause changes in biomarkers suggesting pathological events in certain organ systems, including the gastrointestinal system, which can usually normalize within a week 5
- Acute kidney injury and electrolyte imbalances are relatively common amongst marathon finishers, which can also contribute to GI symptoms 5
- Intestinal cell injury, as indicated by increased I-FABP levels, can occur after a marathon, and can be correlated with post-race GI symptom scores 3