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Dear Sir:
Fields et al (1) compare results obtained with the "new" method of air-displacement plethysmography (BOD POD; Life Measurement Inc, Concord, CA) with those obtained by hydrodensitometry and other reference methods to determine the fat content of human subjects. They conclude that air-displacement plethysmography is a reliable, valid, and safe technique but advocate further research to explain the differences between the estimates obtained by these methods. In a recent paper, Demerath et al (2) reached a similar conclusion.
A major problem when methods for measuring human body composition are validated by comparison with other methods is that the other methods also have quite large errors and there is no gold standard with which to compare results other than whole-body chemical analysis, which is unacceptable for living subjects. However, there are accurate (but tedious) methods for measuring change in body fat in volunteers confined to a closed metabolic ward and who consume a low-energy diet. If energy intake and output are carefully measured over several weeks and a value for the energy density of fat and fat-free mass are assumed, the proportion of fat and fat-free mass in the weight lost can be calculated. Similarly, this calculation can be made if nitrogen balance is measured and a value for the nitrogen content of fat-free mass is assumed.
Long ago we reported a study on 19 obese women studied in a closed metabolic ward for a total of 408 person-days (3). The women’s mean (± SD) initial weight was 97.55 ± 19.81 kg and in 3 wk they lost 5.43 ± 1.83 kg, of which 2.77 ± 0.71 kg was estimated by energy balance to be fat. Estimates of fat loss by nitrogen balance and by change in body density, total body water, and total body potassium were also made. Body density was measured by a novel plethysmograph, which was not mentioned in the review by Fields et al (1). The subject stood in a tank of water up to the neck with his or her head in an airtight cover in which the pressure was altered by a reciprocating pump. The volume of air around the head and in the lungs and gut was calculated by the pressure change relative to a given volume change, as in the BOD POD system. However, because this air volume is small compared with that in the BOD POD system, the errors relating to the isothermal or adiabatic behavior of air are similarly reduced. The estimated fat loss by density change measured in our novel plethysmograph was 2.83 ± 2.32 kg. This was closer to the measurement obtained by energy balance than was either the water or potassium measurement.
The purpose of this letter is not to bemoan the neglect of an old piece of research, but to make 2 constructive suggestions concerning the future development and validation of air-displacement techniques for measuring body composition. First, the smaller the volume of air around the subject, the more precise the measurement of body volume will be. Partial immersion of the subject in water achieves this desirable situation. Second, validation of air-displacement methods against a change in body fat measured by metabolic balance is more reliable than one obtained by comparison with other methods that have errors of similar magnitude to those of hydrodensitometry.
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