Underwater Weighing: If you jump into a very big graduated cylinder filled with
a known amount of water and then breathe out as hard as you can and we
carefully measure your underwater weight and also measure the amount of water
that your body displaces, then we can, by simple calculation determine your
body's density. Knowing that fat is less dense than water, we can, from your
body density arrive at an estimate of how much fat you carry. Basically, the
lighter you are underwater, the fatter you are (fat floats).

Air Displacement Plethysmography: If we measure your weight and the
amount of air your body displaces in a small tightly closed container, we can
calculate your density and infer your body fat in the same manner as above.

Bioelectric Impedance: Salty water found in lean body tissue carries electric
current very well. Fat does not. With a lot of assumptions and calculations, we
can use a person's body's resistance to alternating current (Impedance) to
estimate body fat percentage. This is the basis for the sort of expensive scales
that are sold in boutique stores and claim to measure body fat. The problem is
that the error is extremely high. In my opinion, bioelectric impedance isn't worth
the trouble or the expense.

Skinfold thickness: Most excess body fat is just under the skin. If we pull at
your skin with a known amount of force at various locations on your body and
then use a calipers (thickness gauge) to measure your folded skin thickness, we
can also estimate your body fat, and we can do so generally much better than
with bioelectric impedance and slightly less well than with displacement.

Infrared Transmission: If we shine infrared light through your fat and measure
how much of what 'color' gets through the other side, we can, based on the way
fat absorbs light differently than lean tissue, again estimate body composition.

Low dose X-Ray: Here we can come a bit closer to actually seeing how much
fat a body holds.

Whole body CT or MRI scan: This is as close to an autopsy as we can get on a
living person. Here we use modern medical machinery to literally take sequential
sliced pictures of the inside of the whole body. Looking at each slice and the
way tissue displayed within reacts to the scan, we can come very close to
directly measuring fat content. These techniques are the very best we have,
but they are extremely expensive and in the case of CT scanning, they expose
people to very large doses of X-rays.

So that is the current state of body composition analysis. Basically any
technique except bioelectric impedance can, when performed correctly, give a
fairly accurate estimate of a person's body composition.
How much fat do you really have in your body?

How much muscle or bone?

The truth is that we can only make estimates but we can never know for sure. The reason for this
uncertainty lies with the fact that we can't remove all the fat from your body to weigh it. Even at
autopsy, a very skilled pathologist would need days to carefully tease every shred of fat off a
cadaver. So every "measurement" of body composition that we try to perform on living people isn't
really a direct measurement at all. At best, body composition analysis involves measuring
parameters from which we believe we can deduce or calculate a reasonable estimate of body
composition. Without the ability to compare these techniques against a mound of flesh stripped
off the body, we can never know how accurate our deductions actually are.

Nevertheless, a number of techniques exist to approximate body composition and these
procedures, done correctly at least have the virtue of being reproducible from time to time, person
to person and technician to technician
Human Body Composition
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