Being obese can increase the risk of Alzheimer's Disease by as much as 80 percent, according to a study published Wednesday in the May issue of Obesity Reviews.

But it's not just weight gain that poses a risk. People who are underweight also have an elevated risk of dementia, unlike people who are normal weight or overweight, said the research papers.

U.S. researchers carried out a detailed review of 10 international studies published since 1995, covering just over 37,000 people, including 2,534 with various forms of dementia. Subjects were aged between 40 and 80 years when the studies started, with follow-up periods ranging from three to 36 years.

All kinds of dementia were included, with specific reference to Alzheimer's Disease and to vascular dementia -- where areas of the brain stop functioning because the blood vessels that supply them are damaged by conditions such as high blood pressure or heart disease.

"Our analysis showed that obesity increased the relative risk of dementia, for both sexes, by an average of 42 percent when compared with normal weight," says Youfa Wang, lead researcher at Johns Hopkins Bloomberg School of Public Health. "And being underweight increased the risk by 36 percent."

"But when we looked specifically at Alzheimer's Disease, the increased risk posed by obesity was 80 percent. The increased risk for people with vascular dementia was 73 percent," said the authors.

The detailed analysis clearly shows a "U-shaped" relationship between weight and dementia, with people who are obese or underweight facing a greater risk, they concluded. "We believe that our results show that reducing the prevalence of obesity is a promising strategy for preventing the progression of normal aging into Alzheimer's Disease."

(Xinhua News Agency May 8, 2008)