Measurement of Total Carbon Dioxide Production by Means of D2O18

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Abstract
A simple formula was derived by which the total CO2 output of an animal can be calculated from the total body water and the difference between fractional turnover rates of the D and O18 of body water labeled with D2O18. In applying the formula isotopic fractionation factors were taken into account, which involved a very crude estimate of urinary volume. Turnover rates of D and O18 were estimated by initial and final sampling. In a series of 15 mice the calculated CO2 outputs were compared with the observed CO2 outputs. The average absolute discrepancy between calculated and observed CO2 outputs was 7 ([plus or minus] 7)%. The average algebraic discrepancy was -3 ([plus or minus] 10)%. The sources of error, assumptions, and variations of the method are discussed.