A wide range of digestive symptoms have been reported during intense physical activity, and some of the most important include: nausea, vomiting, dry heaves, abdominal pain, diarrhea, and heartburn. Some of those events are present in 20 - 50% of professional athletes, and it has been shown that women are affected more often than man and that runners suffer from these problems more often than other athletes. The symptoms also tend to be more pronounced when exercising with inappropriate intensity and without the necessary level of hydration.
The Connection between Nervous System and Digestive System
Intense physical activity activates the part of our nervous system called Sympathetic Nervous System which helps us to increase the heart rate and breathing capacity during endurance training, and also to enable the appropriate blood supply to the active muscles. In fact, this system prepares the organism for the activities called "Fight or Flight", and it is the same mechanism that activates when a person is scared or attacked. In these situations, the digestive system doesn't play an important role, so it is intentionally left behind without enough blood supply, as all the resources are redirected to the active muscles, brain, and lungs.
This is basically the cause of most digestive symptoms during the endurance training. Digestive organs, especially guts, are very susceptible to the lack of oxygen, so the disturbances in their function easily occur in these circumstances.
Other important effect of sympathetic nervous system is the stimulation of adrenalin secretion, which lowers the thirst, although the water loss is already increased because of sweating. In addition, if the person takes beverages with high sodium content before or after exercises, than the hydration will be seriously impaired, which will also contribute to the digestive disturbances.
In some sports such as running, weight lifting, and cycling, the digestive symptoms might be caused by bouncy movements of internal organs, as well as by increased pressure in the abdominal cavity. Namely, the increased abdominal pressure pushes the content of the stomach towards the esophagus, which can initiate nausea, vomiting, or dry heaves.
Eating Habits Impairment
As many of you know from personal experience, physical activity eliminates the hunger. The mechanism is the same as above stated - inactivation of digestive functions during the training by sympathetic nervous system. Scientists claim that long term and repetitive (>15 days) intense physical activity can lead to low calorie intake because of appetite loss, thus causing hormonal disturbances and consequently the digestive symptoms.
If however your symptoms do not subside, please contact your doctor for further examination.
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