Does BMI measure up?

From ABC Health and Wellbeing, 19 June 2014

Do you know your Body Mass Index, also known as BMI? It’s easy to calculate – simply take your weight in kilograms and divide it by the square of your height in metres.

If the number you reach is between 20 and 25, congratulations. You are one of a minority of Australians who fall into the normal weight range.

If the number you reach is between 25-30, then you can count yourself among the 35 per cent of adult Australians who are overweight. If the number you reach is over 30, then you are among the 28 per cent of adult Australians who are classified as obese.

Or are you?

The Body Mass Index was first described by Belgian mathematician Adolphe Quetelet in 1832, who had an interest in the cross-sectional study of human physical characteristics. It was known as the Quetelet Index until American scientist Ancel Keys coined the term Body Mass Index in 1972, when it become clear that BMI was clearly linked with obesity and other health conditions.

Since then, BMI has become the standard method of working out whether someone is of healthy or unhealthy weight.

“By definition, a large BMI means you’ve got more weight for height than most people, or than you should have, and a low BMI means you haven’t got enough,” says Associate Professor Jonathan Shaw, Head of CardioMetabolic risk at the Baker IDI Heart and Diabetes Institute.

The general assumption with a high BMI is that the extra weight is fat, but this is not always the case.

“Arnold Schwarzenegger, at the peak of his career had a high BMI and everybody could see that there wasn’t an ounce of fat on him,” says Shaw. Arnie’s extra weight came from his muscle, and many professional sports players are technically overweight or obese without carrying much extra fat.

This reveals the major flaw in BMI – it cannot distinguish fat from muscle. Individuals who have a lot of muscle may find themselves being described as unhealthy when in fact the opposite is true. Read more. 

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