Also, Barclay and Tumberg (1988) reported that mild (heart rate = 103 beats per minute) exercise significantly reduced water and electrolyte absorption compared with rest; however, the solution they used contained no carbohydrate, which reduced the rate of intestinal absorption to 2 ml per hour per cm compared with a value of 13 to 15 ml per hour per cm for a carbohydrate-electrolyte solution (Gisolfi et al., 1991).
In contrast to these observations, Fordtran and Saltin (1967) found no effect of exercise (74 percent ) on either active or passive absorption using the more direct technique of segmental perfusion. Moreover, this author's most recent results (Gisolfi et al., 1991), also using the segmental perfusion technique, show no effect of either exercise intensity (30 to 70 percent ) or duration (60 to 90 minutes) on fluid absorption (see Figure 4-2 on following page).
Although there is much to be learned about GI function during exercise-heat stress, the following suggestions are offered to help prevent or manage GI distress under such conditions:
GI symptoms, GI bleeding, and endotoxemia seem to be related to exercise intensity, exercise duration, high thermal stress, and sharp increments in training. They also seem to occur among individuals who are poorly trained and who engage in endurance exercise. Thus, it would be prudent to be well conditioned and heat acclimated if thermal stress is anticipated. Also, sharp increments in physical work performed in the heat should be avoided.
Nonsteroidal antiinflammatory drugs have been known to produce upper GI lesions and should be avoided 12 to 24 hours prior to hard exercise in the heat. Aspirin has a potent and long lasting antiplatelet action and should be avoided for 2 or 3 clays prior to severe exercise in the heat. Aspirin is often taken 30 minutes before exercise by individuals with joint pain. If taken immediately before exercise, aspirin can produce marked cramping and related GI discomfort. High doses of ascorbic acid (vitamin C), which are sometimes taken by athletes, can produce diarrhea and should be avoided.
Prefeeding an elemental semihydrolyzed diet might reduce the incidence and severity of intestinal discomfort in endurance athletes. If gastrids or upper GI ulceration is the source of GI symptoms, therapy with antacids or H2 blockers may provide relief and allow soldiers to function normally.
When GI symptoms do occur as a result of exercise per heat stress, they usually abate quickly (within days) with rest.