Objective: Total body water can be calculated using stable isotopes by
two alternative approaches known as the plateau method and the back e
xtrapolation method. These methods treat the isotopic data in differen
t ways and consequently do not necessarily produce identical values fo
r total body water. Total body water was therefore calculated using bo
th methods using two different tracers, H-2 and O-18. Design: A cohort
of infants was studied at three ages in the first year of life, and t
he two methods and two tracers compared at each age. Results: The plat
eau method was found to produce values for total body water greater th
an those obtained by the back extrapolation method at all three ages u
sing both isotopes (P < 0.01 in all cases). Conclusions: This differen
ce is due to the different time points at which total body water is ca
lculated. Back extrapolation values correspond more closely to values
of total body water obtained by alternative techniques.