• What is assessed?
• How is the measurement conducted?
• When is this method used?
• How are estimates of body composition derived?
• Strengths and limitations
• Populations
• Further considerations
• Resources required
• Instrument library
• References

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.

Dry bodyweight

After having measured or estimated residual lung volume, the participant's dry weight is measured wearing a swimming suit or running shorts.

Underwater weight

1. The participant climbs into the underwater weighing tank and sits in a seat that hangs from a force load-cell.
2. The procedure involves the participant performing a maximal exhalation and slowly leaning forward in the chair until the top of his/her head is under water.
3. After the underwater weight stabilizes (approximately 5 seconds), the participant is instructed, though yelling or pounding on the side of the tank, to return to an upright position.
4. During weighing, the participant is always free to stand up or return to the upright position.
5. Several trials may be required before the participant becomes accustomed to the procedure and consistent readings recorded. Four to eight trials are usually performed.
6. The depth of the water is approximately 140cm, and temperature maintained between 30 and 34 C. The density of water is dependent on the temperature and should always be factored into the equation. After each testing session, the water is drained and the tank allowed to dry.

Pre-test Guidelines

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.
Source: http://nutritionfirstfitness.com

Body density is required to calculate percentage body fat. It is calculated using the following:

1. Participant’s weight outside water (Ma)
2. Participant’s weight entirely submerged in water (Mw)
3. Density of water in which participant is immersed (Dw)
4. Residual volume (estimated or actual) (RV)

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.

Strengths

1. It is a reliable and accurate technique

Limitations

1. Time intensive
2. Labour intensive
3. Considerable participant cooperation required
4. Participant discomfort
5. Inaccessible for some populations such as elderly, disabled and chronically ill
6. Requires specially trained professionals to setup and conduct the testing procedure
7. Participant’s hydration level may affect the error in test results significantly
8. Being submerged underwater might be a little difficult for some (e.g. elderly)
9. Participant must be able to perform complete air exhalation
10. There is a possibility of considerable error introduced by the miscalculation of residual volume (RV)
11. The test does not identify the exact parts of the body where the fat is located (e.g. unable to assess fat distribution)
12. The accuracy of body density measurements can be affected by the consumption of food and carbonated beverages shortly before the test, fluid loss during intensive exercise, fluid retention during menstruation, and the degree of forcible exhalation while submerged
13. Like all the 2 component model methods, it assumes when calculating total body density that the relative amounts and densities of bone, muscle, and water comprising the fat-free mass are essentially the same for all individuals, regardless of age, sex, ethnicity or fitness level

Table 1 Characteristics of hydrostatic underwater weighing.

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.

Refer to section: practical considerations for objective anthropometry

1. Hydrostatic stainless steel weighing tank
2. Underwater mounted chair and scale
3. Weighted belt and nose clip
4. A simpler set up may include a chair and scale suspended from a diving board over a pool
5. Trained research staff

1. Ball SD. Interdevice variability in percent fat estimates using the BOD POD. Eur J Clin Nutr 2005;59:996-1001
2. Brozek J, Grande F, Anderson JT, Keys A: "Densitometric Analysis of Body Composition: Revision of some Quantitative Assumptions", Academic Science 1963: 110; 113
3. Ellis KJ: Human Body Composition: In Vivo Methods. Physiological Rev. 2000: 80; 649
4. Heyward VH: Advanced Fitness Assessment & Exercise Prescription. 2nd ed. Champagne, Ill.: Human Kinetics; 1991
5. Katch F, Michael ED, Horvath SM: Estimation of body volume by underwater weighing: description of a simple method J Applied Physiol 1967; 23: 811
6. McArdle WD, Katch FI, Katch VL: Essentials of Exercise Physiology. 3rd ed. Philadelphia: Lippincott Williams & Wilkins; 2006
7. Norgan NG: Laboratory and field measurements of body composition Public Health Nutrition 2005: 8; 1108
8. Silva D, Ribeiro A, Pavou F et al: Validity of the methods to assess body fat in children and adolescents using multi-compartment models
as the reference method: a systematic review Rev Assoc Med Bras 2013; 59: 475
9. Siri, SE: "Body composition from fluid spaces and density: analysis of methods", in Brozek J,Henschel A, Techniques for measuring body composition, Washington, DC; 1961
10. Wilmore, JH: "The use of actual predicted and constant residual volumes in the assessment of body composition by underwater weighing". Med Sci Sports 1969; 1: 8
11. Wells JC, Fewtrell MS: Measuring body composition Archives Dis Child 2006: 91; 612
12. Widen EM, Gallagher D: Body composition changes in pregnancy: measurement, predictors and outcomes Eur J Clin Nutr 2014; 68: 643