WEDNESDAY, Aug. 17 (HealthDay News) -- Elderly people with extra body fat may not live as long as those who maintain a normal weight, according to a new study that contradicts previous research.

In following seniors over an extended period of time and accounting for changes in their weight, researchers found a higher body mass index (BMI), or height-to-weight ratio, is associated with a shorter life expectancy.

"We had a unique opportunity to do 29 years of follow-up with a cohort that was also followed for mortality outcomes," said study lead author Pramil N. Singh, associate professor in the School of Public Health at Loma Linda University, in a university news release. "Across this long period of time, we had multiple measures of body weight, which provided a more accurate assessment."

For the study, recently published in the Journal of the American Geriatrics Society, researchers examined 6,030 healthy adults who never smoked. They found that men older than 75 years with a BMI greater than 22.3 would live nearly four years less than those with a lower BMI.

Similarly, women older than 75 years with a BMI greater than 27.4 would live roughly two years less than other women their age who were of normal weight.

A BMI between 18.5 and 24.9 is considered normal weight. A BMI of 25 to 29.9 is considered overweight, and a BMI of 30 or higher is considered obese.

The study pointed out, however, that the negative effects of excess weight kick in for men and women at different BMIs. Men experienced a greater risk of dying beginning with a BMI of 22.3, while this risk did not appear for women until they had a BMI of 27.4.

The study authors suggested this difference may be because in postmenopausal women body fat is the main source of estrogen, which may help protect them from heart disease and hip fractures.

These findings contradict previous studies, which concluded that overweight elderly people live longer than their thinner peers. The authors of the current study said previous findings are limited because they do not account for participants' weight changes over an adequate length of time and consider how these fluctuations in weight might affect their life expectancy.

"This suggests that elderly individuals of normal weight should continue to maintain their weight," said Singh.

The authors said additional research is needed to explore how lifestyle patterns help people maintain a healthy body weight over many years.

More information

The U.S. National Library of Medicine provides more information on weight control.