Bioelectrical Impedance Analysis

Bioelectrical impedance analysis (BIA) is a noninvasive method used to estimate body composition by measuring how the body opposes the flow of a small alternating electrical current. This opposition is called electrical impedance. Because electrical current is conducted mainly through body water and dissolved electrolytes, tissues with different water and electrolyte content contribute differently to the measured impedance .^1,2

Impedance has two components: resistance, which represents opposition to current flow, and reactance, which reflects electrical effects associated largely with cell membranes. BIA devices may use a single electrical frequency or multiple frequencies, and measurements can be made across the whole body or individual body segments .^1,3

BIA does not directly measure body fat, muscle mass, or fat-free mass. Instead, it measures electrical properties and uses mathematical models or prediction equations, often together with variables such as height, weight, age, and sex, to estimate quantities such as total body water and fat-free mass. Fat mass may then be derived from these estimates. The accuracy of the estimates depends partly on the assumptions and equations used and on how well they apply to the person being measured .^1,2

BIA measurements can be influenced by factors such as hydration and fluid distribution, body position, electrode placement, recent food or fluid intake, and physical activity. Standardized measurement conditions are therefore important, especially when comparing measurements over time. BIA is best understood as an indirect body-composition assessment method rather than a direct measurement of individual body tissues .^1,3