BMI: IT'S FLAWED, SO WHY DO WE USE IT?

A report published by the Women and Equalities Committee says that the use of the Body Mass Index should be scrapped. Is there a better way to calculate if you’re a healthy weight?

- DR GILES YEO

Barely a day goes by without a news piece or television documentary breathlessly reporting on the current pandemic of obesity. Nearly 60 per cent of us in the UK and US are either overweight or living with obesity. Where does this ominous 60 per cent figure originate from? Well, it has emerged from population-wide BMI statistics.

BMI, or Body Mass Index, is the ratio of body weight in kilograms divided by the square of one’s height in meters, and hence is represented as kg/m2. A ‘normal’ BMI is 20-25kg/m2. Anything below a BMI of 18 would be considered underweight, a BMI of 25-30kg/m2 is considered overweight, and if you have a BMI north of 30kg/m2, you would be classed as living with obesity. The reputation of BMI however, has in recent years been tarnished. At best, it’s considered a poor proxy for fat mass and health; at its worst, it can and often is used as a cudgel to ‘fat-shame the larger among us in society. But what has BMI done to deserve this, and should it be replaced with anything else?

The problem is that BMI as a measure of ‘fatness’ is flawed because it is derived using purely your weight and your height. Thus it cannot, for instance, differentiate between a rugby player and a Joe Public of similar height and weight, but carrying substantially more fat. So why not just measure the amount of fat instead?