Having gastrointestinal (GI) issues (i.e. gas, nausea, diarrhea, etc.) during or after endurance activities is common—studies suggest 30 to 50% of athletes may be affected, with greater prevalence in longer duration events. Current research has boiled down the cause of these stomach problems into three potential explanations: physiological, mechanical, and nutritional. Your cause may be from one, or a combination of these factors.
Physiological: As the duration or intensity of your run increases, the more blood flow is directed towards your working muscles—and away from your gut. Heat and humidity (related to dehydration) and altitude could increase this shift as well. This can lead to slowed digestion and/or impaired absorption of food or fluids, leading to an increased chance of GI distress. Staying hydrated and temporarily reducing your training intensity may help restore blood flow.
Mechanical: The repetitive impact of running, especially if you tend to bounce up and down a lot, could be jostling your gut, leading to increased incidences of gas and the need to go to the bathroom. Poor posture could be creating additional pressure on your gut from above, and swallowing excess air when drinking from cups or water bottles may lead to belching. Having your running mechanics reviewed by a qualified running coach, fitness professional, or physical therapist may be a good option.
Nutritional: The fuel you consume around training can affect your stomach, and is often related to the physiological factors described above. While individual responses vary, GI symptoms have been associated with excess consumption of fiber, fat, protein, fructose (some fruits and agave), highly-concentrated carbohydrate sources (i.e. gels), and dehydration. Additionally, aspirin and non-steroidal anti-inflammatory (NSAID) agents like ibuprofen may increase the risk of GI issues, especially if you take it right before running.
To minimize your risk of getting an upset stomach, choose pre- and post-run foods that you know your body tolerates well, and be sure to drink water with any gels, jelly beans, or chews you consume during your run. If you mix your own sports drinks from a powder, you may want to ease off how much you put in, especially if you add more than the recommended amounts. Keep track of what and when you eat along with your running duration and pace to develop your own personal happy gut fueling strategy.
De Oliviera EP, Burini RC, Jeukendrup A. Gastrointestinal complaints during exercise: prevalence, etiology, and nutritional recommendations. Sports Medicine. 2014;44(1):79-85. Accessed online on 8/30/2015 at: http://link.springer.com/article/10.1007/s40279-014-0153-2/fulltext.html
Jason Machowsky is a board-certified sports dietitian and certified strength and conditioning specialist at the Tisch Sports Medicine Performance Center at Hospital for Special Surgery. He currently serves our nation’s athletes as an active member of the United States Olympic Committee Sports Dietitian Registry. Machowsky received his Master of Science degree in applied physiology and nutrition from Teachers College at Columbia University.