When doubly labeled water is given orally, it mixes with body water in about three hours. The two isotopes then leave the body at different rates; 2H leaves as water, mainly in urine, whereas 18O leaves both as water and exhaled CO2. Thus the turnover rate of isotopic hydrogen and oxygen labeled differ, the difference proportional to the rate of CO2 production. Monkeys were dosed (3-5 ml) three days preflight. Urine samples (3 ml) were collected following the dose to determine body water pool size, as well as pre- and postflight. Animals were redosed with 18O postflight to determine any changes in body protein and fat content.

Results:

The data from this study demonstrate the viability of this technique as an inflight measure of metabolism, although accuracy could be enhanced by collecting more urine samples. The total body water (TBW) in the flight study of 2.986 corresponds to 77.76% of the animal's body weight, suggesting that the animal was in a dehydrated state when the measurements were made. The flight energy expenditure value of 266 kcal/monkey/day or 69.3 kcal/kg/day appears physiological. Due to a problematic sample, the postflight TBW could not be determined.