Focus on Fitness: GI Symptoms in Athletes and Active Clients
By Kate Evans, MS, RDN
Today’s Dietitian
Vol. 25 No. 6 P. 48

Learn what nutrition strategies will help get them back on track.

Gastrointestinal (GI) symptoms such as abdominal cramping, diarrhea, and nausea often are experienced in individuals while participating in a range of athletic activities such as running, cycling, triathlon and team sports, and other endurance exercise.1,2 Physiological and psychological stress as well as dietary factors can cause symptoms during these physical activities.2-4 In addition, GI disorders, eating disorders, or relative energy deficiency in sport (RED-S) may contribute to GI symptoms.5,6 However, by focusing on hydration and fueling strategies and addressing underlying conditions as needed, dietitians can help their active clients keep GI symptoms at bay.

Common Exercise-Induced GI Symptoms
Other GI symptoms associated with endurance sports may include acid reflux, bloating, and vomiting.1 Up to 90% of distance runners have reported GI disturbances during exercise, most commonly abdominal pain or cramping and urges to defecate.7

Research in team sports is limited, although in a recent survey-based study, 14% to 38% of a sample group composed of primarily soccer, rugby, and American football athletes reported GI symptoms that impacted their performance.2 The most common symptoms were abdominal cramping, uncomfortable stomach fullness, nausea, and bloating—and, notably, women were more likely to report symptoms than men.2 This may be related to hormonal changes during menstruation, slower motility in women, and/or differences in willingness to report discomfort. 2 Identifying a client’s preferred physical activity can help RDs anticipate potential GI symptoms and provide a starting place to develop a client’s individualized fueling plan. Nonetheless, the exact causes of GI symptoms during exercise can be complex and varied.

Causes of Exercise-Induced GI Symptoms

Physiological Triggers
A variety of physiological changes occur during exercise that may contribute to the development of GI symptoms. These include reduced blood flow to the intestines while blood flow to working muscles is prioritized, delayed gastric emptying during high-intensity workouts, and physical jostling of the GI tract during high-impact sports.3

Dietary Factors
RDs will want to discuss preworkout dietary practices with clients, as it’s not only what they eat that should be considered but also when they eat. Inappropriate timing or nutrient composition of preworkout and intraworkout fuel increases the likelihood of GI symptoms. Fat and fiber take longer to empty from the stomach than other nutrients and, therefore, may contribute to discomfort or nausea if consumed too close to a workout.7 Some sports drinks and fueling products contain sugar alcohols or excess fructose, which are poorly absorbed and may contribute to abdominal cramping and diarrhea.3 Furthermore, while fueling and hydrating during endurance  activities is key for optimal performance, individuals who don’t practice intraworkout fueling and hydration are more likely to have symptoms when they introduce it.8 Dehydration is positively associated with the severity of GI symptoms experienced during exercise, so RDs should remind clients about the importance of adequate fluid intake before and during physical activity.1

Inadequate energy intake also can be an underlying cause of GI symptoms. Low energy availability, RED-S, and eating disorders can contribute to changes in GI transit time, leading to symptoms of diarrhea, constipation, acid reflux, or nausea.5,6 RED-S is a condition in which there’s inadequate energy intake to support activity levels, leading to impaired function of multiple organ systems.5 This includes microbiome changes that negatively impact gut health, delayed stomach emptying, and increased intestinal transit time.5 Treatment involves restoring energy balance by increasing energy intake, decreasing activity level, or both.5

Stress and Anxiety
Anxiety levels have been associated with upper and lower GI symptoms during exercise, particularly on competition days, in both team sports and endurance athletes.2,4 Stress and anxiety can impact motility and cause heightened sensitization in the intestines by impacting the brain-gut axis, the communication system between the emotional and cognitive centers of the brain and the intestines.9

GI Disorders
Underlying GI disorders such as irritable bowel syndrome (IBS), which affects 10% to 15% of Americans, also may cause GI symptoms during exercise.10 While research has shown that walking is beneficial in constipation-predominant IBS (due to its association with increased GI motility) and yoga is linked to improvements in IBS (possibly due to its role in reducing stress and anxiety), some forms of exercise may exacerbate IBS symptoms.11,12 gastric motility, autonomic and somatic symptom scores, and physical functioning High-intensity exercise such as running and interval training may put additional stress on an already sensitive GI tract in those with IBS or other GI disorders. Therefore, appropriate management of GI disorders is essential in avoiding symptoms during exercise. Patients with progressively worsening symptoms or alarming symptoms such as rectal bleeding, anemia, unintentional weight loss, nocturnal bowel movements, or family history of inflammatory bowel disease, celiac disease, or colorectal cancer should be referred to a gastroenterologist for further workup.

