Bioelectrical Impedance analysis?

[kimberlee]

Does this type of test have a half decent level of accuracy? - googling I keep coming up with definitions..
I had to have one done today and just wondered what kind of ranges the results actually come with - didn't think to ask the kinesiologist at the time.
Cheers,
Kimberly

[Chris_78]

I forget the exact +/- error percentages, but I want to say that it is generally greater than 5% in either direction. Basically, bioelectrical impedance sends a small electrical current through the body and the assumption is that if you have more lean tissue (muscle, bone, etc.), the current will be less impeded. Lean tissue, particular muscle, is composed largely of water. Fat tissue, on the other hand, creates greater impedance to the current.

To begin, there are no body comp techniques that are completely error-free. Even the "gold-standard" of hydrostatic weighing generally has a +/- 2% error and that is largely dependent on having an experienced tester as well as someone who is comfortable under water. DEXA is also a very accepted and accurate way of measuring, but I'm not sure the error percentages on that technique. The only true way to know your body composition is to kill you, grind you up, and separate the fat tissue from the fat-free tissue. Of course, that won't do you much good after you're dead :)

There are several other potential sources of error with bio-impedance...

1) If you aren't well-hydrated, that can definitely affect the results...

2) All of these techniques are dependent upon statistical equations that have been derived from cadaver research...so, the results can be skewed if the wrong regression equation is used or dependent upon which regression equation is utilized in the particular automatic measurement instrument that you are using.

3) This method sort of assumes that the human body is a cylinder and we all know that it is not. We have arms, legs, a trunk, etc. So, individual structure can have an impact on the conductivity and the end results.

There are some other more technical influences, but this should give you some type of idea as to the accuracy of this technique. It definitely has its limitations.

[Espi]

The main thing to remember is that a BIA uses conductivity of the skin as its feedback. Apparently there's a good correlation between body fat and conductivity, as long as others factors don't change.

As a rule of thumb a BIA that goes through your legs (Tanita for instance) will be more accurate for determining that part of the body while a hand-held one will be more accurate for your torso.

When you take into account that there will be errors in measurements, I've always found them to be a good tool to measure progress when using a 30 day average. For women, this also erases the fluctuation due to hormones within a cycle.

Since you are only being measured once, I'd not put too much faith in that single number .. it can be off quite a bit.

[char-dawg]

A lot depends on the type of machine that was used. There's one here that I think is actually pretty damn accurate... It consistently gives numbers that match pretty well what I think should be the case just "eyeballing" someone's physique. You stand on a platform and have current running through your legs for the lower body and at the same time are holding a handle in each hand for the upper body. I've had it done several times and always gotten a pretty clear result. It's good enough that it picks up the difference between my right and left legs (I'm pretty right-dominant), which nothing in the past ever did.

That said, you can affect the results in various ways, drinking a lot of water being the main one. So if you're going to go back and have another test done in the future, be sure to try to get it at the same time of day, after the same number of meals, in the same "exercise state" (ie, did you just work out?) and so on.