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

2
Giorgio Bedogni, Claudio Tiribelli and Stefano Bellentani
Quételet’ s Index
The ratio of body weight to squared stature (W/S 2 ), nowadays known as the body mass
index (BMI), was proposed as a measure of body shape by the Belgian anthropologist
Adolphe Quételet in the late 19 th century [1-4]. W/S 2 was then just one of the many indexes
used by anthropologists to control body weight for stature, i.e. to measure body shape [5], and
was the option preferred by Quételet for adults [2, 3]. Of the plethora of weight-stature
indexes developed in the last two hundred years [5], W/S 2 is however the only one still in use
today. Surprisingly, it is also the only one not to be remembered with the name of its inventor
[3, 5]. Even if Quételet did not use W/S 2 as an adiposity or prognostic index as we do today,
the lack of acknowledgement for its discovery is surprising in view of the immense
contributions given by this man to the fields of anthropology and statistics (not to mention
sociology and astronomy) [2, 4, 6]. For instance, by showing that the heights of French
conscripts and the chest circumferences of Scottish soldiers tended to be normally distributed,
Quételet made the fundamental discovery that the normal curve could be used other than an
error law applied to the movements of planets. He literally brought the normal curve from the
skies to the earth, in the hope of building a new science of man [1]. This finding was central
to the development of both anthropometry and statistics. Thus, even if W/S 2 is not used today
as it was by Quételet, we should always recall the great contributions of its inventor to
anthropometry when using “his” index [3].
Body Mass Index as an Index of Adiposity
W/S 2 might have been one of the many forgotten weight-stature indexes if it had not been
rediscovered as an index of adiposity more than one hundred years after its introduction. The
demonstration of an association between W/S 2 and body fat gave a new life to Quételet’ s
index and the new name of BMI [5, 7-10]. As a demonstration of the success encountered by
W/S 2 , the term “body mass index” entered the Medical Subject Heading of the National
Library of Medicine in 1990. Here, BMI is defined as “one of anthropometric measures of
body mass; it has the highest correlations with skinfolds and body density”. This definition
reflects the fact that skinfolds [11] and body densitometry [12] were used to measure body fat
in the first validation studies of BMI.
The search for anthropometric indexes of adiposity has been fuelled by the hypothesis
that excess body fat could be a risk factor for chronic disease [13]. Due to its simplicity, BMI
offered the opportunity to study the relationship between body fat and disease in population
studies. However, even if BMI is associated with fat-free tissues less than other weightstature
indexes [5, 13], its numerator (weight) is the sum of fat and fat-free tissues so that
BMI cannot be considered a pure index of adiposity (Figure 1). It should also be noted that,
while the association of BMI with morbidity and mortality is now very clear (see next
paragraph), only recently the association between body fat and chronic disease started to be
investigated. The measurement of body fat requires in fact methods that are difficult to use in