Body-composition assessment using underwater weighing techniques.

The growing popularity of physical activity for enhancing health and fitness has sharpened the health care professional's perspective on techniques for evaluating body composition. A major goal of many adult fitness programs, in addition to improving aerobic fitness, is to govern body weight and an individual's percentage of body fat with regular exercise and proper nutrition. Health care professionals require accurate measurements of body composition in order to consult with patients about desirable body weights and appropriate proportional body compositions for optimal health.

Ideal body weight is defined as the weight at which an individual's body fat percentage is equal to lower than the recommended fat level for his or her sex and age. [1] Recognizing that there is a natural increase in fat content with age, investigators [1,2] have suggested that fat content should be in the generalized range of 14% to 19% in adult men and 205 to 24% in women aged 20 to 60 years. Standard values and categorization of body fat content by age and sex are listed in the Table. The "recommended" level of fat is taken as the lowest value of the "average" range of values for a given age. [1]

Obesity is defined as a fat-content level that exceeds one standard deviation above the mean body fat levels for a particular age and sex. [1] The "poor" category in the Table represents the obesity classification.

For most individuals, attaining an ideal body weight is usually synonymous with weight reduction. It is estimated that 60 to 70 million American adults and at least 10 million teenagers are overweight. [3] The importance of adjusting an individual's weight to the recommended weight is evidenced by various epidemiological studies that have shown a prevalence of higher levels of blood lipids, blood pressure, physical inactivity, and risk of cardiovascular disorders in men with more than 20% to 25% body fat and in women with more than 30% to 35% body fat. [4] A recent analysis of prospective epidemiological surveys has identified overweight as an independent risk factor for coronary artery disease. In the Framingham study, [5,6] a 20% increase in relative weight was associated with a life-span reduction of 2.5 years in men and 5 years in women. In addition to cardiovascular disorders, obesity also predisposes individuals to other disorders, notably diabetes mellitus, degenerative joint diseases, and gallstones. [4] Some forms of cancer in women are also associated withe obesity, possibly because of abnormal steroid metabolism in fact cells.

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Whereas many individuals are concerned with adjusting or maintaining theri fat levels iwthin the average range, athletes are often attentive to lowering fat levels beyond the "good" category. The mean percentages of body fat of 12% and 18% have been suggested as the appropriate fat levels for male and female athletes, respectively. [7,8] In some sports such as gymnastic where higher fat levels may impede performance, women may exhibit percentages of body fat ranging from 13% to 18% and men may exhibit even lower values ranging from 45 to 10%. The athlete should be aware that even with a concern for lowering body fat, there appears to be a biologically lower limit beyind which a person's body weight cannot be reduced without impairing health status. The minimal recommended fat content that can be sustained and not impair normal physiological function or capacity for exercise is 3% to 7% in men and 10% to 14% in women. [7-9]

Assessment of

Body Composition

There are a number of techniques for the measurement of body composition. The most valid procedures are those that use some form of "wholebody" or "direct" measurement such as the K40 gamma-emission procedure, the helium-dilution technique, radiographic analysis, chemical analysis of cadavers, and hydrostatic weighing. [10,11] Currently, hydrostatic weighing is considered the most reliable technique for the measurement of fat and lean body tissue in a clinical setting. [11] The equipment requirement is minimal (ie, access to a water source such as a pool or water tank or a recessed whirlpool).

The basis of the hydrostatic, or underwater weighing, technique is Archimedes' principle. Arhimedes' principle states that the volume of an object submerged in water equals the volume of water the object displaced. If a mass is weighed in air and in water, the difference, corrected for the density of the water corresponding to the water temperature at the time of the underwater weighing, is the volume of the mass. In other words, fat tissue is less dense and lean tissue (mainly bone and muscle) is more dense than water. Hence, fat will float and fat-free tissue will sink. When determining body density (mass/volume) with hydrostatic weighing, the mass is commensurate with the weight of the body out of the water. The volume of the body is determined by calculating the difference in body weight when weighed underwater, corrected for the density of water. With this technique, it is necessary that the calculations also be coreected for the buoyancy of the air trapped in teh lungs and the gastrointestinal tract during submersion. The air remaining in the lungs after a maximal expiration is called the "residual lung volume," and the air in the intestines is called the "gastrointestinal gas volume." The gastrointestinal gas volume is generally considered to be minimal, with a constant value of 100 mL. The residual air volume left in the lungs is either determined directly or estimated from indirect assessments derived from vital capacity measurements or assuming set values of 1.3 L for men and 1.0 L for women. The preferable method of determining residual lung volume is the nitrogen washout procedure developed by Wilmore. [12] This procedure is basically a modification of the closed-circuit, oxygen-dilution method in which nitrogen is "washed out" of the lungs during a specified period of oxygen breathing.

