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 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 with whether that weight comes from fat or muscle.
The Mayo Clinic uses the (frightening) term "normal-weight obesity" to capture the issue. The scales may treat you kindly, in other words, but if too much of your weight comes from fat, as opposed to lean tissue, you run some of the same health risks as those who are obese.
For men, the aim is to keep body fat under about 20% of total weight; for women, under about a third, though the numbers vary with age.
Not content with that statistic, however, competing companies are loading their machines with lots of other stuff: estimates of how much muscle you have, how many pounds of bone, hydration levels, the amount of "visceral fat" larded around your vital organs, how many calories you need to eat in a day, and, based on all of the above, how "old" you are.
Which prompts the question: If the age estimate given by two monitors can be so divergent, what about the rest of the stuff the machines are supposed to measure?
To get a sense of the accuracy of retail-grade monitors, at least when it comes to that basic measure of body fat, I gathered five models from three companies and matched them against two clinical methods for taking the same measurement: a hydrostatic "dunk tank" test often used in research and the hand-calipers pinch test often performed in health clinics and gyms.
The two clinical measures, taken by doctoral student Andy Ludlow at the University of Maryland, raised a point that representatives for the monitor companies like to emphasize: that even accepted standards such as the dunk tank, which uses formulas related to the displacement of water and the comparative density of muscle and fat, are still only estimates of body fat. In the case of the monitors, the mathematical models involved have many built-in assumptions -- particularly when it comes to things such as visceral fat -- and if your body strays from that norm, results will be less accurate.
"The only true way to measure body fat is through an autopsy," said Herb Conroy, group marketing manager for Homedics, one of several companies that sell body fat monitors. (Thanks, but I'll pass for now.)
The dunk tank put me at just under 21%, a bit higher than it should be, while the pinch test registered 18%, comfortably within the recommended range.
So should I panic, or have a beer?
That's the kind of anxiety this test can prompt. Company representatives seem to understand and are generally careful to say that their machines should be used more to establish trends than for precise measurements.
The devices, in other words, will give you a rough sense of where you stand but are better used to see whether your body fat percentage is going up, down or staying steady over time. If you're losing weight, for example, and your body fat is creeping higher, that's a sign of unhealthy dieting -- weight loss through dehydration or the destruction of muscle tissue. By contrast, if your body fat readings are going down but the scales are not budging, that's a sign that you're building muscle and getting stronger.
"What we try to tell everybody is: Don't get hung up on 'Am I really 20%?' " said Keith Erickson, director of North and South American sales for Tanita. "It's a tool. It's not an absolute."
From that perspective, the monitors -- two from Omron, two from Tanita and one from Homedics -- came reasonably close to Ludlow's estimates. They ranged from a low of 18% (Tanita's higher-end Ironman Innerscan model, which also gave me credit for a heck of a lot of muscle, and a 25-year-old's metabolism), to a high of 20.9% (Tanita's less expensive model UM061). The two Omron models came up with similar numbers: 20.7% from the more expensive Full Body Sensor (which, alas, also put me near the end of my life expectancy, based on estimates of muscle content and metabolism) and 20.1% from the company's hand-held HBF-306 Body Logic, a model popular among personal trainers and health clubs because of its low cost and small size.
The Homedics 540 Health Station was the outlier, putting me at a whopping 29% fat. Switched to "athlete mode," a feature on many monitors that uses different assumptions about muscle tissue, it registered a lean 12%.
Anyone considering a purchase should keep an important point in mind: They are twitchy, especially when it comes to hydration levels.
Given all the caveats, I wonder how much value the machines really add. If you're attracted to technology and like to quantify things, this is a reasonable purchase, used correctly. But here's an alternative: Contract your abs and grab your belly. Anything in your hands doesn't need to be there.
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How some products compare
Here is how five "body composition" monitors compared. Dunk tank and caliper tests were used for purposes of comparison.
*--* -- -- -- Touch -- Body Monitor Price Statistics points Memory fat* Homedics $60 Weight, body fat, feet 12 people 29% 540 percent water, -- -- percent muscle, bone mass, -- -- daily calorie estimate Omron $60 Body fat, body mass hands 9 people 20.1% Body index Logic Omron $120 Weight, body fat, feet and 4 people 20.7% Full muscle, Body Sensor -- -- daily calorie estimate, hands visceral fat, -- -- "metabolic age," body mass index Tanita $300 Weight, body fat, feet and 4 people 18% Ironman -- -- percent water, muscle, hands daily -- -- calorie estimate, bone mass, -- -- visceral fat, "physique rating," -- -- "metabolic age" Tanita $32 Weight, body fat, feet none 20.9% UM061 percent water *--*
*Compared with readings of 18% and 21% from two clinical tests