This conversation is not new: Body Mass Index (BMI) is junk science. For the past ten years, experts in the media have repeatedly pushed against this diagnostic tool, labeling it as a scam and a terrible measure of health.
Established in the nineteenth century by the Belgian social scientist and statistician Lambert Adolphe Jacques Quetelet, the body mass index, formerly referred to as Quetelet’s index, is a mathematical construct that was derived based on a population of White European (French and Scottish) participants. Notably, Quetelet was not a medical doctor or a health practitioner, and he had no interest in obesity. He is known for introducing the concept of ‘‘social averages.’’ His work describes “l’homme moyen” or “the average man”, and the distribution of various human characteristics around the ‘‘average man” to obtain a bell-shaped curve or normal distribution.
BMI is derived by dividing the weight in kilograms by the height in meters squared. The result falls into one of six categories. A BMI of 25 is considered “pre-obese”, while a BMI of 30 is in the “obesity I” category. These cutoff values are arbitrary, and it is unclear why those ranges were chosen or what they represent.
| BMI | Nutritional Status |
| Below 18.5 | Underweight |
| 18.5–24.9 | Normal weight |
| 25.0–29.9 | Pre-obesity |
| 30.0–34.9 | Obesity class I |
| 35.0–39.9 | Obesity class II |
| Above 40 | Obesity class III |
There is an agreement in the medical literature that BMI is inaccurate and inherently flawed. It does not distinguish fat from muscles and bones (fat-free mass) and does not indicate the location or distribution of fat. Since weight and adiposity are different measures, BMI overestimates fatness in individuals who have a high muscle mass. For example, Sprinter Usain Bolt’s BMI, calculated as 24.5, is only just within the “normal weight” range – bordering on “pre-obese”. This is clearly nonsensical, and results like this could actually harm public health policies on obesity: when BMI is ridiculed for classifying athletes as obese, the public may become increasingly skeptical that obesity is a public health issue.
Yet, BMI is deeply integrated into healthcare systems around the world. The Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) define obesity in terms of BMI, despite the debate among experts on its effectiveness as a measure and concerns that is not applicable to all people – for example, BMI produces unhelpful results when applied to people with high muscle-mass, women, and seniors.
BMI also does not reflect ethnic variations in body composition. Several studies indicate that BMI inaccurately classifies African Americans in particular as obese, thus overestimating the prevalence of obesity and overstating the difference in obesity rates between white people and African Americans. A measure built by and for white people is not necessarily accurate for people of colour, and can even lead to misdiagnosis and mistreatment. A 2003 study on obesity showed that, compared to white people, Black people are at a lower risk and Asians are at a higher risk at the same BMI. Consequently, BMI overestimates health risks for Black people, and underestimates health risks for Asian communities.
BMI is not a measure of the current health risks of individuals. Quetelet’s work was clearly intended to be used as a population evaluation tool, rather than to assess the health of individual people. Its prevalence in clinical settings can mostly be attributed to its convenience in offering a measurement via a simple formula based on easily collected data (height and weight of an individual). While it can be a useful tool in epidemiologic research, BMI does not accurately describe individual health.
Science has repeatedly debunked several myths about weight, diet, and health. Yet, BMI is still the current criterion used to label individuals as “healthy” or “unhealthy” in popular discourse, and in media further reinforcing “the stigma of fat bodies as diseased bodies”. Being a convenient metric contributes to its widespread use, despite its theoretical limitations that challenge its reliability as a “proxy for healthy weight”.
As a single measure, BMI is a misleading indicator of health and must be used in context with other data for any given individual. It is time to look beyond the BMI and adopt a more accurate metric that encompasses confounding variables such as gender, ethnicity, age, genetic factors, comorbidities, and other factors. Rooted in racist assumptions, BMI lacks demographic and cultural generalisability. BMI is a product of its time – it is not a valid diagnostic of fatness, or of an individual’s overall health.
