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Meta-analysis and systematic review: Low vitamin D linked to cognitive decline

Posted on: June 28, 2013   by  John Cannell, MD

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Recently, Doctor van der Schaft, working under senior author Professor Emmelot-Vonk, both of the University Medical Center Utrecht in the Netherlands, published a systematic review and meta-analysis on vitamin D and cognition.

van der Schaft J, Koek HL, Dijkstra E, Verhaar HJ, van der Schouw YT, Emmelot-Vonk MH. The association between vitamin D and cognition: A systematic review. Ageing Res Rev. 2013 May 29.

Cognition is how well your brain is working and includes attention, memory, language, learning, reasoning, problem solving, and decision-making. In dementia diseases, these aspects decline to varying degrees. Dementia, either Alzheimer’s or other dementias, are what many of us fear as we age.

Where does vitamin D fit in? The researchers here update us on what we currently know about vitamin D, cognition and dementia. Here are some of the highlights:

  • In the United States, the reported prevalence of severe vitamin D deficiency [25(OH)D < 20 ng/ml] in adults is anywhere between 25% and 57% depending on the study.
  • Biosynthetic and degradative pathways for vitamin D are found in both the neurons and the glial cells in the cerebral cortex. Glia cells are non-neuronal cells that provide support and protection for neurons in the brain.
  • Studies have shown that vitamin D may protect the structure and integrity of neurons, through detoxification pathways and neurotrophin synthesis, necessary for neuronal survival. Thus, researchers are hopeful that vitamin D may play a role in cognition and cognitive decline.
  • In their meta-analysis, they included more than 48,000 individuals from 25 cross-sectional studies. The main finding of the cross-sectional studies was a statistically significant worse outcome on one or more cognitive function tests or a higher frequency of dementia with lower vitamin D levels or intake in 18 out of 25 (72%) studies.
  • In their meta-analysis, more than 10,000 individuals were included from 6 prospective studies. The main finding of the prospective studies was a statistically significant decline on one or more cognitive function tests or a higher frequency of dementia in participants with lower vitamin D levels or intake compared to participants with higher vitamin D levels or intake in 4 out of 6 (67%) studies.
  • It remains unclear if vitamin D supplementation can prevent cognitive decline in people with low vitamin D levels since there is a lack of placebo-controlled intervention studies.

The authors concluded:

“In conclusion, this systematic review pointed out that the majority of the cross-sectional as well as prospective studies found that hypovitaminosis D is associated with a statistically significant worse outcome on one or more cognitive function tests or a higher frequency of dementia. Further studies should focus on the role of vitamin D supplementation in the prevention of cognitive decline in participants with low vitamin D levels.”

It is important that a randomized controlled trial study older adults at risk of dementia who have a low 25(OH)D level to see if vitamin D is preventative. Likewise, in patients with dementia who have low 25(OH)D levels, does vitamin D improve cognition? However, that raises difficult ethical questions. Would you want to be in the placebo arm of either study?

It is also important to remember that 25(OH)D levels are often a marker of sun exposure. Sunlight may have effects on cognition that are independent of vitamin D.

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