January 1, 2024
3 min learn
The physique mass index is flawed, and drugs now has higher choices to measure weight problems
In response to commonplace calculations, my husband’s physique mass index (BMI) is just too excessive. But he’s the fittest individual I do know—an athlete carrying loads of muscle and little or no fats.
Therein lies the issue with BMI. Derived by dividing somebody’s weight in kilograms by the sq. of their peak in meters, a BMI quantity classifies an individual as underweight (lower than 18.5), regular weight (18.5 to 24.9), obese (25 to 29.9) or overweight (30 or extra). However that straightforward method obscures crucial particulars such because the distinction between muscle and fats. In the case of particular person well being dangers, these particulars inform the true story.
The shortcomings of BMI have been acknowledged for many years. But physicians stored utilizing it as a fast method to diagnose weight problems and as a proxy for total well being. “It made life very easy,” says scientific psychologist Cynthia Bulik, founding director of the College of North Carolina Middle of Excellence for Consuming Issues. “It additionally led to a kind of tunnel imaginative and prescient.”
Equating a barely excessive BMI with poor well being is not at all times correct. The American Medical Affiliation acknowledged as a lot when it introduced final June that BMI alone is an imperfect measure and that scientific follow wants to vary.
The brand new considering on BMI doesn’t negate the necessity to deal with the well being dangers related to a excessive body-fat proportion. In 2013 the AMA acknowledged weight problems as a illness and famous that it typically results in many harmful circumstances, together with most cancers, diabetes and coronary heart illness. The dangers of weight problems have not modified. In the interim, nonetheless, insurers nonetheless depend on BMI to find out folks’s eligibility for bariatric surgical procedure and most weight-loss medicine, together with standard new choices akin to Wegovy.
A current examine confirmed simply how imprecise BMI could be. Yftach Gepner, a physiologist and epidemiologist at Tel Aviv College, and his colleagues checked out knowledge on about 3,000 Israeli women and men. Roughly one third of these whose BMI positioned them within the regular vary had been discovered to be overweight when their precise physique fats was measured. And a 3rd of those that had been recognized as obese by their BMI had regular quantities of physique fats. “In case you are combining the misclassification on each side,” Gepner says, utilizing BMI to find out weight problems “is like flipping a coin.”
Not solely does BMI fail to differentiate between muscle and fats, nevertheless it says nothing about the place that fats sits in your physique, says Priya Jaisinghani, an endocrinologist and specialist in weight problems at N.Y.U. Langone Well being in New York Metropolis. With fats, as with actual property, location issues. Stomach fats confers greater danger, as does fats round very important organs. A 2018 examine checked out magnetic resonance imaging scans—the gold commonplace for body-composition analysis—collected by the U.Okay. Biobank, a big biomedical database. The outcomes confirmed that folks with fats concentrated of their stomach had greater danger for kind 2 diabetes, coronary heart illness and metabolic illness than did folks with the identical BMI and of the identical age whose fats was unfold by means of different elements of their physique.
An extra downside is that BMI relies on peak and weight tables developed utilizing knowledge from non-Hispanic white folks, largely males. But researchers now know that race, ethnicity, intercourse and age have an effect on physique composition and well being dangers otherwise. As an example, Black folks are inclined to have higher muscle mass and thus could also be misclassified as overweight on the idea of BMI. The other is true for Asians, who are inclined to have extra physique fats at decrease BMIs, so their precise illness dangers could also be missed.
Though BMI has worth for assessing weight problems at a inhabitants degree, higher strategies exist for people. “The secret is to not use BMI by itself as an index of well being,” Bulik says. To correctly assess well being, medical doctors ought to mix the index with measures akin to waist circumference, blood stress and levels of cholesterol. Bioelectrical impedance evaluation, which makes use of electrical indicators to inform fats from muscle within the physique, is turning into extra widespread in medical workplaces. “In a only a few years it’s going to change into commonplace,” Gepner says. DEXA scans, a kind of x-ray imaging that may distinguish between muscle and fats, and MRI additionally can be utilized to measure physique fats, though they are typically extra pricey and are subsequently much less accessible.
With so many options accessible, nobody, together with health-care staff, ought to give BMI an excessive amount of weight.
That is an opinion and evaluation article, and the views expressed by the writer or authors will not be essentially these of Scientific American.