Siri et a [13] used these concepts of body composition in developing the following equation for determining the percentage of body fat once the body density has been determined:

(1) & body fat = [(4.95/body density - 4.5) x 100]

Ideal Body Weight

If it is assumed that lean body weight can be maintained with regular exercise and proper nutrition, attainment of an ideal body weight requires a reduction in body fat. The quantity of body fat that is required to be lost in order to attain an ideal body weight is determined by partitioning the present body weight into fat weight and lean body weight. Then, using the recommended percentages of body fat, the ideal body weight and the required fat loss can be determined by subtracting the ideal weight from the current body weight.

Computer Program

The personal computer is an ideal device for the computation task involved in determining the various components of the body-composition equations. The use of a microcomputer and the accompanying program can significantly reduce the time necessary to calculate the different body compositions and the desired body weight. The rapid ciphering of the data with an almost instant access to summary can be invaluable for providing immediate results that can be used by the clinician for timely consultation with the patient.

The program uses a flexible menu-based framework with specific queries for input of patient data. In addition, the program allows the option of using various methods of estimating or measuring residual volume. Results are summarized in a numerical as well as graphical format for easy interpretation (Figure). The program is written in Microsoft BASIC for IBM-PCs* and compatible computers; however, the program can easily be adapted for other computers with minimal changes. REMark statements are included to facilitate the adaptation to other BASIC dialects.

Determination of Body Density

The step-by-step procedure used for the assessment of body fat and the derivation of the values required by the program from underwater weighing is described below:

1. The subject is weighed on land to the neares 0.1 kg, which represents the weight in air.

2. The subject is suspended in the weighing chair assembly from a hanging scale and is placed in the water so that his or her head will remain above water.

3. As the subject is seated in the chair assembly, he or she forcibly expires as much air as possible from the lungs.

4. The subject then places his or her head underwater for 3 to 5 seconds as the scale is read to the nearest 0.1 kg, which represents the weight of the subject in water.

The body density (BD) is determined by the program using the following formula:

(2) BD =Wt[sub.A.]/((Wt[sub.A.]-Wt[sub.w.])/DW) -(RV + GV)]

where Wt[sub.A.] represents the weight of the subject in air (in kilograms), Wt[sub.w.] is the weight of the subject in water (in kilograms), DW is the density of the water, RV is the residual volume (in liters), and GV is the gastrointestinal volume (in liters).

Determination of Residual

Volume Using Nitrogen-Washout

Procedure

The step-by-step procedure for the nitrogen-washout method of determining residual volume developed by Wilmore [12] is as follows:

1. The subject is seated in front of a respirometer that is filled with oxygen. The oxygen is analyzed for the presence of nitrogen, which represents the impurity of the gas.

2. The subject performs 3 or 4 normal respirations with the breathing valve turned to the outside air.

3. The subject then inhales and exhales as deeply as possible. A reading of the nitrogen concentration, which represents the initial alveolar nitrogen concentration, is recorded at the end of the expiration.

4. The breathing valve is then turned to the spirometer, and the subject inspires and expires oxygen at two thirds of the maximum level at a rate of one respiration every 3 seconds for five to eight breaths. The percentage of nitrogen at the end of expiration of the last breath is recorded as the equilibrium nitrogen level.

5. The subject again inspires and expires maximally. At the termination of the maximal expiration, the nitrogen is measured and the breathing valve is turned back to room air. The nitrogen concentration in the expired air represents the final alveolar nitrogen concentration.

