While previous studies had raised the possibility that excess flab may increase the risk of dementia, a new study that followed several hundred elderly Swedish people for 18 years clearly showed that women who were heavy at age 70 were markedly more likely to develop Alzheimer's in their eighties.

The findings add Alzheimer's to the long list of serious ailments associated with being overweight or obese, a problem that is skyrocketing in the United States and other parts of the world. But the new study offers perhaps the most compelling reason to stay slim even into old age: reducing the danger of an agonizing loss of mental abilities.

"A lot of times, as people age, they say, 'I don't have to be worried about my weight anymore.' But clearly having excess body fatness isn't healthy, in particular for Alzheimer's," said Deborah Gustafson, who conducted the study while at Utah State University. "Since more people are living into their eighties and nineties, I think it's a significant public health impact of being overweight and obese."

The study found the link only for women, but Gustafson and other experts said that was probably because there weren't enough men in the study who lived long enough to develop Alzheimer's.

"We just didn't have enough men to look at in the sample," Gustafson said. "It very well may be that obesity is so toxic in men that they die from it before they can develop Alzheimer's," said Bill Thies, vice president of medical and scientific affairs for the Alzheimer's Association.

Previous research had found that people suffering from conditions associated with being overweight or obese, such as high blood pressure, a high cholesterol level and cardiovascular disease, were at increased risk for Alzheimer's. But this is the first large, long-term study to specifically examine the connection between weight itself and Alzheimer's.

Alzheimer's is a progressive, devastating disease that damages and destroys brain cells, for reasons that remain unclear, causing a loss of thinking abilities. About 4 million Americans are estimated to have Alzheimer's, a figure that could increase to 14 million by 2050, according to some estimates.

Gustafson and her Swedish colleagues studied 392 people participating in a broad, ongoing project investigating various health issues in Sweden. The subjects underwent extensive physical and cognitive examinations, and answered detailed questions about their health and lifestyles at age 70 and then periodically over the next 18 years. Over the course of the study, 59 women and 34 men developed dementia.

Women who were overweight at ages 70, 75 and 79 were more likely to develop dementia by age 88, the researchers found. For every one-point increase in their body mass index (BMI) at age 70, there was a 36 percent increased risk for Alzheimer's, the researchers reported in yesterday's issue of the Archives of Internal Medicine. BMI is a standard body weight measurement based on height and weight. A 5-foot-6-inch-tall woman who weighs 155 would have a BMI of 25. A BMI of 25 or above is considered overweight; 30 or above is considered obese. The women in the study who eventually developed dementia had a mean BMI of 29 when they were 70. In comparison, the women who did not develop dementia had a mean BMI of 25 when the study started.

Being overweight may increase the risk for Alzheimer's by raising blood pressure and narrowing arteries, restricting blood flow to the brain, Gustafson said. It could also have a direct effect, perhaps because fat cells secrete substances that are harmful to neural cells.

"The obesity issue is important because it is fundamental to all those other risks. It raises people's blood pressure, it raises serum lipids," Thies said.

"We probably don't have to know the exact mechanism to suggest that a healthy long life will be associated with trying to control these risk factors. We have all sorts of good reasons for doing that," Thies said.

Neil Buckholtz, chief of the Dementias of Aging Branch at the National Institute on Aging, said the findings are intriguing because they could offer one of the few ways people might be able to reduce their risk of Alzheimer's.

"We're always looking for potentially changeable risk factors, and this is changeable," Buckholtz said. "There are some things we can't change, like age. If this is true, then there are ways of modifying it."

A study released last month reported that playing chess, bridge or a musical instrument, or engaging in other mind-stretching activities, significantly lowers the risk of developing Alzheimer's.