Strategies for Counseling Clients

Screen for RED-S and Eating Disorders
Since RED-S and eating disorders can impact GI function, screening clients with GI complaints for these conditions and making referrals to specialists as needed is key.5,6 The SCOFF Questionnaire and the 26-Item Eating Attitudes Test are validated screening tools for eating disorders,13,14 and the RED-S Clinical Assessment Tool can be used to identify the signs and symptoms of RED-S.15

There’s also an increased prevalence of GI symptoms in eating disorders. A high frequency of constipation has been noted in patients with anorexia nervosa, while a high frequency of both constipation and diarrhea has been noted in patients with bulimia nervosa and binge eating disorder. 6 The mechanisms for these changes in intestinal transit haven’t been fully elucidated but may be related to erratic eating patterns, laxative abuse, changes in hormone secretion, and microbiome changes. Delayed stomach emptying has been noted in both anorexia nervosa and bulimia nervosa, which may contribute to symptoms of nausea and bloating.6

Provide Preworkout Snack Tips
Advise clients to choose lower fiber carbohydrates before a workout. They’re readily absorbed, will provide more immediate energy for muscles, and are less likely to contribute to GI symptoms. For preworkout snacks consumed 15 to 45 minutes before a workout, choosing carbohydrates only such as fruit, pretzels, or toast with jam will be best tolerated. For preworkout snacks consumed one to two hours before a workout, advise clients to pair carbohydrates with a moderate amount of protein or fat to slow absorption and provide longer-lasting energy, such as toast with peanut butter, yogurt with fruit, or a protein bar. Tolerance and timing needs will vary depending on the client.

Ensure Adequate Hydration
Given that the severity of GI symptoms during exercise increases with dehydration, fluid intake is a critical topic of discussion with clients.1 Fluid needs are highly individualized, but most athletes need approximately 5 to 10 mL of fluids per kg of body weight within two to four hours before exercise and 400 to 800 mL of fluids per hour during exercise to maintain adequate hydration.16

Integrate Intraworkout Fuel
The Academy of Nutrition and Dietetics recommends 30 to 60 g of simple carbohydrate per hour of endurance activities such as running and cycling.16 This may come from sports fueling products such as gels, gummies, and electrolyte drinks. Evidence suggests that practicing intraworkout fueling can improve tolerance, highlighting the importance of incorporating fueling strategies into clients’ training plans.17 Advise clients to experiment with a variety of sports fueling products on longer duration training days (>1 hour) to see what they tolerate best and then increase quantity gradually. For example, have clients start with one sports gummy with water or 4 oz of a sports drink per hour, which will provide 7 to 10 g carbohydrate. Then, increase carbohydrate intake in 7 to 10 g increments until they’re in the 30 to 60 g per hour range and feel energy levels are optimal. It’s important to build up gradually to ensure tolerance and give the GI tract time to adjust to the demands of absorbing carbohydrate during exercise.

Offer Mental Health Resources
Managing anxiety is another key element in preventing GI symptoms during physical activity, particularly on competition days. Diaphragmatic breathing exercises before competitions can help activate the parasympathetic nervous system, potentially reducing the severity of stress and GI symptoms.18 To perform diaphragmatic breathing, also known as “belly breathing,” instruct clients to inhale deeply through the nose for four seconds while allowing the lower abdomen to rise, then exhale gradually through the mouth for six seconds, and repeat for at least five minutes. It’s also important to reinforce the need for adequate fueling even if stress or anxiety is suppressing appetite. Consider referring clients to therapists or sports psychologists if more extensive mental health support is needed.

Final Thoughts
Helping clients address GI symptoms they experience during exercise can improve sports performance and quality of life, whether they’re professional athletes or active individuals. When counseling these clients, consider the role that hydration, meal and snack timing, nutrition adequacy, and mental health plays in supporting GI function and create an individualized fueling plan to meet their nutrition needs without exacerbating GI discomfort.

— Kate Evans, MS, RDN, is a clinical dietitian at the UCLA Vatche & Tamar Manoukian Division of Digestive Diseases and a consultant dietitian at Kelly Jones Nutrition, a performance nutrition private practice that supports athletes at every level.


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