- Rafael Abuchaibe
- BBC world news
This is how a doctor’s appointment usually starts in 2022, but it can also be like this in 1972 or 1922. Most of the time, this basic information about our anatomy is fed into a mathematical formula to define what is called a 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 their height (in meters), has helped doctors, insurance brokers and even gym instructors to classify people in one of four camps: underweight, normal, overweight and obese.
But despite the BMI’s simplicity and convenience as a measurement tool, a growing number of experts are questioning the index’s value as a diagnostic tool.
“You can’t interpret anything about a person’s health just by looking at their BMI,” Kendrin Sonneville, a professor of nutritional science at the University of Michigan, told BBC News Mundo, the BBC’s Spanish-language service.
A statistical measure
To understand the arguments of this debate, we must 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”, explains Garabed Eknoyan, a nephrologist at Baylor College of Medicine in Houston, Texas in a study from 2007.
“When he created his index, Quetelet was not interested in obesity,” he adds. “His concern was to define the characteristics of the ‘normal man’ and adapt the cast 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.
It was then that what was called the Quetelet index was born for over a century.
The role of insurance
In 1972, American insurers began estimating the risk level of their customers by comparing their weight with the average weight of similar individuals.
Customers considered “high risk” paid more.
Furious at this situation, 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 just one “brand” change – from the Quetelet Index to the Body Mass Index – the formula has become the standard measure for establishing any individual’s healthy weight. This title is still valid today.
“The reason we use it is very silly,” Rekha Kumar, an endocrinologist at Weill Cornell University Hospital in New York, told BBC News World.
“It’s a very cheap and quick tool to calculate, and the alternatives to performing 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 the individual level,” Kendrin Sonneville, professor of nutritional science at the University of Michigan (USA), told BBC News World.
“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 range of factors that affect a person’s weight and health.
“People feel 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 a person’s health.”
New Zealand nutritionist Lucy Carey told BBC News World that BMI “does not provide any kind of information that can be used to determine an individual’s health status”.
“This can be useful at the population level, to identify trends over time.”
Other critics have even more radical views.
“Calculating a person’s BMI doesn’t really do any good,” says Jeffrey Hunger, an associate professor of psychology at the University of Miami, USA.
We must move away from this “Manichean” view which only perpetuates the belief that such a measure could be valid. BMI deserves to be in the big basket in history,” says Hunger.
The dark side of BMI
“A father told me about his experience with a nurse,” says nutritionist Carey.
“The nurse told her that her 4-year-old was too heavy for his height. This 4-year-old started to stop eating, saying he was ‘too big’.”
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 at a healthy weight.
But the effects aren’t just about image issues, says Carey. 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 giving up the good habits they’ve created.” told the BBC.
“I see it all the time. Patients who stop exercising because they haven’t lost weight, although they have considerably improved their cardiorespiratory capacity.”
Professor Sonneville agrees. “A lot of evidence shows that when you focus on weight and make people feel bad about their weight, there is a counterintuitive effect on behavior.
What can we do ?
The experts we interviewed 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-based. Our medical school training is based on metrics like BMI. But just because we’ve always done it that way doesn’t mean it’s good,” says Sonneville.
“We have enough information to know that we need to change, and I think those who have practiced medicine from a unique perspective will have to adapt to the new ideas and recognize the damage they have done. it says.
Nutritionist Carey says the change in approach will require less reliance on digital tools like BMI.
“There’s definitely a link between body fat and health. But in the real world, if we can get people to sleep better, cook at home, eat meals with loved ones, all of that will improve their health and well-being, with or without changes of weight.”