The residual volume (RV) is calculated using the following formula:

(3) RV=[Vm X (En[sub.2] - IN[sub.2]/ (AiN[sub.2 - AFN[sub.2]) - DS] x BTPS factor

where IN[sub.2] is the percentage of nitrogen, or impurity, in the oxygen; EN[sub.2] is the percentage of nitrogen at equilibrium; AiN[sub.2] is the percentage of nitrogen in the alveolar air initially; AFN[sub.2] is

[TABULAR DATA OMITTED]

the percentage of nitrogen in the alveolar air at termination of testing; Vm is the volume of oxygen in the spirometer; DS is the dead space volume of the spirometer system; and BTPS is the correction factor for body temperature ambient pressure saturated with water vapor.

Estimation of Residual

Volume

Residual volume can be estimated from measurements of vital capacity by multiplying the vital capacity by 0.24 for men and by 0.28 for women. [14] If a respirometer is not available, the residual volume can be estimated with little loss of accuracy by assuming a value of 1.3 L for men and 1.0 L for women. [14]

Summary

Clinically, it is beneficial to assess body composition as an aid in developing an overall health promotion program. Athletes and nonathletes alike need information about body fatness and the weight changes necessary to achieve a body composition that is compatible with optimal health and participation in physical activity. Of the techniques commonly used for assessing body composition, underwater weighing is the most widely accepted procedure. In hydrostatic weighing, percentage of body fat is computed from body density (the ratio of body weight to body volume). The recent introduction of the microcomputer into the clinical environment greatly facilitates the numerous calculations associated with this technique. The accompanying program provides a means of rapidly determining the various characteristics of body composition as well as providing individualized recommendations for body-composition changes for health enhancement.

References

[1] Gettman L. Fitness testing. In: Blair S, et al, eds. Resource Manual for Guidelines for Exercise Testing and Prescription. Philadelphia, Pa: Lea & Febiger; 1988:161-170.

[2] Bray G. Overweight is risking fate: definition, classification, prevalence, and risk. Ann NY Acad Sci. 1987;499:14-28.

[3] Lohman TG. Skinfolds and body density and their relation to body fatness: a review, Hum Biol. 1981;53:181-225.

[4] Nestel P. Some medical aspects of obesity. Med J Aust. 1978;2:478-480.

[5] Higgins M, Kannel W, Garrison R, et al. Hazards of obesity: the Framingham experience. Acta Med Scand Suppl. 1988;723:23-27.

[6] Gordon T, Castelli W, Hjortland M. Diabetes, blood lipids, and the role of obesity in coronary heart disease risk for women. Ann Intern Med. 1977;87:393-397.

[7] Lohman TG. Body composition methodology in sports medicine. The Physician and Sportsmedicine. 1982;10:47-59.

[8] Behnke A. The estimation of lean body weight from skeletal measurements. Hum Biol. 1959;31:295-315.

[9] McArdle W, Katch F, Katch V. Exercise Physiology. 2nd ed. Philadelphia, Pa: Lea & Febiger; 1986:483-512.

[10] Garrow J. New approaches to body composition. Am J Clin Nutr. 1982;35:1152-1156.

[11] Buskirk ER. The 1986 CH McCloy Research Lecture. Body composition analysis: the past, present and future. Research Quarterly for Exercise and Sport. 1987;58:1-10.

[12] Wimore J. A simplified method for determination of residual lung volume. J Appl Physiol. 1969;27:96-100.

[13] Siri W. The gross body composition of the body. In: Tobias C, Lawrence J, eds. Advances in Biological and Medical Physics IV. New York, NY: Academic Press Inc; 1956;239-280.

[14] Wilmore J. The use of actual predicted and constant residual volumes in the assessment of body composition by underwater weighing. Med Sci Sports. 1969;1:87-90.

[15] Jackson AS, Pollock ML. Generalized equations for predicting body density of men. Br J Nutr. 1978;40:497-504.

[16] Jackson AS, Pollock ML, Ward A. Generalized equations for predicting body density of women. Med Sci Sports Exerc. 1980;12:175-182.

K Francis, PhD, is an exercise physiologist, Division of Physical Therapy, The University of Alabama at Birmingham, SHRP Bldg, Rm B41, Birmingham, AL 35294 (USA).

This article was submitted January 9, 1990, and was accepted June 21, 1990.