YWM Engage: Our Rocky Relationship with BMI

We have a relationship problem with BMI – the body mass index. As with any rocky relationship, we can’t seem to live with it and we can’t seem to live without it either. Professor Robert Kushner described the the good, the bad, and the ugly of BMI better than ever in an opening session of the YWM Engage Convention in Orlando yesterday. He told us:

“We have medical students coming into training with serious questions about BMI that they pick up in social media. So I’ve had to do a really deep dive into the history and the science of it. Medical faculty everywhere are facing these questions.”

We are the beneficiaries of his struggles. He offered us four important thoughts to take home.

1. BMI Is Simply Math

BMI is nothing more or less than than a mathematical formula for expressing the relationship between height and weight as humans age from birth to adulthood. But people have a way of exaggerating what it really is. It is just weight in relationship to height.

2. It’s Useful, but Needs Adjustments

It really does a pretty good job of estimating adiposity and health risk at a population level – with one big caveat. For different racial and ethnic groups, the levels of BMI that predict health risk can be different. Thus, to interpret BMI appropriately, we need adjustments to benchmarks for different populations.

3. BMI Has Real Limitations

As useful as BMI can be for epidemiology or for a screening tool, it has very real limitations as an individual measure. It does not measure excess body fat, fat distribution, health, or fitness. It is not sufficient for a clinical diagnosis of obesity. Anyone who tries to tell you otherwise is badly mistaken.

4. Defining Clinical Obesity Requires More Than BMI

Taking all of this into account, we are delighted to know that we will soon have a better clinical definition of obesity that takes these facts about BMI and more into account. Francesco Rubino is leading an effort with Lancet to make this happen, and fortunately, Kushner has lent his expertise to this effort. We expect to see the results early next year.

Until then, if someone tries to tell you that BMI is a scam or BMI defines obesity, respond gently, with facts.

Click here for the slides from Kushner’s presentation.

Couple Walking Arm in Arm with a Child in the Rain, pencil drawing by Vincent van Gogh / WikiArt

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September x, 2023