BMI’s failure as a health indicator for all

How a measure of health actually harms black people

Writer: Isabella Boyne
Editor: Ebani Dhawan
Artist: Sophie Maho Chan

The invention of BMI

Body Mass Index (BMI) is a value derived from the weight and height of a person. It is defined as body mass divided by square of the body height, and is expressed in kg/m2. It is used to categorise individuals as ‘underweight’ (BMI of 18.5), ‘normal weight’ (18.5 to 25), ‘overweight’ (25 to 30) and ‘obese’ (over 30). BMIs outside of the ‘normal range’ are often associated with higher rates of mortality and various cardiovascular illnesses.

BMI was devised by Adolphe Quetelet between 1830 and 1850. Quetelet, a mathematician and statistician, created the BMI measurement as a part of what he called “social physics”, a field of science in which mathematics is used to understand the behaviour of groups of people. As such, Quetelet’s intention had nothing to do with the measurement of obesity, but rather he wanted to find an equation that fitted the weight to height relationship to indicate the standard proportions of an “average man”. This meant that throughout Quetelet’s life, and even after his death, his equation indicating the ratio of weight to height of the average adult, had minimal impact on the medical community.

The term “body mass index”, as we now use in its modern sense, was first introduced in a paper published in the Journal of Chronic Diseases’ 1972 edition by Ancel Keys, in which Keys suggested that BMI was a satisfactory indicator of relative obesity. However, in his paper, Keys explicitly said that BMI was to be used to observe the general population and the increasing obesity rates in wealthy Western countries, and that it was inappropriate for individual evaluation and diagnosis. 

BMI as a measure of individual health

This new measure became very popular amongst researchers as a cheap and easy test that allowed them to perform new studies with hundreds of thousands of participants, retrospectively estimating obesity levels throughout history. The measurement eventually spread into the medical community, as doctors wanted to implement BMI as a simple way of measuring body fat for individuals. In 1985, the National Institutes of Health began to define obesity according to BMI, based on the idea that doctors would use BMI cutoffs to caution patients who were at higher risk for obesity-related illness. At the time, the cutoff for men was 27.8 and for women 27.3. But in 1988, the NIH consolidated the measurement for men and women, even though BMI and body fat is greatly influenced by sex. 

BMI, furthermore, is a poor indicator of an individual’s health. There are plenty of people with normal BMI measurements that are unhealthy, and plenty of people with BMIs correspond to being over- or underweight who are completely healthy. American Football players are commonly used to show the failure of BMI as a health measure. Despite being professional athletes, there is not a single player for the Denver Broncos who has a normal BMI. On average, the Broncos’ players have a BMI of 30+ which would be considered obese. A new study has shown that BMI is also a terrible measure of cardiovascular health – almost half of those who were considered “overweight” had a healthy cardiometabolic profile (i.e. normal blood pressure, blood sugar, and cholesterol levels), and about a third of people with normal BMI measures had an unhealthy cardiometabolic profile. 

How does this affect black people?

Individuals with high BMI are often unfairly penalised in healthcare systems. For example, provisions set out by the Affordable Care Act and the Equal Employment Opportunity Commission in the USA allow employers to charge their employees up to 30% of healthcare cost due to being overweight or obese. Similarly, in the UK, life insurance providers categorise being overweight as a ‘high risk’ condition, leading to higher insurance premiums. This disproportionately affects Black people: a study has shown that compared to African Americans on average have a lower body fat percentage than Caucasians of the same sex, age, waist circumference, weight and height. This has resulted in obesity being overestimated in African Americans. Furthermore, a separate study concluded that White and Hispanic women have 2.9% greater percentage of body fat than black women for a given BMI. Therefore, body fat is actually likely to be lower in black people than white people, although BMI measures may not indicate this. 

It seems logical that BMI is especially inaccurate as a health measure for black people considering that Quetelet’s formula was derived only from the proportions of French and Scottish people, so was exclusively geared towards white Western Europeans. Similarly, Keys’ work in forming the modern BMI measurement used subjects from predominantly white countries, such as the USA, Finland, and Italy; the only non-Western participants in the study were from Japan and South Africa. In South Africa, Keys discovered that his conclusions applied to all “but the Bantu men” – so did not generalise to, and were not reflective of, the black participants studied.

Black bodies matter

While the medical community’s heavy reliance on BMI as a measurement of health is problematic for everyone, it clearly has a disproportionate impact on black people; it inaccurately identifies health problems and presents an unfair financial burden. This is further exacerbated by the fact that British-born black African employees earn 7.7% less than UK-born white British employees with a similar educational background and occupation. Social media accounts such as @medschool_racism and the disproportionate number of black people dying from COVID-19 have highlighted that racism is ingrained into medicine, and reevaluating BMI as a health measure would be one step towards alleviating it.

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