Bioelectrical Impedance Analysis (BIA)

For those of you who know what to do if you've in a swimming pool and begin to hear thunder nearby, you already know the principle behind bioelectrical impedance analysis (BIA). Water is a good conductor of electricity, and most body water is found in the lean mass. Fat, which has almost no water in it, is such a poor conductor of electricity that it actually impedes the electrical flow. BIA equipment comes in two basic forms. In one form, the subject lies down and the right wrist and right ankle are fitted with electrodes, which produce an electrical current that runs from the wrist to the ankle. In another form, the subject stands on a platform with bare feet, and an electrical current runs from the right foot, up the right leg, down the left leg, and out the left foot. Regardless of the BIA equipment used, the principle behind the technique is the same. If you know the beginning level of energy (electricity) that enters the system and you can measure the level of energy that exits the system, you know how much of the energy has been impeded in the system. Since muscle is an efficient conductor of electricity (because of the water and electrolytes it contains) and fat is an efficient insulator of electricity, the greater the impedance, the greater the level of fat. If you start with 100 units of electricity going into your system, and 80 units of electricity come out of the system, you have more water and muscle than someone who has 100 units going in and 60 units coming out.

Of course, a number of adjustments to the prediction are necessary. The electrical current would run a longer distance in a taller person, so a taller person would automatically have a greater level of impedance. The ratio of weight to height is also important because it helps predict the distance the current is running and the composition of the tissues it is running through. Since body composition commonly changes with age (people become less lean and more fat as they get older), age is also an important predictor of body composition. At the initiation of the adolescent growth spurt, males and females begin to differentiate themselves on body composition, with women having relatively more fat than men do. So gender is also an important consideration in this prediction. Therefore, when performing a BIA, the variables age, height, weight, and gender are included in the equation that predicts body fat percentage.

Although BIA has an excellent theoretical basis for making good body composition predictions, several important protocols must be followed for the results to be accurate and repeatable. Since the technique is dependent on electrical conductivity through the lean mass, the hydration state of the subject can alter the results. If someone having a BIA test is not well hydrated, the electrical current will not be conducted through the lean mass as well, so the subject will appear to have more fat mass than they actually do. Therefore, it is critically important that the person being measured be in a well-hydrated state. It is generally believed that drinking alcohol, exercising, consuming large amounts of coffee, and spending time outside in hot and humid weather within 24 hours of a BIA test lead to sufficient dehydration that the results will not be accurate. Since serious athletes exercise most days, this technique may provide results that indicate more body fat than they really have. Therefore, athletes who are measured with this technique should wait until after a day of rest and should make certain they are well hydrated. An easy hydration check is to see if the urine is clear. The more clear it is, the better hydrated you are.