*Body Composition: Hydrostatic (Underwater) Weighing*

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Underwater (hydrostatic) weighing, based on
Archimedes' Principle, is generally regarded as the "gold standard"
for body composition assessment, although this claim is being made less of late
because of newer and more sophisticated procedures. Because body fat is less
dense than water, it increases one's buoyancy while the fat-free mass, which
has a density greater than water, makes one sink. After correcting for residual
volume–which increases buoyancy and decreases the underwater weight–percent fat
can be calculated based on the underwater weight. The largest source of error
in underwater weighing is thought to be the determination of residual volume
(RV; the amount of air remaining in the lungs following maximal expiration).
When RV is estimated rather than measured, the precision of underwater weighing
is little better than anthropometric (skinfold) determination.

The procedure for measuring underwater weight
is used to determine the body density. Using body density, percent fat can be
estimated using the Siri or Brozek formula.

There is an excellent interactive web site on
body composition and underwater weighing that was produced at the University of
Vermont (http://nutrition.uvm.edu/bodycomp/uww/uww-toc.html). Included is a simulated underwater weighing
procedure.

- Weigh
the subject to the nearest 0.1 kg.
- Measure
or estimate the subject's RV. If possible, directly measure RV using the
diluted helium or oxygen techniques. Otherwise, RV can be estimated by
measuring vital capacity (VC) and multiplying it by 0.24 or 0.28 for males
and females, respectively.

- VC is the amount of air that can be maximally
exhaled. To measure VC, place a noseclip on the subject. Have the subject
maximally inhale, place the mouthpiece from the spirometer in his/her
mouth, and then maximally exhale. Perform 2-3 trials and use the highest
volume. Also, correct the volume to BTPS by multiplying the volume by the
correction factor (see an instructor for this value).

- Tare
(zero) the scale in the underwater weighing tank. Doing so automatically
subtracts the weight of the chair from the subject's underwater weight.
While seated in the tank, instruct the subject to blow out all the air
that he/she can exhale, bend slowly forward until the top of his/her head
is underwater, and to remain motionless until the scale has settled and
the computer indicates that it has a reading. After the subject has
settled underwater, it takes the computer approximately 5-10 sec to
determine the weight. Have the subject perform 6-10 trials and average the
heaviest 2-3 readings.

**Determination of Body Density **

*Body
Density = dry weight / [((dry weight - wet weight) / water density)- RV - 0.1]*

Note: Units for all weights are in kg and RV
is in L. The 0.1 represents an estimated volume (L) of gas in the GI tract.

**Estimation of Percent Fat**

The two most commonly used equations for
estimating percent fat from body density are the Siri (1961) and Brozek (1963)
formulae. A limitation to these formulae is that they assume the density of
fat-free mass to remain a constant across the population when in fact is
varies. Thus, the actual percent fat tend to be slightly higher than the
measured percent fat in the lean, muscular individual and the opposite effect
in obese individuals.

Siri |
Percent Fat = [(495 / Body Density) -450] * 100 |

Brozek |
Percent Fat = [(4.570 / Body Density) - 4.142] * 100 |