Tuesday, 19 December 2017

Defining obesity

The government's official obesity statistics were released last week. There was noting very exciting in them. Male obesity fell a bit and female obesity stayed the same. As is traditional, I have updated my graphs to see how the Lancet's widely reported predictions from 2011 are looking.

On current trends, the Lancet's predictions for 2030 look about as plausible as the Foresight committee's prediction that 36 per cent of men would be obese by 2015.

The women's figure is not too far off the lowest end of the Lancet estimate. One of the weird things about the estimates is that they consistently predict higher rates of obesity for men than for women, despite men having never had a higher rate of obesity ever since obesity started being measured properly in 1993.

As for childhood obesity, it continues to fail to 'spiral'. After falling from 17 per cent to 14 per cent last year, it ticked up to 16 per cent in this week's figures - the same as it was in 1999.

The rate of obesity among 11 to 15 year olds is an implausibly high 23 per cent. Why implausible? Because the rate among 16 to 24 year olds is only 10 per cent. Are we supposed to believe that most obese kids suddenly lose weight once they leave school?

No, the difference is that adult obesity is measured in a semi-objective way whereas childhood obesity is entirely arbitrary. Adult obesity is having a BMI over 30. Childhood obesity is having a BMI that is above what the 95th centile was in 1990 (hardly any kids have a BMI over 30).

In clinical practice, the 98th centile is considered to be a more appropriate cut-off. If you measure it from the 98th, you get less of a gap between the 11-15 year olds and the 16-24 year olds, and yet we persist with a measurement that is clearly wrong.

As Public Health England says...

Children with a BMI above the 98th centile are considered clinically obese. For population monitoring those above the 95th centile are classed as obese.

I have never understood why they do this, but the effect is to greatly exaggerate the scale of childhood 'obesity'. Rather than accept that the excessively broad definition has led to millions of false positives, we accuse parents of not recognising that their children are obese.

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