TBWpercentage

TBWpercentage, or total body water percentage (TBW%), is the fraction of a person’s body weight that consists of total body water. Total body water is the sum of intracellular fluid and extracellular fluid, with roughly two-thirds inside cells and one-third outside cells in a healthy adult.

Measurement and estimation methods vary. The gold standard for TBW is isotope dilution, using labeled water (such as deuterium oxide) to determine dilution space. Noninvasive surrogates include bioelectrical impedance analysis (BIA), which estimates TBW from how body tissues conduct electrical current, and other body composition techniques that relate hydration to lean mass. TBW estimates depend on hydration status, recent food and fluid intake, and electrolyte balance, and results can differ between devices and methods.

Typical values and determinants. In healthy adults, TBW% generally ranges from about 45 to 60% of body weight. Men typically have higher TBW% than women, reflecting greater lean mass, while infants have markedly higher TBW% that declines with growth. Obesity lowers TBW% because adipose tissue contains less water than lean tissue. Disease states such as dehydration, edema, or fluid overload can alter TBW% readings and their interpretation.

Applications and interpretation. TBW% is used to assess hydration status, guide fluid therapy, and inform dosing of water-soluble drugs. It also supports estimation of lean body mass and evaluation of nutritional status in clinical and research settings, and is a component of body composition assessment.

Estimation. TBW% can be calculated as (TBW in liters / body weight in kilograms) × 100. TBW in liters can be estimated from anthropometric equations (for example, the Boer or Watson models) that use height, weight, and sex.