Insulin appears to shield the brain from toxic proteins associated with Alzheimer's disease, US researchers said, supporting a theory that Alzheimer's may be a third form of diabetes.
And they said GlaxoSmithKline's diabetes drug Avandia, or rosiglitazone, which increases sensitivity to insulin, appeared to enhance this protective effect.
"Our results demonstrate that bolstering insulin signalling can protect neurons from harm," William Klein of Northwestern University, whose study appears in the Proceedings of the National Academy of Sciences, said in a statement.
Klein said the findings support a new idea that Alzheimer's is a type of diabetes of the brain.
Insulin receptor problem in brain
"In Type 1 diabetes, your pancreas isn't making insulin. In Type 2 diabetes, your tissues are insensitive to insulin because of problems in the insulin receptor. Type 3 is where that insulin receptor problem is localised in the brain," Klein said.
In some people, this can occur with age, he said.
"As you get older, some individuals start to have less effective insulin signalling, including in the brain," he said, making the brain more vulnerable to toxins that cause Alzheimer's disease.
Large sticky plaques of amyloid beta protein are a hallmark of Alzheimer's, which causes memory loss, confusion, the inability to care for oneself and eventually death.
Diabetics at higher risk for Alzheimer's
Recent studies by Klein and others have suggested that short strands of the protein, known as amyloid beta-derived diffusible ligands or ADDLs, attack memory-forming brain cells, causing memory loss.
Klein and colleagues treated rat nerve cells with insulin. "It blocked all of the effects of ADDLs," Klein said.
The effect was amplified when they added the drug rosiglitazone, which increases insulin sensitivity.
Several studies have found that diabetics have a higher risk of getting Alzheimer's than the general population.
Last July, researchers at Mount Sinai Medical Centre in New York reported that diabetics who take insulin plus a diabetes pill have a lower risk of developing Alzheimer's than diabetics who only take insulin.
That study included a range of anti-diabetic medications, including an older pill known as a sulfonylurea.
Measures to protect against Alzheimer's
Klein said the findings suggest that measures to protect people from diabetes - including a healthy diet and exercise - are also important for avoiding Alzheimer's disease.
In a separate study of more than 6 500 people, US researchers found that having more education does little to slow progression of Alzheimer's disease once people show signs of becoming forgetful.
"While education is associated with the memory's ability to function at a higher level, we found no link between higher education and how fast the memory loses that ability," said Robert Wilson of Rush University Medical Centre in Chicago, whose study appears in the journal Neurology.
Alzheimer's affects 26 million people globally, according to the Alzheimer's Association. - (Julie Steenhuysen/Reuters Health)
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