T Nation

The Poop on Tanita Bodyfat Monitors

There have been a few posts about bodyfat scales measuring incorrectly.I originally saw one and the salesman (not a Company salesman)let me try it out. He told me it was accurate to within plus or minus 0.5%BF. After conducting a little research I came to the conclusion his product knowledge was lacking.
To understand the trueness of measurement I will have to go into a little detail, for those of you not interested in details, go for a protein break.
The method that these scales use to measure BF are by impedance(resistance to current flow).
They measure from where your feet touch the scale so the current path is only through the lower body. I don’t have my notes with me for exact values, but muscle contains more water than fat so provides less impedance to the path of current. The scale uses your height, weight and the impedance to calculate your BF% from a co-efficient constant that they obtained by surveying many people. There is a child, adult and athlete mode that set different co-efficients. To their credit the state that the scale is not designed for pro athletes and body builders (any of these people out there?). Ok…the nitty gritty.

In understanding accuracy you have to look at how it is expressed. They say this monitor is within +/- 5 percentage points of the DEXA method of determining bodyfat(Dual x-ray). Autopsy is the most accurate. How accurate is the Dexa method? I don’t know, within 1 %BF? within 2%BF? Higher?
Let’s say the Dexa method is within 1%BF of the true value (mainly cause I don’t know, I think it would be higher). Now this monitor measures within 5% points of the Dexa method… That is +/- 5%BF not 5% of the Dexa method. Ok… now you have the error from DEXA +/- 1%BF we’ll call this error “A” the stated accuracy of this type of BF monitor is +/- 5%BF from the Dexa value (error “B”) There are several specifics I could go through about these errors, but let’s keep it simple. Remember that the bladder should be empty, you should be hydrated and take off your stockings before measuring yourself Big Guy! For the sake of argument we’ll even forget about the errors that may be introduced here. Let’s just say we have error “A” and “B”… Pretty anal stuff right? Anyone figure out how much total error their scale has so far with just “A” & “B” ?

I recently purchased one of the Tanita body fat scales, the model with the athlete mode. A realistic (estimate from previous caliper-measurements) of my current bodyfat level is about 9%. The scale typically measures me at 6-7%, that’s first thing in the morning, on an empty stomach. One thing I’ve found interesting, I’ve been alternating between a low carb (about 180 grams per day) and a T-dawg (about 50 grams per day) diet. On the weeks when I’m eating more carbs, the bodyfat % DROPS to 4%, while my weight increases(typical, due to retention of water from the carbs). I’m guessing that the excess water conducts better, and results in a lower overall impedence. Makes it kinda tough to guage progress on my ‘low carb’ diet!

I think you’re onto something there Mark.
It is suggested that you don’t measure yourself first thing in the A.M. due to the likelyhood that you will be de-hydrated. I believe they suggest the best time is between 5-6 P.M. and it is a good idea to take measurements at the same time each time.