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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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