WASHINGTON - People who have big bellies in their 40s are much more likely to get Alzheimer's disease and other forms of dementia in their 70s, according to new research that links middle-age spread to fading minds for the first time.

The study of more than 6,000 people found that the more fat they had in their guts in their early to mid-40s the greater their chances of becoming forgetful or confused or showing other signs of senility as they aged. Those who had the most impressive midsections faced more than twice the risk of the leanest.

Surprisingly, a sizable stomach seems to increase the risk even among those who are not obese, or even overweight, the researchers reported in a paper published online Wednesday by the journal Neurology.

"A large belly independent of total weight is a potent predictor of dementia," said Rachel Whitmer, a research scientist at the Kaiser Permanente Division of Research in Oakland, Calif., who led the new study.

The findings are alarming in light of America's growing girth, Whitmer and other experts said.

"If these findings are replicated and better understood, it looks like an unhealthy brain could be another consequence of this epidemic of obesity," Lenore Launer of the National Institute on Aging said.

Belly fat oozes toxins

The research is the latest evidence that fat in the abdomen is the most dangerous kind. Previous studies have linked the apple-shaped physique to a greater risk of diabetes, heart disease and even cancer. Researchers suspect that those fat cells are the worst because of their proximity to major organs. They ooze noxious chemicals, stoking inflammation, constricting blood vessels and triggering other processes that might also damage brain cells.

"There is a lot of work out there that suggests that the fat wrapped around your inner organs is much more metabolically active than other types of fat right under the skin," Whitmer said. "It's pumping out toxic substances. It's very potent toxic fat."

Whitmer and her colleagues analyzed data from 6,583 members of Kaiser Permanente of Northern California who had their belly fat carefully calculated as part of a broad health study between 1964 and 1973. The researchers examined whether there was a link between abdominal obesity between the ages of 40 and 45 and the chances of developing Alzheimer's and other forms of dementia by the time they hit their 70s between 1994 and 2006.

The risk for dementia, the researchers found, increased steadily with the amount of fat in the abdomen, even after accounting for alternative explanations, such as other diseases, bad habits and lower education. They found no such association for fat in the thigh.