Depending on which Japanese conglomerate you believe, either I have the body of a 25-year-old or I'm pushing 70. Which is disconcerting either way, because I was a mess when I was 25, and I'd prefer to let 70 wait its turn.

But according to the statisticians at such companies as Omron and Tanita, my "metabolic age" lies at one of those extremes.

What's "metabolic age"? It is a statistical construct that, along with a growing number of other bells and whistles, is being built into the "body composition" monitors proliferating on store shelves.

There are a good half-dozen or so of these machines on the market now, at places including Target, with prices from about 30 bucks to nearly 300. Typically similar in shape and use to bathroom scales, their main purpose is to measure body fat.

They use a technology called bioelectrical impedance, which passes a small current through conductive foot pads or handheld electrodes (and, in some cases, both). The current can pass easily through water-rich muscle fiber, but it bogs down in fat. Based on a measure of impedance (how much of the current gets through from one electrode to the other), the machines use mathematical models to estimate the amount of fat that got in the way en route.

Why is this a good idea? It is pretty widely acknowledged that people should be less concerned with what they weigh than on whether that weight comes from fat or muscle. ...continued