WASHINGTON (Reuters) - A gene already linked to Alzheimer's and heart disease can also cause everyday memory loss in healthy people as they age, US researchers said on Tuesday.

A study of people 55 and older found that the nerve cell activity in the front of the brain declined three times faster among those who had a gene variation called APOE4 when compared to those who did not have the gene.

The findings, presented at the American Association for Geriatric Psychiatry in Orlando, could allow researchers to understand why some people age faster than others, and design drugs to slow the process.

"This gene appears to be very closely related to how a nerve cell can repair itself or survive after different types of insults," Dr. P. Murali Doraiswamy of Duke University in North Carolina said in a telephone interview. "If you carry this gene, your nerve cells appear less capable of repairing themselves."

Doraiswamy and his colleagues compared the levels of frontal lobe brain activity in 165 healthy people between the ages of 55 and 85. The frontal lobe is important to memory.

The subjects were then separated depending on whether or not they carried the APOE4 gene.

Brain activity declined 28% in people who had active copies of APOE4 gene compared to 9% in people who did not have the gene, the study found.

APOE4 is found in 25% of the population and has been linked to higher levels of Alzheimer's, cardiovascular disease and memory loss.

Doraiswamy said even though the discovery of APOE4 could lead to the development of drugs to slow the aging process, more experiments must be done to find what this gene does to cause the brain to age faster.

"But we have a ways to go and clearly this finding is not going to have any clinical applications right away in people," he added.

A separate study published by Duke researchers has identified the average age at which people begin to develop memory loss. Alzheimer's affects people beginning at 72.8 years old and Parkinson's at 60.1, researchers found.

The study, published in the American Journal of Human Genetics, studied 449 families with a history of Alzheimer's and 174 with multiple cases of Parkinson's disease.

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