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By Robert Bazell Chief science and health correspondent
NBC News
updated 9/19/2011 4:02:01 PM ET 2011-09-19T20:02:01

The evidence linking the most common form of diabetes and the risk for Alzheimer's disease has been growing for years. New research released Monday in the journal Neurology makes the case even stronger.

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For the study, Japanese doctors recruited volunteers, ages 60 and older, and followed them for 11 years. In addition to checking the participants' medical history, the scientists gave them mental exams to determine whether they had symptoms of dementia and a glucose tolerance test to measure whether they had Type 2 diabetes.

Of 150 people with confirmed Type 2 diabetes at the beginning, 41 developed Alzheimer’s, compared to 115 who developed dementia out of 559 non-diabetics. That adds up to 27 percent rate of Alzheimer’s for diabetics, compared to a 20 percent rate for non-diabetics -- or a 35 percent increased risk. Those with the most severe diabetes at the beginning had a more than threefold increase in the rate of dementia.

The Alzheimer’s Association estimates that 5.4 million Americans have been diagnosed with the mind-robbing condition. As the population in the U.S. continues to age, that number could climb to 16 million within 40 years.

Story: Insulin nasal spray may slow Alzheimer's

“The fact that Type 2 diabetes is increasing, and it's a risk factor for Alzheimer's would only make those numbers bigger," says Dr. William H. Thies, medical and scientific officer for the association. “It's already a disaster that's going to come to us, if we don't do something better about treating Alzheimer's disease.”

How are diabetes and Alzheimer’s, two of America’s biggest public health problems, related?  All the connections are not known, but last year researchers found a gene that increases both the risk for Type 2 diabetes and for Alzheimer’s.

And scientists recently presented results of a small pilot study showing that insulin, the common treatment for diabetes delivered directly to the brain through a special inhaler seemed to slow the progression of Alzheimer’s. However, it will be years before insulin or any other treatment can be clinically verified to slow the progression of Alzheimer’s.

The actions need to reduce the risk of Type 2 diabetes -- and all of its side effects, including heart trouble, amputations, kidney failure and blindness --  are well known: Exercise more and lose weight. It's possible that adding the threat of Alzheimer’s disease to that list could end the reluctance of many people to make those lifestyle changes.

Story: Early Alzheimer's disclosures to become more common

Alzheimer's disease "is one of the most feared conditions for people who are entering their later years," says Thies. "[The new study] gives us an extra piece of information that may move people from considering changes in their life to actually making those changes."

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