Commentary: Origins and evolution of body mass index (BMI): continuing saga

We reflect upon Ancel Keys’ classic article, reprinted here, which dealt with a leitmotif of his long career: body mass, its composition, measurement, function and meaning for health, disease and survival. This preoccupation was reflected in some 50 of Keys’ 500-odd publications. Along this historical vein, our colleague, Warren Winkelstein, in a note just before his death, reminded us that the mid-19th century Belgian polymath, Adolphe Quetelet, under the premise that ‘the transverse growth of man is less than the vertical’, derived the function most used today to characterize relative body weight, that is, the ratio of weight (kg) over height (m) squared. It was in the 1972 article, reproduced here partly because of its voluminous citations, that Ancel Keys gave Quetelet’s calculation its modern name, body mass index (BMI), along with evidence to support its current wide usage. To begin, we particularly recommend Keys’ short introduction to the article for the characteristic clarity and parsimony of his writing and for his lucid thinking about the measurement and meaning of body mass. In the text, he compared historic indices of relative weight for their independence of weight from height and for their representation of body fatness. The latter validation was based on correlation of an index with direct measures of body fat, either by the practical method of skin-fold thickness or the ‘platinum meter’ of laboratory-determined body fat. We then refer to the example of epidemiological design in which Keys tested the several indices across populations of men varying widely in body size and build. We happen to know that Keys himself did all these analyses and wrote this article, while his co-authors, all long-term colleagues, provided the cohorts and made the field measurements. This effort prevailed among larger attempts to relate obesity and body build to incident disease, death, or survival, which were among the core pursuits of Ancel Keys’ career. Keys’ interest in and contributions to the detailed anthropometry involved in the indices tested here date to his earlier work on nutrition, body composition and performance in studies of semi-starvation and other stresses imposed among conscientious objectors to war and among army personnel during and after World War II. We believe that Keys’ observations, in his Minnesota semi-starvation experiment, of dramatic changes in body composition and Sheldonian body typing, and in numerous anatomical and functional characteristics previously regarded as fixed ‘constitutional’ traits, were central to his development of concepts, beyond physiology, in physiological hygiene and public health. This led, in turn, to his ideas and enterprise about the potential for modifying characteristics of risk and the prevention of common diseases. The quantitative measures of body fatness Keys used here to validate the relative weight indices derive from another monumental study carried out earlier with Joseph Brozek in the Laboratory of Physiological Hygiene at Minnesota and published in 1953 as ‘Body fat in adult man’. That study cross-validated various measures of body composition in fat and bone, ‘lean mass’, ‘fat-free’ mass and the plasma and tissue fluid or water ‘compartment’. One of us (H.B.) arrived on the scene in the Laboratory at the tail end of this extensive study and was promptly immersed, not only in the actual underwater