The most common cause of belly cramps during exercise is having a full colon. The best way to prevent exercise-induced belly cramps is to completely empty your colon before you start to exercise. Belly cramps are rarely caused by having food in your stomach. To meet their energy needs, bicycle racers have to eat lots of food before and during long-distance races and they almost never suffer belly cramps.
Exercise-induced belly cramps are caused by contracting colon muscles that cannot meet their needs for oxygen. Intense exercise increases the frequency of contractions of your colon that move stool, gas and fluids toward the end of the colon (American Journal of Physiology – Gastrointestinal and Liver Physiology, April 1, 1991;260:G646-G652; and Neurogastroenterology & Motility, September 1992;4(3):173–177). Stool in your colon causes colon muscles to contract even more which requires extra blood flow to supply them with oxygen. However, during intense exercise, your skeletal muscles require so much extra blood flow that your heart decreases blood flow to the intestines. The contracting intestinal muscles suffer from lack of oxygen, and hurt.
In one study, 12 athletes (cycling, long-distance running and triathlon) were referred to a gastroenterologist to evaluate debilitating cramps during exercise (Br J Sports Med, October 20, 2011). Sophisticated tests showed that all had decreased blood flow to their intestines during maximal-intensity exercise and 50 percent had it during sub-maximal exercise. Other studies show that during intense exercise, blood flow to the colon can be reduced by more than 80 percent (Alimentary Pharmacology & Therapeutics, March 2012; 35(5):516-28).
How to Prevent Belly Cramps During Exercise
• Empty your colon. To prevent belly cramps, you should empty your colon prior to your most intense workouts and races.
• Eat a high-fiber diet with lots of fruits and vegetables. The extra fiber leaves a residue in your intestines that holds water to keep the contents of the colon soft. This will make it easier for you to empty your colon before exercise.
• Restrict the great constipators – bread and pasta. Foods made from flour are usually low in fiber and bulk and therefore increase your risk for constipation.
• Try to empty your colon after you have had breakfast. Food entering your stomach causes the gastro-colic reflex that causes your colon to contract and push stool toward its opening. Thus for many people, it is easiest to go to the bathroom half an hour after breakfast. If you want to exercise or compete in the morning, get up earlier so you can empty your colon after you eat breakfast.
• Don’t let stool stay in your colon. The longer food remains in your colon, the harder it becomes. After you eat, food passes to your stomach. Then the pyloric sphincter at the end of your stomach closes, and food is not allowed to pass into your intestines until it is turned into a liquid soup. Then your stomach contracts and squeezes the liquid soup into your intestines. The liquid soup does not solidify until it reaches your colon, where large amounts of fluid are absorbed. The longer stool remains in your colon, the more fluid is absorbed and the harder it becomes, making it more difficult to pass. You want to get the soft stool out before it hardens.
• Stay regular. Get in the habit of trying to empty your colon at the same time of day every day.
• Prevent constipation. Limit low-fiber foods such as those made from flour, and high-fat foods such as cheese, eggs, and meats. Eat plenty of vegetables, whole grains, beans, nuts and fruits, the foods that hold extra water in your colon. Dried fruit such as prunes or apricots are particularly effective. All carbohydrates are made up of combinations of sugars, and before they can be absorbed, they must be broken down into single sugars that are almost always absorbed before they reach your colon. Skins of dried fruits contain fiber that your body cannot break down, and sugars are imbedded so deeply in the fiber that they cannot be absorbed in the small intestines. When these unabsorbed sugars reach the colon, bacteria ferment them rapidly and break them into 1) small particles that draw large amounts of fluids into the colon, and 2) gas that dilates the colon and pushes stool toward the opening.
• Drink plenty of fluid. Dehydration increases the rate that fluid is absorbed from your colon to harden your stool.
If you still suffer belly cramps, check with your doctor. Difficulty moving your bowels can be caused by an underactive thyroid gland, a hidden cancer or other intestinal obstruction, or diabetic nerve damage. You may need thyroid tests, a colon exam and tests for diabetes. Most likely these tests will be normal and you will just need to make a greater effort to change your diet.