Total Body Water (Watson, and paediatric fractions)
Total body water with the intracellular and extracellular split — Watson regression in adults, age-banded fractions in children
Why use it
Body water is the volume of distribution for water-soluble drugs, the V in dialysis adequacy, and the compartment that changes first in dehydration and in fluid overload. The age dependence is not a detail: giving a neonate adult assumptions about hydration misreads both the drug volume and the fluid deficit.
About
Estimates total body water and its division between the intracellular and extracellular compartments. In adults it uses the Watson equations, regressed on age, height and weight against isotope dilution, and still the basis of the volume of distribution used in dialysis adequacy and in the dosing of water-soluble drugs. In children it reports body water as the age-specific fraction of body weight, because water falls from about 75% of weight at term to adult values through childhood, and because that fall is mostly in the extracellular compartment.
When to use
- Estimating the volume of distribution for water-soluble drugs
- Dialysis adequacy calculations that require V
- Assessing hydration and planning fluid replacement, particularly in infants
- Interpreting bioimpedance output, which reports water and infers the rest
- Body-composition work where fat-free mass is derived from water
Formula
Pearls & pitfalls
- The newborn's extra water is extracellular, not intracellular. That is why neonates dehydrate quickly, why their fluid turnover per kilogram is so high, and why the extracellular fraction is the one that falls through infancy.
- Women have a lower water fraction than men at the same weight because they carry more fat, and fat is only about 10% water — the difference is body composition, not physiology of the water itself.
- Watson's female equation has no age term; this is a feature of the original regression, not an omission here.
- Body water is roughly 73% of fat-free mass in a healthy adult, which is the relationship bioimpedance uses to get from water to lean mass.
- In obesity the Watson estimate tends to overestimate, because the extra weight is largely fat, which holds little water.
Next steps
If the clinical question is hydration rather than volume of distribution, the estimate is a starting point and not a substitute for weight change, clinical signs and electrolytes. In an infant, remember that the large extracellular fraction is why a given percentage loss of weight represents a much larger proportional loss of circulating volume than the same figure in an adult.
Evidence
Watson, Watson and Batt (Am J Clin Nutr 1980) derived the equations from deuterium and tritium dilution measurements in 458 men and 265 women, and they remain the most widely used anthropometric estimate, embedded in dialysis adequacy software. The paediatric fractions are the standard values of developmental body-fluid physiology, where total body water falls from roughly 80% in the preterm infant and 75% at term to adult proportions during childhood, with the decline occurring in the extracellular compartment.
References
- Watson PE, Watson ID, Batt RD. Total body water volumes for adult males and females estimated from simple anthropometric measurements. Am J Clin Nutr. 1980;33(1):27-39. — link
- Friis-Hansen B. Body water compartments in children: changes during growth and related changes in body composition. Pediatrics. 1961;28:169-181. — link
- Chumlea WC, Guo SS, Zeller CM, et al. Total body water reference values and prediction equations for adults. Kidney Int. 2001;59(6):2250-2258. — link