The Body Mass Index (BMI) is a numerical measure derived from a person’s weight and height. It is calculated by dividing an individual’s mass in kilograms by the square of their height in meters (kg/m²). This simple metric provides a general classification of weight status, categorizing individuals into ranges such as underweight, normal weight, overweight, and obese. While it offers a quick and inexpensive screening tool, BMI does not directly measure body fat but rather estimates it.
Adolphe Quetelet and the Quetelet Index
The concept for BMI originated in the mid-19th century with Adolphe Quetelet. A Belgian mathematician, astronomer, and statistician, Quetelet developed this formula, initially called the “Quetelet Index,” around 1832. His primary interest lay in “social physics,” a field studying population characteristics and human measurements. Quetelet aimed to describe the “average man” through statistical analysis, observing how human physical attributes like weight distributed across populations.
He discovered weight tends to increase as the square of height. This observation formed the basis of his index, a tool for broad statistical analysis of populations, not for assessing individual health. Quetelet’s work was rooted in analyzing data primarily from European men. The “Quetelet Index” was conceived as a demographic and sociological tool to understand human populations.
From Quetelet Index to Body Mass Index
The Quetelet Index transitioned to the Body Mass Index later, in the 1970s. American physiologist Ancel Keys and his colleagues played a central role in this rebranding and popularization. In a 1972 paper, Keys formally coined the term “Body Mass Index” (BMI). Keys’ research involved analyzing data from thousands of men across multiple countries, demonstrating that this particular ratio was a reliable indicator for population-level studies of obesity.
He argued that the BMI was “at least as good as any other relative weight index” for assessing relative obesity in populations. Keys primarily advocated for its use in large-scale epidemiological studies, emphasizing its convenience and ease for analyzing health trends across groups. This work led to its widespread adoption in public health and medical research. The simplicity and cost-effectiveness of BMI made it an attractive tool for researchers studying the increasing rates of obesity in Western societies.
Evolution of BMI’s Application
Following its popularization by Ancel Keys, BMI’s application underwent a historical shift. Initially, its primary use remained as an epidemiological tool for population studies, tracking trends in weight and health across large groups. Over time, its application broadened. By the 1980s and 1990s, organizations like the U.S. National Institutes of Health (NIH) began to define specific weight categories using BMI, such as “underweight,” “normal weight,” “overweight,” and “obese.”
This development led to BMI becoming a more commonly applied metric for individual health screenings and classifications in clinical settings. The World Health Organization (WHO) further standardized these categories in 1997, declaring obesity a global epidemic and setting universal thresholds for overweight (BMI over 25) and obesity (BMI over 30). This standardization facilitated consistent research and intervention programs globally, making BMI a routine measure in doctors’ offices and medical records. While originally intended for populations, the BMI evolved to inform individual health discussions and assessments.