Fitbit Aria 2 question regarding the accuracy/preciseness of lean and fat mass percentages.

[jerrrrrrrrbear]

I was wondering how Fitbit's Aria 2 measures the lean and fat mass percentages? I know that a small electrical signal (that one can't feel) goes through the body (and back onto the scale). And because the conductivity of lean muscle and fat are different, the Aria 2 can estimate the lean and fat mass percentages by the time it takes to go through the body and back to the Aria 2 scale (and I presume by also taking into account one's height, sex, age, etc...).

My suspicion is that if it IS an electrical signal going through the body, wouldn't it only take into account only the lower half of one's body (i.e. the legs), due to the fact that the electrical "circuit" of the body (from the feet) would find the quickest way back by skipping the upper half of the body?

If that's the case, then what if a person had more fat concentrated in his belly area compared to his legs? Wouldn't the measurement be kinda off? Say he somehow reduced his belly fat by a lot, but the fat in his legs stayed roughly the same, wouldn't the Aria 2 scale show pretty much the same body fat percentage as before (even though he did lose belly fat)? I know that the Fitbit Aria 2 scale is really meant to show trends rather than accurate/precise measurements, but the fact that it only takes the lower half of the body into account (by, I presume, measuring the time it takes to get from one foot to the other) would mean that if somehow fat in the upper half of the body changed by a lot, it wouldn't even be shown up in the trend.

The above argument takes into account fat, but lean mass is pretty much the same; just with a different calculated electrical signal rate. If that makes any sense.

Hopefully that makes some sense? If you read this far, I appreciate it! I would just want to wrap my head around the accuracy of my scale, and what I should expect.

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I actually found this page online that confirmed my suspicion: https://inbodyusa.com/blogs/inbodyblog/39971073-bia-once-flawed-not-anymore/

But yeah, my question/concern still stands. And I'd like to see what someone with some knowledge of bioelectrical impedance analysis, and in particular the Aria 2's implementation of it (as well as the other Fitbit scales), has to say! Thanks again!

[LZeeW]

Hello @jerrrrrrrrbear and welcome to the Community.  All BIA scales use proprietary algorithms to make their calculations.  Community Council members aren't Fitbit employees, so I have no knowledge of Fitbit's algorithms.  Even if I knew them, I couldn't share them because they are proprietary.  😉

I have an Aria 2, but never owned an Aria.  So I can't even speculate if Fitbit changed the algorithms between the scales.

Your link has a lot of good information about the limitations of BIA.  I think accuracy is a serious issue with all brands, not just Fitbit.  Some users need to follow the trends and not believe the raw values.