The use of a deuterium tracer technique to follow the fate of fluids ingested by human subjects: Effects of drink volume and tracer concentration and content

Deuterium oxide (²H₂O) has been added to drinks as a tracer for water to estimate the availability to the body water pool of ingested fluids, but doubts have been raised as to the reliability of the method. The present investigation evaluated the effects of systematic variations in the volume of fluid consumed and the amount and concentration of added tracer on the rate of accumulation of tracer in arterialized blood after ingestion of a labelled drink. Three separate experiments were undertaken. In expt 1, six healthy men ingested on separate occasions 200, 400 and 800 ml of a dilute glucose-electrolyte solution: all test drinks contained the same concentration (40 g l^-1) of ²H₂O. In expt 2, six healthy men ingested 200, 400 and 800 ml of the same glucose-electrolyte drink: each drink contained 8 g of ²H₂O so that the concentration, but not the amount, of ²H₂O differed between treatments. In expt 3, six healthy men ingested 300 ml of the same drink on three separate occasions: each drink contained 8, 16 or 32 g of tracer so that amount and concentration of ²H₂O both varied. Arterialized venous blood samples were collected for the determination of deuterium (²H) concentration before ingestion of the test drink and at intervals for 120 min after ingestion. All trials for each of the experiments were conducted in the morning after an overnight fast and trials were in randomized order and separated by 7 days. In expt 1, the blood ²H concentration at all time points from 2 min after ingestion of the test drink onwards was higher for the drink containing 32 g ²H₂O than for the drink containing 16 g ²H₂O, which in turn was higher than after ingestion of the drink containing 8 g of ²H₂O. In expt 2, no significant differences between treatments were observed at any time. In expt 3, the rate of ²H accumulation was greater after ingestion of the drink containing 32 g of ²H₂O than after either of the other two drinks, and the ²H accumulation rate was greater after ingestion of the drink containing 16 g of ²H₂O than after the drink containing 8 g of ²H₂O. When data from all three experiments were combined, significant correlations were observed between the rate of accumulation of ²H in the circulation (p.p.m. min^-1) and the amount (r(s) = 0.75, P < 0.001) and concentration (r(s) = 0.69, P < 0.001) of ²H₂O in the test drink, but there was no relationship (r(s) = 0.09), P = 0.5) between the rate of ²H accumulation in the blood and the volume of the drink consumed. The results suggest that the rate of tracer accumulation in the blood after ingestion of different volumes of test drinks is not a reliable indication of the availability of the ingested fluid, but that the method gives at least a qualitative measure of the sum of the effects of gastric emptying and intestinal water absorption.