How can bathroom scales be used to measure body fat?
There are two main approaches to figuring out how much fat you have: one is by calculating your Body Mass Index, or BMI, from your weight and height. This requires nothing more than a regular scale and tape measure.
The other is via a "smart" scale that can do BIA, or Bioelectrical Impedance Analysis.
Let's discuss the BMI method first. Divide your weight in kilograms (that's your weight in pounds divided by 2.2) by the square of your height in meters. For example, if your weight is 100 kg, and your height is 2 meters, then your BMI is 25. That's a bit higher than the normal range of 18.5 to 24.9, so possibly a sign that you could lose a pound or two. Of course, muscle is denser than fat, so if you were very athletic, the higher rating might just reflect your extra measure weight.
The second method - which is more accurate - is to use a "smart" scale that can do a Bioelectrical Impedance Analysis.
A BIA smart scale passes a small alternating electrical current - which is small enough that you do not feel it - through your body, usually using foot and hand contacts as electrical entry points to your body.
Higher frequencies tend to go through fatty tissue more easily than do lower frequencies, while water lets all frequencies through with equal ease. So if as you raise the frequency and more current goes through, it would indicate fatty tissue. But if the increase in frequency does not change the rate of the current, it would indicate water, not fat.
The comparison of currents at different frequencies then gives you a relative measure of fat vs. water.
While both techniques may help you track progress with a diet and exercise regimen, they're no substitute for either proper medical consultation or common sense!
This Dr. Knowledge entry was written by physicist John Swain of Northeastern University. E-mail questions to drknowledge@globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819. 