The BMI thresholds to determine overweight and obese levels may be overly restrictive for older people, researchers say. (CBC)

Mildly overweight adults over the age of 70 were less likely to die in their 80s than those considered normal weight, Australian researchers have found.

"Our study suggests that those people who survive to age 70 in reasonable health have a different set of risks and benefits associated with the amount of body fat to younger people," lead researcher Leon Flicker, of the University of Western Australia, said in a release.

The World Health Organization's categories for body mass index or BMI use heights and weights to assess obesity in populations. The categories were mainly based on evidence in younger and middle-aged adults, and it's unclear whether they apply to predicting mortality in older people, the researchers said.

The study included more than 9,200 Australians aged 70 to 75 in 1996 who were followed for 10 years.

The risk of death during the study was 13 per cent or 0.87 times less for overweight participants than for normal-weight participants, the study's authors found, after taking into account factors such as education, smoking, alcohol consumption and self-reported history of conditions such as high blood pressure.

Nutritional reserves

"In conclusion, these results add further credence to claims that the WHO BMI thresholds for overweight and obese are overly restrictive for older people," the study's authors wrote.

The reasons are unclear, but lower mortality in those who are overweight has also been observed in older people with chronic conditions. It may be that excess weight is linked with better survival "because of the importance of metabolic and nutritional reserves for sick older people," the researchers said.

During the course of the study, there were 1,369 deaths in the men and 939 in the women, mainly from cardiovascular disease, followed by cancer and stroke.

Being sedentary doubled the mortality risk for women at all levels of BMI and increased the risk by 1.28 times for men.

Its unclear what could be behind this difference between men and women. For example, women who are sedentary might be unable to exercise because of ill health, but men who are sedentary might be able to exercise but choose not to, the study's authors suggested.

The researchers acknowledged several limitations of the study, such as collecting heights and weights from participants only at the beginning of the study, and the use of BMI, an imperfect measure of body fat.