. "4. Effects of Excercise and Heat on Gastrointestinal Function." Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press, 1993.
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Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations
PARTICIPANT: This damage is associated with running or is it associated with other types of activity as well?
DR. GISOLFI: It is primarily associated with running, but has been observed in triathletes and elite cyclists.
I think it is important that over at least a 90-minute period of intense exercise, we had one subject reach a core temperature of 40°C and observed no reduction in intestinal absorption of either active or passive solutes nor did we observe any change in fluid absorption.
If you exceed 90 minutes of strenuous exercise, especially if it is performed in a warm environment, I don't know the consequences on the GI tract. There is no data.
PARTICIPANT: Would you care to speculate on the differences between indirect and direct evidence on the deuterated water versus the sampling from the intestine?
DR. GISOLFI: This is a good point. Using the direct method of segmental perfusion you are looking at absorption from just a segment of the intestine. Using the indirect method of D2O accumulation in the blood, you are looking at absorption from the entire intestine.
Some studies have demonstrated that deuterium oxide is taken up by the stomach. How much this contributes to overall absorption is not clear.
When you look at the accumulation of a substance in the blood, you need to know the rate at which the substance is coming into the vascular compartment and the rate at which it is leaving. How is it being distributed to different organs? At what rate is it moving from the vascular compartment into the interstitial fluid compartment and at what rate is it being filtered off by the kidney? Without knowing the dynamics of that situation, it is difficult to say what the accumulation in the vascular compartment really means.