MONDAY June 23, 2008 (foodconsumer.org) -- A plague of beta-amyloid in the brain is believed to be the cause of Alzheimer's disease.   But new research suggests that not all forms of beta-amyloid play a role in the disease, The Associated Press reports.

Researchers found from a study of rats that only a two-molecule form of beta-amyloid causes the characteristics of Alzheimer's, but not other forms.

Dr. Ganesh M. Shankar and Dr. Dennis J. Selkoe of Harvard Medical School manipulated extracts from the brains of people who died from Alzheimer's disease and injected different forms of beta-amyloid in the brains of rats to see what happens.

They found the insoluble plaque or the soluble one-molecule or three molecule forms did not cause any detectable effect on the rats who received the injections.

Only the rats that received two-molecule form of soluble beta-amyloid exhibited signs of Alzheimer's such as impaired memory function, particularly newly learned behaviors, the researchers reported in Sunday's online edition of the journal Nature Medicine.

A study of mice also showed that the density brain cells were reduced by 47 percent, the AP reports. The beta-amyloid seemed to have an impact on synapses.

Now the researchers want to know why one form of beta-amyloid is harmful while others are not.

Alzheimer's disease, a major form of dementia, affects an estimated 4.5 million Americans. The disease is believed to begin after age 60. About 5 percent of men and women age 65 to 74 have the disease while of those age 85 or older, half have the disease.

There is no cure for the fatal Alzheimer's disease.