Gastrointestinal (GI) symptoms affect as many as 65% of long-distance runners.1 Exercise-related complaints for young athletes can include abdominal pain “stitch in the side,” diarrhea “runner’s trots.” Exercise-related ischemic colitis can lead to GI blood loss in young endurance athletes under extreme conditions. Constipation, gastroesophageal reflux, and gastroenteritis can affect young athletes. Traveler’s diarrhea can affect young athletes that travel for competition. Exercise in can uncover underlying conditions such as irritable bowel syndrome or inflammatory bowel disease.

Exercise is associated with reduced gastric emptying, malabsorption of water and nutrients, delayed transit time, and a decrease in splanchnic blood flow. Although small bowel transit time may be decreased, there is no clear effect on overall gastrointestinal (GI) transit time. Esophageal peristalsis is altered by exercise, but there is no consensus of effect on motility (Table 17-1).

Gastric emptying has been shown to decrease with increasing intensity of activity. Gastric emptying is impeded by exercise intensity of greater than 70% VO2max.2 Consumption of a high-carbohydrate load (>7%) can slow gastric emptying as well.2 Dehydration and hyperthermia will both further impair gastric emptying. Slowed gastric emptying may lead to symptoms such as nausea vomiting, reflux, heartburn, side ache, and chest pain.

A direct mechanical effect of exercise has been suggested. Muscle hypertrophy, for example, of the psoas muscle may alter GI motility directly.3 The role of physical activity on motility is not clear. Only heavy activity may have an impact on motility. No significant change in motility has been found because of transmitted movements with running.3 However, increased intra-abdominal pressures related to exertion of lifting and increased activity of the abdominal muscles can lead to reflux of gastric contents.

Overall, there is no change in small colon transit time or absorption related to exercise. Small bowel transit time and propulsion decline with increasing exercise intensity.3 Exercise may have no effect on large bowel transit time. Therefore, it is likely that exercise has no overall effect on GI motility. One study showed small bowel and colonic transit times were similar in trained and sedentary subjects at the rest and with exercise. The diarrhea seen in this study did not result from accelerated colonic transit.4 Small colon transit time has been shown to increase in some studies, decrease in other studies. No total reduction in absorption in spite of decreased blood flow, is probably because of length of colon. Small colon distension may feedback to stomach to decrease emptying.

Sympathetic tone relaxes colon and decreases blood flow to the GI tract. Hormones such as catecholamines, secretin, glucagon, motilin, gastrin, beta-endorphin, and vasoactive intestinal peptide (VIP) are altered by exercise and play a role in GI motility.3 Decreased splanchnic blood flow is the result of increased sympathetic tone. Visceral blood flow is therefore significantly impacted by exercise. Heavy exercise can decrease blood flow to the point of bowel ischemia. Hyperthermia can further increase the likelihood of GI ischemia. Dehydration can lead to even further stress and ischemia to the gut. Sympathetic tone is increased by hyperthermia, hypovolemia, hypoglycemia, and exhaustion. The GI tract is resilient to a decrease in blood flow of 75% up to 12 hours.2 However, exercise can lead to ischemia, hypoxia, and hyperthermia leading to loss of functional integrity, necrosis, and increased permeability. There have been reported cases of ischemic colitis for triathletes under extreme conditions.

There is an evidence that training can improve gut functioning (Table 17-2).2 The stomach has great ability to increase volume to accommodate larger intake. Gastric emptying can improve with training. There may be a “learning effect” of the stomach where athletes become accustomed to training with fluid in stomach. Endurance athletes have shown enhanced gastric emptying and GI transit time. Splanchnic blood flow improves with training. Further, loose stools often improve with adaptation to training. Rehydration with beverages of higher carbohydrate and electrolyte content will lead to GI symptoms because of delayed gastric emptying.

Endotoxins may play a role in exercise-related GI symptoms. With increased trauma, the mucosal barrier of the colon can be compromised in response to activity. Breakdown of the mucosa may allow entry of the lipopolysaccharide (LPS) from the cell wall of the normal bowel flora. The resulting endotoxemia may either directly or indirectly lead to GI complaints.5

Although scientific evidence for an ergogenic effect of probiotics is lacking, probiotics may provide athletes with secondary health benefits. Enhanced recovery from fatigue, improved immune function, and maintenance of healthy gastrointestinal tract function have been attributed to probiotics. They may also play a role in preventing GI conditions such as traveler’s diarrhea. Improved GI function could positively affect athletic performance.6

