Measurement Tools — Bioelectrical Impedance


What It Is

bioelectrical impedance
bioelectrical impedance

Bioelectrical impedance measures the resistance of body tissues to the flow of a small, harmless electrical signal.

  • Current flows more easily through the parts of the body that are composed mostly of water (blood, urine and muscle) than it does through bone, fat or air.
  • Bioelectrical impedance measures the strength and speed of the electrical signal sent through the body (impedance measure). It then uses this measurement and information such as height, weight and gender to predict how much body fat a person has.

Equipment Used

  • body fat analyzer for home use
    body fat analyzer
    for home use
    Home Use: The Tanita home use machine resembles a conventional bathroom scale. It is a precision electric scale that has two built-in footpad electrodes (pressure contact.)
  • body fat analyzer for professional use
    body fat analyzer
    for professional use
    Professional Use: Spot electrodes are connected to a bioelectrical impedance machine. The electrodes are placed on the hands and bare feet. There is also a professional model available with foot pad electrodes.

How It's Done

  • Home Use: The person stands in bare feet on the machine. Computer software in the product measures impedance and then uses this and the person's gender, height, fitness level, weight, and, in some cases, age to determine body fat percentage. First weight appears on the machine and then percent body fat appears.
  • Professional Use:
    • Spot Electrode Model. A person lies on a cot and spot electrodes are placed on the hands and bare feet. An electrical signal is introduced and an impedance value is obtained. This is then entered into an equation to predict percent body fat.
    • Foot Pad Model. The person stands in bare feet on the machine. Computer software in the product measures impedance and then uses this and the person's gender, height, fitness level, weight, and, in some cases, age to determine body fat percentage. First weight appears on the machine and then percent body fat appears.

Cautions

  • In both home and professional uses, people need to be adequately hydrated. This is because this method assumes that the body is within normal ranges for its water content.
  • Changes in a person's hydration levels can be due to failure to drink enough water, hormonal changes, food, caffeine or alcohol consumption, strenuous exercise, stress or illness, or taking prescription drugs.
  • If a person is dehydrated, the amount of fat will likely be overestimated.
  • Measuring under consistent conditions (proper hydration and at the same time of day) gives the best results.

Pros

  • It's safe.
  • Some, but not all Bioelectrical Impedance systems, are simple and fast.
  • It can measure long-term changes in body fat.
  • The home measurement does not require a technician so repeated measures over time can be more accurate and thus more comparable to one another.
  • Home measurement is private.

Cons

  • It tends to over-predict body fat in lean and athletic people unless the machine is equipped with an "athlete" mode with appropriate predictive formulas.
  • It does not take into account the location of body fat.
  • People with pacemakers are not candidates for this method.

Illustration courtesy Tanita Corporation of America Inc.

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