Bioelectrical Impedance Analysis (BIA) estimates your body composition by measuring resistance to the flow of a weak electrical current that runs through the water in your muscle and fat tissue. When BIA technology is included in a bathroom scale, the electrodes are located in the foot sensor pads. BIA is safe, but your hydration status influences the accuracy of the analysis. Hydration is affected by the intake of fluids, sodium, alcohol, caffeine, and prescription drugs, as well as by strenuous exercise, illness, and hormonal changes during the menstrual cycle. Since the ratio of fat to muscle changes very slowly, measure your body composition once every six months.