“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 analysed data from 6,583 members of Kaiser Permanente of Northern California who had had their belly fat carefully calculated as part of a broad health study between 1964 and 1973. The researchers examined whether there was 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 the patients 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.

The researchers used a complicated method for measuring fat known as sagittal abdominal diameter (SAD). Those with a SAD score above 25 — roughly equivalent to a waist of at least 39 inches — had the biggest bellies and the greatest risk.

Previous studies have shown that people who are overweight are at increased risk for dementia. But when the researchers examined patients’ body mass index (BMI), which is the most common way to determine whether someone is overweight or obese, they found that people with big bellies were still nearly twice as likely to develop dementia, even if they had BMIs that were considered healthy. In fact, their risk was about the same as for those who were overweight or obese.

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