Millions of adults are being wrongly branded too fat because of misleading BMI tests, researchers claim.
The body mass index does not accurately measure whether someone is carrying excess weight or is more likely to die.
Academics also point out that many ‘slim’ adults whose BMI is in the normal range are carrying unhealthy fat without realising it.
The findings are further evidence that the measurement widely used by the NHS and schools is an unreliable and blunt tool.
But it does not distinguish muscle from fat, which means that rugby players and other well-built athletes are classified as obese.
Dr William Leslie, of the University of Manitoba in Canada, compared the records of almost 50,500 middle aged men and women, including their BMIs, fat percentages – and whether they had died.
He found that the two groups most likely to have died had either a low BMI or a high body fat percentage.
By comparison, those with a high BMI – classified as overweight or obese – were actually in relatively good health and often very fit.
Dr Leslie, whose study is published in the Annals of Internal Medicine, said: ‘The simple messaging that we’re all too fat and should be thinner is based on BMI.
‘It’s an oversimplification of a more complex story.
‘There are some people who are in the overweight or obese range but they may not necessarily be an unhealthy weight because they have large amount of muscle tissue.
‘I’m not saying BMI isn’t helpful, clearly it is. But it’s a crude measure and body composition is not accurately captured by BMI alone.
‘To say someone is obese implies that they have too much fat. BMI may misclassify some people as having excess fat, which isn’t the case.
‘At the other end of the scale just because you’re not in the obese range, doesn’t mean you don’t have excess fat.
He added that some adults with normal BMIs have hidden fat stored around organs, which is actually extremely harmful.
Around 60 per cent of adults and one in five children aged 11 are either obese or overweight as classified by their BMI.
In January researchers from the University of California, Los Angeles called for the measurement to be scrapped, claiming it was wrongly giving adults a ‘death sentence’.
They also pointed out that many were being wrongly penalised by health insurers, who considered their risks of dying to be so much higher.
Last month the mother of four-year-old Honey Rose Stroud was sent a letter explaining she was overweight and should go on a diet.
Samantha Stroud, 29, who has a relative battling anorexia, said labelling children by their weight at such a young age is dangerous and can trigger eating disorders.