“What that tells you is the effect of the belly is over and above of being overweight,” Whitmer said. “One of the take-home messages is it’s not just your weight, but where you carry your weight in middle age, that is a strong predictor of dementia.”

Stomach fat may increase the risk for dementia in the same ways it promotes heart disease -- by boosting blood pressure and constricting blood flow, said Jose A Luchsinger of Columbia University. It may also promote the accumulation of amyloid, a substance found in the brains of Alzheimer’s patients.

“We think the buildup and clumping of this material is an important risk factor,” said Sam Gandy of the Mount Sinai School of Medicine, who chairs the Alzheimer’s Association’s medical and scientific advisory council. Previous research has found that people who are obese have higher levels of amyloid in their blood, he said.

Some experts remained skeptical, saying such a study cannot rule out the possibility that whatever is making people gain weight in their bellies in their 40s also puts them at risk for dementia in their 70s.

“There could be a connection. I’m not saying there couldn’t be,” said Barbara Corkery, director of Boston University’s Obesity Research Center. “But it could be those two things are caused by the same root cause.”

While acknowledging that more research is needed, Whitmer said the findings provide one more reason to try to maintain a healthy weight, noting that this type of fat is the most easily shed by dieting and exercise. “It’s not as stubborn as the fat under the skin,” she said. “It’s a modifiable risk factor.”