Researchers at the Salk Institute and UCSD said they have uncovered one reason why diabetics are up to 65 percent more likely to develop Alzheimer's, the degenerative disease mainly associated with memory loss in old age.

In a newly published study, they said some of the factors that cause diabetes might also trigger Alzheimer's. In particular, their report found that the inability to process insulin caused swelling of the brain, damaged blood vessels there and hastened the death of neural cells in certain diabetic mice.

They believe a similar process might be taking place in humans.

The work provides yet another motivation for combating obesity and the growing diabetes epidemic it is fueling in the United States. It also suggests that onset of Alzheimer's could be delayed or even prevented in some people if they don't become diabetic earlier in life, said David Schubert, the lead investigator and professor at the institute's Cellular Neurobiology Laboratory.

At least 21 million people in the United States have diabetes and about 54 million have prediabetes. Nearly 5 million Americans suffer from Alzheimer's, and virtually all of them are 65 or older.

"The important point to make is not to get obese and develop diabetes," Schubert said. "It's a warning signal. It emphasizes that you don't want to get diabetes when you're young."

The analysis led by Schubert is published in the current online edition of the medical journal Neurobiology of Aging. The study was co-authored by fellow Salk researchers Joseph Burdo and Qi Chen and diabetes expert Nigel Calcutt, who teaches in the pathology department at the University of California San Diego. The researchers are based in La Jolla.

Besides offering an explanation for the combination of diabetes and Alzheimer's in some individuals, the report suggests that people at risk of developing Alzheimer's could benefit from consuming foods and supplements rich in antioxidants, including fish, beans, berries, vegetables, fish oil and vitamin E, said researchers such as Greg Cole, associate director of the UCLA Alzheimer's Center.

Antioxidants can help fight advanced glycation, the buildup of certain toxic protein molecules that can harm some blood vessels and lead to destructive inflammation of the brain.

Developing drugs to treat and cure Alzheimer's requires huge investments of time and money, Cole said, so studies that rely on animal subjects - like the one by the Salk and UCSD researchers - fill an important gap.

"There's sort of an urgent need to move ahead with prevention," he said. "Since we have an obesity epidemic driving diabetes rates up and intersecting with an aging population, we're looking at having even more cases (of Alzheimer's) than we might have expected."

For more than a decade, scientists have been building an increasingly convincing case for a relationship between diabetes and Alzheimer's in some patients.

Several recent studies have shown that people with Type 2 diabetes, the version most associated with obesity, have a 30 percent to 65 percent higher chance of developing Alzheimer's than non-diabetics.

Diabetes centers on insulin, a hormone produced by the pancreas that helps to move glucose - a simple sugar - to cells throughout the body. Glucose is the body's main fuel, and cells die when they can't get enough of it.

In diabetics, the insulin process breaks down and sugar levels build up in the bloodstream. Type 1 diabetics can't produce insulin, while Type 2 diabetics often don't make enough of the hormone and can't process it properly.

Some researchers have suggested that in a way, Alzheimer's might be another type of diabetes.

In October, for example, researchers at Northwestern University said they had found cells in the brains of Alzheimer's patients that were resistant to insulin.

While most previous studies focused on people already suffering from Alzheimer's, the Salk researchers sought to investigate the effects of diabetes before symptoms of the brain disease appear.

Schubert and his colleagues triggered diabetes in young mice that were genetically predisposed to develop Alzheimer's later in life. Seven weeks later, the mice were placed in a shallow pool of water with an escape platform hidden 1 centimeter below the surface.

Researchers measured the time it took the mice to find the platform. The test was repeated twice a day for eight days to measure the rodents' memory and learning ability.

Over the course of the study, the mice that were both diabetic and prone to developing Alzheimer's had a more difficult time finding the platform than those with one disease or the other.

When the researchers examined the brains of all the rodents, they found significantly greater amounts of the toxins linked to advanced glycation in the mice with diabetes and the predisposition for Alzheimer's. These toxins are typically found in large quantities in the brains of mice with fully developed Alzheimer's.

Now that the Salk and UCSD researchers have identified the mechanics of how diabetes triggers the onset of Alzheimer's symptoms in young mice, they hope to use the rodents to test a pair of drugs being developed to treat Alzheimer's in people, Schubert said.

"If a drug worked in a (research) model that we have," he said, "then I think it would be very useful for adult onset Alzheimer's."
Diabetes facts
  • As of 2005, nearly 15 million Americans were diagnosed as diabetic.
  • An additional 6 million had the disease at the time but didn't know it.
  • bout 21 percent of diabetics are 60 or older.
  • An estimated $174 billion was spent treating the disease last year.
Alzheimer's facts
  • About 5 million Americans have the disease.
  • Roughly 96 percent of them are 65 or older.
  • About 10 million baby boomers will develop Alzheimer's.
  • More than $148 billion was spent treating the disease in 2005.
SOURCES: American Diabetes Association; Alzheimer's Association