MIAMI (AFP) - Being on the heavier side may not be as dangerous as it was in the 1970s, according to a study on Tuesday that points to the protective effects of a few extra pounds.
The optimum ratio of weight and height - known as body mass index or BMI - is now on the upper side of the healthy range, according to the report in the Journal of the American Medical Association (Jama).
"Compared to the 1970s, today's overweight individuals have lower mortality than so-called normal weight individuals," said Borge Nordestgaard, clinical professor at the University of Copenhagen and Copenhagen University Hospital.
"The reason for this change is unknown."
The report was based on more than 100,000 people in Denmark.
The study spanned three groups, or cohorts, whose risk of dying for any reason was examined in 1976-78, 1991-1994, and in 2003-2013.
Currently, doctors define the normal range for BMI - calculated by weight in kilograms divided by height in meters 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.
This would be the equivalent of a 1.83m tall man who weighs 77kg, or a 1.65m woman who weighs 65kg.
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 the a person who stands 1.83m, or 9kg to the frame of someone 1.65m.
Researchers also found that obese people in the 1970s were more likely to die than normal weight people, but this association disappeared in the 2000s.
"The increased risk of all-cause mortality associated with obesity compared to normal weight decreased from 30 per cent 1976-78 to 0 per cent in 2003-13," said principal investigator Shoaib Afzal, of Copenhagen University Hospital in Denmark.
The health authorities have long warned of the risks of being overweight, including diabetes and cardiovascular disease.
Nordestgaard said the current findings suggest "a need to revise the categories presently used to define overweight, which are based on data from before the 1990s."
However, researchers cautioned that the biological mechanism behind their finding remains poorly understood, and their results should not be interpreted to mean that people can cease caring about what they eat.
"Maybe overweight people need not be quite as worried about their weight as before", said Nordestgaard.