A new study published today suggests that having a larger belly in middle age may increase the risk of Alzheimer's disease and other forms of dementia.

Extra weight around the midsection is already known to increase the risk of diabetes, heart disease, stroke, and early death.

The new study, published today in Neurology, the journal of the American Academy of Neurology - suggests that the effect of fat on the body is more complicated than just a number on a scale

The study was based on data from more than 6,500 members of Kaiser Permanente in California, shows that "where you carry weight, more than your total body weight, is a good predictor of dementia," says lead author Rachel Whitmer, a research scientist at Kaiser Permanente.

"You can have a healthy body mass index but have a risk of dementia due to belly fat."

People with pear-shaped bodies tend to carry most of their weight below the waist, in the hips, buttocks and thighs. People with apple-shaped bodies are at greater risk of having too much visceral fat surrounding internal organs deep in the abdominal cavity.

Visceral fat is more common in people who are overweight, but even a very lean person with a small pot belly could have hidden visceral fat. in fact, fifty percent of adults have "central obesity," or fat that has accumulated around their midsections, according to the new study.

Study participants were ages 40 to 45 when they entered the study between 1964 and 1973, and some were followed until 2006, when they were as old as 87.

Among patients with the most visceral fat in middle age, the rate of dementia was 324.3 cases per 10,000; patients with the least belly fat had a rate of dementia of 214.6 cases per 10,000.

Even thin people could be at risk if they have a large pot belly. In the study, patients who were of average weight but in the category with the most visceral fat were 89 percent more likely to develop dementia than people of average weight with little or no belly fat.