Those who were overweight and had a large belly when measurements were taken were 2.3 times more likely to develop dementia in old age than those who had a healthy weight and belly size when they were younger.
The researchers categorized subjects as overweight if they had a body mass index of 25 to 29.9. The index, also known as BMI, is a ratio of height and weight. For example, a man who is 6 feet tall and weighs 184 pounds is overweight.
People who were obese -- with a BMI greater than 30 -- and had a large belly in middle age were 3.6 times more likely to develop dementia later in life than those whose weight and belly size had been in the healthy range.
Increases in thigh fat did not add to the risk of dementia, according to the report.
Lead author Rachel A. Whitmer, a Kaiser Permanente researcher, said the study underscored the need for doctors "to check not just weight but how much fat patients are carrying around their middles."
Whitmer said scientists did not know how abdominal fat might contribute to the risk of dementia.
One theory is that hormones and proteins released by abdominal fat spur the buildup of amyloid plaques in the brain, which are associated with Alzheimer's. Abdominal fat is more biologically active than fat located in other areas of the body, she said.
Another possibility, Whitmer said, is that dementia is not directly related to abdominal fat but is linked to obesity-related diseases such as stroke, diabetes or cardiovascular disease.
Although losing weight can be a challenge, she said, abdominal fat is easier to lose than other kinds of fat.
"The good news is it goes away with diet and exercise," Whitmer said.
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denise.gellene@latimes.com
