The Athlete and the GI Tract

by R. Rafoth MD
Sun Mountain Lodge - 1/2004

Gastrointestinal Physiology During Exercise

The etiology of digestive system symptoms with exercise is almost certainly multifactorial. The most two significant factors are felt to be mechanical trauma (esp in running) and changes in blood flow most likely moderated by changes in the autonomic nervous system (sympathetic tone increase). In contrast to the cardiovascular and musculoskeletal systems, the gastrointestinal system does not change or adapt to exercise "loading". Instead, any decrease in symptoms with training is thought to be indirect and related to the fact that the %VO2max for a given level of performance decreases with training and is associated with a decrease in sympathetetic (autonomic nervous system) activity and a decrease in the shift of blood flow away from abdominal viscera to heart and muscles. The result is less physiologic change in digestive system activity. In other words, for a given level of exercise, blood flow to the gut is much less reduced with training. Mechanical trauma to the abdominal viscera, however, remains unchanged.

Direct mechanical effects of exercise on intra abdominal organs are significant factors affecting functioning of gastrointestinal system organs. The effects may be:

A mechanical etiology is supported by the observation that GI symptoms are much more common in the running portion of a triathlon than the swimming or cycling portions. A controlled study in which athletes alternated running and cycling demonstrated symptoms increasing from cycling (the first leg) to running, but then decreasing again when switching back to cycling as the third leg while the intensity (measured by % VO2max) remained unchanged.

Another studies with an abdominal wall accelerometer documented twice the accel/deceleration with running compared to cycling (again compatible with the mechanical hypothesis as significant in the etiology of those symptoms which are more common in runners).

It has been speculated (but unproven) the jarring of small intestine may lead to an increase in the release of small bowel hormones. As levels have been shown (not reproduced) to be higher in runners, this could be related to the increase in symptoms in runners as well.

Next we'll review

Gastrointestinal System Changes with Exercise

1). Autonomic nervous system

In the resting state, digestion is accompanied by high parasympathetic neural activity and low sympathetic activity. During exercise, sympathetic activity is increased. How this relates to change in splanchnic blood flow is unknown (i.e. is there a cause and effect relationship?). We do know that there are lower basal and exercise catecholamine levels for any specific level of performance with the adaptation of training, and this has been proposed as a possible explanation as to why training leads to fewer GI symptoms.

Changes in autonomic nervous system activity are most likely the culprit in the pre exercise stress response. 57% of athletes in one questionnaire based study noted pre competition symptoms, generally cramps and diarrhea, which were similar to symptoms they experienced when emotionally stressed at other times in their lives.

2) Splanchnic Blood Flow

Intestinal blood has been demonstrated to decrease by up to 80% with exercise. Although this was based on a single study (with questionable methodology) other studies support at least a 20 - 50 % decrease. Visceral blood flow decreases with The teleological explanation is that a decrease in visceral blood flow preserves blood flow to the muscles, heart, and brain.

Several studies have demonstrted that splanchnic blood flow is inversely proportional to the intensity of exercise or %VO2max. This would support the concept that for any set level of activity, GI symptoms decrease with training as one functions at a lower %VO2max.

Blood viscosity changes with exercise which is accentuated with dehydration. this is presumed to explain why marathoners with a weight loss of >3.5% of body weight have more symptoms than those with less weight change. Interestingly, when stratified, a weight change of <2 %, 2-3 %, 3 - 3.5% demonstrated no significant difference in symptoms, indicating this may be a threshhold phenomena.

How can a decrease in blood flow cause symptoms? One speculation is that the countercurrent exchange process in the villi leads to a disproportionate anoxia at the tip of the villus with the hypoxia of slow flow. This idea is supported by a histologic picture of ischemic damage.

3) Immune System

While there is suggestive evidence that recreational/leisure exercise may improve immune system functioning, endurance exercise appears to decrease immune resistance. The incidence of URIs increases in direct proportion to training and in the post marathon periods. Although it has been theorized that these changes could impact GI system function (an increase in cramps and diarrhea from a break in the immunological barrier), there is no proof.

4) Motility

Changes in intestinal motility are generally felt to be moderated via an increase in sympathetic nervous system tone with a resultant decrease in GI motility as exercise increases. But other factors may play a role as well:

But even though motility effects can be individually measured with each of the above factors, attempts to correlate them with an overall change in orocecal or total GI transit time have been equivocal.

5) Absorption

One careful study done at 70% VO2 max on a treadmill demonstrated no change in small bowel (where most absorption takes place) water/glucose/electrolyte solution absorption. Other studies have provided mixed results. Realistically, one would speculate that there is so much excess absorptive capacity in the small bowel, there would have to be a significant change in absorption to be clinically significant. But on the other hand, measures of breath hydrogen levels (formed from the bacterial fermentation of unabsorbed carbohydrate that passes into the colon) rise with exercise. The authors of that study speculated that this might be explain a significant proportion of the flatulence and cramps associated with exercise.

6) Secretion

There has been no hard evidence of changes in secretion of digestive enzymes or evidence of abnormal secretion of water.

II. Effects of Exercise on the Esophagus, Stomach, Small Intestine, & Colon

1) The Esophagus (UES, body, & LES)

The lower esophageal sphincter serves as the primary barrier between the stomach and esophagus and a decrease in pressure with exercise has been documented with manometry in healthy subjects. There has been the suggestion that the tendency to reflux is aggravated by:

Mechanical factors (jostling) are definitely part of the pathophysiology. Using 24 hour ambulatory pH monitoring, 12 trained, asymptomatic athletes were evaluated during running, weight lifting, and cycling. Runners showed the most reflux, weight lifters an intermediate amount, while cyclists had the least. All were worse when exercising after eating demonstrating that gastric distention plays a contributing role.

2) The Stomach

Gastric emptying is stimulated at low exercise levels but is definitely prolonged at higher %VO2max - with the cross over point is about 70% VO2max . In addition to fluid osmolarity and fat content, the following can be important factors in the exercise setting:

Vascular changes may have a more direct effect as a hemorrhagic gastritis has been described in association with vigorous exercise.

3) The Small Intestine (duodenum, jejunum, and ileum)

Of the four parts of the GI tract, the small intestine is the least, if at all, clinically affected by exercise. To date, only equivocal effects on small bowel transit have been demonstrated and overall stomach to cecal transit times of 4 to 8 hours remain in the normal range. No definitive effects on absorption or secretion have been proven.

4) The Colon (ascending, transverse, descending, sigmoid, and rectum)

As opposed to the small intestine, the effects of exercise on the colon are marked and the results (diarrhea, urgency, and incontinence) readily evident.

Next section - Exercise Related Gastrointestinal Symptoms

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