Hydrostatic underwater weighing is a form of densitometry (another being air displacement plethysmography), which derives body composition from body density and body volume. It uses Archimedes' principle of displacement.
It is based upon the classic two-component (2-C) model of body composition which assumes that body weight is composed of fat free mass (FFM) with a constant density of 1.10 kg/L, and fat mass (FM) with a constant density of 0.90 kg/L. The density of the whole body, therefore, depends upon the relative size of these two components.
As bone and muscle are denser than water, a person with a larger percentage of fat free mass will weigh more in the water, and have a lower percent body fat. Conversely, fat is less dense than water. Therefore, a large amount of fat mass will make the body lighter in the water and have a higher percent body fat.
Body density is expressed as mass per unit volume, where mass is the weight of the body in air (Ma) and volume is the difference between the weight in air and the weight of the body submersed during underwater weighing (Ma – Mw). Once body density is calculated, it is possible to convert this to a percentage body fat.
The process is divided into three steps: 1) measurement of residual volume; 2) measurement of dry bodyweight; and 3) measurement of underwater weight. Figure 1 provides an illustration of one method used for hydrostatic underwater weighing.
Two methods to determine total body water
An allowance must be made for residual lung volume (the air that remains in the lungs after a maximal exhalation) as this air increases the participant's buoyancy. It is measured using methods such as closed circuit dilution (including helium dilution) or nitrogen washout. If indirect measurement cannot be conducted, it can also be estimated from vital lung capacity or using published equations that are based on age, height.
Hydrostatic underwater weighing typically requires the participant to be completely submerged underwater while exhaling maximally to minimize the effect of buoyancy from lung air. Differences in residual volume determination have been reported to contribute the largest sources of variation.
After having measured or estimated residual lung volume, the participant's dry weight is measured wearing a swimming suit or running shorts.
Participants must not eat or engage in strenuous exercise for at least 4 hours before their scheduled appointment. They should avoid ingesting any gas-producing beverages for at least 12 hours before the test. Participant must wear swim suits.
If each test is performed correctly according to the recommended guidelines, hydrostatic underwater weighing has small percentage error (+/- 1.5% error), and has been labelled the gold standard assessment of fat mass/fat free mass. However, as it is mostly only available in research laboratories or university settings, it is generally used for research purposes and not applicable in clinical practice or in large-scale population studies.
It is used less frequently since the introduction of air displacement plethysmography as this technique is better tolerated by participants.
Figure 1 Illustration of measurement of underwater body weight.
Body density is required to calculate percentage body fat. It is calculated using the following:
The substitution of these values in the given formula estimate the body density:
Body density = Ma / [(Ma – Mw) / Dw – RV]
Once body density has been calculated from the data obtained, body composition can be estimated. The most commonly used equations for estimating the percent of body fat from density are as follows:
Siri Percent fat = [(495 / body density) - 450] * 100
Brozek Percent fat = [(4.570 / body density) - 4.142] * 100
An overview of the characteristics of hydrostatic underwater weighing is outlined in Table 1.
Table 1 Characteristics of hydrostatic underwater weighing.
|Number of participants
|Researcher burden of data collection
|Researcher burden of coding and data analysis
|Risk of reactivity bias
|Risk of recall bias
|Risk of social desirability bias
|Risk of observer bias
|Suitability for field use
|Participant literacy required
Considerations relating to the use of hydrostatic underwater weighing for anthropometry in specific populations are described in Table 2.
Table 2 Anthrompometry by hydrostatic underwater weighing in different populations.
|Suitable but unable to disentangle the mother-fetal unit
|Infancy and lactation
|Toddlers and young children
|May not be suitable as older individuals have less dense bones, resulting in over-estimation of body fat
|Athletes tend to have denser bone and muscle tissue, resulting in under-estimation of body fat
Refer to section: practical considerations for objective anthropometry