The study found exercise particularly benefited elderly women
Moderately overweight elderly people may live longer than those of normal weight, an Australian study suggests.
But being very overweight or being underweight shortened lives.
The report, which was published in the Journal of the American Geriatric Society, said dieting may not be beneficial in this age group.
But the study of 9,200 over-70s also found that regardless of weight, sedentary lifestyles shortened lives, particularly for women.
The study by the University of Western Australia set out to find out what level of body mass index (BMI) was associated with the lowest risk of death in the elderly.
For younger people, there is a well established health risk from being overweight or obese.
The team tracked the number of deaths over 10 years among volunteers who were aged 70 - 75 at the start of the study.
It found that those with a BMI which classed them as overweight not only had the lowest overall risk of dying, they also had the lowest risk of dying from specific diseases: cardiovascular disease, cancer and chronic respiratory disease.
The overall death rate among the obese group was similar to that among those of normal weight.
But those who were very obese had a greater risk of dying during the 10 year period.
Lead researcher, Professor Leon Flicker said: "Concerns have been raised about encouraging apparently overweight older people to lose weight.
"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."
The conclusion of this study, that being overweight may be less harmful for elderly people, corroborates the findings of previous research.
Sedentary lifestyles shortened lives across all weight groups, doubling the risk of mortality for women over the period studied, and increasing it by 25% for men.
Physical exercise "really matters", said Professor Flicker.
As well as helping to build muscle mass, it has broader health benefits for elderly people, he said.
The authors believe BMI may give a poor reflection of fatty mass in elderly people.
"It may be time to review the BMI classification for older adults," says Professor Flicker.
Professor Kay-Tee Khaw from Cambridge University agreed, noting that optimal weight appears to be higher in older age groups.
"This is important since under-nutrition is an important problem in older people.
"Waist circumference, which assesses abdominal obesity, appears to be a better indicator of health consequences of obesity" she said.