Body Mass Index: From Quételet to Evidence ... - Giorgio Bedogni

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Giorgio Bedogni, Claudio Tiribelli and Stefano Bellentani
Figure 2. Relationship between percent body fat and BMI in a sample of 1423 Italian elderly women.
Abbreviations: R 2 = coefficient of determination; SEE = standard error of the estimate; SEE% = percent
standard error of the estimate. Modified from Bedogni et al. [18].
The traditional approach to establish the accuracy of BMI as an index of body fat is to
perform a regression analysis as described above. Another approach, which is more suitable
for screening purposes, is based on the calculation of the sensitivity (true positive rate) and
specificity (true negative rate) of BMI in detecting excess body fat [19]. BMI has generally a
low sensitivity and a high specificity in detecting excess adiposity. For instance, in a study of
934 Italian children aged 8-12 years [20], sensitivity and specificity of BMI in detecting a
value of percent body fat greater than the 85 th percentile were 0.39 and 0.99 for the 95 th
percentile and 0.65 and 0.95 for the 85 th percentile of BMI, respectively. When a direct
measure of adiposity is needed, skinfold thicknesses offer a low-cost and easy way to
evaluate subcutaneous adipose tissue [17]. In the study performed on Italian children [20], the
sensitivity of the log-transformed sum of four skinfolds (biceps, triceps, subscapular and
suprailiac [14]) in detecting excess adiposity were 0.75 and 0.94 for the 85 th percentile. Thus,
the use of the 85 th percentile of the sum of four skinfolds instead of the 85 th percentile of BMI
may increase the sensitivity of screening programs of excess adiposity in children without any
relevant loss in specificity.
The use of BMI as an index of adiposity is more difficult in children than in adults
because of varying growth rates and maturity levels [21]. Standardization of BMI for age is
always necessary in children to avoid spurious interpretations [21]. Likewise, similar values
of BMI may reflect a different body composition in elderly as compared to young adults [22].
For instance, when matched with young women of similar weight and stature, elderly women
have higher body fat and lower fat-free mass, total body water and bone mineral content [23].
There is also increasing evidence that the relationship between body fat and BMI may differ
among individuals of different ethnic background [24, 25]. This evidence, together with that
derived from epidemiological studies of morbidity and mortality (see next paragraph), has
suggested the need of ethnic-specific BMI cut-off points [26].