Running is good for maintaining regular bowel movements. This is one theory on why there is a decreased incidence of colon cancer among those who partake in running. 
BUT, it can also be said that it may be too much of a good thing.
Quite common : An estimated 20-50% of distance runners have “runner’s trots” with a range of symptoms from cramping and nausea to bouts of flatulence and diarrhea. This can occur during or after their exercise.
Why : The reasons are not known for sure, and may be due to factors affecting people differently. One reason considered, is that the up and down motion of running stirs the bowels. The flow of blood to the intestines is diverted to your legs and which may ‘trigger’ the cramping and/or diarrhea. The underlying possibility of irritable bowel disease may be brought to light by the additional stress of the exercise. The added chance of dehydration on long training runs may also cause diarrhea. Lactose intolerance effects enhanced by the exercise.
Symptoms: Cramping, nausea, flatulence, diarrhea during or after exercise. This may produce painful cramping and the compulsory need to defecate.
Prevention :
- Try to avoid eating for at least two hours before you exercise – the presence of food in the stomach will make things worse or contribute to the problem.
- Try to avoid the intake caffeine and warm fluids as it is possible for this to speed up the movement of wastes through the intestines.
- Make sure to limit your intake of dairy products, particularly if susceptible to diarrhea
- Limit high-fiber foods in the days before a long race.
- Avoid the foods that you know produce flatulence or loose stools.
- Drink plenty of fluids. It is best to drink a full 16 oz. of water an hour before your workout, giving the excess fluid time to pass through, and start off well-hydrated.
- Be aware of your bowel habits and try to time your workouts for after such movement times.
- If all other precautions fail, for races or special events where you know toilets will not be readily available, consider the use of an over-the-counter anti-diarrhea product such as Imodium. Studies have shown this has reduced problems related to those prone to exercise-induced diarrhea. It is not recommended to use this on a daily basis.
- Design your training routes to include a restroom. If you develop the urgency while exercising, you will be able to plan your route accordingly.
- Consider a medical check-up for irritable bowel syndrome and be open in discussing your problem with your medical practitioner.
Author: Gavin Doyle
View other articles of interest under the Sports Doctor section:
- When a bug strikes
- Exercise-induced anaphylaxis: ‘Why do I itch so badly?
- A run a day keeps you out of the doctor’s office
- Runner’s Trots
- Bloody urine: Why?
- Human body – Heart and Lungs
- Upper Respiratory Infections
- Exercise and the Immune system
With more topics to be added.
Other Topics of interest :
Food sensitivities can also play a big role. I suffered – and I mean suffered – for almost 10 years with GI distress mid-run and afterward. Through a series of events and an elimination diet I discovered that I’m sensitive to soy. Not allergic, just have a terrible time digesting it, which means my GI is highly irritated anytime it has to deal with soy. Of course, being a health-aware runner, I ate loads of soy, therefore my gut was in a constant state of inflammation and irritation. Running for as little as 30 minutes was enough to set off a nasty (nasty!) chain reaction in my intestines. I stopped consuming all soy and within 2 weeks problem was gone. Miraculous.
Also, in regard to Imodium, if the diarrhea is caused by an irritation such as a food allergy/sensitivity that may not do very much to help since the irritation is always present. I’ve had great luck with cimetidine (Tagamet). It addresses the inflammation and irritation that may be present in your gut before you start running. A stop-it-before-it-starts approach as opposed to make-it-stop-quickly which loperamide (Imodium) operates from.
This is from our flight response. A gazelle will evacuate its bowels when it is running from a lion. 1 to make itself lighter. 2 to keep the lion away. Humans are very simliar.