By Dennis Thompson
"Just because you have amyloid [proteins] in the brain doesn't mean you're going to get dementia tomorrow. It doesn't mean you're going to get dementia in five years," said lead researcher Ron Brookmeyer. He's a professor of biostatistics with UCLA's Fielding School of Public Health.
"It could be many years, and it could be longer than your natural life expectancy," Brookmeyer added.
Further, a person's dementia risk is even lower if the amyloid or shrinkage is detected at a more advanced age, the study authors noted.
On the other hand, a person's risk does rise if early biological signs of Alzheimer's start stacking up, the investigators found.
For instance, a 60-year-old man with high levels of amyloid protein has just a 23 percent chance of developing dementia in his lifetime, while a 60-year-old woman has a 31 percent risk, the researchers said.
Shrinking of specific brain regions confer similarly low lifetime dementia risks at age 60; about 23 percent for men and 30 percent for women.
But if men or women have both high amyloid levels and brain shrinkage at age 60, their dementia risk increases to 34 percent and 42 percent, respectively, according to the report.
People face the most risk if they have high amyloid levels, brain shrinkage and early signs of mild cognitive impairment, the researchers explained. This impairment usually crops up as memory loss.
With all three signs present, 60-year-old men and women have a 93 percent and 96 percent risk of developing Alzheimer's, respectively.
Age and life expectancy also play a strong role in Alzheimer's dementia risk, Brookmeyer said.
Dementia risk decreases with age because it's less likely a person will develop Alzheimer's during their lifetime, he explained. But life expectancy also plays a part.
"Females will generally have higher risks, because females have a longer life expectancy. With a longer life expectancy, there's more time for the preclinical disease to progress," Brookmeyer said.
For this study, the researchers analyzed data on just over 2,000 people who participated in two long-term studies on Alzheimer's disease. These studies included scans for amyloid, brain shrinkage and cognitive impairment, all of which can be observed before a person falls victim to dementia.
This study could be considered good news for the 46.7 million Americans who carry some early signals of Alzheimer's disease, Brookmeyer said.
"It may actually provide some reassurance to people that despite testing positive on some screening tests, their chances of developing Alzheimer's dementia may remain low," Brookmeyer said.
However, he noted that the model needs to be further refined to include other factors -- such as genetics -- that affect dementia risk.
James Hendrix, director of global science initiatives for the Alzheimer's Association, agreed that it's too early to take much hope from this study.
"I think we probably do ourselves a little bit of a disservice if we focus so much on the overall conclusion that Alzheimer's risk is less than we thought, because I don't think we know that for sure," Hendrix said. "We need to dig into the numbers a little bit more and understand what is happening in the continuum of Alzheimer's disease as it progresses."
Hendrix is particularly concerned that these findings might prompt complacency in people, making them "think this is a solved problem."
As a result, people might slack on advocacy, volunteer efforts in their community, or volunteering for clinical trials that are critical to understand Alzheimer's, he said.
"The problem isn't diminished," Hendrix said. "It's still there, and we need people to remain engaged."
The findings were published online May 22 in Alzheimer's & Dementia.