So You Think You Know What Your BF% is Do You?

[Guest guest]

Hi folks:

A newly-opened local drug store is running sessions offering sort-of-

useful free services to try to drum up business. These include a

total cholesterol test, an ultrasound heel bone 'density' scan, and a

BF% measurement, among others.

The latter two are of interest to me, although I also had the

cholesterol test done, which came out about where I had expected. I

turned up for the BF% event recently, just agog waiting to see what

the number would be.

Soon after I arrived and was discussing general health issues with

the RN who adminsters the test, I said that I wanted to drop my

weight another five pounds to get to a BMI of 20·6. She

replied: " You're slim, you certainly don't want to lose any more

weight. " I explained that I had my reasons and she proceded with

the test. It was done with a hand-held bioelectrical impedence

gadget which has two handles. The patient holds one handle of the

gadget in each hand, thumbs up, with arms extended out in front,

horizontally, level with the shoulders.

After entering some data in the gadget, which I believe included age,

height and weight, the digital readout came up with the number. Ta

daaaaaa: BF% = 20·4 !!! " You don't LOOK like a body fat of

20·4% " , she said.

A few months ago the US Navy calculation had been saying my body fat

was 14%. So after returning home from the drug store, I decided to

do a Google search and calculate my BF% using all the methods

available at the sites Google listed.

The readings came in at anywhere between 9·7% and 23·7%. Here is a

listing of the methods used and what they came out at most recently:

US Navy: 17·5%

-Pollock 3-circumference: 11.1%

Biofitness.com: 23·7%

Bio-impedence at drug store: 20·5%

mac.com:

Simple method: 9·7%

Four skinfolds: 18·6%

Csgnetwork.com - three circumferences: 20·5%

Lowcarbdiets.about.com:

A) 15·5%

14·1%

Healthcentral.com: 15·3%

Bmi-calculation.net: 18·14%

NOTES

As for the US Navy method, I notice huge variations in my neck

measurement, measured exactly the same way, from week to week.

Lately I have put on about four pounds of weight according to the

scale, and my abdomen circumference has increased, yet my neck

measurement has *dropped* from 15 " to 14 1/4 " !!! That change alone

makes about 1.5% difference to my US Navy BF% result.

A couple of the sites referenced by Google used skinfold

measurements. Recently a report was posted (author Dr. Jimmy Bell)

at to the effect that different people accumulate body

fat in different places. Some people, according to this report, have

much more visceral fat than others. I have since had that

information confirmed by a radiologist I know, who says he sees this

variation in full body CT scans. So it seems that this large

variation in visceral fat among different individuals renders

skinfold methods unreliable, since they are presumably unable to take

account of the extent of visceral fat?

How does a bioelectric impedence device measure visceral fat when it

is held in both hands, with the electric current presumably running

up one arm, across the body just below the neck, and down the other

arm? Or fat any place, for that matter, except where the current

flows?

It seems to me all these methods rely on an assumption that fat in

one specific location - the one measured - accurately reflects the

amount of fat buildup everywhere. If Dr. Jimmy Bell is right, that

assumption is wrong! In their introductory write-ups, most of the

above sources claim their method is " highly accurate " . I suspect

that none of them are reliable. For that matter is immersion

reliable?

Clearly this matters for people on CRON. If my body fat is really

less than 10% then I have already exceeded my target and am beginning

to approach the 'DANGER - do not go below' level. But if it is over

20% then I would have to lose another ~thirty pounds to get down to

my tentative target of 10%.

So how confident are *you* that you know what your BF% is? I am very

confident that I do not know what mine is.

Sigh.

Rodney.