Body Mass Index (BMI), the standard measure of obesity, is badly flawed and a more accurate gauge should be developed, according to doctors in the United States.

Writing in Friday’s Lancet medical journal, the researchers from the Mayo Clinic College of Medicine, Rochester, Minnesota, found that patients with a low BMI had a higher risk of death from heart disease than those with normal BMI.

At the same time overweight patients had better survival rates and fewer heart problems than those with a normal BMI.

This apparently perverse result, drawn from data from 40 studies covering 250,000 people with heart disease, did not suggest that obesity was not a health threat but rather that the 100-year-old BMI test was too blunt an instrument to be trusted.

“Rather than proving that obesity is harmless, our 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,” said lead researcher Francisco Lopez-Jiminez.

About 30,000 people in Britain die due to obesity every year and 300,000 in the United States where the condition is now thought to have overtaken smoking as the main cause of preventable death.

Body Mass Index, invented by Belgian polymath Adolphe Quetelet between 1830 and 1850, 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.

Intended as a broad indicator of general health, it has become a standard diagnostic tool of heart disease risk.

Maria Grazia Franzosi from the Istituto Mario Negri in Milan, writing in the same issue of the Lancet, noted that a 52-country study comparing four different tests - BMI, waist-to-hip ratio, waist measure and hip measure - found that waist-to-hip was the best predictor of heart attack risk.

“BMI can definitely be left aside as a clinical and epidemiological measure of cardiovascular risk,” she said.

“Uncertainty about the best index of obesity should not translate into uncertainty about the need for prevention policy against excess bodyweight,” she cautioned.