Doubts over obesity measure

Millions of people who think their weight is normal are fatter than they think, scientists said this week.

But there's some good news: an equal number of people who think they are overweight are actually carrying less fat than they believe.

The standard tool for deciding whether someone is overweight or obese - the body mass index (BMI) - has come under fire from researchers who say it is inaccurate in one out of four cases.

The composite measure of weight and height has been used for decades to demonstrate soaring rates of obesity across the Western world. Almost one in four adults in Britain is obese - with a BMI of over 30 - and two thirds are overweight (BMI 25-30), a threefold increase since 1980.

According to Ministry of Health figures for 2002-03, one in three adult New Zealanders was overweight, and another one in five adults was obese. But research published this week shows that the BMI fails to distinguish lean muscle from fat and therefore fails to identify those people carrying most fat.

Scientists led by Professor Andrew Prentice of the Medical Research Council's International Nutrition Group at the London School of Hygiene and Tropical Medicine have pioneered a new method of measuring body fat directly using specially modified scales.

Professor Prentice said: "The problem with BMI is it measures weight not fat. But fat is what matters because it produces molecules in the body that cause high blood pressure, heart disease and diabetes.

"If you look at the weights and heights of American football players, a very large proportion of them would be rated clinically obese on the basis of their BMIs. But it would take a brave doctor to go and tell them so.

"They are clearly not obese - they have got huge lean muscle mass and very little fat and the BMI has got it wrong."

In a study of 2000 school children aged 5 to 18, Professor Prentice and colleagues found that girls had 60 per cent more fat than boys. The average 18-year-old girl had 25 per cent body fat compared with 15 per cent for boys.

The extra fat on girls appears at puberty and is necessary for reproduction and breast feeding, Professor Prentice said. By adulthood, women carry twice the body fat of men, relative to their weight.

Gender-specific growth charts, based on the research, showing expected levels of fat at different ages are being sent to every Primary Care Trust by the Child Growth Foundation charity. Over 30 per cent of children in Britain are considered overweight or obese.

Susan Jebb, head of nutrition and health at the Medical Research Council, said: "Existing BMI charts do not reveal these important gender differences and natural changes during growth and development. Centile charts for fat go beyond BMI to give a more accurate assessment of body composition and hence associated health risks."

The research was presented at an international symposium on body composition studies at the University of Southampton and will be published in the International Journal of Obesity.

Under the new approach, body fat is measured by a special set of scales that sends an electrical pulse through the body which distinguishes lean muscle from fat.

Weighty issues

Working out your body mass index:

* Take your weight in kilograms and divide it by your height (in metres) squared. So, if you're 1.8m tall and weigh 100kg, the calculation is: 100 divided by (1.8 x 1.8). That's 100 divided by 3.24. Answer: 30.8.

* For adults, a BMI of 18.5-24.9 is considered "normal". Adults with a BMI of 25-29.9 are considered overweight. Adults with a BMI over 30 are considered obese.

* For adult Maori and Pacific Islanders, the numbers are a little higher; anything above 32 is considered obese.

* Different figures apply for children.

* The BMI has limitations. For example, very muscular people may have a high BMI but only a low percentage of body fat.

- INDEPENDENT