Scales That Calculate Body Fat

Bioelectric Impedance Analysis, or BIA, is the method used by scales that measure body composition. Developed in the 1980s, BIA consists of harmless electrical currents passed through the body; how well the electric current is conducted in the body depends upon the body's water content and the type of tissue the current encounters. Fat is a poor conductor of electricity compared to muscle. Therefore, the impedance, or disruption, of the current is used to calculate the relative amount of body fat.

Accuracy

Due to the multitude of body types, scientists have encountered difficulty developing scales that accurately measure each body. To measure the accuracy of BIA measurements, scientists use techniques that provide accurate measurements of body composition such as skin-fold measurements; hydrostatic testing, an under-water air displacement procedure; and dual-energy x-ray absorptiometry, or DEXA, which calculates the density of body tissues -- typically used for bone density evaluation. The scales are programmed with a mathematical equation to calculate fat percentage based on predicted demographic data such as sex, age, height and weight. Modern BIA scales attempt to address multiple body types by including a custom profile for body type, activity levels, sex, height, age, etc. The various profile settings would have different mathematical equations for calculating the data measured. BIA scales for consumers can have a 1 to 2 percent margin of error.

Diverse Populations

With the aim of validating BIA measurements for use with a large population, Canadian researchers conducted an analysis, published in the September 2008 issue of "Nutrition Journal," on data collected from 235 studies between 2004 and 2006. They found that for large populations, BIA is not accurate. Calibration equations are "population-specific" and do not include specific ethnic groups including South Asians, Middle Easterners or Africans. This is because body characteristics such as fat distribution, bone length and muscle mass vary along different ethnic groups. The researchers assert that BIA for clinical purposes in developed countries, however, is a useful tool.

Scale Comparison

Researchers compared two different models of BIA scales in a study published in the July 2007 issue of the "International Journal of Body Composition Research." The participants included 169 Caucasian, Hispanic and African-American, adolescent girls between the ages of 10 to 15 years. The girls were measured by DEXA scan for a baseline measurement, then by the two BIA devices. The first scale required participants to recline on a table with electrodes attached at four various locations on the body; the second device was a floor scale with hand-held sensors. Results varied across ethnic groups. The table-model BIA device was more accurate for Caucasian and Hispanic girls while the floor scale model was more accurate for African-American girls. Researchers concluded that the type of BIA model, as well as ethnicity, influenced the data collected.

Pros and Cons

BIA scales are inexpensive, easy to use and provide quick, noninvasive measurements. Compared to other measuring types, BIA is more accurate when specific body type profiles are included in the analysis. Scales designed for a particular body type will be more accurate than a scale for a wide range of bodies, such as scales marketed for women or athletes. When using a BIA scale, monitoring changes between consecutive measurements may be more beneficial than the accuracy of an individual measurement. Measuring at the same time of day and wearing little clothing, metal or jewelry will yield the best results from one measurement to the next. Fluid balance can interfere with the accuracy of measurements, stay well hydrated to provide consistent results.