Intense physical exercise can disrupt gastrointestinal function, sometimes causing an urgent need to defecate or even faecal incontinence during competition, medical experts say. This reaction, experienced by athletes such as Australian Joanna Wietrzyk during a Hyrox fitness race in Beijing and British runner Paula Radcliffe during the 2005 London Marathon, highlights common digestive challenges linked to prolonged or vigorous workouts.

Physical activity typically benefits digestion by stimulating peristalsis, the coordinated muscular contractions that propel food through the intestinal tract, said Gabriel Cher, head of gastroenterology at Khoo Teck Puat Hospital (KTPH). Exercise also improves blood circulation and supports a healthy gut microbiome, contributing to more regular bowel movements. However, when exercise intensity moves from moderate to high, particularly over extended periods exceeding 90 minutes, blood flow prioritizes working muscles at the expense of digestive organs, leading to reduced digestive activity and gut disturbances.

High-impact exercises such as running are more strongly associated with gastrointestinal symptoms than lower-impact activities such as cycling or swimming. Melvin Look, a gastrointestinal surgeon at PanAsia Surgery, explained that the repetitive mechanical jarring from running accelerates bowel movements. Psychological and physical stress from competitive or interval training can further exacerbate symptoms.

Reduced blood supply to the gut may also cause a shift of water and electrolytes into the intestines, precipitating cramps, nausea, and urgent, loose stools. This can be worsened by the types of foods and sports nutrition consumed during exercise, including concentrated sugars and high-fructose drinks, which draw more water into the digestive tract. Cher noted that heavy meals before intense exercise, such as the pasta and salmon dish Radcliffe ate pre-marathon, can provoke bloating, reflux, discomfort, and nausea.

Continence during exercise depends on the strength and endurance of the pelvic floor and anal sphincter muscles, which maintain control over bowel movements. Fatigue, illness, and muscle weakness—due to ageing, childbirth, or chronic coughing—can impair this control, increasing the risk of involuntary soiling, especially during events like Hyrox that combine running with weightlifting and elevate abdominal pressure.

Experts recommend that athletes “train the gut” by practicing pre-competition nutrition and hydration strategies during training, avoiding new foods or supplements on race day, and allowing time for digestion. Consuming pre-race meals three to four hours before competition and performing a light jog or stretch 45 minutes prior may encourage a timely bowel movement. Stress reduction techniques can also help, as anxiety contributes to gastrointestinal upset.

While exercise-induced digestive issues are common and usually benign, doctors advise consultation if symptoms include blood in the stool, severe or persistent diarrhoea, unexplained weight loss, significant abdominal pain, or ongoing nausea. Mark Lim of SingHealth Polyclinics stressed the importance of aligning exercise type and intensity with individual digestive tolerance and cautioned that serious endurance athletes should dedicate as much attention to gut training as they do to physical conditioning.