Reply to JS Garrow

¹ Energy Metabolism Laboratory, Jean Mayer USDA Human Nutrition Research Center on Aging, Tufts University, Boston, MA, E-mail: megan.mccrory{at}tufts.edu
² Department of Preventive Medicine, Institute for Preventive Research, Keck School of Medicine, University of Southern California, Los Angeles, CA
³ Department of Internal Medicine, Center for Human Nutrition, Washington University, St Louis, MO

Dear Sir:

We thank Garrow for highlighting an inherent and almost unavoidable characteristic of body-composition validation studies: that the reference methods against which we compare the test methods have their own associated errors. The most common techniques against which air-displacement plethysmography (BOD POD; Life Measurement Inc, Concord, CA) has thus far been validated are hydrostatic weighing and dual-energy X-ray absorptiometry, each of which, as we point out in our review (1), has multiple inherent errors. When evaluating these studies, therefore, we must keep in mind that any differences in body-composition measurements between the reference method and the BOD POD should be attributed not only to the BOD POD but also to the reference method. Whenever possible, measurements made by any new body-composition method should be evaluated against techniques with the highest achievable accuracy and precision. These include any number of multicompartment models that take into account individual variations in body compartments (2–4). However more precise and accurate these may be, they still have some associated error (4–7). In addition, they may not be completely independent of the method under evaluation; such is the case when the BOD POD, which ultimately is a densitometric method, is evaluated against a 4-compartment model that also requires a measurement of densitometry. Examples of methods that could serve as independent reference methods are in vivo neutron activation analysis (8) and total-body carbon analysis (9). Neither of these methods is widely available, however; thus, the BOD POD will continue to be evaluated against commonly used techniques such as hydrostatic weighing and dual-energy X-ray absorptiometry. Because of this, when judging whether the BOD POD (or any other method) is reliable and accurate, we believe it is important to keep in mind the big picture. It may be reasonable to expect and also accept that differences in measurements between methods exist. When the totality of studies shows that these differences between methods are small and randomly distributed among studies, the method under evaluation can be deemed acceptable and reasonably valid.

Garrow also states in his letter that a smaller air volume around the subject will lead to a more precise measurement of body composition. In theory this should be true; however, as stated in our review (1), evidence thus far suggests that, in practice, the size of the air volume makes only a small difference in the BOD POD. Recently, Wells and Fuller (7) reported that the absolute precision for percentage body fat (%BF) measurement was 0.88%BF for adults and 0.91%BF for children aged 5–14 y. The difference in precision between adults and children increased only slightly when expressed relative to the mean %BF in each group (4.2%BF for adults and 5.8%BF for children). Additionally, there was no relation between the difference in 2 consecutive body density measurements and body volume (r = -0.20,P = 0.14). One major advantage of the BOD POD over hydrostatic weighing is that individuals do not need to get wet. Whereas Garrow’s plethysmograph (10), in which subjects are immersed to the neck in water, is likely a precise research tool, we feel that for everyday clinical use the gain in precision achieved by having a smaller air volume is offset by the inconvenience of the subject’s having to get wet.

REFERENCES

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