Patterns of bioelectrical impedance vector distribution by body mass index and age: implications for body-composition analysis.

Background: Bioelectrical impedance analysis (BIA) gives resistance (R) and reactance (Xc). R and Xc normalized for body height (H) can be plotted as a bivariate vector (H 2 /Xc versus H 2 /R). Vector BIA is useful for studying the determinants of BIA results. Objective: We investigated the effect of age on BIA results and its relevance to body-composition analysis in a large database of impedance vector distributions stratified by age, sex, and body mass index (BMI). Design: Mean bivariate vector distribution patterns (95% confidence ellipses) were examined in a German population of 15 605 children and adolescents and 213 294 adults. Children and adolescents were divided into 3 age groups with up to 5 BMI categories. In adults, 5 BMI categories were stratified into 7 age groups. Results: Mean impedance vectors were shorter in children than in adults. The vector distribution pattern was influenced by sex, BMI, and age, with shorter vectors in females than in males and longer vectors with increasing BMI. Consistent with a decrease in body cell mass with increasing age, there was a downward slope in the mean vector with age as a result of a decrease in the H 2 /Xc vector component. By contrast, there was no age-dependent increase in the H 2 /R vector component. In women of the same BMI at different ages, H 2 /R and percentage fat mass tended to decrease with age. Conclusions: The lack of an age-dependent increase in the H 2 /R vector component renders conventional BIA unsuitable for an examination of the age-related increase in body fat mass. By contrast, the increase in the H 2 /Xc vector component with advancing age suggests the potential of BIA to depict the age-related decrease in body cell mass.