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Effects of exercise on the digestive tract

Serious athletes often develop gastrointestinal (GI) disorders during training and competition that include cramps, diarrhoea, and nausea. Cramps and diarrhoea are a reflection of over activity of the lower intestinal tract or colon, and are much more common in runners than in cyclists.

Studies have demonstrated a reduced blood flow to the digestive system during vigorous exercise - an 80% reduction after 1 hour cycling at 70% VO2max. Plus there was a direct relationship in that individuals with the most severe symptoms had the greatest decrease in blood flows. The type of exercise also plays a role, and it is speculated that the mechanical trauma (a jostling effect) to the abdominal organs may explain why runners have more symptoms than cyclists. Changes in GI hormone levels have been noted with vigorous exercise, but a cause and effect relationship to symptoms has not been proven. Stress factors are probably more important as a cause of pre competition symptoms such as nausea, vomiting, and diarrhoea.

Heartburn (or oesophageal reflux) is more frequent when exercising within 2 hours of eating. The current feeling is that this increase in reflux is related to a combination of meal effects (especially fats) on the oesophageal sphincter pressure (which prevents reflux of stomach contents into the oesophagus), the increased volume of food and acid in the stomach available to reflux, and the mechanical jostling that occurs.  This is usually a minor problem for cyclists and is best handled by delaying exercise after eating.

Exercise delays stomach emptying, and the more vigorous the exercise, the greater the delay. Running once again appears to have a greater effect than cycling, presumably because of the mechanical jostling of the stomach as well as other abdominal organs. In addition to the increase in oesophageal reflux the delay in stomach emptying can cause a sensation of fullness and nausea as well as limiting the immediate availability of Calories from the food eaten. 

An increase in small and large intestinal activity is the cause of abdominal cramps and is reflected in an increase in the frequency of defecation as well. It has been speculated that there might be changes in digestive hormones associated with exercise that then stimulate the colon. But it is more likely that once again the mechanical factor of jostling the bowel is a more important factor. A fibre rich, pre race meal can also play a role. In a recent post race survey, almost all the triathletes who had eaten a high fibre meal suffered from cramps. Minimizing cramps requires a focus on:

  • avoiding electrolyte imbalance (including dehydration)
  • avoiding riding too soon after eating
  • training at a level closer to your event (the more your event exceeds the maximum levels of your training, the more likely you will develop cramp abdominal pain).

Most of these issues are more problematic for runners (and thus triathletes) than cyclists. Except for competitive cyclists, the effects of exercise on the GI tract are minimal.

  • If heartburn is a problem, timing of the ride to assure an empty stomach needs to considered (and for the competitive rider a 3 to 4 hour fasting period is already the recommendation to minimize a feeling of fullness and nausea).
  • Slow gastric emptying is generally not a problem for a recreational rider, but those with an especially sensitive stomach should plan to eat their last pre ride meal at least 3 to 4 hours before the ride. Small, frequent snacks while on the bike are recommended for rides of greater than 2 hours, and if it is going to be a vigorous workout, avoiding hypertonic sports drinks is recommended.
  • Stay hydrated. If you are dehydrated, the stomach will empty more slowly and there will be an accentuation of the decrease in blood flow to the small intestine.
  • Although some racers will eat a low residue diet for several days before an event to minimize cramps and the "call to stool", this greatly complicates diet planning, and for the rest of us, slowing the pace will usually decrease the urge until a bathroom is located.

So let's review the tips to decrease GI problems:

  • pace yourself - the stomach empties better at <75%VO2max
  • hydrate - dehydration leads to decreased stomach emptying and nausea
  • avoid concentrated (hypertonic) solutions
  • determine which foods work for you on your training rides
  • eat on your training rides - your digestive tract will adapt to eating while exercising

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