WASHINGTON (AP) -- Scientists have found yet another reason to slim down: The high blood sugar so common among the overweight may contribute to the fogged memory of old age.

A small study, published Monday in the Proceedings of the National Academy of Sciences, showed that middle-aged and elderly people with high blood sugar actually had a smaller hippocampus, the brain region so crucial for recent memory.

The good news: If the findings are confirmed, simple diet and exercise could help many people protect their brains. Maybe the threat of memory loss will provide the final push for aging baby boomers to take those steps, said lead researcher Dr. Antonio Convit of New York University.

"That's a great motivator to stay off the calories and stay off the couch," he said.

For every Alzheimer's patient, there are eight older people who suffer enough memory loss to significantly harm their quality of life even though they have no dementia-causing disease, said Convit, an NYU psychiatry professor who set out to uncover the causes.

Blood sugar was a natural suspect because scientists have long known that diabetics are at higher-than-normal risk for memory problems. Diabetes harms blood vessels that supply the brain, heart and other organs.

The new study found that people's memory may be harmed long before they ever develop full-fledged diabetes - and that it's a problem of fuel, not plumbing.

Convit studied 30 non-diabetic middle-aged and elderly people. He measured how they performed on several memory tests; how quickly they metabolized blood sugar after a meal; and, using MRI scans, the size of the hippocampus.

The slower those outwardly healthy people metabolized blood sugar, the worse their memory was - and the smaller their hippocampus was, Convit found.

Unlike most other tissues that have multiple fuel sources, the brain depends on blood sugar for almost all its energy, Convit explained. The longer that glucose stays in the bloodstream instead of being metabolized into body tissues, the less fuel the brain has to store memories.

Convit's research found no specific threshold at which memory automatically worsened. Overall, though, the slower the glucose metabolism, the worse people did.

Once that metabolism reaches certain levels, it becomes a condition called "impaired glucose tolerance" or pre-diabetes, thought to afflict 16 million Americans. It strikes mostly in middle age, although people of any age who are overweight and sedentary are at risk. Without treatment, pre-diabetes usually turns into full-fledged diabetes, which in turn brings deadly heart attacks, kidney failure and numerous other ailments.

Why did only the memory-crucial hippocampus seem harmed? Previous animal and human research shows it's the region most likely damaged by any brain insult, Convit said. Conversely, it's also a very adjustable region, with the potential for some recovery if people bring their blood sugar under control, he said.

Convit's study sheds important light on yet another risk of bad blood sugar, said Dr. Fran Kaufman, president of the American Diabetes Association.

She cautioned that it was a small study that requires confirmation before doctors test glucose solely for memory complaints.

But if confirmed, the same advice for lowering people's overall diabetes risk - drop a few pounds and do exercise as simple as walking 30 minutes a day - apparently would help protect people's brains, too, Kaufman said.

Meanwhile, the diabetes association already recommends pre-diabetes testing for everyone 45 or older, and for younger people who are significantly overweight and have one other risk factor: a diabetic relative; bad cholesterol; high blood pressure; diabetes during pregnancy; birth to a baby bigger than 9 pounds; or belonging to a racial minority.

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