Get out the tape measure.

According to a new study in today's online issue of Neurology, your waist circumference at midlife may be a good indicator of your risk of developing Alzheimer's disease in your 70s or beyond.

People who were overweight and had a large belly were found to be most at risk, but — and this may cause more than one MinnPost reader to head immediately to the gym — an increased risk (albeit a small and perhaps insignificant one) was found even among people of normal weight with flabby abdomens.

And a lot of us are lugging around that excess flab. American waistlines have steadily expanded over the past two decades. Research [pdf] has shown that more than half of adults in the United States now have waistlines that can be officially classified as abdominally obese.

The SAD facts
The Neurology study involved more than 6,500 people in northern California whose sagittal abdominal diameter (SAD) had been measured and recorded when they were 40 to 45 years old. SAD is the girth of the abdomen at a point midway between the lower ribs and the top of the pelvis. It's thought to be much more reliable than a standard waistline measurement at determining the density of visceral fat (the "hidden" fat that surrounds your internal organs — not the fat under your skin that you can pinch).

When the medical records of the study's participants were reviewed and analyzed three decades later, some 16 percent of the people had been diagnosed with dementia.

Abdominal fat played a big role in who ended up among that 16 percent. The data showed that overweight people with a large belly (a SAD of 25 centimeters or greater) were 2.3 times more likely to develop dementia than people with a normal weight and belly size. Obese people with a large belly were 3.6 times more likely to develop dementia.

Study participants who were either overweight or obese but who didn't have a large belly, however, were only 1.8 times more likely to have dementia.

"Where one carries the weight — especially in midlife — appears to be an important predictor for dementia risk," according to a statement from the study's lead author, Rachel Whitmer, Ph.D., of the Kaiser Permanente Division of Research in Oakland, Calif.

Three possible explanations
Does the study prove that abdominal obesity causes dementia? 

No, although it may be one possibility, said Dr. David Knopman, a professor of neurology at the Mayo Clinic in Rochester and associate editor of Neurology. Excess abdominal fat, he said, could "somehow promote the development of low-level inflammation that is deleterious to the brain."

Another explanation, said Knopman, is that excess abdominal fat is simply a marker for diseases, such as diabetes and high blood pressure, which are known risk factors for dementia.

A third possibility — and the one that Knopman leans toward — is that obesity, including a large belly, is a marker for poor health behaviors, such as smoking and lack of exercise, that increase the risk of dementia.

"The bigger picture," Knopman emphasized, "is that health behaviors in midlife do impact late life conditions like Alzheimer's and heart disease in a major way."

The bottom line
What's the take-home message from this study?

Maintain a healthy weight — and, particularly, watch your paunch. The National Institutes of Health recommends that women keep their waist circumference (not SAD, which is more difficult to measure accurately) under 36 inches and men under 41 inches.  Depending on your height and build, however, you may need to reduce those numbers even more.

Don't rely on sit-ups and crunches alone to whittle down your waistline. You must hit the gym or lake paths and walk, run, bike or do other cardiovascular exercises to burn up that visceral fat.

So get moving and keep moving. A fat abdomen is not just bad for your heart, it's bad for your brain, too.