The Effect of Exercise on the Gastrointestinal Tract

Summary

Surveys of athletes, primarily runners, have shown that digestive disorders are common, associated both with training and racing. Women, in particular, seem to suffer most commonly. Nearly half have loose stools and nausea and vomiting occur frequently after hard runs. Diarrhoea, incontinence and rectal bleeding occur with surprising frequency. Runners may use medications prophylactically to minimise some of these symptoms. Upper digestive symptoms seem to occur more commonly in multisport events such as triathlons or enduro. The published literature is difficult to analyse and the basic intestinal physiology not well studied. Most gastroenterologists are accustomed to evaluating the fasting patient at rest and exercise physiologists are seldom experienced with digestive techniques.

Digestive symptoms occurring with exercise referable to the oesophagus include chest pain, gastro-oesophageal reflux symptoms, or symptoms related to alterations in motility. While little is known of the oesophageal physiology during exercise, it is believed that only minimal changes occur in most subjects. Gastro-oesophageal reflux occurs more frequently with exercise than at rest and may produce symptoms of chest pain suggestive of ischaemic disease. Acid exposure may be reduced by pretreatment with histamine H2-receptor antagonists. Oesophageal symptoms, though common, are rarely disabling to the athlete, and the clinical importance lies in confusion with ischaemic disease.

Cases of acute gastric stasis following running have been reported and gastric physiology during exercise, particularly bicycling, has been more actively investigated. Gastric emptying during exercise is subject to a number of factors including calorie count, meal osmolality, meal temperature and exercise conditions. However, it is generally accepted hat light exercise accelerates liquid emptying, vigorous exercise delays solid emptying and has little effect upon liquid emptying until near exhaustion. Gastric acid secretion probably changes little with exercise although some have postulated that ulcer patients may increase secretion with exercise.

Some exercise-associated digestive symptoms, such as diarrhoea and abdominal pain, have been attributed to changes in intestine function. Small bowel transit is delayed by exercise when measured by breath hydrogen oral caecal transit times and motility may be reduced as well. Intestinal absorption during exercise has not been well evaluated but probably changes little in ordinary circumstances. Passive absorption of water, electrolytes and xylose are not affected by submaximal effort. Colonic transit and function is even more difficult to evaluate and published results have been conflicting. However, it is likely that many of the lower digestive complaints of runners such as diarrhoea and lower abdominal cramps are due to direct effects of exercise upon the colon.

Gastrointestinal bleeding is the most dramatic digestive disorder associated with exercise. While ‘runner’s anaemia’ may often represent a pseudonaemia from expanded plasma volume, runners may develop haematemesis or melaena after competitive or training events or present only with symptoms of profound iron deficiency and anemia. Many surveys have demonstrated that approximately 20% of marathon runners will convert to guaiac positivity following the race. While cases of presumed ischaemic colitis occur and anorectal sources of bleeding have been identified, by far the most frequently reported lesion of running associated bleeding has been haemorrhagic gastritis. The aetiology is felt to be ischaemic though other possibilities have not been excluded. The lesion is transient, resolves quickly with rest, and is not recognised if endoscopy is not done within 72 hours of the event. While exercise-associated intestinal bleeding is common, individual cases must be evaluated clinically. Exercise, while of obvious benefit to the general health of many, does not offer assurance against other, more mundane causes of intestinal bleeding.

The study of the digestive tract during the stress of exercise is in its infancy, it is hoped that the awareness of symptoms and clinical difficulties encountered by active subjects will provoke additional study of the GI physiology of the active individual in health and disease.