The researchers say their results should not be used as an excuse for people to allow themselves to become overweight.
In a JAMA study paper, a team from the University of Copenhagen describes how it analyzed data on over 100,000 individuals to reveal that in 1976-1978, the risk of premature death from all causes was higher in obese individuals than in normal-weight individuals, but this was no longer the case in 2003-2013.
Lead investigator Dr. Shoaib Afzal, of Herlev Hospital, Copenhagen University Hospital, says:
"The increased risk of all-cause mortality associated with obesity compared to normal weight decreased from 30 percent in 1976-78 to 0 percent in 2003-2013."
In many countries, public health policies recommend that people keep to a healthy weight to avoid chronic conditions such as diabetes and cardiovascular diseases, and thereby hopefully avoid dying prematurely.
To define healthy weight, experts refer to the normal weight range of body mass index (BMI), the ratio of a person's weight in kilograms divided by the square of their height in meters.
For adults, BMI ranges are defined as follows:
- 30 and over is obese
- 25-29.9 is overweight
- 18.5-24.9 is normal or healthy weight
- Under 18.5 is underweight.
The Danish study also found the BMI range linked to the lowest rate of death from all causes was different in 1976-78, 1991-1994, and 2003-2013. All participants were followed until 2014.
'Not a reason to become overweight'
Dr. Afzal notes they found the BMI linked to the lowest risk of death went up from 23.7 in 1976-1978 to 24.6 in 1991-1994, and then climbed again to 27 in 2003-2013.
However, individuals whose BMI was below or above these values had a higher risk of death.
Senior author Børge G. Nordestgaard, a clinical professor at the University of Copenhagen and Copenhagen University Hospital, says the reason for this change is not known, but he suggests the findings indicate a need to revise the categories used to define overweight. However, he adds:
"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."
Commenting on the study, Naveed Sattar, professor of metabolic medicine at the University of Glasgow, United Kingdom, says while the results are interesting, they do not change the advice on obesity and its treatment and prevention.
He explains that with modern medicines we can help people with risk factors such as obesity live longer, but he suggests it would be wrong to conclude that we need to redefine obesity cut-offs based on the results of the study.
For example, the higher availability of cheap, preventive drugs, means many more overweight and obese people are receiving treatments for type 2 diabetes, high blood pressure, and high cholesterol, which is lowering their risk of death.
"Obesity and overweight categories also signal risks for many diseases, such as type 2 diabetes, liver disease, cancers, sleeping problems, multiple pregnancy complications, to name but a few, many of which we can manage much better these days but of course such complications also impair quality of life, impair self-esteem, and increase health costs for societies."
Prof. Naveed Sattar