What is Bioelectrical Impedance Analysis?

Bioelectrical impedance analysis (BIA) is a simple, quick way to calculate estimated body fat. In bioelectrical impedance analysis, a very small electric current is passed through the test subject’s body. BIA monitors actually measure the total body water in a person’s body.

The total body water number is used to calculate a person’s fat-free body mass. The lean body mass is then subtracted from the test subject’s weight. The remainder is the total body fat.

The body fat measurement is expressed as a person’s total body mass index (BMI). BMI indicates the ratio of lean muscle to body fat. A BMI of 18.5 to 24.9 is generally considered normal.

If a person’s BMI is under 18.5, that person could be underweight. Generally, a BMI of 25 to 29.9 means that the test subject is overweight. A BMI of 30 or higher typically indicates obesity.

To complete bioelectrical impedance analysis, the impedance of the small current passed through the body is measured. Water conducts electricity and is found in lean body tissue. Body fat, or adipose tissue, contains no water and is a poor conductor of electricity. As a result, lean body mass will have a lower impedance than body fat.

The amount of the current used in BIA is so small that the test subject will not feel it. In the United States, a National Institute of Health conference concluded that the electric current used in BIA is generally safe for most people. The conference statement noted that no testing was completed on people with implanted cardiac defibrillators, better known as pacemakers.

Along with the impedance measurement, other factors about the test subject are needed to calculate BMI. These include physical information such as height, weight, body type, ethnicity, and age. Lifestyle information, such as exercise levels and overall health, is also required.

Several other factors can change the outcome of a bioelectrical impedance analysis. The person’s current level of hydration and skin temperature can alter test results. A test taken lying down can have different results than a test completed with a person standing up.

BIA monitor types include hand-held versions and scales. These portable options have been widely available since the 1980s. Earlier versions of these commercially available BIA models were regarded as unreliable.

Technological improvements allow today’s bioelectrical impedance analysis monitors to provide more accurate results. Portable models are commonly found in health clubs, and can be easily purchased by individuals. More complex models that attach electrodes to the hands and feet are usually found in a medical office, or other professional setting.