CORRESPONDENCE: LETTER TO THE EDITOR
Is Body Mass Index Really the Best Measure of Obesity in Individuals?
Daniel J. Green*
* Research Institute for Sport and Exercise Science, Liverpool John Moores University, 15-21 Webster Street, Liverpool Merseyside L3 2ET, United Kingdom (Email: d.j.green{at}ljmu.ac.uk).
In the August 19, 2008, edition of the Journal, Gelber et al. (1) present data from the Physician's and Women's Health studies and conclude that, although waist-to-height ratio (WHtR) has the strongest gradient in the association with incident cardiovascular disease (CVD), "differences between BMI [body mass index] and WHtR in association with CVD...[are] small and likely not clinically consequential." They suggest that "BMI may remain the most clinically practical measure of adiposity." Litwin (2), in an accompanying editorial, makes the general argument that "clinical practicality is frequently overshadowed by statistical significance" and that, in the interests of clarity and to avoid equivocation, BMI should be retained as the principal and universal measure of obesity.
A critical point is being missed here, which relates to the translation of measures from large population studies to the management of the individual. In large population studies performed in adults, BMI provides a useful surrogate index of obesity because it corrects for individuals who are heavy by virtue of the fact that they are also tall. In large population studies, a higher BMI might reasonably be assumed due to excess fat mass. Of course it is accepted that BMI provides no information regarding the composition of the weight, or its distribution, but this does not matter so much when the study is performed in 49,000 subjects. However, populations are comprised of individuals, and it is individuals we treat. In adults, body height does not change over time, so BMI reverts to a measure of gross body weight in the individual. Is it valid to simply measure body weight as an index of fatness when following up patients? If so, why do we measure BMI in the clinic at all?
Moreover, changes in BMI in the individual cannot be appropriately used as an index of change in obesity or cardiovascular risk. To make the point using a relevant example, exercise is an intervention that is widely prescribed for obese subjects. It is also an intervention that can increase skeletal muscle and lean body mass. Exercise studies often result in no change in body weight or BMI, whereas sensitive imaging modalities reveal significant decreases in fat mass, including abdominal fat mass (3). Exercise has countervailing impacts on fat and lean body mass (4), which render BMI meaningless as an index of obesity, adiposity, or risk in individuals.
Consideration of changes in body composition, rather than measures of body weight, including BMI, remains important, and this may be one reason to use the WHtR index, in preference to BMI, as a simple, uncomplicated measure that moves patients "efficiently through our offices."
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1. Gelber RP, Gaziano JM, Orav EJ, Manson JE, Buring JE, Kurth T. Measures of obesity and cardiovascular risk among men and women J Am Coll Cardiol 2008;52:605-615.[Abstract/Free Full Text]2. Litwin SE. Which measures of obesity best predict cardiovascular risk? J Am Coll Cardiol 2008;52:616-619.[Free Full Text] 3. Watts K, Beye P, Siafarikas A, et al. Exercise training normalises vascular dysfunction and improves central adiposity in obese adolescents J Am Coll Cardiol 2004;43:1823-1827.[Abstract/Free Full Text] 4. Watts K, Davis E, Jones T, Green DJ. Effect of exercise training in obese children and adolescents Sports Med 2005;35:1-18.[CrossRef][Web of Science][Medline]
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