Scientists already know that the first lesions of the degenerative brain disease tend to show up in an area of the brain that is home to olfactory senses.
Doctors at the Rush University Medical Center in Chicago tested 589 people aged between 54 to 90, and asked them to identify 12 common odors such as lemon, chocolate, black pepper, banana, and soap. The subjects chose the best multiple choice answer for each.
At the start of the study in 1997, none of the subjects had cognitive impairments. The participants were tested every year for five years, and over the course of the study, 30.1 percent developed mild cognitive impairment.
"Risk of developing mild cognitive impairment increased as odor identification decreased," said the study which is published in the July issue of the Archives of General Psychiatry.
"Those who scored below average on the odor identification test were 50 percent more likely to develop the condition than those who scored above average," the authors of the study wrote.
"The findings suggest that olfactory dysfunction can be an early manifestation of Alzheimer's disease."
However, "the neurological bases of age-associated olfactory dysfunction are uncertain," and since "difficulty identifying odors is associated with other neurological diseases, including Parkinson's disease," further research was needed, the authors said.