Rich Jarvis and Dr. Larry Tucker, Physical Education

Obesity will probably replace cigarette smoking as the major killer of Americans in the next century.1 Research shows that excess body weight leads directly to many health problems, and it often contributes indirectly to other serious diseases by precipitating dangerous metabolic abnormalities.2 In short, obesity plays a central role in the development of numerous diseases, including Type II diabetes, coronary heart disease, high blood pressure, hyperlipidemia, various cancers, osteoarthritis, gallbladder disease, stroke, and all cause mortality.

The prevalence of obesity has increased steadily throughout recent history. Population-based surveys indicate that the age-adjusted prevalence of obesity among adults in the United States has increased from about 25% in the 1970’s to 33% in 1988-1991.3 This increase in obesity has been evident in all race and sex groups. In 1990, it was estimated that 58 million American adults were obese.3

Because of the multitude of health problems associated with obesity, and because of the increasing prevalence of this disorder, accurate assessment of obesity is critical. Two of the most valid and accepted methods used to measure obesity, or more specifically, estimate body fat percentage, are hydrostatic weighing and air displacement plethysmography (Bod Pod). However, very little research has been conducted to compare the results of these two common methods. Hence, the purpose of this study was to compare the extent to which these methods produce equivalent body fat percentage results in 185 women.

After signing an informed consent form, residual volume was measured using the helium-dilution method for use with hydrostatic weighing.4 Subjects where then asked to use the restroom to eliminate all waste. Subjects then changed into a one-piece nylon swim suit supplied by the lab. Subjects were weighed to the nearest 0.005 kg (0.01 lb) using a calibrated electronic scale, and height was measured without shoes to the nearest mm using a wall-mounted sliding gauge. Subjects were then tested in the Bod Pod until two body fat percentages were within one percentage point of each other. The Bod Pod system was calibrated with a known-volume cylinder before each subject was assessed.

After the Bod Pod assessment, subjects were oriented to the hydrostatic weighing procedure. Hydrostatic weighing was performed using a computerized hydrostatic strain gauge system. The system was calibrated before each subject was measured. After practicing, subjects were weighed underwater until two body fat percentages were within one percentage point of each other. For both methods, percent fat and lean body mass were calculated using Siri’s formula.5

Of the 185 subjects, 35 were invited to return one week later for a retest. The same methods and procedures were used for the retest as the original assessment. This allowed the test–retest reliability of each method to be assessed and compared.

The hydrostatic weighing and Bod Pod methods produced highly correlated results, however, the average body fat percentage produced by the two methods were statistically different. Specifically, the Pearson Product-moment correlation between the results derived using the two methods was 0.92 (p < 0.001). The mean fat percentage derived using hydrostatic weighing was 30.6 + 7.1 and 28.4 + 6.6 for the Bod Pod, a statistically significant difference of 2.2 + 2.7 (p < 0.001).

Test–retest performed on 35 subjects with one week between tests using hydrostatic weighing resulted in an intraclass correlation of 0.94. The test–retest intraclass correlation using the same sample of 35 subjects and the Bod Pod was 0.98.

Apparently, assessment of body fat percentage using hydrostatic weighing and the Bod Pod result in highly related figures. However, hydrostatic weighing seems to produce body fat percentage results that are consistently higher than the Bod Pod. Which procedure is more accurate is difficult to know—they are both considered excellent methods to estimate body fat percentage.

Because the Bod Pod method seems to produce slightly better test–retest results compared to hydrostatic weighing, and because subjects seemed to enjoy the Bod Pod procedure more than hydrostatic weighing (with the Bod Pod subjects did not have to get wet or exhale all their air while under water), the Bod Pod method appears to be the better method for measuring body fat percentage.

References

1. Grundy SM. Multifactorial causation of obesity: implications for prevention. American Journal of Clinical Nutrition. 1998;67 (suppl):563S-572S.
2. National Institutes of Health. Consensus Development Conference Statement. Health implications of obesity. Annals of Internal Medicine. 1985;103:981-1077.
3. Kuczmarski RJ, Flegal KM, Campbell SM, Johnson CL. Increasing prevalence of overweight among U.S. adults. The National Health and Nutrition Examination Surveys, 1960–1991. JAMA. 1994;272:205-211.
4. Åstrana PO and Rodahl K. Textbook of Work Physiology: Physiological Bases of Exercise (3rd ed.). New York: McGraw-Hill Book Company. 1986:222-224.
5. Siri WE. Body composition from fluid spaces and density: analysis of methods. In: Brozek J and Hensel A (eds.). Techniques for Measuring Body Composition. National Academy of Sciences: Washington DC. 1961:223-244.