If your stomach goes south after taking a mid-run gel, chew or sport drink, this could be the reason why.

shutterstock_146312903 (1)

Do tummy troubles ever get in the way of your run? Pip Taylor knows the feeling. The sports nutritionist and professional triathlete battled GI issues for years—a struggle that prompted her to write The Athlete’s Fix. In her new book, Taylor outlines a three-step program to “identify food intolerances, navigate popular special diets and develop your own  customized clean diet that will support better health and performance.”

A common issue for athletes is fueling during exercise. Sport foods often trigger symptoms of intolerances, because they are consumed when you are most vulnerable—during exercise. You are dehydrated and blood is pulled away from your stomach and intestines, so that it can support working muscles. If these problems sound all too familiar, read this excerpt from Taylor’s book to help you tame your pesky gut.

Q: Why do I have gas, bloating and other GI issues when I eat sports foods and drinks?

When we eat foods containing carbohydrates, a portion is not absorbed or digested in the small intestine and instead passes right on through to the large intestine, where it ferments and produces short-chain fatty acids and gas. This is a normal process that occurs in everyone. In fact, the short-chain fatty acids are an important part of a healthy digestive system, because they provide fuel for gut bacteria and help protect the lining of the intestines.

However, in some people, certain carbohydrates can lead to symptoms such as bloating, gas, distension, abdominal discomfort and either diarrhea or constipation or a mix of both. The types of carbohydrates that are most commonly malabsorbed in the intestine are known as FODMAPs, and fructose is an example of one that’s prevalent in sports foods. Under normal circumstances, fructose is absorbed through the gut wall and transported to the liver for processing. Sometimes a particular protein needed for this normal digestion is missing, and fructose sugars end up in the large intestine instead. Here they ferment, producing gas with bloating, diarrhea, flatulence and the urgency to rush to a bathroom.

Some degree of fructose malabsorption may be present in as much as 30 to 40 percent of the population. When you consider that sports foods and drinks often use fructose as a carbohydrate source, they could potentially be of concern for many athletes and to blame for some instances of GI distress.

Related: Try These Tips To Avoid Race Day Stomach Issues