Low Vitamin D and Its Association with Cognitive Impairment and Dementia

Vitamin D is a neurosteroid hormone that regulates neurotransmitters and neurotrophins. It has anti-inflammatory, antioxidant, and neuroprotective properties. It increases neurotrophic factors such as nerve growth factor which further promotes brain health. Moreover, it is also helpful in the prevention of amyloid accumulation and promotes amyloid clearance. Emerging evidence suggests its role in the reduction of Alzheimer's disease hallmarks such as amyloid-beta and phosphorylated tau. Many preclinical studies have supported the hypothesis that vitamin D leads to attentional, behavioral problems and cognitive impairment. Cross-sectional studies have consistently found that vitamin D levels are significantly low in individuals with Alzheimer's disease and cognitive impairment compared to healthy adults. Longitudinal studies and meta-analysis have also exhibited an association of low vitamin D with cognitive impairment and Alzheimer's disease. Despite such evidence, the causal association cannot be sufficiently answered. In contrast to observational studies, findings from interventional studies have produced mixed results on the role of vitamin D supplementation in the prevention and treatment of cognitive impairment and dementia. The biggest issue of the existing RCTs is their small sample size, lack of consensus over the dose, and age of initiation of vitamin D supplements to prevent cognitive impairment. Therefore, there is a need for large double-blind randomized control trials to assess the benefits of vitamin D supplementation in the prevention and treatment of cognitive impairment.


Background
Vitamin D is a fat-soluble steroid vitamin with a definitive role in bone health. Beyond its role in the regulation of bone health, it also plays an important role in the functioning of other systems such as cardiovascular, endocrine, and nervous systems [1]. Ultraviolet radiation (UVR) is the major source of vitamin D. e two forms of vitamin D are ergocalciferol (D2) and cholecalciferol (D3). It undergoes two hydroxylation processes, first in the liver by enzyme 25 hydroxylase to produce 25(OH)D and second in the kidney to produce active form of 1,25(OH)D [2,3]. An estimated one billion people worldwide suffer from hypovitaminosis D. ere is no worldwide consensus regarding the cutoff value for definition of vitamin D deficiency. Typically, vitamin D deficiency is defined as a 25(OH)D level of less than 50 nmol/L, with severe deficiency defined as less than 25 nmol/L and insufficiency between 50 and 75 nmol/L [4]. Vitamin D can reach the brain by crossing the blood-brain barrier (BBB) through passive diffusion. e active form, 1,25(OH)D, binds to the vitamin D receptor (VDR) and influences gene expression. Vitamin D exerts its action via VDR present in neurons, glial cells of the hippocampus, orbitofrontal-cortex, cingulate, amygdala, and thalamus [5][6][7]. Its neuroprotective, anti-inflammatory, and antioxidant effect on neurons promotes brain health [8][9][10]. Vitamin D promotes the production of neurotrophic factors such as nerve growth factor (NGF). Many studies have consistently reported the increase in neuronal growth in rat hippocampal cell cultures enriched with vitamin D [8,9]. e NGF and other neurotrophic factors promote the survival of both hippocampal and cortical neurons [10,11]. Vitamin D is also implicated in regulating the gene expression of various neurotransmitters such as acetylcholine, dopamine serotonin, and gamma butyric acid [12,13]. Vitamin D reduces age-related tau hyperphosphorylation, the formation of amyloid-beta oligomers, increases amyloid clearance, and prevents neuronal death [13]. Although it promotes amyloid phagocytosis and clearance, correlation of serum vitamin D with CSF (cerebrospinal fluid) biomarkers of amyloidosis such as phosphorylated tau and amyloid-beta is, by far, not investigated except very few studies [13]. Nevertheless, vitamin D has also shown its neuroprotective activity by curtailing the glutamate-induced neurotoxicity [13] and upregulating genetic expressing of various proteins required for new synapse formation, thus promoting neurogenesis especially in the hippocampus [14]. Neuroimaging has suggested a positive association between low vitamin D levels, white matter hyperintensities, and enlarged frontal horn of the lateral ventricle [15]. Significant positive correlation of serum 25(OH)D with total hippocampus volume and disrupted structural connectivity between hippocampus, cortical, and subcortical areas in the right hemisphere was found in patients with mild cognitive impairment [16]. However, small sample size, cross-sectional design, and lack of detailed data on potential covariates (hypertension and diabetes) were the limitations of the study. Furthermore, prospective studies of longer duration exploring neuroimaging outcomes will provide useful insights into potential mechanisms as most neuroimaging studies have been crosssectional resulting in the possibility of reverse causation.
is review aims to provide an overview and discussion of the current state of evidence regarding vitamin D and dementia-related outcomes.

