Olfactory function is associated with cognitive performance: Results from the population-based LIFE-Adult-Study

Abstract

Background: Studies in older adults or those with cognitive impairment have shown associations between cognitive and olfactory performance, but there are few population-based studies especially in younger adults. We therefore cross-sectionally analyzed this association using data from the population-based LIFE-Adult-Study. Methods: Cognitive assessments comprised tests from the Consortium to Establish a Registry for Alzheimer's Disease (CERAD): verbal fluency (VF), word list learning and recall (WLL, WLR), and the Trail Making Tests (TMT) A and B. The "Sniffin' Sticks Screening 12" test was used to measure olfactory performance. Linear regression analyses were performed to determine associations between the number of correctly identified odors (0 to 12) and the five cognitive test scores, adjusted for sex, age, education, and the presence of depressive symptoms. Receiver operating characteristic (ROC) analysis was carried out to determine the discriminative performance of the number of correctly identified odors regarding identification of cognition impairment. Results: A total of 6783 participants (51.3% female) completed the olfaction test and the VF test and TMT. A subgroup of 2227 participants (46.9% female) also completed the WLL and WLR tests. Based on age-, sex-, and education-specific norms from CERAD, the following numbers of participants were considered cognitively impaired: VF 759 (11.2%), WLL 242 (10.9%), WLR: 132 (5.9%), TMT-A 415 (6.1%), and TMT-B/A ratio 677 (10.0%). On average, score values for VF were higher by 0.42 points (p < 0.001), for WLL higher by 0.32 points (p = 0.001), for WLR higher by 0.31 points (p = 0.002), for TMT-A lower by 0.25 points (p < 0.001), and for TMT-B/A ratio lower by 0.01 points (p < 0.001) per number of correctly identified odors. ROC analysis revealed area under the curve values from 0.55 to 0.62 for the five cognitive tests. Conclusions: Better olfactory performance was associated with better cognitive performance in all five tests in adults - adjusted for age, sex, education, and the presence of depressive symptoms. However, the ability of the smell test to discriminate between individuals with and without cognitive impairment was limited. The value of olfactory testing in early screening for cognitive impairment should be investigated in longitudinal studies.