One of the most popular ways of telling if you're a healthy weight is bogus - here's what you should do instead




  • In Science
  • 2022-10-03 16:10:32Z
  • By Business Insider
BMI is not the best predictor of ill health.Getty Images
BMI is not the best predictor of ill health.Getty Images  
  • BMI was invented in the 1800s and says next to nothing about health.

  • Waist measurement gives you a better idea of whether you may be at risk.

  • A leading obesity expert explains why BMI is bogus and how to better assess risk of overweight.

This scene happens in doctor's offices every day. A doctor takes a patient's measurements. Their weight is put into a nifty chart that estimates the Body Mass Index (BMI), a metric based on the ratio of weight to squared height.

If the weight falls above the cut off for obesity or severe obesity, the prognosis will be terrifying. The doctor may inform the patient that they are at risk of developing cancer, diabetes, and all other matters of health issues. The only answer: lose weight.

The problem is, BMI was never really meant to be used that way.

"It doesn't tell you anything about the health of a person," Dr Fatima Cody Stanford, an obesity medicine physician and associate professor of medicine and pediatrics at Harvard Medical School, told Insider.

Here's why BMI is a bogus way to evaluate your health and what you should be paying attention to instead, according to an expert.

A woman in activewear looks at stairs before running up them
A woman in activewear looks at stairs before running up them  

Higher weight is not a death sentence

Having a higher body weight increases the risk of developing a life-altering disease such as diabetes or high-blood pressure. But it is not a diagnosis.

"You can be lean and have significant disease and you can have obesity as characterized by BMI and not have as much significant disease," she said.

Stanford says the problem with BMI is that it puts too much emphasis on a target weight and that can be misleading.

Stanford recalls the case of a patient who weighed 550 pounds and had obstructive sleep apnea and low testosterone levels, in spite of an active lifestyle.

He was able to bring his weight down to 300 pounds, at which point he no longer had those health issues.

"His BMI is still in a very high category, right?" she said, "But is severe obesity causing any health problems at this point? I would say no."

A woman smiles while carrying a yoga mat
A woman smiles while carrying a yoga mat  

BMI is an arbitrary and outdated metric

When BMI was created, it had nothing to do with medicine.

It dates all the way back to Albert Quetelet, a statistician and sociologist from the early 1800s whose life work was to find the dimension of the perfect or "average" human.

His thoughts on what an ideal human looks like were later used to justify racist and eugenic policies and have been discredited.

He was the first to define a metric relating weight to height squared, which later formed the basis of BMI. To do so, Quetelet only looked at white French and Belgian men, a very narrow representation of humanity.

Quetelet never intended the ratio to be linked to health. That came in the 1900s when the Metropolitan Life Insurance Company used Quetelet's formula to build actuarial tables based on height and weight. Again, they used data from their mostly white policyholders.

So the metric was only designed to measure "your risk of dying if you're white," and it is deeply biased, said Stanford.

Studies, including one led by Stanford, have shown that Black and Asian people may not be at the same risk as white people with the same BMI.

body diversity
body diversity  

Waist size is a better, but not perfect, predictor of ill health

Scientists are evaluating the risk factors behind cardiometabolic issues like diabetes, heart disease, or stroke - altogether called the metabolic syndrome - and it's much more complicated than just having excess weight.

For instance, "where we carry our adipose or fat is much more important than how much fat we have," Stanford said.

That's because not all fat cells are created equal. Fat that wraps around internal organs is much more likely to lead to disease.

On the other hand, fat wrapped around the hips may have a protective effect, said Stanford, for instance against heart disease.

So while BMI can be a useful early screening tool, a more reliable measure of risk associated with weight is the size of the waist, said Stanford.

Women with a waist size over 35 inches, and men over 40 inches, are at higher risk of developing metabolic diseases, says Stanford.

Regardless, without other measures of ill health - such as blood pressure, fasting blood sugar, or cholesterol, insulin, and testosterone levels - it is very difficult to tell if a person is unhealthy, says Stanford.

A healthcare provider measures the waist size of a patient.
A healthcare provider measures the waist size of a patient.  

What if your doctor only looks at BMI?

Some doctors will use BMI to assume ill health and prescribe weight loss, said Stanford.

Stanford advises patients whose doctor only focuses on BMI to challenge them.

"Say: 'Okay doctor, I hear that. And I do see that BMI does exceed these guidelines, but how does this relate to my current health status?', which will probably challenge them because they've only thought to think of it in terms of BMI," she said.

Erin Brodwin contributed reporting to a previous version of this article 

 

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