Kind of (well in fact actually) bringing up a somewhat old and dead horse, but found something new on this:
At least one model of Tanita sold as a “bioelectric impedance” fat monitor, the Model 2204 and I would not be surprised if it’s all of them, in fact does NOT derive its bodyfat percentage value principally if even significanly from impedance.
This device is as mentioned above incredibly inaccurate. But not only this, actually it derives its value based on your programmed height and your weight. At most it makes a slight tweak on this from the impedance.
So, if let’s say you’re 200 lb at 5’11", the thing figures you must be 23% bodyfat or so, else you wouldn’t weigh that at that height, right?
That might statistically be true for most of the population.
But to claim to be measuring the bodyfat from impedance and taking people’s money on this basis when in fact it’s principally or almost entirely from height and weight seems to potentially even raise fraud questions.
To see for oneself that the device does this: take a reading, then hold a DB overhead or any other position really, which cannot significantly affect impedance between the feet, and compare the new “bioelectric impedance” fat value. In the case of my Tanita the alleged bodyfat percentage went up 15 points from my holding a 45 lb db, clearly not doing so from impedance change.
On the other hand, changing the value for my height by one foot drops the claimed bodyfat percentage by 14-15 points, or to only about 1/3 the value claimed at true height.
Alternately, supporting 60 lb of my weight with my hands on towels on the countertop resulted in a claimed “bioelectric impedance bodyfat percentage” 17 points lower while I doubt that in fact impedance differed at all.
We’re not talking about the weight tweaking the impedance calculation – if there is an impedance calculation at all – but weight-for-height completely dominating the results to the tune of plus or minus 17 points or more.
So the results clearly are dependent far more on weight and the height value one enters, not on any real electrical measurement.
Last further example: in recent months I’ve dropped at least 30 lb of fat unfortunately gained while regaining slightly more muscle than this. So weight for height has been almost unchanged while for example ab skinfold dropped from 35mm to 4mm, and thigh from an unknown value to 3mm. The Tanita shows no change whatsoever. At least the reason now is clear: because weight for height did not change and that is the type of device that this actually is.
So not only is it wrong in absolute terms, it is completely wrong in tracking terms as well if muscle mass changes.
No wonder the things are so incredibly inaccurate for weight trainers. Added weight for height is figured to be fat even if there is no change in impedance. Their mathematical model assumes a relatively lean person 6’ tall will weigh only about 140, and the impedance measurement – if it’s even a factor at all instead of marketing gimmick – will NOT overcome or significantly go towards correcting this assumption they make.
