Bioelectrical impedance analysis (BIA) measures the impedance or opposition to the flow of an electric current through the body fluids contained mainly in the lean and fat tissue. Impedance is low in lean tissue, where intracellular fluid and electrolytes are primarily contained, but high in fat tissue. Impedance is thus proportional to body water volume (TBW). In practice, a small constant current, typically 800 uA at a fixed frequency, usually 50 kHz, is passed between electrodes spanning the body and the voltage drop between electrodes provides a measure of impedance. Prediction equations, previously generated by correlating impedance measures against an independent estimate of TBW, may be used subsequently to convert a measured impedance to a corresponding estimate of TBW. Lean body mass is then calculated from this estimate using an assumed fluid status fraction for lean tissue. Fat mass is calculated as the difference between body weight and lean body mass.
The impedance of a biological tissue comprises two components, the resistance and the reactance. The conductive characteristics of body fluids provide the resistive component, whereas the cell membranes, acting as imperfect capacitors, contribute a frequency-dependent reactive component. Impedance measurements made over a range of low to high (1 MHz) frequencies therefore allow development of prediction equations relating impedance measures at low frequencies to extracellular fluid volume and at high frequencies to total body fluid volume. This is known as multi-frequency bioelectrical impedance analysis (MFBIA).
One way to assess your weight is to calculate your Body Mass Index (BMI). Your BMI estimates whether you are at a healthy weight but provides no specific information regarding your body composition. BMI is not always an accurate way to determine whether you need to lose weight. Being overweight puts strain on your heart and can lead to serious health problems. These problems include type 2 diabetes, heart disease, high blood pressure, sleep apnea, varicose veins, and other chronic conditions.
|BMI = Weight (kg) / [Height (m)] 2|
|To calculate |
|For example, a person who weighs 56 kg and is 1.63 meters (163 cm) tall has a BMI of 21.1|
|US and Imperial System|
|BMI = Weight (lb) x 703 / [Height (in)]2|
|To calculate |
|For example, a person who weighs 123 pounds and is 64 inches tall has a BMI of 21.1|
BMI is then used to place the individual into one of five general categories:
|19.0 – 25.0|
|25.0 – 29.9|
|30.0 – 39.9|
BMI is not always an accurate way to determine whether you need to lose weight. Here are some exceptions:
Because muscle is more dense than fat, people who are unusually muscular may have a high BMI.
In the elderly it is often better to have a BMI between 25 and 27, rather than under 25. If you are older than 65, for example, a slightly higher BMI may help protect you from osteoporosis.
While an alarming number of children are obese, do not use this BMI calculator for evaluating a child. Talk to your child’s doctor about what an appropriate weight is for his or her age.
Doctors use a few different methods to determine whether you are overweight. Waist circumference, waist-to-hip ratio and body composition using bioimpedance are other means. A lot of other factors influence your health. These include diet, physical activity, smoking, blood pressure, cholesterol levels, and blood sugar levels.
Where BMI provides a general categorization of weight based on height, body composition assessment by bioimpedance provides more detailed information regarding tissue composition (fat versus fat-free mass) and fluid distribution (total body water, intracellular and extracellular fluid levels). In addition, a bioimpedance spectroscopy (BIS) device such as ImpediMed’s SFB7 is a more robust means of assessing body composition in body types where BMI has limitations (body builders, elite athletes, the elderly and in children).