Alzheimer’s disease begins years before mental deterioration is detected, suggests a new study that could result in doubling the estimated number of people with the neurodegenerative disease.

Cognitively normal individuals who had elevated levels of a toxic brain protein called amyloid experienced more rapid declines in thinking than those with normal levels. The 445 subjects were followed for as long as 10 years after their assessment to detect any changes. The median time was 3.1 years, and the subjects’ average age was 74.

The study, published in the Journal of the American Medical Association, adds to evidence that Alzheimer’s actually begins years before symptoms appear, said Dr. Paul Aisen, senior author of the study. Aisen directs the University’s of Southern California’s Alzheimer’s Therapeutic Research Institute, located in San Diego.

The finding has vast implications for health care, according to an accompanying JAMA editorial. It poses unsettling questions about how to tell people they may be on the path to Alzheimer’s well before any impairment occurs.