Body Composition: Hydrostatic (Underwater) Weighing

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.

1. Weigh the subject to the nearest 0.1 kg.
2. 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).
1. 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