fat you have. This is because fat impedes, or blocks, the signal. The signal
travels quickly through muscle because muscle is 70 percent water and water
conducts electricity. Fat, on the other hand, is just 5 to 13 percent water.Similar technology is used in body-fat scales and handheld gadgets that
resemble a car steering wheel and are even less accurate than BIA.Bioelectrical impedance can have a huge margin of error, especially if you’re
extremely fat or extremely lean. In one study, world-class female distance
runners were found to average 20 percent body fat, when more reliable meth-
ods actually show that they were closer to 10 percent. Dehydration also can
skew the results wildly; the signal slows down, and you appear to have more
fat than you really do. Don’t drink alcohol or caffeine for at least 24 hours
before the test because they can lead to dehydration.Getting dunked (underwater weighing) ...........................................
Underwater weighing is the most cumbersome method of body-fat testing,
but it’s also the most accurate method that’s anywhere near affordable. You
sit on a scale in a tank of warm water about the size of a Jacuzzi. (When
Suzanne did this, she felt like a giant piece of tortellini floating in a big pot.)
Then comes the unnerving part: You blow all the air out of your lungs and
bend forward until you’re completely submerged. If there’s air trapped in
your lungs, you score fatter than you really are. Knowing this fact makes you
try really, really hard to blow out your air, which makes you feel like you’re
about to explode. You stay submerged for about five seconds while your
underwater weight registers on a digital scale. The result is then plugged into
a mathematical equation.This method of testing is based on the premise that muscle sinks and fat
floats. The more fat you have, the more your body wants to float when
dunked under water. The denser you are, the more you sink, and the more
water your body displaces.The margin of error for this test is 2 to 2.5 percent for young to middle-aged
adults. The results are less accurate for children, older adults, and extremely
lean people. This is because lean body tissue is made up of other things
besides muscle. Bone, for example, isn’t fully formed in children, and it
may be somewhat porous in older adults and somewhat denser in super-fit
people. You can get this test done at sophisticated sports-medicine clinics or
labs for $50 to $100.24 Part I: Getting Your Butt off the Couch