Bioelectrical Impedance Analysis (BIA)
What is body fat percentage?
Body fat percentage is the proportion of fat in an individual's body.
Excess body fat has previously been determined by measuring weight against
height, but body fat is not always visible and cannot be measured on an
ordinary scale. Obesity, which indicates a high degree of excess body fat, has
been linked to high blood pressure, heart disease, diabetes, cancer, and other
Body fat percentage measurement
Body fat scales use the Bioelectrical Impedance
Analysis (BIA) technique. This method measures body composition by sending a low,
safe electrical current through the body. The current passes freely through the
fluids contained in muscle tissue but encounters difficulty/resistance when it
passes through fatty tissue. This resistance of the fat tissue to the current is
termed 'bioelectrical impedance' and is accurately measured by body fat
scales. When set against a person's height, gender and weight, the scales can then
compute their body fat percentage.
Calculation of Body Fat %
Lukaski & Bolonchuk's (1988) formula for total body water (TBW) is:
- TBW = 0.372(S²÷R) + 3.05(Sex) + 0.142(W) - 0.069(age)
- S = Height in centimetres
- R = Resistance
- W = Weight in Kg
- Sex Male =1 Female = 0
- Age in years
An athlete is a male, 25 years old, height 170cm, weight 65kg - measured resistance is 382
- TBW = 0.372(170² ÷ 382) + 3.05(1) + 0.142(65) - 0.069(25)
- TBW = 38.7kg
The hydration constant of the fat-free mass (FFM) is 0.73 so we can determine the FFM
- FFM = TBW ÷ 0.73 = 38.7 ÷ 0.73 = 53.01 kg
The Fat Mass = Weight - FFM = 65 - 53.01 = 11.99kg
Body Fat % = Fat Mass ÷ Weight x 100 = 11.99 ÷ 65 x 100 = 18.5%
Body fat percentage fluctuations
Our eating habits, lifestyle and the amount of exercise we
perform all affect our weight and levels of hydration. Since BIA relies on the
fluid levels of 'fat-free mass', such as muscle tissue, certain factors can
alter an individual's body fat reading. For example:
- the reading may be lower than normal if you measure your body
fat just after a bath or exercising
- the reading may be higher than normal if you have just woken up
or eaten a meal
In general, there is less fluctuation in a person's body weight
and hydration levels between the late afternoon and the early evening (two
hours after eating lunch and before the evening meal). The graph above shows the fluctuations of my Body Fat % throughout the day.
However, as everyone's daily routine and eating habits are
different, you should determine the most suitable time to measure your body
Why monitoring body fat is important
Body fat is vital to daily body functions; it cushions the joints
and protects the organs, helps regulate body temperature, stores vitamins and helps the body sustain itself when food is scarce. Everyone needs some body-fat to be active and healthy.
Most people think that body weight, and not body fat, is a direct
indication of fitness. Yet during a diet and exercise regime, while someone's
absolute weight may fluctuate, their body fat will decline in a slow but steady
rate to the desired level.
Using body fat scales to measure changes in both body fat and
weight gives a more dependable picture of fitness.
The use of body fat scales only measures the resistance of the lower part of the body and therefore may not provide an accurate reading of your body fat.
- LUKASKI and BOLONCHUK (1988) Formula for total body water. Aviation Space and Environmental Medicine, 59, p. 1163-1169
The following references provide additional information on this topic:
- KUSHNER, R. F. (1992) Bioelectrical impedance analysis: a review of principles and applications. Journal of the American College of Nutrition, 11 (2), p. 199-209
- KUSHNER, R. F. and SCHOELLER, D. A. (1986) Estimation of total body water by bioelectrical impedance analysis. The American journal of clinical nutrition, 44 (3), p. 417-424
- JANSSEN, I. et al. (2000) Estimation of skeletal muscle mass by bioelectrical impedance analysis. Journal of Applied Physiology, 89 (2), p. 465-471
If you quote information from this page in your work, then the reference for this page is:
- MACKENZIE, B. (2004) Bioelectrical Impedance Analysis (BIA) [WWW] Available from: https://www.brianmac.co.uk/fatbia.htm [Accessed
The following Sports Coach pages provide additional information on this topic: