Steve Stannard: BMI skewed by body shape but health is one size fits all

If you start exercising, your body reaps the benefits immediately.Photo / Thinkstock
If you start exercising, your body reaps the benefits immediately.Photo / Thinkstock

A World Health Organisation report lists New Zealand as the 12th fattest nation on earth out of 197 countries. Why? According to its statisticians 27 per cent of Kiwis are classed as obese because their body mass index is 30 or greater.

What does this really mean? Should we be worried?

Body mass index (BMI) is a calculation based on your weight and height, not a measurement of body fatness. Are we really porkers or the victims of a statistical beat-up?

What BMI best represents is your shape. A high BMI, say 35, indicates you possess a large volume to surface area ratio, making you wide in the middle like a rugby ball. A low BMI, say 18, means you're most likely long and thin. Much like a pencil. What public health scientists know is that if you resemble a rugby ball, you're more likely to carry a lot more body fat than a 'pencil'.

Carrying too much body fat is associated with a greater chance of dying from a heart attack, kidney failure, or getting diabetes and some cancers. When our fat stores become too full, the fat molecules spill over into the circulation, causing other tissues like muscle, the liver and even the heart to accumulate fat, upsetting their normal function. One of the most common results is that your muscle stops using glucose from the blood while your liver is encouraged to produce glucose. That's why obesity makes it tough to control blood glucose, leading too often to diabetes.

But that's not the whole BMI story. Some people can be wider (and heavier) because they are very muscular. And unlike fat, muscle protects against diabetes and heart disease.

So what does this mean for Kiwis? Does our population contain a plethora of muscle-bound Polynesians or rugged country folk falsifying the stats? Well no, because the same World Health Organisation report reveals New Zealand has a problem with blood glucose control, despite spending a fortune on medications to correct it.

Here's the good news. Whether you're a rugby ball or a pencil, you can improve your health prospects by being physically fit. Exercise scientists have very good evidence that making your muscles fitter can improve your health regardless of whether your exercise programme produces weight loss. It's got a lot to do with the ability of fitter muscles to burn that fat which they accumulate.

This means that if you start exercising, your body reaps the benefits immediately.

That's very good news for all of us. We can go out and enjoy that game of neighbourhood touch rugby or that walk on the beach for the sheer joy of it, knowing that we'll be healthier - whether we're pencils or not.

Professor Steve Stannard is the head of the School of Sport and Exercise at Massey University's College of Health.

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