Vitamin D and Brain-Evidence through Animal Studies
Developmental vitamin D deficiency and inactivated vitamin D receptor gene affect brain functioning and behavioral outcome in rodents. e studies which support this hypothesis were conducted on mice with prenatal deficiency of vitamin D and vitamin D knock-out mice. Rats born to vitamin D3-deficient mothers demonstrated a reduction in the nerve growth factor and glial-derived neurotrophic factor compared to control rats [17]. Similarly, a study on 10week-old rats with transient vitamin D deficiency during the early developmental stage demonstrated enlarged lateral ventricle volume and reduced nerve growth factor compared to controls [18]. e evidence concerning the impact of vitamin D deficiency on the behavior of mice which developed later in life is sparse. However, a study investigating the effect of vitamin D-deficient diet for 10 weeks on 20week-old mice reported behavioral and neurochemical changes [19]. Similarly, another study reported a subtle effect on attentional tasks in 16-20-week-old rats with a vitamin D-deficient diet given for 10 weeks compared to control rats [20].

Evidence through Cross-Sectional and Longitudinal
Studies. e association of low vitamin D and global cognitive deficit is established through many cross-sectional and longitudinal studies. Nevertheless, the issue of reverse causality remains to be answered [21]. Table 1 shows a summary of evidence demonstrating an association between serum 25(OH)D, CI, and dementia [9,21,22,[23][24][25][26]. Many studies have shown an association of low vitamin D with CI at a cross-sectional level although the same studies with longitudinal follow-ups did not replicate the association [21,23,24]. All included studies showed a difference in the study population, sample size, participants' age, follow-up time, vitamin D exposure, method used for estimation for vitamin D, criteria used to diagnose dementia and CI, and methods of assessment of cognition. Most studies adjusted for confounders like age, education, physical activity, diabetes, hypertension, hypercholesteremia, and season. However, most studies did not consider confounders like depression. Four studies [9,22,23,25] found no association between low serum vitamin D levels and CI and dementia in a longitudinal follow-up, whereas two studies [21,24] found a significant association (P � 0.001). However, the studies which found a significant association were smaller in size and duration of follow-up. A Swedish study [22] done on a large sample (2,841) for a longer follow-up (18 years) did not find an association. is study took into account the confounders like common dietary intake of vitamin D, physical activity, and sun exposure. However, repeated blood sampling and dietary assessments improve the precision of exposure information, the study lacked in doing so. Similarly, another American study [25] done on a large sample (13,044) with a long follow-up (20 years) did not report any such association. e previously reported associations between 25(OH)D concentrations and cognitive impairment may be a result of reverse causation-whereby low 25(OH)D is a marker of poor health (resulting from those in poor health (e.g., those with cognitive impairment) doing less physical activity and having less sun exposure and thereby having lower vitamin D concentrations) rather than a causative factor in cognitive impairment and dementia pathogenesis. is study can be considered less susceptible to reverse causation as 25(OH)D was measured in midlife, and cognitive change was evaluated over 20 years. Another methodological shortcoming compromising the validity of the data is the use of single serum 25(OH)D measurements taken at baseline to represent long-term exposure in all studies [9,21,22,[23][24][25]. A prospective study with two follow-ups, each at 5 years, conducted to examine the association of dietary and supplemental vitamin D intake and cognitive decline showed an association between high intake and a slower decline in the cognitive domains of verbal fluency.
ose with supplemental intake also exhibited a slower decline in the cognitive domain of verbal fluency although the effect on visual and verbal memory was less in magnitude [26]. Similarly, a study on participants (age 55-67 years) with levels >25 nmol/l has demonstrated better verbal fluency and executive functioning both at baseline and at a 10-year follow-up [27].
In contrast to the existing body of literature demonstrating a positive correlation between cognitive function and vitamin D status, Lam et al. [28] reported a negative association between vitamin D levels and verbal episodic    memory. In a prospective (3-year follow-up), populationbased study of older adults aged 85+, it was found that both low and high season-specific quartiles of 25(OH)D were associated with higher odds of prevalent cognitive impairment (assessed by MMSE), poorer attention reaction times/ processing speed and focused attention/concentration, and greater attention fluctuation [29].

Evidence through Meta-Analysis and Systematic Review.
Several systematic reviews and meta-analysis of cross-sectional studies, case-control studies, and observation prospective studies have suggested an association between low vitamin D, cognitive impairment, and dementia. Moreover, a meta-analysis on vitamin D levels and specific cognitive domains have suggested a strong association between low vitamin D and a range of executive dysfunction, such as impaired processing speed, mental shifting, and information updating. Only a modest association was noted with episodic memory [30]. Several such systematic reviews and metaanalysis in the last 6 years are depicted in Table 2 [30-34].

