Worrying about your festive BMI? You may not need to

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Roula Khalaf, Editor of the FT, selects her favourite stories in this weekly newsletter.

The writer is a science commentator

The post-Christmas period is often a time for weighing things up, whether taking stock at the end of the year or preparing for the one ahead. For others, it is simply a time for weighing: stepping on the bathroom scales while reflecting mournfully on the quantity of turkey dinners, mince pies and sherries consumed.

The seasonal phenomenon of diner’s remorse might well include a panicked estimate of body mass index, calculated by dividing weight in kilogrammes by height squared (height measured in metres). The reassuring news is that some of those with a BMI above the World Health Organisation’s “healthy” range might have less to worry about than previously thought. This year, the American Medical Association declared the index an “imperfect measure” of clinical health and “misleading about the effects of body fat mass on mortality rates”. Nor, the medics asserted, should BMI alone be used to deny insurance reimbursement.

The thinning popularity of BMI reflects an expanding recognition that the measure is saddled with both scientific and historical baggage, making re-evaluation a sensible move. The index is intended to reflect adiposity, or levels of body fat, which in theory should translate neatly into risk for weight-related conditions such as heart disease and diabetes.

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

“I do think it’s time to stop using BMI on its own,” says David Stensel, a professor of exercise metabolism at Loughborough University in England and editor-in-chief of the International Journal of Obesity. Some of those deemed overweight (BMI of 25-30), he told me, may be disease-free throughout life; some studies even suggest a health advantage. Adding in blood pressure or cholesterol level, Stensel adds, gives a truer picture of health.

To complicate matters, not all populations show the same risks at the same weights. For those of South Asian heritage, including me, the upper healthy cut-off is lower, at 23, reflecting increased diabetes risk. For African-American women, it may be closer to 28. For these reasons, the so-called Edmonton Obesity Staging System, which prioritises weight loss for those with severe obesity, is gaining a medical following.

Still, Stensel does not believe BMI should be totally junked; its use in thousands of studies across decades allows for large-scale comparisons. And that sums up its value: BMI captures the big picture on population-level risk reasonably well but loses its power when pinned to an individual. “If your BMI is in the 40s or 50s, your odds of developing diabetes are much higher than if it’s in the 20s,” Stensel says. “But someone with a BMI of 30 might never develop diabetes at all.” It is about probabilities, not destiny. 

Today, waist circumference (or waist-to-hip ratio) is seen as a useful alternative or supplementary metric, because carrying fat close to vital organs is believed to be risky; interestingly, the paunch of the middle-aged male might explain why men are more vulnerable to heart disease than women. Other ways of gauging body fat include using calipers to measure skinfolds; bioelectrical impedance analysis, which involves passing a current through the body (more fat equals more resistance); underwater weighing (fat is more buoyant than bone or muscle); MRI; and dual X-ray absorptiometry (Dexa), which scans fat tissue, lean mass and bone density.

The science has certainly moved on from the 1830s, when the Belgian polymath Adolphe Quetelet began collating statistics on the l’homme moyen, or the average man. He calculated adult weight and height could be linked formulaically — and his anthropometric research attracted the attention of Francis Galton, who founded the 19th-century eugenics movement. The Quetelet Index also appealed to 20th-century actuaries, who sought to quantify the link between corpulence and early death for insurance companies. In the 1970s, the formula was rediscovered and renamed BMI; the WHO adopted guidelines for it in 1995.

Nearly three decades later, the wisdom is changing once again. Lose weight not for vanity or to chase an arbitrary number, Stensel urges, but instead “to engage with life and to live the life you want”.

It is a timely message of balance, moderation and hope — to which I shall gladly raise a glass.