New research conducted by the Mayo Clinic in the US suggests body mass index is not the best measurement of obesity. John von Radowitz reports.
Doctors have questioned a common measure of obesity after research found that "overweight" heart patients had better survival rates than those described as "normal".
They say the finding exposes shortcomings in the use of the body mass index (BMI), which has formed the basis of defining healthy and abnormal weight for more than 100 years.
Many experts now say that waist circumference or waist-to-hip ratio, which indicate levels of abdominal fat, are more accurate guides.
BMI, invented by the Belgian statistician and sociologist Adolphe Quetelet in 1869, is calculated by dividing a person's weight in kilograms by their height in metres squared.
It is universally relied upon in clinical trials designed to assess the health risks associated with weight. 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.
However, the measurement has thrown up anomalies. For example, former New Zealand rugby star Jonah Lomu has a BMI of 32, yet he could hardly be described as "obese".
The new research conducted by American researchers from the Mayo Clinic in Rochester, Minnesota, pooled data from 40 studies involving about 250,000 people with heart disease.
It found, as expected, that severely obese patients had a higher risk of heart-related death - but it also showed that overweight patients, as defined by their BMI scores, had better survival and fewer heart problems than those with a normal BMI.
People with normal BMI were, in turn, less likely to die than people with a low BMI.
The better outcomes for overweight patients were most likely due to muscle, which weighs more than fat, the researchers reported in The Lancet medical journal.
Dr Francisco Lopez-Jimenez, who led the study, says: "Rather than proving that obesity is harmless, our data suggest that alternative methods might be needed to better characterise individuals who truly have excess body fat, compared with those in whom BMI is raised because of preserved muscle mass."
Research has shown that visceral fat that builds up around the internal organs is a good predictor of heart disease and diabetes.
Visceral fat produces an extended abdomen - classic "middle-aged spread" - and one way of assessing it is simply by placing a tape measure round the stomach.
One large study in 52 countries found that waist-to-hip ratio was the most reliable predictor of heart attack risk.
Raised waist-to-hip ratio increased the number of people at risk of having a heart attack more than three-fold, compared with BMI.
Commenting on the latest findings in The Lancet, Italian heart disease expert Dr Maria Franzosi, from the Mario Negri Institute in Milan, wrote: "BMI can definitely be left aside as a clinical and epidemiological measure of cardiovascular risk for both primary and secondary prevention."
Dr Colin Waine, chairman of the UK's National Obesity Forum, says: "The thing about BMI is that it's been used in nearly all the studies, so you can't just drop it. But its big fault is that it says nothing about body composition.
"That's why we say that as well as reading BMI, people should recognise waist circumference. We think it should become a normal clinical tool used to complement BMI, because it does correlate quite closely with visceral fat, which is the dangerous fat."
Meanwhile, a separate US study suggests that exercise may be more important than diet in helping to reduce fat cells around the waistline.
Among a group of obese women who were placed on a regimen of calorie-cutting alone or diet plus exercise, those who exercised showed a reduction in the size of fat cells around the abdomen. Women who only dieted showed no such change.
In contrast, both groups trimmed about the same amount from fat cells in the hip area. The findings suggest that exercise may "preferentially increase" the body's breakdown of fat cells in the abdomen, says study author Dr Tongjian You. While both exercise and diet are important, exercise may be key in the distribution of a person's body fat, he says.
The researcher and his colleagues at Wake Forest University School of Medicine in Winston-Salem, North Carolina, report the findings in the International Journal of Obesity. - (PA; Additional reporting: Reuters)