The BMI Explained: How Useful Is It Really?

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The BMI is a measure used to determine if you are a healthy weight for your height. It is calculated by dividing your weight in kilograms by your height in metres squared, to place you in a category of underweight, healthy weight, overweight or obese.

But is it useful or does it give us misleading and inaccurate information about our health?

Professor Garry Jennings, National CEO of The Heart Foundation, told The Huffington Post Australia the BMI is an effective measure to determine whether a person is overweight or obese.

“Most people with a high BMI are overweight or obese,” he said.

However, this should be taken with a small pinch of salt. Margaret Hays, Accredited Practising Dietitian and Spokesperson for the Dietitians Association of Australia, told HuffPost Australia the BMI is more useful for measuring populations rather than individuals.

“The BMI is useful just for a very quick appraisal, but it's not anything you can really get your teeth into,” she said.

The problem with the BMI is that it doesn’t account for where the fat is stored, which provides a far greater indication of a person’s likelihood of developing obesity-related diseases than weight alone.

Hays also said different body shapes must be considered.

“The ‘apple’ shape is much more susceptible to chronic disease than the ‘pear’ shape,” she said.

Professor Jennings agreed. “You can be pretty skinny and have a pot belly and be at a high metabolic risk,” he told HuffPost Australia.

Another problem with the BMI is that it does not take into account a person's muscle mass or fat mass. This means someone with a high muscle mass, like a rugby player, may be categorised as overweight or obese, when, in fact, they are heavy because they are strong. On the other hand, elderly people or those who do not exercise regularly may have a low muscle mass, which categorises them in the healthy weight range despite being overweight.

As a result, the BMI does not work for children and teenagers, elderly people, pregnant women and some races, as they don’t fit into the stock standard measurements.

However, Jennings said it is very important for people to know which weight category they belong to.

“The first step in doing something about obesity is measuring it,” he said.

He said combining the BMI with a waist circumference measurement is the best way to start, and then if you’ve got any concerns you can see your doctor.

“We’d encourage people to keep an eye on their BMI and their waist circumference. It is much harder to get weight off than keep it off in the first place,” he told HuffPost Australia.

Measuring around the waist (at the belly button) provides a better estimate of visceral fat -- the ‘bad fat’ stored in the abdominal cavity and therefore around many important organs.

For women, measurements of above 80 cm put you at an increased risk of developing diseases, and measurements over 88 cm put you at a high risk. For men, it is 94 cm and 102 cm respectively.

Another alternative to the BMI is a DEXA scan (or DXA scan) which measures body composition and therefore provides more information about where the fat is stored inside your body. DEXA scans usually cost between $90 and $120 for an initial consultation.

Jennings also advises people to consider other factors that place them at increased risk of developing diseases, such as smoking, poor diet and excess alcohol consumption.

If maths isn't your strong suit, you can use the BMI calculator below to see how you measure up.

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