Having a large gut in midlife increases the chance of dementia in old age, according to new research published Wednesday that suggests that abdominal fat is a bigger risk factor than even family history.
The study of 6,583 adults found that people with the highest amount of abdominal fat between the ages of 40 and 45 were about three times more likely to develop dementia than those with the lowest amount.
By contrast, people who have parents or a sibling with Alzheimer's face twice the risk of developing the disease.
The report in the journal Neurology was the latest to show that belly fat can pose serious health risks, even for those who are not obese. Previous research has shown that people with large abdomens face a greater chance of diabetes, stroke and heart disease.
"This ought to be a wake-up call to baby boomers in terms of diet and exercise," said Dr. Sam Gandy, a spokesman for the Alzheimer's Assn. who was not involved in the study. "If they are not frightened enough about heart disease, maybe they will worry about losing their mental function."
Dementia is an age-related condition that involves the loss of memory and other cognitive functions. It affects 5.7 million Americans, or about 1 in 10 people over age 65. Alzheimer's is the most common form of dementia, accounting for 60% to 80% of cases.
Being overweight has also become a significant health problem in the U.S. About 50% of the nation's adults have an unhealthy amount of belly fat, according to the latest report.
People who tend to pack on abdominal fat are often described as apple-shaped and have a waist-to-hip ratio greater than 1 to 1 if they are male and greater than 0.8 to 1 if they are female.
The other major overweight group has a body type described as pear-shaped, characterized by fat around the thighs and lower body.
Participants in the study were members of Kaiser Permanente of Northern California who had their belly fat measured between 1964 and 1973.
Clinicians measured belly fat by placing one end of a tong-like instrument on the back of each subject and the other end of the tong on the subject's abdomen. A person had high belly fat if the distance between the two ends -- the subject's diameter -- was more than 25 centimeters, or 9.8 inches.
An average of 36 years later, 16% of the participants had been diagnosed with dementia.
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.