The Quetelet Index: A Statistician's Work Used to Make Money

The Quetelet index, later renamed the BMI, has made several companies a lot of money. What's behind it?

Written and verified by the psychologist Cristina Roda Rivera.

Last update: 21 December, 2022

The Quetelet index is an obsolete reference. An erroneous ‘health’ measure that was never intended to be.

When we want to assess our weight or monitor the effectiveness of a diet, we generally use the body mass index (BMI). Often presented as a medical tool, BMI is really just one way among many to ‘measure and classify the human body’ (anthropometry), but it’s certainly not the most effective of all.

In fact, many health professionals today question the usefulness of this index. Indeed, it’s not universal and it could even be more harmful than beneficial. Not only is it based on an unrepresentative sample, but it has deep methodological flaws.

So, have we been trapped in the wrong measurements of our bodies for more than a century? What implications for our physical and mental health could this have?

The Quetelet Index

Although BMI is used today to assess health status and disease risk in a population, its designer didn’t intend to measure health, even less to use it on an individual scale.

In 1832, when Adolph Quetelet (astronomer and statistician) developed the Quetelet index, he did so to define the characteristics of a ‘normal human’. He wasn’t interested in measuring health or obesity, the index was related to patterns within a population.

Quetelet developed the index using data exclusively from a male and Caucasian population. Therefore, it’s clear right from the start that it’s an index with a questionable scientific methodology.

Nevertheless, despite its biases and limits, the formula was successful, giving rise to the BMI we know today. Different variables, different name, different objective, but the same approach.

An outdated benchmark used by insurers to make more money

At the beginning of the 20th century, in the United States, American insurance companies were looking for a simple and fast way to earn more money. From the little data collected from subscription forms, they noted a higher death rate in men who were ‘overweight’.

For decades, doctors couldn’t agree on an index. Some divided weight by height, while others used weight times height cubed. Finally, a physiology professor, Ancel Keys, and his colleagues who were researching obesity published Indices of Relative Weight and Obesity. These were the results of a study in which they analyzed more than 7,400 men in five different countries.

Keys and his researchers went through all the different equations and it was Quetelet’s formula (weight divided by height squared) that turned out to be the best at ‘detecting’ excess weight. Keys renamed it the ‘body mass index’.

A good business, but a society with worse health

At the same time, the popular culture of the diet to lose weight was born and people looked for different formulas to achieve this goal. However, the (re)birth of the BMI and the slim/fit culture didn’t have the expected effects. In fact, many people went on diets, but this didn’t mean they had better habits or improved health.

Therefore, the BMI hasn’t acted as a support for physical health. As a matter of fact, in the last 50 years, the use of this index has contributed to the increase in metabolic diseases and the construction of a fat-phobic society, with the appearance of many eating disorders and all their associated deaths.

A problematic measure

BMI has been widely recognized as a problematic measure for several reasons:

• Its controversial origins. As we mentioned earlier, the BMI was developed by an astronomer and a statistician, not by a health researcher or a medical professional. He studied groups of men from extremely specific origins and not diverse populations.
• Weight or BMI is the same thing. BMI is directly related to weight and since height doesn’t change in an adult, only weight changes the BMI. So, tracking your weight with kilograms or with your BMI is exactly the same.
• Ethnocentrism. BMI is based on height and weight data from mostly upper-middle-class white Europeans. This means it’s not a representative sample of the general population and doesn’t take into account differences in average body size in other ethnic groups.
• Age, gender, and body composition. BMI doesn’t take into account muscle composition and body fat. A muscular person will be considered obese (because muscle weighs more than fat for the same volume) and yet their figure will be slimmer. Furthermore, their risk of metabolic complications will be lower.
• Misclassification. Differences between BMI classifications (normal weight, overweight, obesity, etc) are arbitrary. They’re not based on any scientific data but are defined by a handful of people with an idea of what a ‘normal’ weight should be.
• Misdiagnosis: This index assumes that we know the eating behaviors and health status of a person based solely on their height and weight.

An obsolete reference that we must stop using

Labeling someone with an overweight or obese BMI leads to discrimination, shame, and blame. It doesn’t motivate people to make better life decisions and does nothing to help them get out of the spiral of being overweight, if, indeed, this is necessary beyond purely aesthetical reasons.

In fact, public authorities, the general population, and health professionals have a great deal of substantive work to do to stop pigeonholing people and judging the health of others based on limited data or popular belief.

BMI misrepresents health. Its assumptions made about people in different categories are actually scientifically inaccurate.

Confusion regarding people’s health

A recent large-scale study found that nearly half of people classified as overweight and one-third of people classified as obese are actually metabolically healthy. This means they have completely normal levels of blood pressure, cholesterol, and blood sugar.

In contrast, almost a third of people in the ‘normal weight’ category were found to have metabolic diseases. What emerges from this study is that there is no disease that only affects people of high weight. People of all shapes and sizes are affected by these health problems.

Because it’s multifactorial, obesity must be redefined, particularly when we know that the first cause of death among obese people isn’t weight, isn’t BMI, but fatphobia. Anxiety caused by rejection and stigma causes inflammation and increased adiposity. It’s the reaction of others that increases anxiety and enforces people to play a game of Russian roulette.