Is our ability to accurately identify smells a possible test to predict our likelihood of developing Alzheimers' disease (AD) as we age? A group of researchers think that it is.

Nearly 3000 older adults, aged 57 to 85, were asked to identify five odors, such as oranges and leather for the study. All lived at home and had normal cognitive functioning. Five years later, researchers interviewed the participants to see who among them had been diagnosed with dementia. Because previous studies that had shown that olfactory dysfunction precedes the development of AD, the researchers wanted to see if people with more impaired sense of smell, or olfaction, at the start of the study would have a higher likelihood of developing dementia after five years.

Almost half of the adults with a new dementia diagnosis had been classified as having olfactory impairment at the start.

The study group participants came from all geographic regions in the U.S., and the statistical analysis controlled for other factors that can contribute to olfactory dysfunction and to an elevated risk for dementia. These included age, sex, race, ethnicity, education, other diseases and conditions, and baseline cognitive skills.

The participants were classified as having normal olfaction if they could correctly identify four or five odors, and having impaired olfaction is they identified three or fewer correctly. The five odors, in order of increasing difficulty, were peppermint, fish, orange, rose and leather.

Twenty-two percent of the study group had olfactory dysfunction at the start of the study. At the five-year follow up 4.1 percent of the total study group had been diagnosed with dementia by a physician. Almost half of the adults with a new dementia diagnosis had been classified as having olfactory impairment at the start. This means that those with olfactory impairment from the beginning had twice the odds of developing dementia five years later. Conversely, more than three quarters of those who had had normal olfaction at baseline were cognitively normal at the five-year follow up.

The authors suggest that the amyloid plaques that have been seen in the brains of patients with AD may affect the olfactory system in the brain long before they affect the cognitive centers and cause noticeable impaired thinking, reasoning and memory.

It may also be that the olfactory system is more vulnerable to other mechanisms that lead to AD — such as not being able to regenerate or repair damaged cells as well as other brain centers can. This would also lead to dysfunction of the sense of smell before altered cognition was noticed by the individual.

“We think smell ability specifically, but also sensory function more broadly, may be an important early sign, marking people at greater risk for dementia...Loss of the sense of smell is a strong signal that something has gone wrong and significant damage has been done,” said the study's lead author, Jayant M. Pinto, MD, a professor of surgery at the University of Chicago, in a statement. “This simple smell test could provide a quick and inexpensive way to identify those who are already at high risk.”

The study is published in The Journal of the American Geriatrics Society.