Essential Oils Enhance Cognition in Healthy Older Adults During Sleep

Minimal overnight exposure to essential oils may be a cost-effective method to improve cognitive and neural functioning in healthy older adults, according to study findings published in the journal Frontiers in Neuroscience.

Previous research has shown that olfactory enrichment alone, defined as the daily exposure of multiple odorants in individuals, could improve both memory and neurogenesis in mouse models. With cognitive loss becoming more common among older adults, researchers have acknowledged the need for an inexpensive low-effort method to improve cognition in this patient population. This prompted a group of researchers from at the University of California, Irvine to assess whether the use of olfactory enrichment at night could improve cognition in healthy older adults.

The researchers enrolled 132 older adults aged 60-85 who had normal cognition and good general health into a randomized controlled trial, Sensory Enrichment for Older Adults (ClinicalTrials.gov Identifier: NCT03914989), to assess the impact of olfactory enrichment on cognition. Each participant was randomly assigned to 1 of 4 groups: olfactory enrichment with magnetic resonance imaging (MRI), olfactory enrichment without MRI, control condition with MRI, and control condition without MRI. Research disruption due to the COVID-19 pandemic caused some attrition, and 44 participants were ultimately excluded from analyses. The final count included: 20 participants in the olfactory-enriched with MRI group, 21 participants in the olfactory-enriched group without MRIs, 23 participants in the control group with MRI, and 24 participants in the control group without MRI scans.

Individuals in both olfactory-enriched groups were provided with an odorant diffuser and 7 different essential oil odorants. The diffuser released 1 scent for 2 hours per night, cycling through the scents each week. Individuals in the 2 control groups were given the same testing instructions, but the essential oil odorants were replaced with sham trace odorants. All groups used their odorant diffusers for 6 continuous months.

Study participants underwent assessments for cognitive and olfactory functioning at baseline and after 6 months of odorant exposure. These assessments included the Mini-Mental State Examination (MMSE) to confirm normal cognitive functioning as well as the Rey Auditory Verbal Learning Test to evaluate verbal learning and memory, and the completion of 3 subsets of the Weschler Adult Intelligence Scale-Third Edition (WAIS-III). The researchers used the “Sniffin’ Sticks” test to assess olfactory system function as a way to determine whether olfactory enrichment enhanced olfactory performance.

Study participants in the 2 MRI groups had functional (fMRI) scans at baseline and after the 6-month exposure. This allowed the researchers to use diffusion-weighted imaging to analyze bilateral changes in the white matter of limbic regions in the brain, specifically the uncinate fasciculus and the hippocampal cingulum of the brain. These particular regions were of interest because of their documented association with learning and memory.

After 6 months, individuals in the olfactory-enriched groups demonstrated improved cognitive functioning on the Rey Auditory Verbal Learning Test (Timepoint x Group interaction, F=6.63; P =.02; Cohen’s d=1.08). Compared with the control groups, the adults in the olfactory-enriched groups demonstrated a 226% difference in average performance on this neuropsychological test.

Additionally, compared with individuals in the corresponding control group, those who were exposed to the essential oils overnight and underwent MRI demonstrated increased diffusivity of the left uncinate fasciculus (Timepoint x Group interaction, F=4.39; P =.043; η_p²=.101) after 6 months.

The researchers wrote, “We have shown that minimal olfactory enrichment at night using an odorant diffuser results in significant improvements in both verbal memory and the integrity of a specific brain pathway.” They concluded, “Our findings should stimulate larger scale clinical trials systematically testing the therapeutic efficacy of olfactory enrichment in treating memory loss in older adults,” the researchers concluded.

Study limitations included attrition due to COVID-19, which further reduced the small sample size. Additionally, the design limitations of the diffusor did not allow for testing multiple odorants per night.

Disclosures: Several study authors declared affiliations with biotech, pharmaceutical, and/or device companies. Please see original source for full list of disclosures.