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LETTER TO THE EDITOR |
The University of Georgia
Department of Foods and Nutrition
Athens, GA 30602
E-mail: rlewis{at}fcs.uga.edu
Medical College of Georgia
Department of Cellular Biology and Anatomy
Augusta, GA 30912
The University of Georgia
Department of Statistics
Athens, GA 30602
Dear Sir:
Cole et al state that our study's (1) conclusion is fallacious. We disagree with the authors' assertion. The basis for their statement is that they take the position that percentage body fat (BF) is an inappropriate index of adiposity. We elected to use percentage BF as an index of adiposity rather than body mass index (BMI) because of the limitations associated with BMI (2) and, more importantly, because it is the excess fat, not the excess weight, that predisposes metabolic disorders (3-7).
Using percentage BF cutoffs associated with cardiovascular disease risk factors (3) was the basis for grouping our participants. Cole et al did not take into consideration or comment on the group comparisons; however, our conclusion that "excess weight in the form of fat mass does not provide additional benefits, and may potentially be negative, for adolescent bone" was derived primarily from the group comparisons. Although no differences were found between the normal- and high-fat groups in mean values of fat-free soft-tissue mass, statistically significant differences were detected between mean levels of fat mass. This additional 9-kg of fat mass in the high-fat group provided no advantage in the peripheral quantitative computed tomography–derived bone measurements at the tibia and radius.
In addition to the group comparisons, we sought to determine relations between the bone measurements and percentage BF and fat mass. Whereas Cole et al state the limitations of using the ratio, fat mass/body weight x 100, as an index of adiposity in denoting meaningful relations with other health outcomes, their same argument can be applied to the ratio, weight/height2 (8), which is the most commonly used index of adiposity. Nevertheless, in Table 3 of our article, if we had just reported fat mass adjusted for muscle cross-sectional area and limb length, the negative relations with cortical bone area and cortical bone mineral content at the tibia and radius still provide important information and support our conclusion.
The issue of normalization of body composition for body size is complex and controversial; accordingly, there is no universal agreement on the correct choice of statistical adjustment. Our decision, however, for adjustments of body size, was based on functional biological mechanisms related to bone strength (9, 10). More specifically, the study's conclusion stems from "adjustments" relative to mechanical loading generated by muscle forces at the site of bone measurement. Our findings are consistent with the theory that bone adapts its strength mainly to dynamic (muscle), rather than to static (fat), loads. We acknowledge the points of caution made by Cole et al with respect to the use of percentage BF as a proxy for adiposity when examining relations with health outcomes. Our data collectively, however, support the study conclusion.
ACKNOWLEDGMENTS
None of the authors declared a conflict of interest.
REFERENCES
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