Definitions and Epidemiology

Localized pain in the abdomen associated with exercise may be referred to as the “stitch in the side” or exercise-related transient abdominal pain (ETAP). Abdominal pain is common in endurance athletes, especially runners. Surveys have reported that one-third of runners experience the stitch in the side.7 This pain is most common among younger, possibly inexperienced, athletes. Incidence decreases with age. There appears to be no differences in incidence based on gender, but a majority of athletes experience the pain on the right side compared to the left.7

Pathogenesis

There is no direct evidence of the cause of exercise-related transient abdominal pain. Several hypotheses have been proposed. One of the most accepted theories is that hypoperfusion and ischemia of the viscera or diaphragm leads to pain. Direct, frequent jolting of the abdominal contents along with ischemia may lead to diaphragmatic spasm. Motion of the visceral organs during activity could lead to strain of peritoneal ligaments at the site of attachment to the diaphragm, leading to abdominal pain.8 An exertional peritonitis from irritation and friction between the relatively mobile abdominal organs has been hypothesized as a cause for ETAP. Psychologic factors may also play a role.

Clinical Presentation

Affected athletes complain of abdominal pain often localized to the lateral aspect of the midabdomen. This pain may be described as sharp, stabbing, cramping, aching, or pulling. The intensity can vary from intense and severe to a mild annoyance. Symptoms are often worse when a meal is consumed just prior to activity.

Differential Diagnosis

Other conditions to consider include: splanchnic pain, renal colic, rib stress fracture, abdominal wall strain, and intestinal ischemia. Pain that localizes to the left upper quadrant would implicate the spleen as a source of pain. This could include enlargement of the spleen caused by infectious mononucleosis or splanchnic infarct. Acute pain in the flank could indicate renal colic possibly because of nephrolithiasis. Pain localized to the lower ribs at the attachment of muscle could indicate stress fracture of the rib. Stress fracture should be considered especially in rowers. Strain of congenital supernumery ligament of the abdominal wall has been reported in the literature.8

Diagnosis

The diagnosis is based on a detailed history and physical examination. Symptoms of ETAP classically occur only with activity. The physical examination is without findings in the office setting. New complaints that occur during exertion in an experienced, previously asymptomatic young athlete should raise concern. Absence of triggering factors such as trauma or dietary changes could suggest an underlying disorder.

Based on significant findings on physical examination, laboratory studies such as complete blood count, liver function tests, and pancreatic enzymes may assist in diagnosis of an underlying GI disorder. Abdominal ultrasound or CT scan can identify structural abnormalities of the internal organs or masses. Endoscopy may be necessary to identify upper or lower GI tract abnormalities. Rarely, exploratory laparoscopy may be necessary in the case of severe persistent abdominal pain.9 All studies will be negative in athletes with ETAP.

Treatment

Lay literature is replete with anecdotal treatments for ETAP. There are no systematically studied treatments. Young athletes should avoid large meals just prior to activity, especially competitions. Fluid should be consumed regularly, but in small amounts. Techniques such as bending forward while tightening the abdominal muscle, breathing through pursed-lips or stopping activity, and raising the arms over the head have been suggested to relieve the stitch.7 Some authors have recommended switching breathing patterns with running so that inhalation and exhalation occurs while striking alternating feet. A program to strengthen abdominal muscles may also relieve the symptoms on a long-term basis. Symptoms will resolve with discontinuation of activity, but most young athletes prefer to participate.

Diarrhea

Definitions and Epidemiology

Diarrhea is defined as an increase in frequency and/or looser consistency of stools. This disorder of motility is often related to disorders of the lower GI tract. Runner’s diarrhea, or “runner’s trots” is a disorder related to increased motility of the lower GI tract. The prevalence of diarrhea reported among runners is 8% to 60% compared to 40% in control subjects.10 Diarrhea is more common in runners than cyclists, swimmers, skaters, or skiers. The mechanisms of this disorder are not clearly understood.

Pathogenesis

Diarrhea is most often associated with gastroenteritis, which is usually caused by a viral infection. Strenuous exercise may not have a net effect on bowel motility. Meals high in fat, protein, and fiber taken prior to exercise can worsen symptoms. Regional decrease in blood flow may cause ischemia leading to GI bleeding and diarrhea. This may only be of significance for ultraendurance athletes under extreme environmental conditions. Mechanical stimulation of the intestinal mucosa during running may result in a release of vasoactive intestinal peptide (VIP) and prostaglandins, which promote colonic contraction and could cause abdominal cramps and secretory diarrhea.3 Underlying bowel pathology may be uncovered in young athletes.