h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I had an analysis done yesterday at the gym which involved sticking little pads to my feet and hands. Afterwards, when I had provided my weight and height, it gave me a readout of my %fat, %lean tissue and %water.

Anyone know what these machines actually measure?

I don't know the details, but it's probably measuring 'bioimpedance'. Different tissues in the body affect electrical currents by a different amount, so by passing an electrical current through the body and seeing what happens to it you can estimate the amounts of muscle, fat etc. Apparently it's quite reliable these days - I've known it to be used in clinical trials of growth hormone, for example.

Danny's right, it's measuring resistance.

The most accurate measure involves dunking the person, but this method doesn't require a tank or pool.

How can it give values for the different bits if it is just measuring resistance?

Basically that is what puzzled me - how it did water, fat *and* lean tissue.

Would it have given different percentages if I had said I was much heavier than I am, or much lighter?

Electrical current moves easily through water, but very poorly through fat. (Fat provides greater resistance.) Bodies with greater muscle mass hold more water than bodies with less muscle mass.

The reading obtained provided a measure of the current that passed through your body. This reading gives an estimate of the water in your body, which in turn provides an estimate of the amount of lean tissue in your body. Of course that depends on the operator selecting the correct equation and using the correct values for height and weight.

There are equations for males, females, children, and very athletic people. These equations have been refined over the years using the data from many test subjects.

So the answer is, the method isn't measuring all those other values, it is only measuring resistance. The other values are calculated/estimated, and are considered fairly accurate.

>>> Would it have given different percentages if I had said I was much heavier than I am, or much lighter?

Yes.


HN