A gene that predisposes its carriers to Alzheimer’s disease - APOE4 - appears to be active only in old age and has nothing to do with mental declines that are sometimes seen early in life, Australian researchers report in the current issue of Neuropsychology.

They evaluated the cognitive powers of 6,560 people regularly over the last 20 years, testing their memory, mental speed, reaction time and reading vocabulary - functions that are normally affected by Alzheimer’s disease.

Twenty-seven percent of the participants were APOE4 carriers but the researchers could find nothing differentiating them from non-carriers. The subjects, living in Canberra and neighboring Queanbeyan, were between 20 and 64 years old.

“Performance on all tests declined across age groups, a sign of normal cognitive aging. However, APOE4 did not affect performance at any age,” they wrote.

Lead researcher Anthony Jorm at the University of Melbourne’s psychiatry department said people begin to lose cognitive functions as early as in their 20s. This applied to both carriers and non-carriers of the APOE4.

“The memory changes that occur from 20 to 60 do not seem to be connected to that (APOE4) at all. I don’t know what they are associated with, but other brain processes must be involved,” Jorm told Reuters in a telephone interview.

“Carriers show the same declines as non-carriers…People with APOE4 are no more likely to have that early on than the people without. It is much later in their life spans that that effect begins to kick in.”

He stressed, however, that drastic declines - which happen over a year or two and affect the quality of life - were very rare in both carriers and non-carriers.

Clear signs of Alzheimer’s disease usually begin to show when a victim is in their 80s. While many researchers agree that APOE4 seems to be the predominant risk factor for Alzheimer’s disease, its function is not entirely clear.

“It may have a role in repair of nerve cells. When there is nerve damage, people with APOE4 version seem less able to repair and less resistant to a whole lot of diseases or damage to the brain,” Jorm said.