Body mass index: why method of defining obesity may be wrong

“Weight height…”

This is how a medical appointment usually starts in 2022, but it could also be like this in 1972 or 1922.

Most of the time, this basic information of our anatomy is introduced into a mathematical formula to define the so-called body mass index (BMI).

For over 40 years, the simple formula for establishing BMI, which consists of dividing a patient’s weight (in kilograms) by the square of height (in meters), has helped doctors, insurance brokers and even gym instructors classify people into one of four fields: underweight, normal, overweight and obese.

But despite the simplicity and practicality of the BMI as a measurement tool, a growing number of experts question the value of the index as a diagnostic tool.

“You can’t interpret anything about someone’s health just by looking at their BMI,” Kendrin Sonneville, professor of Nutritional Sciences at the University of Michigan, tells BBC News Mundo, the BBC’s Spanish service.

A statistical measure

To understand the arguments in this debate, it is necessary to go back to the origins of the BMI.

The formula for measuring BMI was born thanks to the Belgian mathematician, astronomer and statistician Lambert Adolphe Quetelet, in 1832.

Quetelet is considered one of the “founders of the social sciences,” says Garabed Eknoyan, a nephrologist at Baylor College of Medicine in Houston, Texas, in a 2007 study.

“At the time he created his index, Quetelet had no interest in obesity,” he added. “His concern was to define the characteristics of the ‘normal man’ and fit the distribution to the rule.”

Quetelet was obsessed with understanding how probabilistic trends were reflected in human populations and began to study the relationship between height and weight.

“His pioneering studies of human growth led him to conclude that, with the exception of the growth spurts that follow birth and puberty, ‘weight gain is equal to the square of height,'” Eknoyan said.

At that moment, what was known for more than a century as the Quetelet index was born.

The role of insurance

In 1972, insurance companies in the United States began estimating the risk level of their customers by comparing their weight with the average weight of similar individuals.

Customers deemed “higher risk” paid more.

Infuriated by this, physiologist Ancel Keys conducted a study of 7,000 healthy people to show that the formula used by insurers was cumbersome and ineffective. The tool used by Keys was the Quetelet index.

With only one “brand” change – it went from Quetelet index to body mass index – the formula became the standard measure for establishing any individual’s healthy weight. That title remains to this day.

“The reason we use it is very silly,” explains Rekha Kumar, an endocrinologist at Weill Cornell University Hospital in New York, to BBC News Mundo.

“It’s a very cheap and quick tool to calculate, and the alternatives for making a similar measurement are expensive, complicated and difficult to access.”

But does the BMI work or not?

“BMI is not designed to be applied at an individual level,” Kendrin Sonneville, professor of nutritional sciences at the University of Michigan (USA), told BBC News Mundo.

“It’s a measure designed to characterize populations, such as, on average, what the body size of a given population is and how much that has changed over time.”

And there are a wide variety of factors that affect a person’s weight and health.

“People feel really bad if they’re told their BMI is too high, or people assume they’re healthy if their BMI is normal,” says Rekha Kumar.

“And none of that is necessarily true. BMI is just a tool to draw attention to what might be a health problem, but it’s not a good tool used in isolation to make assumptions about someone’s health.”

New Zealand nutritionist Lucy Carey told BBC News Mundo that BMI “does not provide any kind of information that can be used to determine an individual’s health status”.

“Maybe it’s useful at the population level, to identify trends over time.”

There are other critics with even more radical views.

“There’s really no benefit to calculating a person’s BMI,” says Jeffrey Hunger, an associate professor of psychology at the University of Miami in the US.

“We have to move away from this ‘Manichean’ view that only serves to perpetuate the belief that this might be a valid measure. The BMI deserves to be in the great dustbin of history,” says Hunger.

The dark side of BMI

“A father was telling me about his experience with a nurse,” said nutritionist Carey.

“The nurse told him that his 4-year-old was too heavy for his height. This 4-year-old started to stop eating, saying he was ‘too fat’.”

Carey encounters these types of cases all the time, which is why she wrote a column for the New Zealand Medical Journal about the risks of using BMI to determine whether or not a child is a healthy weight.

But the effects aren’t just about image problems, Carey argues. There is also the risk of affecting other lifestyle habits.

“If we put too much emphasis on their bodies and they don’t lose weight, we run the risk of saying ‘that didn’t make sense’ and they might end up dropping the good habits they’ve created,” he told the BBC.

“I see this over and over again. Patients who stop exercising because they haven’t lost weight even though they were dramatically improving their cardiorespiratory capacity.”

Professor Sonneville agrees. “There’s a good amount of evidence to show that when you focus on weight and make people feel bad about their weight, there’s a counter-intuitive effect on behavior. They feel less motivated.”

What can be done?

The experts we spoke to agree that we need to change the way medicine is practiced, focusing less on trends and more on the individual.

“Our systems and structures are very weight-focused. Our medical school education is based on metrics like BMI. But just because we’ve always done it that way doesn’t mean it’s right,” says Sonneville.

“We have enough information to know that we need to change, and I think those who have practiced medicine from a single point of view will have to adapt to new ideas and recognize the damage they’ve done by focusing so much on weight and BMI,” he says. .

Nutritionist Carey ensures that the change in approach will require less reliance on numerical tools such as BMI.

“There’s definitely a relationship between body fat and health. But in the real world, if we can get people to sleep better, cook at home, have meals with their loved ones, all of that will improve their health and well-being, regardless of whether their weight changes.” or not.”