Vitamin D is now known to be important in brain development and brain function. There is a wide distribution of vitamin D receptors in the brain, and many are directly involved in learning and memory, as well as motor control. A large study of older men suggests attention and speed of processing may be particularly affected by vitamin D deficits. Infants and the elderly, and those with darker skin living at northern latitudes, are particularly at risk of vitamin D deficiency. Vitamin D is received mostly from sunlight, but can also be found in some foods such as oily fish.
A study involving 382 older adults (average age 75) followed for around five years, has found that those who don’t get enough vitamin D may experience cognitive decline at a much faster rate than people who have adequate vitamin D.
Participants included 17.5% with dementia at the beginning of the study, 32.7% with MCI, and 49.5% cognitively healthy.
Those with dementia had lower levels of vitamin D than the other two groups.
While some people with low vitamin D didn’t show any cognitive decline and some with adequate vitamin D declined quickly, people with low vitamin D on average declined two to three times as fast as those with adequate vitamin D, in two crucial cognitive domains: episodic memory and executive function. Semantic memory and visuospatial ability were not significantly affected.
Factors such as age, gender, education, BMI, season of blood draw, vascular risk, and presence of the 'Alzheimer's gene', ApoE4, were controlled for.
Unlike previous studies of vitamin D and dementia, the participants were racially and ethnically diverse and included whites (41%), African Americans (30%), and Hispanics (25%). Nearly two-thirds (61%) had low vitamin D levels in their blood, including 54% of the whites and 70% of the African-Americans and Hispanics.
Vitamin D is primarily obtained through sun exposure. Accordingly, people with darker skin are more likely to have low levels of vitamin D because melanin blocks ultra-violet rays.
It remains to be seen whether Vitamin D supplements could slow cognitive decline.
 . VItamin d status and rates of cognitive decline in a multiethnic cohort of older adults. JAMA Neurology [Internet]. 2015 . Available from: http://dx.doi.org/10.1001/jamaneurol.2015.2115
The study involved 104 healthy older adults (average age 87) participating in the Oregon Brain Aging Study. Analysis of the nutrient biomarkers in their blood revealed that those with diets high in omega 3 fatty acids and in vitamins C, D, E and the B vitamins had higher scores on cognitive tests than people with diets low in those nutrients, while those with diets high in trans fats were more likely to score more poorly on cognitive tests.
These were dose-dependent, with each standard deviation increase in the vitamin BCDE score ssociated with a 0.28 SD increase in global cognitive score, and each SD increase in the trans fat score associated with a 0.30 SD decrease in global cognitive score.
Trans fats are primarily found in packaged, fast, fried and frozen food, baked goods and margarine spreads.
Brain scans of 42 of the participants found that those with diets high in vitamins BCDE and omega 3 fatty acids were also less likely to have the brain shrinkage associated with Alzheimer's, while those with high trans fats were more likely to show such brain atrophy.
Those with higher omega-3 scores also had fewer white matter hyperintensities. However, this association became weaker once depression and hypertension were taken into account.
Overall, the participants had good nutritional status, but 7% were deficient in vitamin B12 (I’m surprised it’s so low, but bear in mind that these are already a select group, being healthy at such an advanced age) and 25% were deficient in vitamin D.
The nutrient biomarkers accounted for 17% of the variation in cognitive performance, while age, education, APOE genotype (presence or absence of the ‘Alzheimer’s gene’), depression and high blood pressure together accounted for 46%. Diet was more important for brain atrophy: here, the nutrient biomarkers accounted for 37% of the variation, while the other factors accounted for 40% (meaning that diet was nearly as important as all these other factors combined!).
The findings add to the growing evidence that diet has a significant role in determining whether or not, and when, you develop Alzheimer’s disease.
 . Nutrient biomarker patterns, cognitive function, and MRI measures of brain aging. Neurology [Internet]. 2012 ;78(4):241 - 249. Available from: http://www.neurology.org/content/78/4/241.abstract
Another study has come out showing that older adults with low levels of vitamin D are more likely to have cognitive problems. The six-year study followed 858 adults who were age 65 or older at the beginning of the study. Those who were severely deficient in vitamin D were 60% more likely to have substantial cognitive decline, and 31% more likely to have specific declines in executive function, although there was no association with attention. Vitamin D deficiency is common in older adults in the United States and Europe (levels estimated from 40% to 100%!), and has been implicated in a wide variety of physical disease.
 . Vitamin D and Risk of Cognitive Decline in Elderly Persons. Arch Intern Med [Internet]. 2010 ;170(13):1135 - 1141. Available from: http://archinte.ama-assn.org/cgi/content/abstract/170/13/1135
A study of over 3,100 older men (49-71) from across Europe has found that men with higher levels of vitamin D performed consistently better in an attention and speed of processing task. There was no difference on visual memory tasks. Although previous studies have suggested low vitamin D levels may be associated with poorer cognitive performance, findings have been inconsistent. Vitamin D is primarily synthesised from sun exposure but is also found in certain foods such as oily fish.
 . Is there convincing biological or behavioral evidence linking vitamin D deficiency to brain dysfunction?. FASEB J. [Internet]. 2008 ;22(4):982 - 1001. Available from: http://www.fasebj.org/cgi/content/abstract/22/4/982
A review described as “definitive” has concluded that there is ample biological evidence to suggest an important role for vitamin D in brain development and function, and that supplementation for groups chronically low in vitamin D is warranted. Vitamin D has long been known to promote healthy bones, but more recently has been found to have a much broader role — over 900 different genes are now known to be able to bind the vitamin D receptor. Evidence for vitamin D's involvement in brain function includes the wide distribution of vitamin D receptors throughout the brain, as well as its ability to affect proteins in the brain known to be directly involved in learning and memory and motor control. Because we receive most of our Vitamin D from sunlight (UV from the sun converts a biochemical in the skin to vitamin D), those with darker skin living in northern latitudes are particularly at risk of vitamin D deficiency. Nursing infants and the elderly are also particularly vulnerable. It has also argued that current recommendations set the recommended level of vitamin D too low. This review is the fourth in a series that critically evaluate scientific evidence linking deficiencies in micronutrients to brain function. Earlier reviews have looked at DHA, choline, and iron.
 Association between 25-hydroxyvitamin D levels and cognitive performance in middle-aged and older European men. Journal of Neurology, Neurosurgery & Psychiatry [Internet]. 2009 ;80(7):722 - 729. Available from: http://jnnp.bmj.com/content/80/7/722.abstract