The doubly labeled water expenditure method for measuring energy expenditure is simple, non-invasive, and highly accurate. If doubly labeled water is given orally, it mixes with the body water in about three hours. The two isotopes then leave the body at different rates. (Labeled hydrogen leaves as water, mainly in the urine, whereas labeled oxygen leaves both as water and exhaled labeled carbon dioxide). Thus the turnover rate of isotopic hydrogen and oxygen labeled water differ, and that difference is proportional to the rate of carbon dioxide production. Urine was used to sample the body water. Energy expenditure was measured for the two flight monkeys over a four day period preflight. Three days preflight the monkeys were redosed with doubly labeled water. The urine was sampled again preflight and immediately postflight.

Results:

In this experiment, the values for mean inflight energy expenditure were significantly less than the preflight values. The approximate 30 percent decrease between the flight subjects and ground controls was found to be statistically significant.