Vitamin D Supplementation and Cognition.
Five studies have investigated the effects of vitamin D supplementation on cognitive outcomes in elderly individuals (see Table 3); three were RCT's [36][37][38] and two had pre-post study design [8,37]. Overall, three studies found that vitamin D supplementation did not improve either cognitive outcomes [36,38,39] or reduce the risk of dementia/MCI compared to controls. A prospective pre-post interventional study [39] on nursing home residents with a mean age (86 years) reported no significant change in cognitive outcome with oral vitamin D2 (50,000 IU 3 times/week) for 4 weeks. On the contrary, another prospective pre-post interventional study [37], which included 80-year-old subjects from memory clinic, found that those who received oral vitamin D3 supplementation (800 IU per day or 100,000 IU per month) experienced improved global cognition and executive functioning abilities over a 16-month follow-up period compared to controls [40]. Nevertheless, the pre-post design (without randomization) of the study and small sample size and shorter duration of treatment limit the exploration of cognitive effect of vitamin D. A randomized trial [38] found that visual memory improved in the high dose group (4,000 IU per day for 18 weeks of oral vitamin D supplementation) when compared to the low dose group (400 IU per day) in healthy adults, although verbal memory and other cognitive domains did not improve. On the contrary, Stien et al. [36] reported no significant change in cognition with higher doses of vitamin D followed by intranasal insulin (nasal insulin improves cognition, and vitamin D increases insulin receptor expression) when compared to lower dose of vitamin D and intranasal insulin in subjects diagnosed with mild-to-moderate AD. A more recent double-blind, randomized, placebo-controlled trial showed no significant difference in cognition over time (3 years) according to the MMSE score (assessed every 6 months) between older postmenopausal African-American women who took vitamin D (orally in doses of 2,400, 3,600, and 4,800 which maintained serum level of >30 ng/mL) than those who did not [37]. However, methodological weaknesses such as small sample sizes [36,38,39], short follow-up periods [36,38,39], and lack of participant randomization [39,40], as well as heterogeneous doses of vitamin D supplementation and baseline vitamin D levels make it difficult to interpret the results of the interventional studies. Another limitation found in most studies was the use of MMSE for cognitive testing. is test is best used as a screening tool and not for diagnosis. ere is no clear idea of when vitamin D is most effective in the pathogenesis of cognitive decline and particularly the advent of AD. erefore, the supplementation of vitamin D after the advent of CI or AD might not have helped the already existing neurological insult which could have been the reason for the failure of such a treatment. Larger trials over a longer period in patients at risk for, but has not yet progressed to cognitive decline or dementia, may be more capable of demonstrating an impact. Identifying such individuals using CSF biomarkers such as amyloid-beta and phosphorylated tau may help. Future studies directed towards finding the effect of vitamin D on biomarkers of AD would further clarify the role of vitamin D and its disease-modifying effect. Pharmacogenomic studies to identify the individuals who could benefit from such a therapy may further help.

Conclusion
Evidence from animal and cellular studies suggests that vitamin D has multiple functions throughout the central nervous system and could be implicated in the prevention  (18-20 years) follow-up time did not find such an association. Future neuroimaging studies may uncover a link with specific abnormalities that could explain the observed associations between vitamin D concentrations and dementia-related disorders. Clinical trials investigating the effect of vitamin D supplementation on cognitive outcomes have produced mixed findings; however, a variety of methodological weaknesses limit the interpretability of these findings. Lack of consensus over the exact dosage of vitamin D to be used and optimal age of treatment initiation of individuals at risk remains unidentified. Furthermore, large double-blind, randomized, placebo-controlled trials with appropriate dosage and duration may provide conclusive results. Taken together, this body of evidence suggests that vitamin D may be a new paradigm for therapy in the prevention and treatment of dementia and AD. Although vitamin D may be considered as a modifiable risk factor, the causal relationship between vitamin D deficiency and CI so far remains inconclusive.

25(OH)D: 25 hydroxyvitamin D Aβ:
Amyloid-beta AD: Alzheimer's disease AMSTAR: Assessing the methodological quality of systematic reviews BBB: Blood-brain barrier BDNF: Brain-derived neurotrophic factor CDR: Clinical dementia rating CI: Cognitive impairment CKD: Chronic kidney disease CSF: Cerebral spinal fluid DSM4: Diagnostic and Statistical Manual for Mental Disease DSST: Digit symbol substitution test DST: Digit symbol test DWRT: Delayed word recall test GM: Grey matter HPV: Hippocampal volume ICD: International Classification of Disease ICV: Intracranial volumes IOM: Institute of medicine LTP: Long-term potentiation MCI: Mild cognitive impairment MMSE: Mini-Mental Status Examination NGF: Nerve growth factor NINCDS-ADRDA: National institute neurological and communicative disorders and stroke-Alzheimer disease and related disorders NMDA: N-Methyl d-aspartate RCT: Randomized controlled trials RDA: Recommended daily allowances TMT: Trial making test UVR: Ultraviolet radiation VDR: Vitamin D receptor WFT: Word fluency test WM: White matter WMA: White matter abnormalities WMH: White matter hyperintensities.

Conflicts of Interest
e authors declare that they have no conflicts of interest.

Authors' Contributions
Dr. S. Sultan, MD, substantially contributed to conception or design of the work, finally approved the version to be published, and was responsible for agreement to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. Miss Uzma Taimuri, Miss SA Basnan, Miss W K A Orabi, Miss A Awadallah, Miss F Almowald, and A Hazazi substantially contributed to conception and design of the work. All authors read and approved the manuscript.