New research from Denmark involving more than 100,000 individuals suggests that the excess risk of premature death associated with obesity has decreased over the past 40 years. All-cause mortality was higher in obese individuals than in normal weight individuals in 1976-78, but not in 2003-13.

Many try to lose weight to avoid diabetes and cardiovascular disease and hopefully live longer. This is often driven by recommendations from health care authorities and is further supported by the media and not least, by commercials often presenting normal weight or even thin people as ideal humans.

"The increased risk of all-cause mortality associated with obesity compared to normal weight decreased from 30% 1976-78 to 0% in 2003-13," says principal investigator Dr. Shoaib Afzal, Herlev Hospital, Copenhagen University Hospital, Denmark.

This research has just been published in the Journal of the American Medical Association (JAMA).

What is the optimal BMI for lowest mortality?

The study also revealed changes in the BMI associated with the lowest all-cause mortality in three cohorts from Copenhagen, examined respectively in 1976-78, 1991-1994, and in 2003-2013 (all individuals were followed until 2014).

"The optimal BMI for the lowest mortality increased from 23.7 in 1976-78, through 24.6 in 1991-94, to 27 in 2003-13, while individuals with a BMI below or above the optimal value had higher mortality," adds Shoaib Afzal.

"Compared to the 1970's, today's overweight individuals have lower mortality than so-called normal weight individuals. The reason for this change is unknown. However, these results would indicate a need to revise the categories presently used to define overweight, which are based on data from before the 1990's" says senior author Clinical Professor Borge G. Nordestgaard, University of Copenhagen and Copenhagen University Hospital.

"Importantly, our results should not be interpreted as suggesting that now people can eat as much as they like, or that so-called normal weight individuals should eat more to become overweight. That said, maybe overweight people need not be quite as worried about their weight as before", adds Nordestgaard.

Obesity and overweight are classified using Body Mass Index (BMI), calculated as weight in kilograms divided by height in meters squared. A BMI of 25-29.9 represents overweight, a BMI of 30 or greater represents obesity, while a BMI of 18.5-24.9 is considered normal weight. These categories are often used for recommendations on optimum weight.