A study from The Canadian Journal of Neurological Sciences found that both vitamin D insufficiency and seasonal decline of vitamin D levels are correlated with lower scores related to cognitive performance.
Vitamin D insufficiency has been associated with cognitive impairment and dementia in multiple observational studies. For example, one meta-analysis of longitudinal studies revealed that vitamin D deficient individuals had 2.5 times the chances of developing cognitive impairment as vitamin D sufficient individuals.
Despite the growing body of literature on the role of vitamin D in cognition, there are several questions that remain unanswered. For example, which cognitive domains are most affected by vitamin D insufficiency and how do seasonal changes in vitamin D status relate to changes in cognitive functioning?
Canadian researchers conducted the current study to answer these questions. Specifically, they wanted to determine which cognitive domains are most associated with vitamin D status during both summer and winter and what effect the degree of seasonal change in vitamin D levels has on cognition. Examples of cognitive domains are working memory, pattern recognition, and executive functioning.
Working memory is the system that temporarily holds and manipulates visual or verbal information in the mind to be used towards completion of a goal or task. Executive functioning is the management of various cognitive processes such as planning, reasoning, problem solving, and execution.
The researchers selected participants from the general population of northern British Columbia, Canada. They then had the participants complete a series of standardized cognitive tests that evaluated working memory, pattern recognition, and executive functioning. The participants then had their blood drawn and their vitamin D levels measured. Vitamin D sufficiency was defined as having a level of 30 ng/ml or higher.
Data collection first occurred in the summer and all measurements and tests were repeated again in the winter, about four months after the initial assessment. There were 32 individuals who completed the summer assessment and 19 who completed the winter assessment.
The researchers performed both a cross-sectional and longitudinal analysis of the data. This means that they analyzed the results during winter and summer separately as well as analyzed the results based on the differences between the two seasons.
The cross-sectional analysis yielded the following results:
- For the summer assessment, participants with insufficient vitamin D levels (n=14) scored lower on a test of working memory than participants with sufficient levels (n=18, p=0.018). Scores of other cognitive measures did not differ significantly between groups.
- For the winter assessment, participants with insufficient levels (n=12) again performed worse on the working memory test than participants with sufficient levels (n=7, p=0.05). Scores of other cognitive measures did not differ significantly between groups.
The longitudinal analysis found:
- Vitamin D levels decreased an average 6.8 ng/ml from summer to winter.
- For follow-up testing in the winter, participants with a 6 ng/ml or greater decline in vitamin D levels (n=10) had a decrease in scores on both working memory and executive functioning tests, while participants with less than a 6 ng/ml decline in levels (n=9) had an increase in scores on working memory and executive functioning tests. The difference between these scores was significant.
- Seasonal changes in performance were not significantly different between groups on other cognitive measures.
In their discussion, the researchers concluded,
“Vitamin D sufficiency status was related to cognition and, specifically, working memory, as assessed by the digit span- backwards (DS-B) and CANTAB Spatial Working Memory tasks. In addition, greater declines in vitamin D levels from summer to winter were accompanied by greater declines in executive functioning, as measured by CANTAB One Touch Stockings of Cambridge task.”
While the effect sizes (a measure of the strength of the results) were quite large, the direction of causality can’t be established, so we don’t know if lower vitamin D levels cause a decrease in cognitive performance.
The finding that vitamin D levels are most associated with working memory and executive functioning is consistent with the few cross-sectional studies that have looked at vitamin D and cognition in the past.
The small sample size is the greatest limitation of the study. The researchers call for larger prospective longitudinal studies to be conducted, in addition to more intervention trials that assess the effects of vitamin D supplementation on cognition.