March 26, 2008 -- Having a fat belly at midlife, which is already associated with an increased risk of diabetes, heart problems, and stroke, also increases the risk of getting dementia in your later years, according to a new study.
"This is the first time research has linked central obesity in midlife with dementia later in life," says Rachel Whitmer, PhD, research scientist at the Kaiser Permanente Division of Research in Oakland, Calif., who led the study. Central obesity is the term used for excess abdominal body fat.
"Where you carry your weight is an important risk factor," she tells WebMD. "If you are overweight and carry it in your belly, you are at greater risk [of health problems] than someone overweight who doesn't carry it in their belly."
Belly Fat and Dementia: Study Details
Whitmer and her colleagues followed the medical records of 6,583 members of Kaiser Permanente of Northern California, a large health maintenance organization, for an average of 36 years, from the time they were in their 40s until they were in their 70s.
They looked at medical exam records from visits between 1964 and 1973, when the members were in midlife (40-45 years old), and then again from 1994 to 2006. At the exams, the Kaiser members provided information on lifestyle, height, and weight and had the diameters of their bellies measured.
Whitmer's team divided the participants into five groups -- from those with the least amount of belly fat to those with the most.
The measurement taken, called the sagittal abdominal diameter, measures the belly from front to back, she says, and isn't easily translated into waist size. A sagittal abdominal diameter of 25 centimeters or more is considered high and reflects excess fat. They didn't collect waist circumference measurements.
Belly Fat and Dementia Risk
Over the course of the study, 1,049 participants were diagnosed with dementia, including Alzheimer's disease.
The bigger the belly, the higher the risk of dementia, they found.
"We found that people who were in the upper 20% of the belly size had a 2.72 times greater risk of dementia, compared to people in the bottom 20%," Whitmer says.
Not everyone who is overweight has a big belly, and some normal-weight people carry excess belly fat. So the researchers looked at the effects of both weight and belly fat.
The worst scenario was being obese with a lot of belly fat. "If you are obese with a big belly, the risk of dementia is 3.6 times [the risk of those with normal weight and low belly fat," Whitmer tells WebMD.
People of normal weight but with excess belly fat had a 1.89 times higher risk of getting dementia than those of normal weight and no excess belly fat.
The study is published online in Neurology.
Belly Fat and Health: How Much Is Too Much?
How to tell if your belly fat is a health risk? Men who have a waist circumference over 40 inches and women with a waist circumference over 35 inches are at greater risk, says Whitmer, citing National Institutes of Health guidelines.
While Whitmer's study didn't look at why belly fat increases dementia risk, she can speculate. "We are hypothesizing that the mechanism must have something to do with some of the substances the belly fat secretes," she says, "and those may have an effect on the brain."
The study results make sense to two other experts not involved in the research. "It fits quite nicely with all the other data that suggest vascular risk factors are also risk factors for dementia and Alzheimer's disease," says William H. Thies, PhD, vice president of medical and scientific relations for the Alzheimer's Association in Chicago.
While the study didn't investigate whether reducing belly fat reduces risk, Thies says, it's still a good idea for those with big bellies to work on reducing them through diet and exercise. "Ultimately there is enough data in other places to say changing your central obesity [belly fat] is good for your health."
"It's another piece of information that midlife cardiovascular risk factors are not just bad for your heart, they are bad for your brain," says David Bennett, MD, director of the Rush Alzheimer's Disease Center at Rush University Medical Center in Chicago.
Where you carry your weight is somewhat genetically determined, Whitmer says. Some people are ''apple" shaped, carrying excess belly fat, while others are more "pear" shaped, carrying excess weight in their hips and thighs.
"If you are apple shaped and do carry fat around your middle," she says, "be really aware you are at greater risk. One message is, if you can't lose the weight, lose the belly."