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How to Measure Your Body Fat

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Several methods of measuring your percentage of body fat are fairly quick, simple, cheap, and safe. None of these is particularly accurate for detecting small changes. Still, unless you live in a very small town, its easy to find a health club, health fair (held at your company or at the mall, for instance), or weight-loss clinic that can perform at least one of these measures on you. These simple, safe measures are:

Bioelectric impedance analysis, in which your fat is measured by voltage differences between your fat and lean tissues (home bioelectric impedance analyzers are starting to become available)

Skin-fold measurement, in which your fat is estimated from the thickness of the fat lying right under your skin

Hydrostatic weighing (Archimedes method), in which your fat is estimated by the water overflow from a tank (just like Hieros crown)

Near-infrared interactance, a controversial method in which fat is detected by shining a light through your arm

Ultrasound, in which fat is mapped with sound waves, just as ships map the sea floor by bouncing sound off it

Near-infrared interactance is thought to be fairly error-prone. Underwater weighing is considered the most accurate of these five methods. Still, none of them are 100% reliable, for at least three reasons:

Except for ultrasound, these methods get at your fat content indirectly. First, something other than fat is measured. Then, that measurement is translated into fat percentage by an equation that assumes you are typical. But you may have larger muscles than average, or you may be dehydrated. Bioelectic impedance analysis, skin-fold measurements, and near-infrared interactance tend to give too low a fat percentage in people who are overweight. And bioelectric impedance gives inaccurate results for people with diabetes unless their glucose is under tight control.

Technique matters. With skin folds, for example, both the calipers and the technician wielding them can affect the result. Ultrasound is hard to perform the same way every time. Also, there are a variety of equations for converting measurements to fat. Different equations do not always give the same answers.

Often, estimation equations were developed with young white subjects. But people of different ages and races sometimes differ biologically. For example, blacks tend to have denser bones than whites, and older people tend not to carry half their fat right under their skin as younger people do. So unless the right conversion equation is used correctly, the result may be off.

There are several ways to get a more accurate measurement of your fat (at least in some parts of your body). But these require costly, awkward tests that are used mostly for research studies on fat. Like ultrasound, these tests are based on imaging methods that is, they involve sending waves of some kind through your body and creating a picture of your fat. Three of these methods that you may hear about are computed tomography, dual-energy X-ray absorptiometry (which is becoming more available), and magnetic resonance imaging.