Measuring the exact amount of a person's body fat is not
easy. The most accurate measures are to weigh a person
underwater or in a chamber that uses air displacement to
measure body volume, or to use an X-ray test called Dual
Energy X-ray Absorptiometry, also known as DEXA. These
methods are not practical for the average person, and are
done only in research centers with special equipment.
There are simpler methods to estimate body fat. One is to
measure the thickness of the layer of fat just under the
skin in several parts of the body. Another involves sending
a harmless amount of electricity through a person's body.
Results from these methods, however, can be inaccurate if
done by an inexperienced person or on someone with extreme
obesity.
Because measuring a person's body fat is difficult, health
care professionals often rely on other means to diagnose
obesity. Weight-for-height tables, used for decades, have a
range of acceptable weights for a person of a given height.
One problem with these tables is that there are many
versions, all with different weight ranges. Another problem
is that they do not distinguish between excess fat and
muscle. According to the tables, a very muscular person may
be classified obese when he or she is not. The BMI is less
likely to misidentify a person's appropriate
weight-for-height range.
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