BMI: Why Experts Ask For Better Ways To Assess Health Than A BMI | Obesity

Most of us are no stranger to body mass index: weight in kilograms divided by height in square meters.

At the population level, research tells us that having a higher BMI is associated with a higher risk of certain conditions, such as type 2 diabetes and high blood pressure. Obesity rates, according to the World Health Organization, have tripled globally since 1975.

But despite being enthusiastically adopted in doctors’ rooms and even by average people to quantify their body composition, BMI is far less useful as an individual health indicator.

Australian experts, who discussed the usefulness of BMI and possible alternatives at the International Obesity Congress in Melbourne last month, say health judgments based on BMI alone can be stigmatizing and potentially harmful. Now they are asking for more nuanced measurements beyond just taking a number by face value.

Risks linked to ethnicity

BMI is an imperfect proxy for body fat. “The risks of certain health conditions change as people gain weight, but BMI alone doesn’t tell us much about the individual’s health,” says Dr Priya Sumithran, who leads the Obesity Research Group. at the University of Melbourne. “It is difficult to define a BMI limit for optimal health.”

The concept of BMI originates from Adolphe Quetelet, a 19th-century Belgian astronomer and mathematician whose goal of describing “l’homme moyen” – the statistically average man – influenced the development of eugenics.

Initially called the Quetelet index, the term BMI was coined by researchers in 1972 who described the measure as “not entirely satisfactory” but preferable “over the simplicity of the calculation” and “at least equal to any other relative weight index as an indicator of relative obesity “. Fat as a medical problem only arose in the West in the last century, with American life insurance companies collecting data on weight and height and linking the information to mortality.

According to the official guidelines adopted by the WHO, a BMI below 18.5 is considered underweight. A score between 18.5 and 24.9 produces a “healthy” weight, 25 to 29.9 is overweight, and a score of 30 and above is considered obese. There are several caveats to this classification, including the fact that it is less applicable for tall, short, and muscular people.

The health risks associated with obesity are also influenced by ethnicity. Compared to Europeans, people of Polynesian descent have lower body fat levels for the same BMI. The metric also overestimates obesity in African Americans. For Asians, obesity-related health risks occur at a lower BMI, which has prompted countries like Singapore to revise their guidelines to rank 23 and over as overweight.

Obesity can also be defined as excessive accumulation of fat that presents a health risk, Sumithran says. “There will be people whose BMI is over 30 but who don’t have unhealthy fat accumulation.”

Another flaw of the index is that it does not take into account the distribution of fat. “If it is distributed more centrally, if it is around your internal organs, it will have more health consequences than the classic pear-shaped distribution around the buttocks,” says Professor Louise Baur, president of the World Obesity Federation.

Baur, who is also president of child and adolescent health at the University of New South Wales, says BMI is still a useful measure to illustrate population trends over time.

“We know, for example, that the percentage of people with a high BMI … has increased quite dramatically over the past few decades from what it was, in Australia and many other high-income countries,” he says. “In South Africa, women are more likely to have a high BMI than men … in China, boys are more likely to have a high BMI than girls of the same age.”

A Lancet Commission for Diagnostic Criteria for Clinical Obesity – an international panel of experts including Baur – is currently evaluating the use of BMI and discussing new ways to diagnose obesity, including whether there are better measures of body composition.

“You can’t rate nutrition based on body size”

Dr. Alex Craven, an obesity surgeon at Austin Health Melbourne, is concerned about what he sees as over-reliance on BMI as a single indicator of health for individuals.

“For some reason with obesity, we accept that we can give people advice based entirely on a number … to say, your BMI is this, so you’re automatically sick,” he says.

While BMI can be helpful, using it in isolation “would be the equivalent of your GP taking your heartbeat … and, without looking further, give you a diagnosis and medication based on that,” Craven says. .

The calls for other body composition measures are not new and the persistence of BMI is likely due to the simplicity of its calculation.

“BMI is pretty damn simple – a scale and a meter are all we need,” says Craven. “Just because something is convenient, [it] it does not automatically deduce the quality.

There are alternative metrics, such as the waist-to-hip ratio, which has been found to be a better predictor of cardiovascular disease (the downsides: waist measurements are difficult to measure accurately and Baur says the ratio doesn’t work for children), and the waist-to-height ratio, which can better predict mortality risk.

The Edmonton Obesity Staging Score measures the impact of obesity and takes into account conditions including diabetes, hypertension and osteoarthritis.

“I can teach my patients to use it in less than five minutes, I can teach young doctors, nurses and doctors to use it very quickly,” says Craven. “The only drawback is that you have to approach your patients with a little curiosity and ask questions about things that are often more important to them than their weight anyway.”

Dr Fiona Willer, a dietician and lecturer at Queensland University of Technology, is frustrated that a “panic over body weight” and rising obesity rates in recent decades have led to public health messages prioritizing. to weight control.

“Diet guidelines talk about weight before they talk about food,” says Willer. “You can’t rate nutrition based on body size.”

Focusing so intensely on weight could mean, for example, that the inborn errors of metabolism – rare genetic diseases – that affect weight are overlooked.

Willer’s doctoral thesis, which studied eating behavior and size acceptance, found that people who focused on a healthy lifestyle, regardless of BMI, had more nutritious dietary patterns.

“The poorest diet quality – the narrower diets that least likely meet nutritional requirements – were the ones that focused on weight and didn’t care about health,” he says. “They also had the worst levels of body acceptance.”

Baur says some researchers acknowledge that “by focusing on obesity we unintentionally create an environment that can trigger unhealthy eating behaviors or make people living in larger bodies feel uncomfortable.”

“There is a lot of stigma on the part of health workers that doesn’t help the situation,” he says. Even if a new definition of obesity were to replace BMI, Baur doesn’t believe it would “magically change people’s views on the stigma of weight.”

“I suspect that whatever word we use to describe people of larger build who have health complications as a result of this, that word will be stigmatized in due course, unless we change some of our thinking about this on a social level, and also on a social level. healthcare-professional “.

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