The Big Question: Is the BMI index the right way to tell what our weight should be?

What is Body Mass Index?

It is a calculation to determine the amount of fat in a person's body, and therefore their risk of weight-related diseases such as diabetes. The Body Mass Index was devised in 1840 by Adolphe Quetelet, a Belgian scientist, and has since been adopted worldwide as the standard way to measure obesity. To find your BMI, divide your weight in kilograms by your height in metres squared.

Why is BMI in the news?

The authorities in Madrid have banned designers at the city's annual fashion week from using models who have a Body Mass Index (BMI) of less than 18, to stop the promotion of super-skinny body shapes.

The new rule has made an immediate impact, with one designer, Antonio Pernas, having to change all 18 of his models for his catwalk show on Monday because they failed the BMI test.

Eating-disorder experts in Britain have also called for a law to be brought in that would make it illegal for agencies and bookers to employ models with a BMI of less than 18. The move has triggered a debate about what can and can't be considered a healthy size, with many in the fashion world insisting that some models are being scapegoated. At the other end of the scale, some NHS Trusts now refuse to provide fertility treatment for women who have a high BMI because the success rates of IVF are not as good for obese patients. There are also plans to test the BMI of children at school, with targeted interventions for those who are considered to be overweight.

So what is a good BMI to have?

A BMI between 18 and 25 is considered healthy; less than 18 is classed as underweight, between 25 and 30 and you are overweight, while a score of more than 30 makes you obese.

People with a BMI of more than 40 are considered to be morbidly obese. For children, the rating is more complex. Juvenile BMI is calculated in the same way but is compared against the total range of scores for all children of the same age. A child whose BMI falls below the 85th centile of their age group is underweight, while those in the 85th to 95th centiles are classed as overweight and those in the 95th centile or above are obese.

How do the British measure up?

A study for the Department of Health found that, by BMI status, 43 per cent of men and 33 per cent of women in England are overweight. A further 22 per cent of men and 23 per cent of women are classed as obese. Among children, 643,515 boys and 613,048 girls are overweight - about one in 10 of the juvenile population. A further 746,662 boys and 675,983 girls are classed as obese - around five per cent of youngsters.

Are there any problems with the index?

Yes. First there is the problem of accuracy when assessing children. Three months ago, researchers at Loughborough University said that BMI was a "dreadfully imprecise measuring tool" for youngsters because they grow at different rates.

They warned that the proposed "fat tests" for children as young as four could lead to an increase in bullying and eating disorders. They said a "degree of panic" had set in about the perceived dangers of being overweight. It is not just children; one flaw with the BMI system is that it assumes that everyone has the same ratio of muscle to fat. Because muscle is more dense than fat, someone who is extremely fit and toned may have an "unhealthy" BMI, while another person with little muscle tone but a lot of fat can be rated "normal".

This anomaly was highlighted by a study last year which showed that when American Football players were rated on their BMIs, 50 per cent fell into the "obese" classification despite being highly physically fit and healthy. BMI also fails to take into account gender differences; the average man has 15 to 17 per cent body fat, while the average woman has 18 to 22 per cent. And there are ethnic variants - research suggests that Asian people have smaller frames than Caucasians and therefore higher levels of body fat at similar BMIs, which may explain why weight-related diseases such as hypertension and diabetes seem to occur at a lower level of BMI in people from China and India.

Is a BMI test for models valid?

It is a difficult one. Experts say that with a BMI of 18, a woman is at risk of developing an eating disorder and any rating below that will mean she probably already has one. Dee Dawson, who runs the Rhodes Farm clinic in north London for girls with eating disorders, says that it's a fallacy to say that some models are just naturally very skinny.

"It's just not possible to be 6ft 2 and to weigh eight and a half stone without it being about controlling your eating," she said. Most model agencies, fashion designers and magazine editors insist they will only work with healthy models and that an arbitrary BMI cut-off is unfair. And they point to the fact that the trend for super-slim, "size zero" models on the catwalk has hardly crossed over into "real" high street life; the average British woman is a size 16 and growing.

Is there an alternative for measuring obesity?

Old-fashioned scales have been replaced by electronic machines that measure your exact fat content rather than simply your body mass.

German researchers found that the ratio of waist circumference to height is a more accurate way than BMI of indicating the risk of obesity-related heart disease. The WTR (waist-to-tallness) measurement involves dividing your waist circumference by your height (both in inches). Anything over 0.53 for women and 0.55 for men is considered unhealthy. And the old adage about being able to "pinch an inch" may be the simplest health check; according to private healthcare providers Bupa, a waist measurement of more than 102 cm (40 inches) for a man and 88cm (35 inches) for a woman indicates that you are reaching the tipping point.

As dieticians never tire of telling us, assessing whether you are healthy comes down to counting the calories you are taking in and comparing them with the energy spent on exercise.

The best means of determining whether we are too fat or too thin?

Yes...

* It is a simple and accurate way of assessing weight, which can be used as a basic indicator of overweight and underweight

* Medical evidence shows that a BMI of below 18 or above 30 carries serious health risks

* Critics simply don't like the truth that BMI gives them about being too fat - or too thin

No...

* It is a crude measuring device that does not take into account fitness and other individual facets

* Warnings about an obesity epidemic among children may be alarmist and the obsession with BMI is merely fuelling the panic

* Healthy eating and physical exercise are more important issues than body mass to height ratios