The real culprit is deep belly fat, also known as visceral fat, says study researcher Sudha Seshadri, MD, an associate professor of neurology at the Boston University School of Medicine.
''The greater the amount of visceral fat, the smaller the brain," she tells WebMD. While she didn't follow the participants to see if they developed dementia, she says that ''smaller brain volume is associated with poor cognitive function on testing and a greater risk of dementia on follow-up.''
The study is published online in the Annals of Neurology.
About 5.3 million Americans have Alzheimer's disease, according to the Alzheimer's Association. It's the most common form of dementia -- the loss of cognitive ability affecting memory, language, thinking, and judgment.
Much previous research has looked at the dangers of belly fat, with experts warning it boosts the risk of heart attack and heart disease. More recently, researchers have found the link with brain health.
Before her study, Seshadri says, "It was known that midlife obesity, from age 55 on, was a risk factor for dementia. It's not just your BMI but the central obesity which seems to add increased risk over just the BMI [if it's in the obese range, 30 and higher.]"
While previous research has linked excess visceral belly fat with dementia, Seshadri says many studies have included fewer than 300 participants. One exception is a study published in 2008, involving more than 6,500 participants, finding the more belly fat, the greater the risk of being diagnosed with Alzheimer's or other dementias later in life. Those with the biggest bellies had nearly a three times greater risk of dementia compared to the people with the smallest bellies.
Measuring Belly Fat
Seshadri and her team performed CT scans of the abdomen and MRI scans of the brain of 733 men and women who were participants in the Framingham Heart Study Offspring Cohort. On average, they were age 60; about 70% of participants were women.
Seshadri's team looked at the potential associations of body mass index, waist circumference, waist-to-hip ratio, and the CT measure of abdominal fat with the total brain volume.
The CT measured both visceral or deep belly fat and subcutaneous fat -- the fat that lies right below the skin.
While Seshadri can't quantify the risk of having a high amount of belly fat with a specific brain shrinkage, she says the results she found are linear: the more belly fat, the lower the brain volume.
The deep fat is the culprit, she says. "We found that subcutaneous was not [significantly] associated with any adverse effect on the brain volume, whereas visceral fat was clearly associated with smaller brain volume."
She also found a link between higher BMI and higher waist circumference, but the strongest association was between high visceral belly fat and lower brain volume.
The average BMI of study participants was 28 (30 and above is termed obese, 25 and above overweight.) The average waist circumference was 39 inches. Women should keep waist circumference below 35 and men below 40, according to the National Institutes of Health.
Exactly why the belly fat reduces brain volume isn't known, Seshadri says. Inflammation may play a role, as obesity is linked with inflammation in the body.
Some research has found that people on anti-inflammatory drugs show smaller age-related volume changes in their brain than do those not on the drugs.
Hormones produced by visceral fat tissue could pay a role in brain shrinkage, too, she says.
The study results are another reminder that paying attention to heart disease risk factors is also a good way to preserve your brain health, says William Thies, PhD, chief medical and scientific officer at the Alzheimer's Association.
"The key message in this study is another reason for people to keep good control over the factors that influence their heart health -- such as body weight, blood sugar, and blood pressure -- as an important way to also keep their brain healthy as they age, and possibly reduce their risk for cognitive decline and dementia."
But he also cautions that ''we don't have all the answers yet. You can do everything 'right' and still not prevent Alzheimer's."
Without a CT scan to measure belly fat, people can look to their BMI and waist measurement for a rough estimate of how much belly fat they carry, Seshadri says.
"If your BMI is in the obese range, it's 99% [certain] that you have too much visceral fat," she says. If your waist circumference is above 35 inches for women, above 40 inches for men, that's another good predictor, she says.
"If your BMI is under 25, you are probably OK,'' she says.
To reduce the dangerous visceral belly fat, she says, pay attention to diet and exercise regularly.