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Bioelectrical Impedance Analysis (BIA)

What is body fat percentage?

Body fat percentage is the proportion of fat in a 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 disabling conditions.

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 fat 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.

BIA ScalesBIA Scales

Calculation of Body Fat %

Lukaski & Bolonchuk's (1988)[1] 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

Example

Athlete is 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, life-style 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

Body Fat Graph

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 % through out a day.

However, as everyone's daily routine and eating habits are different, you should determine the most suitable time to measure your own body fat.

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, whilst 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.

Limitations

The use of body fat scales only measure the resistance of the lower part of the body and therefore may not provide an accurate reading of your body fat.


References

  1. LUKASKI and BOLONCHUK (1988) Formula for total body water. Aviation Space and Environmental Medicine, 59, p. 1163-1169

Related References

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

Page Reference

The reference for this page is:

  • MACKENZIE, B. (2004) Bioelectrical Impedance Analysis (BIA) [WWW] Available from: http://www.brianmac.co.uk/fatbia.htm [Accessed

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