BMI Is Racist and Sexist? Yes, Just as Numbers Lie

Oh my, we do love to hate BMI. Monday on Huffpost, Christine Byrne dispensed the latest argument against this villainous measure. BMI is racist. What’s more, she tells us it’s sexist, too. Definitely, this sounds bad, this composite index of height and weight.

Is this a case of an innocent little number worming its way into our psyche and making us into bad actors? Or is it simply a measure that too many people misunderstand and misuse?

The Case Against BMI

Byrne’s argument against BMI is simple. It does not give a full picture of well-being. A Belgian mathematician conceived it in 1832 and he used data from mostly European men to evaluate it. She contends that the medical community uses it as a measure of individual health and concludes:

In short, the way BMI is being used is unscientific because of its origins and the homogenous population it was created from.

Numbers Are Not Racist

To the extent that Byrne is saying that many people misuse BMI, she is right. People have many ways to express bigotry. Stereotypes about race, ethnicity, gender, and size are offensive. Mix them all together and you can get especially noxious attitudes and bigoted behavior. If a healthcare professional uses BMI as an excuse for this, the problem lies with the professional who’s acting unprofessionally. Not with a simple index that is a crude surrogate for adiposity.

BMI Has Its Problems

As a very crude indicator for adiposity, BMI can be useful. But Byrne is correct. It is not a measure of health. Anyone who uses it as a measure for health doesn’t understand BMI and certainly doesn’t understand obesity.

As an epidemiological tool for assessing population health risks, BMI is useful because it is so simple. But thresholds for elevated risks are different in different populations. A recent study by Sean Coffey and colleagues, for example, found that BMI might not be reliable for people of Māori ethnicity. Coffey was looking narrowly at measures of epicardial fat, so this study is hardly the final word.

Many other studies have shown that BMI cutoffs should be different in different ethnic population. In 2004, the World Health Organization advised different Asian populations should consider different thresholds for BMI as a surrogate for health risks. These thresholds differ from the number for European populations.

Crude, But Useful

All of this tells us that BMI is a crude indicator of health risk. Because it’s simple, it can be useful. But for individuals, it’s not and never will be diagnostic. BMI is just a number – it’s not racist. When people misuse it or misunderstand it, the problem lies with the people who are doing that. Not with the index.

Numbers don’t lie, but sometimes liars use numbers to serve their purpose.

Click here for Byrne’s commentary, here for the Coffey study, and here for the WHO guidance on BMI in Asian populations. For an explanation of BMI from mathematician Diana Thomas, who understands it well, have a look at this video.

Out for a Walk, photograph © Obesity Action Coalition / OAC Image Gallery

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July 22, 2020