Australian National University-led research has busted the myth that overweight people live longer than people with a lower body mass index (BMI).
The research examined the way in which data were analysed in a range of studies, some of which had suggested that overweight people can actually live longer and others which had suggested the contrary.
The new study published in the peer-reviewed online journal PLOS ONE provides direct evidence as to why some studies can come up with apparently different results.
"The results are in. The clear truth is that people who are overweight or obese have a higher risk of premature death," said lead author Dr Grace Joshy, from the ANU National Centre for Epidemiology and Population Health (NCEPH).
"It is a pretty simple truth, especially given the clear evidence that the risks of cardiovascular disease and cancer, Australia's biggest killers, are higher for those who are overweight or obese."
The ANU-led study looked at factors such as the width of the BMI categories used and pre-existing illness, to examine why some studies suggested a link between being overweight and longer life expectancy.
Dr Joshy said the studies suggesting that overweight people live longer did not take into account other major health factors.
"These studies failed to take into account the fact that many people with a healthy weight can still be quite unhealthy as they might smoke, or be trim because they are actually sick," she said.
Professor Emily Banks of NCEPH and the Sax Institute, who was a senior author on the study, said that the studies that produced the misleading positive findings about being overweight had compared the risk of death in overweight and obese people to the risk in a large "mixed bag" of thinner people.
"Overweight people looked good by comparison, but when you account for illness and look in detail at how the risk of death varies gradually with BMI, you find that the best BMI to be is relatively lean, at around 22.5-24.9," Professor Banks said.
She said the ANU-led research aimed to clear up public confusion about the health impact of being overweight.
"Although this is difficult news for Australia, where around 63 percent of adults are overweight or obese, it does give us reliable evidence on how to stay healthy in the long term and shows that now is not the time to relax our efforts in the fight against overweight and obesity."