CALCULATION OF TOTAL-BODY WATER IN INFANCY

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.