Niki Bezzant: Obese - but by what measure?

How much of our perception of what is "normal" is shaped by what we see around us? Photo / Supplied

COMMENT: This week came the news that within 15 years, the mean BMI (body mass index) of New Zealanders will be 30 - the cut-off point for obesity.

That means the average Kiwi is likely to be obese - a scary rise from today's 30 per cent. This in turn has tripled since the 1970s.

I mentioned this in a talk I gave recently, and was challenged by a member of the audience. BMI, he said, is not a good measure of fatness or health. It doesn't apply to everyone.

This is a commonly accepted wisdom. People like to put themselves into BMI calculators and dismiss the results. (BMI is your weight divided by your height squared).

"According to this BMI I'm overweight", people say. "That's ridiculous. I look completely normal".

It's true that BMI has some limitations. For a start, people who are very muscular can be falsely identified as overweight or obese according to their BMI, because muscle is denser than fat.

It's been correctly stated that many of the All Blacks would have BMIs that classify as obese, when in fact they are super-fit athletes.

The same would likely be true of highly muscular body builders. And some people are just bigger - for example Pasifika people tend to be naturally larger and there are slightly different BMI classifications to account for this.

Most of us, though, are not All Blacks. We're regular humans, surrounded by other regular humans who look similar to us. We have to wonder how much our perception of what's "normal" when it comes to body size – for better or worse - is shaped by what we see around us.

On the other hand, there are people with a normal BMI who are less healthy than others who are technically overweight.

It's possible to have a BMI in the normal range, but still have high body fat and low muscle mass – the so-called TOFI: thin outside, fat inside. Visceral fat, sitting unseen around the organs, is more dangerous than the fat that sits on hips and thighs, which we may dislike, but is not really dangerous.

A TOFI person may be at higher risk of type 2 diabetes and heart disease than a larger person who's fitter and has a lower proportion of body fat.

So should we forget about BMI altogether?

For health researchers, BMI is useful as a measure of a population. It gives a big picture view of what's going on.

For individuals, BMI can be useful to check – but it's just one thing. Other numbers worth knowing include waist circumference and, if you can, body composition.

But more important is to go back to basics of what we're doing every day to support our health.

Are we eating really well and exercising? Are we choosing whole, unprocessed, plant-based foods? Focusing here - and not on the scales - is likely to keep us healthier in the long run than measuring ourselves against others.

• Niki Bezzant is editor-at-large for Healthy Food Guide www.healthyfood.co.nz