To (B)MI Or Not To (B)MI

The debate around Body Mass Index (BMI) still captures our attention. BMI has long been a crucial tool in the field of health and weight management worldwide. However, questions about its effectiveness and relevance in today’s diverse population have come to light, sparking debates amongst health professionals.

For many decades, BMI has been the primary global metric used in assessing weight-related health risks. The NHS, amongst other health institutions, employs it as the first point of assessment for treatments related to weight. However, in a significant shift, the American Medical Association (AMA) announced last month that it plans to move away from the BMI. Their reasoning? Measurements of waist circumference are thought to provide a more precise insight into the risk of developing conditions such as diabetes, high blood pressure, and certain forms of cancer.

Adding fuel to the fire, recent research from Tel Aviv University indicates that the BMI may not be as reliable as once believed. The study found that a considerable number of people, up to a third, with a BMI in the ‘healthy’ range were, in fact, overweight. According to the study’s lead author, Professor Yftach Gepner, it appears that body fat percentage might offer a much more accurate health indicator than the BMI.

Delving into the history of the BMI, it was first formulated by a Belgian astronomer two centuries ago to gauge the size of the average man. The BMI later transitioned into an obesity metric in 1972, based on the formula of weight divided by height squared. Although it was initially focused on white European men, adjustments were made to include averages for women. Yet, BMI has been less effective for people from some ethnicities and age groups, as it fails to distinguish the nuanced meaning of weight for different individuals.

Take for example highly fit individuals and athletes. The BMI does not differentiate between muscle and fat, leading to inaccurate categorizations. If we were to apply the BMI to Arnold Schwarzenegger during his prime, he would be categorized as obese, which, by any standard, is a misrepresentation. A study published in the British Medical Journal Heart underscored this point by revealing that two-thirds of rugby forwards in the previous year’s Six Nations Championship had a BMI over 30, which according to BMI metrics, classified them as obese.

The BMI’s generic categories are quick to categorize who might need medical attention, but they often misrepresent individuals. As Professor Giles Yeo of the University of Cambridge states, while the principle that “the higher the BMI, the more fat we tend to carry” is generally true, for many individuals, the results are not applicable.

Furthermore, the standard BMI categories fail to reflect the health risks of non-white patients accurately. An Asian person, for instance, is at a much higher risk of developing weight-related illnesses when their BMI crosses 23, a number considered healthy according to the BMI’s scale for a white person.

Waist circumference measurement is emerging as a potential replacement for BMI. Measuring the waist is a relatively simple, cost-effective method of assessing visceral fat — the fat that wraps around organs, elevating the risk of diseases such as type 2 diabetes and heart disease.

Several studies have suggested that waist measurement might be a more efficient tool than BMI in managing weight. A study from Imperial University in 2019 found waist size to be a better predictor of heart attacks in both men and women. Another study published in the journal Circulation noted that women with larger waist sizes had nearly double the risk of dying from heart disease than those with smaller measurements. The British Heart Foundation advises specific waist measurements for men and women to be considered “low risk.”

Another intriguing measurement tool gaining attention is neck circumference, which may indicate obesity more accurately than waist measurements. It signifies excess fat storage in the upper body, linked with increased chances of obesity and coronary artery disease.

For individuals seeking a more precise risk assessment, the ABSI (a body shape index) could be useful. The ABSI incorporates multiple factors like waist circumference, sex, height, age, and weight to predict the risk of premature mortality.

The DEXA (dual-energy X-ray absorptiometry) scan, available in hospitals and private clinics, is considered the gold standard for analysing fat distribution. It could be particularly beneficial for older people as the BMI often underestimates their body fat levels by not considering the decline in muscle mass with age.

Edited from The Sunday Times (July 22, 2023).

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