Background: Most investigations of TBW, ECW and body composition and reports of their intra-body relationships were published prior to 1980. Distributional TBW and ECW relationships within the body have been considered fixed, but there was evidence these relationships were affected by the level of fatness. Body composition models based on past findings and assumptions could produce inaccurate estimates when the majority of the population is overweight to obese.
Methods: TBW and ECW volumes, their proportions of body weight, FFM and percent body fat and associations with age are considered in U.S. children and adults. This review focuses on studies reporting measured body water volumes from large samples except for the national predicted values from NHANES III.
Results: Measured TBW volumes for children and adults are almost exclusively from whites with the exception of the estimated values from NHANES III for non-Hispanic black and Mexican-Americans. Mean adult TBW volumes are as much as 9 liters greater than those reported prior to 1980. Low mean percentages of TBW%WT reflect the greater level of adiposity in children and adults, and this level of adiposity affects the value of TBW% FFM. Mean ECW volumes for white adults are 10 to 12 liters larger than those reported previously. With greater fatness in adults, ECW%TBW has increased to near 60%, and this implies that a calculation of FFM based on 73% and an ECW%TBW of 25-45% could produce an overestimation but more important clinically an underestimation of body fatness.
Conclusion: There is inadequate timely information on measured total and extra-cellular water volumes for the population. Available data indicate a coincident increase in body water with overweight and obesity, and a shifting in the proportion of ECW in TBW. Clinical and pharmacological treatments based upon past assumptions of body water volumes, proportions and relationships could produce inaccurate estimates.