Why does hydration status affect the readings in bioelectrical impedance analysis (BIA)?

Hydration status significantly impacts the readings in bioelectrical impedance analysis (BIA) primarily because electrical conduction varies between different tissues. In BIA, an electrical current is passed through the body, and the resistance encountered is influenced by the amount of water present in the tissues. Hydrated tissues, which contain more water, can conduct electrical signals more efficiently than dehydrated tissues. This variability in electrical conductivity due to differences in hydration levels can lead to changes in the impedance measurements, thus affecting the accuracy and reliability of body composition estimations derived from BIA.

For instance, when a person's hydration levels are optimal, the measurements reflect an accurate body composition assessment. However, if an individual is dehydrated, the resistance increases due to the lower water content, leading to an underestimation of lean body mass and an overestimation of body fat. Understanding this relationship helps in interpreting BIA results and emphasizes the importance of standardized hydration conditions when conducting these assessments.