RE: Re: Extreme CR ...QOL..tanita accuracy

[Guest guest]

First the good news, the weight function appears to be quite accurate. It measures within .5 lbs (low) of my balance beam scale which is spec'd at 1/4 pound accuracy up to 300lbs.

I wish it was that simple to get an accurate reading of BF. The bio-impedance approach begins with an approximation. Since fat is not very conductive, their premise is that they measure the muscle and reverse that out from the total to get fat. Of course you also have to reverse out the bones but I guess they impute that from height and age.

With a homogeneous enough population you can make some fair estimates of %BF from bio-impedance. The paper that was posted a few weeks ago (Thanks AP) confirmed that the Tanita was statistically predictive for a cohort that mimic'd the group used to create the original "sports" algorithm (athletic college age males).

While I would love to discover that my particular unit is defective the wide measurement swings I experience follow a consistent pattern and correlate with logical sources of error.

For example, the method of measuring impedance between your two feet is going to be significantly influenced by conduction in you legs and pretty much going to ignore your arms and upper body. You mention a 1% worst case 24h variation. Today I experienced 0.8% in about 10 minutes as my % BF increased from 10.2 % to 11% after I took a post run shower which removed the "conductive" sweat from my legs (Is that were the sodium goes?) .

While I would have speculated the dominant error source to be general hydration, my experience suggests a stronger influence from water content of the lower alimentary region. My max BF measurements occur upon waking after a full night of water extraction (yes I'm a nocturnal pee'er). My BF measurements actually drop several points between waking and after running 5 miles in midday heat. I often lose 3+ pounds of water in a 50 minute run but still measure lower fat (more conductive) than I did upon waking (food I ate prior to running?).

My absolute minimum BF measurements occur just before retiring when alimentary and all water stores are maxed out.

I would be willing to live with all these error sources and use the Tanita as a relative "Am I moving in the right direction" marker but I'm further concerned by a post from another user who reported he gained muscle mass but measured higher not lower BF. If we assume that he is correct in his experience, then it's logical that a loss of muscle mass would register as drop in BF. This makes the Tanita useless for even relative measurements.

The fact that several posters measure lower than 0% suggest that the algorithm is not very robust. If it isn't accurate to start with, and can't be trusted to move in the right direction with body changes, what is it good for?

Like I said, it's a reasonably accurate weight scale....

JR

-----Original Message-----From: Hue [mailto:kargo_cult@...]Sent: Thursday, August 29, 2002 2:44 PM Subject: Re: [ ] Re: Extreme CR ...QOL

----- Original Message -----From: "john roberts" <johnhrob@...>> Determining my BF is almost as much of an unknown as my set point. Ibought> a Tanita (don't waste your money).I bought a Tanita, and i am *very* happy with it. In fact, i wouldrecommend it to anyone, even folks who would reject CR. Itwould certainly be worthwhile for more folks to be aware ofwhere they stand, in terms of life danger.I started several months back with ~12% BF, now about 7.5%.This does not seem to vary more than about 0.5% over the day.The biggest change i have seen in a short period is 1% over 24 hours.I believe these variances i see, come from not so much actual BF variation,as variability in test conditions, i mean like skin resistance.My daily measurements range from low of> 8% to high of 17%..Something be wrong here.It surely is ridiculous to> have 0.1% resolution and vary that much.Right- the 0.1 digit is somewhat suspect, but we would probably be lesshappy withthe scale if the lowest resolution was 1% and the machine did the roundingand guessworkon its own.Hue

[Guest guest]

Why would one want to know exact body fat?

One reason is not to go below 5% for males (I forget the female number). But then

the 5% number came from Walford’s book with no evidence to support it, so

IMO there’s nothing magical with the 5% value.

Other than that, if one want to know the

relative change in body-fat (BF), than the cheapest and most accurate way is

skin fold. Get a good caliper, choose a spot on your body which is both easy to

measure and has substantial (relatively) skin-fold, and monitor it. One inch right

of the navel or middle of the upper thigh comes to mind.

Tanita and Muscle. I wouldn’t trust Tanita

to correctly report BF while body composition changes in both muscle and fat. It

just cannot possibly know how to calculate it correctly. However, I doubt the

report that muscle gain caused a higher BF measurement (I hope it wasn’t

me…). Reason being: how does one know that only muscle was gained and not

fat? More than that, it is practically impossible to gain muscle without fat

gain (short of using drugs).

Micky.