Gastrointestinal (GI) distress during or after exercise is a very common problem. Athletes often evaluate what they’ve eaten before or during exercise to resolve their GI distress, but they overlook another important factor: hydration.
To understand how hydration plays a role in GI distress, one needs to look at the effects of dehydration on blood volume. Dehydration leads to hypovolemia, a decreased volume of circulating blood, which causes a lowering of blood pressure. As a result of this effect, even mild dehydration can cause upset stomach and nausea. Severe dehydration can sometimes cause vomiting, which leads to more water loss.
Another result of dehydration is impaired digestion, which is already compromised during exercise as blood is diverted from the digestive tract to the skin and muscles to cool the body and provide oxygen for the working muscles, respectively. With less blood flow to the digestive tract, gastric emptying is delayed. Hypovolemia as a result of dehydration compounds this and starts a vicious cycle: once someone is dehydrated, any fluid they drink will sit in the stomach and small intestines, not getting absorbed, and often causing more discomfort.
Dehydration can also increase damage to the intestinal lining. Vigorous exercise, particularly running, causes micro damage to the intestinal tissues, making the intestinal walls more permeable. The lack of oxygenated blood to the digestive tract tissue exacerbates this damage. This can cause GI distress lasting after exercise, such as diarrhea. Remember: the more dehydrated an athlete is, the more their blood volume is reduced, and therefore the more damage to the intestinal lining.
Tips for Hydration
To understand how best to hydrate, consider how our bodies balance the concentration of water in our blood and cells through a process called osmosis. Osmosis is the movement of a solution of lower concentration across a selectively permeable membrane, into an area of higher concentration, to balance the concentrations on both sides.
Water absorption is dependent on absorption of solutes, particularly sodium, glucose, and fructose. Just drinking water alone doesn’t do much to help hydration, but adding a little sodium and sugar to water helps the body retain water and also prevents hyponatremia (dangerously low concentration of sodium).
However, overly concentrated sport drinks, and high carbohydrate drinks can also lead to GI distress and contribute to dehydration. When the highly concentrated fluid gets in the small intestines, water is pulled from the body into the small intestines to dilute the concentrated fluid. This causes bloating and sensations of abdominal cramping. Because more water is pulled into the small intestines, gastric emptying is further delayed, and the fluid isn’t absorbed well into the body.
The key points for hydrating:
· When exercising and sweating, add some sodium, and sugar to water for better absorption.
· Avoid drinks with more than 15-20g of carbohydrates per serving.
· Ensure that you’re hydrated before exercising.
· On particularly hot days, long, or vigorous workouts, pre-load with sodium in your drink to buffer against the effects of dehydration.
There are many more factors to consider in your hydration plan including sweat rate, type of exercise, environmental conditions, age, sex, and menstrual cycle phase. Consult with an expert to create a personalized hydration strategy, and to avoid GI distress during your summer races and workouts.
Lauren Capone is a guest author and owner of Aspen Alley Nutrition.
Address: 745 Poplar Ave, Boulder, CO 80304