Digestion is a process which takes place in resting conditions. Exercise is characterised by a shift in blood flow away from the gastrointestinal (GI) tract towards the active muscle and the lungs. Changes in nervous activity, in circulating hormones, peptides and metabolic end products lead to changes in GI motility, blood flow, absorption and secretion. In exhausting endurance events, 30 to 50% of participants may suffer from 1 or more GI symptoms, which have often been interpreted as being a result of maldigestion, malabsorption, changes in small intestinal transit, and improper food and fluid intake. Results of field and laboratory studies show that pre-exercise ingestion of foods rich in dietary fibre, fat and protein, as well as strongly hypertonic drinks, may cause upper GI symptoms such as stomach ache, vomiting and reflux or heartburn. There is no evidence that the ingestion of nonhypertonic drinks during exercise induces GI distress and diarrhoea. In contrast, dehydration because of insufficient fluid replacement has been shown to increase the frequency of GI symptoms. Lower GI symptoms, such as intestinal cramps, diarrhoea--sometimes bloody--and urge to defecate seem to be more related to changes in gut motility and tone, as well as a secretion. These symptoms are to a large extent induced by the degree of decrease in GI blood flow and the secretion of secretory substances such as vasoactive intestinal peptide, secretin and peptide-histidine-methionine. Intensive exercise causes considerable reflux, delays small intestinal transit, reduces absorption and tends to increase colonic transit. The latter may reduce whole gut transit time. The gut is not an athletic organ in the sense that it adapts to increased exercise-induced physiological stress. However, adequate training leads to a less dramatic decrease of GI blood flow at submaximal exercise intensities and is important in the prevention of GI symptoms.