Rochester: One of the main measurements for obesity, the body mass index (BMI) is being called into question by US doctors.

This follows research by the Mayo Clinic in Rochester, Minnesota, which investigated 40 studies involving 250,000 victims of heart disease. As expected it found that severely obese patients had a higher risk of heart-related death.

But would it didn’t expect was that overweight patients, as defined by BMI scores, had better survival and fewer heart problems than those with a normal BMI. In turn, those with normal BMI were less likely to die than those with a low BMI.

They concluded that many of the so-called BMI defined overweight patients actually had more muscle – which actually weighs more than fat – and that BMI failed to identify the difference.

Many experts now want waist circumference or waist to hip ratio, which indicates levels of abdominal fat, adopted as a more accurate guide rather than the method for BMI which is calculated by dividing weight in kilograms by height in metres squared. Someone with a BMI of less than 18.5 is considered underweight, between 18.5 and 24.9 lies within the ‘ normal’ range, and 25 to 29.9 is classified as overweight. Clinical obesity is defined by a BMI of 30 or greater.

A recent international study in 52 countries found waist-to-hip ratio was the most reliable predictor of heart attack risk. As a rough guide to healthy waist circumference, doctors regard over 40 inches for a man and 35 for a woman as danger signs.