About Bioelectrical Impedance Analysis (BIA)

The use of BIA to predict body composition has had wide acceptance for many years because of its repeatability and simplicity. However, BIA is a statistical tool. It relies on prediction equations that use measured resistance and reactance from a BIA analyzer and other information such as gender, height and weight. The results are estimations that include total body water (TBW), fat free mass (FFM), fat (FAT) and other compartment information. The measured BIA values (resistance and reactance) are constantly changing due to everyday life functions. These changes have little effect on body composition values but are always present.

A one hour BIA test with samples taken every second illustrates a continuous increase of resistance of about 2.5 ohms per hour. This is because body water is being processed by the kidneys and deposited in the bladder. This is less than a one percent change in total body water, but does demonstrate how dynamic BIA is. One hour BIA measurement (pdf) of a 32 year old male who was measured in the morning before showering or any daily activity.

The BIA measurement protocol is very important when creating an accurate body composition report. The first measurement rule is the subject should not have exercised for at least 8 hours before testing. An example of not following this rule is Daily BIA measurements (pdf). The subject was measured daily before or after extreme exercise every other day for 33 days. Body water can change as much a 5 liters or about four percent of weight. This would change all other body composition values including body fat.

More Information

For more information about BIA see the following sections:

• BIA History - The beginning of BIA

• What is BIA - The science of BIA

• BIA Applications - New BIA applications and experiments

• Informative Links - Journals and institutions