Total water content of laboratory animals with special reference to volume of fluid within the lumen of the gastrointestinal tract.

I N RECENT YEARS, there has been much progress in the study of body water and its sub-divisions, and yet, little attention has been given to fluid within the lumen of the gut and its possible significance in body water measurements. The very facts that, a) flow of water across the gut-wall is bi-directional (I), b) that labeled water placed within the gut rapidly reaches equilibrium with other body fluids (2), G) that labeled water in jetted intravenously rapidly reaches an equilibrium in gut fluid (3), and d) that feces contain the same concentration of tagged water as other body fluids at equilibrium (4), suggest that fluid in the gut should be considered, unless of course the amount of this water is so small as to be of no significance. Should, however, gut fluid be of significant volume, then some of our present day in vivo measurements are yielding results which are being incorrectly interpreted, because certain measuring agents enter into the gut while others do not. For example, heavy water and sucrose or inulin have been utilized simultaneously to determine total body water and extracellular fluid, the difference between the two being considered as water within cells. Heavy water enters the &t lumen and hence includes in its volume distribution whatever water may be contained within the gut. On the other hand, neither inulin nor sucrose injected intravenously enter the gut fluids (5, 6). Thus, if body water is partitioned by simple subtraction, the result is an underestimated extracellular fluid volume while the intracellular phase is overestimated by the amount of fluid located within the gut. Since no known study has been made of gut fluid in relation to the subject of body water, it was thought appropriate to investi-