People who have both Alzheimer's disease and diabetes have slower rates of memory loss than people who just have Alzheimer's disease, French researchers said on Tuesday.

They studied 600 Alzheimer's patients for four years and found those who had both Alzheimer's and diabetes — about 10 percent of the total — scored far better on twice yearly memory and thinking tests than those with Alzheimer's who did not have diabetes.

"This result was surprising," said Dr. Caroline Sanz of the French National Institute for Health and Medical Research, whose study appears in the journal Neurology.

"Our initial hypothesis was that diabetes would increase the rate of cognitive decline in people with Alzheimer's disease," she said in a statement.

Prior studies have suggested that diabetics are at higher risk for developing Alzheimer's disease. Sanz and colleagues wanted to see if diabetes influences the rate of mental decline in people who already have mild to moderate Alzheimer's.

At the beginning of the study, Alzheimer's patients with and without diabetes had average scores of 20 points on cognitive tests.

But during each periodic testing period, the group that did not have diabetes declined by an average of 1.24 points, while those who had diabetes slipped a mere 0.38 points per test.

The team said it was not clear why the rate of memory loss was slower in diabetics, but it could be related to some of the drugs diabetics were taking, the team said.

They said older diabetics were more likely to be taking heart medications such as statins, aspirin or drugs to lower blood pressure. "These drugs have been reported to decrease the risk of developing Alzheimer's disease and also the rate of cognitive decline in people with Alzheimer's disease," the team wrote.

Diabetics in the study were also taking drugs to control their blood sugar, such as metformin, sulfonylureas and insulin.

A study in rats last February by researchers at Northwestern University in Illinois suggested that insulin may shield the brain from toxic proteins associated with Alzheimer's disease, and certain antidiabetic drugs, such as GlaxoSmithKline's Avandia, or rosiglitazone, enhance this effect.

Alzheimer's disease is a mind-robbing form of dementia that is expected to affect more than 35 million people in 2010, according to the Alzheimer's Association. There is no cure, and current drugs merely delay symptoms.