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Worried about your festive BMI? You may not need

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The writer is a scientific commentator

After Christmas is often an opportunity to take stock, whether it is to take stock of the end of the year or to prepare for the year to come. For others, it’s simply a time to weigh in: stepping on the scale while reflecting ruefully on the amount of turkey dinners, tarts and sherry consumed.

The seasonal phenomenon of restaurant remorse may well include a panicked estimate of body mass index, calculated by dividing weight in kilograms by height squared (height measured in meters). The reassuring news is that some people with BMIs above the World Health Organization’s “healthy” range may have less reason to worry than previously thought. This year, the American Medical Association said the index was an “imperfect measure” of clinical health and “misleading about the effects of body fat on mortality rates.” Doctors said BMI should also not be used alone to deny insurance reimbursement.

The waning popularity of the BMI reflects a growing recognition that this measure is loaded with both scientific and historical baggage, making its reevaluation a wise decision. The index is intended to reflect adiposity, or body fat levels, which, in theory, should clearly translate into risk for weight-related diseases such as heart disease and diabetes.

But the optimal range of 18.5 to 24.9 does not take into account an individual’s body shape, nor the different proportions of muscle, fat and bone. It is deceptively high, for example, for muscular athletes. It has also historically ignored non-white populations and originated in a scientific preoccupation with what is normal, desirable, or ideal, giving it a dubious association with eugenics.

“I think it’s time to stop using BMI alone,” says David Stensel, professor of exercise metabolism at Loughborough University in England and editor-in-chief of the International Journal of Obesity. Some of the people considered overweight (BMI 25 to 30), he told me, may not suffer from disease throughout their lives; some studies even suggest a health benefit. Adding blood pressure or cholesterol levels, Stensel adds, gives a truer picture of health.

To complicate matters, not all populations present the same risks at the same weights. For those of South Asian descent, including me, the upper threshold for good health is lower, at 23, reflecting an increased risk of diabetes. For African American women, it might be closer to 28. For these reasons, Edmonton’s obesity classification system, which prioritizes weight loss for those with severe obesity, wins field in the medical world.

Still, Stensel doesn’t believe BMI should be abandoned altogether; its use in thousands of studies over several decades allows large-scale comparisons. And that sums up its value well: BMI reflects overall risk at the population level quite well, but loses its power when associated with an individual. “If your BMI is in the 40s or 50s, your chances of developing diabetes are much higher than if it’s in the 20s,” says Stensel. “But a person with a BMI of 30 might never develop diabetes.” It’s a question of probability, not destiny.

Today, waist circumference (or waist-to-hip ratio) is considered a useful alternative or additional measurement, as it is believed that carrying fat near vital organs is risky; Interestingly, middle-aged men’s bellies could explain why men are more vulnerable to heart disease than women. Other ways to assess body fat include using a caliper to measure skin folds; bioelectrical impedance analysis, which involves passing a current through the body (more fat equals more resistance); weighing underwater (fat floats more than bones or muscles); MRI; and dual X-ray absorptiometry (Dexa), which analyzes adipose tissue, lean mass and bone density.

Science has certainly evolved since the 1830s, when Belgian mathematician Adolphe Quetelet began collecting statistics on the average man, or the average man. He calculated that adult weight and height could be linked according to a formula – and his anthropometric research attracted the attention of Francis Galton, founder of the 19th-century eugenics movement. The Quetelet index also appealed to 20th century actuaries, who sought to quantify the link between corpulence and premature death for insurance companies. In the 1970s, the formula was rediscovered and renamed BMI; WHO adopted guidelines on this subject in 1995.

Nearly three decades later, the wisdom is changing once again. Don’t lose weight out of vanity or to reach an arbitrary number, Stensel advocates, but rather “to engage in life and live the life you want.”

It’s a timely message of balance, moderation and hope – to which I will happily raise a glass.

Gn En gealth

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