MIAMI • Being on the heavier side may not be as dangerous as it was in the 1970s, at least when it comes to the risk of dying sooner.

The optimum ratio of weight and height - known as body mass index, or BMI - is now on the upper side of the healthy range, said a report in the Journal of the American Medical Association on Tuesday.

"Compared to the 1970s, today's overweight individuals have lower mortality than so-called normal- weight individuals," said Dr Borge Nordestgaard, a clinical professor at the University of Copenhagen. "The reason for this change is unknown."

A study of more than 100,000 people in Denmark spanned three groups, or cohorts, whose risk of dying for any reason was examined in 1976-78, 1991-1994 and 2003-2013.

Currently, doctors define the normal range for BMI - calculated by weight in kilograms divided by height in metres squared - as between 18.5 and 24.9. A BMI of 25-29.9 is considered overweight, while 30 or higher is obese.

In the 1970s, the optimal BMI for the lowest risk of death was 23.7. By 1991-94, the optimal BMI had risen to 24.6. And in 2003-2013, it reached 27. Compared to four decades ago, that would mean adding 14kg to the frame of a person who stands 1.83m tall.

Researchers also found that obese people in the 1970s were more likely to die younger than normal-weight people, but this association disappeared in the 2000s.

According to Dr Naveed Sattar, professor of metabolic medicine at the University of Glasgow, the study is "of interest", but does not merit changing advice on obesity since many individuals are likely to be better treated for abnormal blood pressure, abnormal cholesterol and, if also present, Type 2 diabetes, leading to lower death risks.

"In other words, the current findings do not mean that being overweight is protecting you from death, far from it - rather, many confounding factors may give the current result and we know from many other studies that being overweight or obese does increase mortality risks, in the same way that it increases risk for many other conditions," he added.

AGENCE FRANCE-PRESSE