The effects of Vitamin D Insufficiency and Seasonal Decrease on cognition.

BACKGROUND Vitamin D3 (cholecalciferol) deficiency has been associated with dementia and cognitive decline. Which cognitive domains are most associated with D3 levels and how seasonal fluctuations in levels relate to cognition is unclear. We addressed these questions using a prospective observational study examining associations between D3 levels and cognition among individuals living in northern latitudes (54°N) in summer and winter. METHODS Healthy adult participants underwent testing in summer and winter of D3 levels and cognition, using the Symbol digit Modalities test, phonemic fluency, digit Span and CANTAB battery. RESULTS Of 32 participants tested in the summer, 46% were D3 insufficient (<75 nmol/L) and performed worse on digit Span Backward (DS-B) (μ=5.8, SD=2) than those who were sufficient (μ=7.9, SD=2), p=0.018. In multivariate analyses, sufficiency status was an independent predictor of dS-B, (b=0.41, p=0.02). The majority (63%) of 19 participants tested in winter were D3 insufficient, with levels declining by a median of 15 nmol/L overall. Those with insufficient levels performed worse (i.e., higher scores) on the CANTAB Spatial Working Memory (SWM) task (μ=36.1, SD=6 versus μ=29.3, SD=8), p=0.05). Those with larger drops in levels (≥15 nmol/L) showed decline/less improvement on the CANTAB one touch Stockings of Cambridge (OTS) task, (μ=0.50, SD=1.9 versus μ=-2.11, SD=2.6, p=0.01), a test of working memory/executive functioning. CONCLUSIONS Vitamin D3 insufficiency and seasonal declines ≥15 nmol/L were associated with inferior working memory/executive functioning. While our findings require confirmation, they suggest that sufficient D3 levels should be maintained year-round, likely necessitating supplementation, at least during winter at higher latitudes.