According to doctors in the United States the standard measure of obesity, the Body Mass Index (BMI), is badly flawed and may not be such an accurate indicator as it lumps muscle and fat into one category.

Researchers from the Mayo Clinic, Rochester, Minnesota, say they have found that people with a BMI of 30-35 were at lower risk of cardiovascular disease than those whose BMI was below 20.

They also say that overweight patients had better survival rates and fewer heart problems than those with a normal BMI.

They have arrived at this seemingly contradictory conclusion after examining data from 40 studies covering 250,000 people with heart disease and say they do not deny that obesity is a major health threat but rather that the 100-year-old BMI test was too inaccurate an instrument to be trusted.

Lead researcher Francisco Lopez-Jiminez says rather than proving that obesity is harmless, the data suggests that alternative methods might be needed to better characterize individuals who truly have excess body fat compared with those in whom BMI is raised because of preserved muscle mass.

It is estimated that as many as 30,000 people in Britain and 300,000 in the United States, die because of obesity every year.

In the U.S. the condition is now thought to have overtaken smoking as the main cause of preventable death.

Body Mass Index was invented by Belgian polymath Adolphe Quetelet in the eighteen hundreds and is a measure of body fat calculated from height and weight.

A figure of less than 18.5 is considered underweight, while from 18.5 to 24.9 is normal, 25 to 29.9 is overweight and anything over 30 is categorized as obese.

Originally intended as a broad indicator of general health, it has now become a standard diagnostic tool of heart disease risk but this latest study has found the test badly flawed.

Another study carried out by researchers at the London School of Hygiene and Tropical Medicine also concluded, after analyzing data on 15,000 patients, that BMI was not a good indicator of health risks for elderly people.

They also favour a calculation which looks at hip-waist ratio.

Other researchers agree and suggest that in a comparative study of BMI, waist-to-hip ratio, waist measure and hip measure, waist-to-hip was the best predictor of heart attack risk and BMI should be disregarded as a clinical and epidemiological measure of cardiovascular risk.

As the researchers point out according to BMI calculations the whole of the English national rugby team would be classed as very-overweight to obese and at high risk of cardiovascular disease along with many other sportsmen.

The research is published in The Lancet medical journal, August 19.