Serum Vitamin D Concentrations and Cognitive Change Over 20 Years: The Atherosclerosis Risk in Communities Neurocognitive Study

The role of nitric oxide, insulin resistance, and vitamin D in cognitive function of older adults

Aging is accompanied by possible cognitive impairment, as well as a reduction in nitric oxide (NO) bioavailability, insulin resistance, and vitamin D deficiency. Among these Nitric oxide exhibits a dual role in brain biology, where both low and high levels can have detrimental effects on memory and neurotoxicity. In this study we aimed to investigate the role of nitric oxide, insulin resistance, and vitamin D in cognitive function of elderly individuals. This case-control study involved participants aged ≥ 60 years who were under observation in the Birjand Longitudinal Aging Study. A total of 40 participants were selected as controls, while 64 participants with cognitive impairment were identified as cases. The cases were further subdivided according to their Mini-Mental State Examination (MMSE) scores into two groups: mild and moderate cognitive impairment. Cognitive function in both cases and controls was assessed using the MMSE. The levels of total NOx, vitamin D, insulin, and blood glucose were measured using ELISA and spectrophotometric immunoassays. Insulin resistance indices, such as homeostasis model assessment of insulin resistance (HOMA-IR) and quantitative insulin sensitivity check index (QUICKI) were calculated. Serum NOx levels were significantly lower in participants with cognitive impairment compared to the control group (p < 0.01). However, the mean (SD) levels of serum vitamin D, insulin, and glucose, as well as the insulin resistance markers, showed no significant differences between the groups. A positive correlation was observed between serum NOx levels and MMSE scores. NO may be an essential factor for normal brain function, as serum NOx levels were significantly higher in controls compared to those with cognitive impairment, and there was a positive correlation with MMSE scores. Other metabolic and nutritional markers, including vitamin D and insulin resistance, did not demonstrate a significant effect.

Serum Vitamin D Status and Cognitive Function in Iranian Older Adults: Evidence from the Birjand Longitudinal Aging Study

BACKGROUND

There is evidence of the role of vitamin D deficiency in cognitive decline. However, the results of studies regarding the relationship between the reduction of vitamin D concentrations and cognitive impairment are heterogeneous.

OBJECTIVES

We aimed to answer the question of whether vitamin D deficiency is associated with cognitive decline in older adults.

METHODS

In this cross-sectional study, the baseline data of the Birjand Longitudinal Aging Study (BLAS) were analyzed. Of 1420 participants in the BLAS, 1219 participants aged ≥60 y old were included in the present study. Serum concentrations of 25-hydroxyvitamin D [25(OH)D] were measured by the enzyme-linked immunosorbent assay method. The 6-item Cognitive Impairment Test (6-CIT), Mini-Mental State Examination (MMSE), Category Fluency Test (CFT), and Abbreviated Mental Test Score (AMTS) were used to assess cognitive function. Multiple logistic regression analysis, adjusted for potential confounders, was employed to estimate the association between cognitive function and 25(OH)D concentrations.

RESULTS

Among study participants, 905 (74.24%) had sufficient vitamin D concentrations (≥20 ng/ml), 209 (17.15%) had insufficient vitamin D concentrations (12-20 ng/ml), and 105 (8.61%) had vitamin D deficiency (<12 ng/ml). There was no significant correlation between serum 25(OH)D concentrations and scores of 6-CIT (P = 0.279), AMTS (P = 0.181), MMSE (P = 0.118), and CFT (P = 0.259). Also, the prevalence of cognitive impairment had no significant relationship with vitamin D status. Finally, in the multiple logistic regression analysis, there was no association between the insufficient or deficient concentrations of 25(OH)D and impaired cognitive function both before and after adjustment for various cofounders.

CONCLUSIONS

The present study found no significant association between vitamin D status and cognitive impairment.

A Biological Index to Screen Multi-Micronutrient Deficiencies Associated with the Risk to Develop Dementia in Older Persons from the Community

BACKGROUND

Low blood status in several nutritional compounds, including long-chain omega-3 fatty acids (LC n-3 PUFA), carotenoids, and vitamin D, have been associated with a higher risk to develop dementia. Nutritional deficiencies may potentiate each other regarding dementia risk; yet the association of multiple nutritional deficiencies with dementia has been little explored.

OBJECTIVE

To develop an index of micronutritional biological status (MNBS) for the screening of multi-micronutritional deficiencies associated with the risk of dementia in a prospective population-based cohort of older persons.

METHODS

We included participants from the Bordeaux Three-City study, who were free of dementia at baseline, had blood measurements of LC n-3 PUFA, carotenoids, and 25(OH)D, and who were followed for up to 18 years for dementia. We used penalized splines in Cox models to model dose-response relationships of each nutritional component with the risk of dementia and construct a risk index.

RESULTS

629 participants with an average age of 73.1 years were included in the study. Each increase of 1 SD of the MNBS index was associated with a 46%higher risk of dementia (HR = 1.46, 95%CI 1.23; 1.73). Participants with highest index ([mean+1SD; max]) had a 4-fold increased risk of dementia compared with participants with a low index ([min; mean-1SD]) (HR = 4.17, 95%CI 2.30; 7.57).

CONCLUSION

This index of assessment of micronutritional biological status is a practical tool that may help identify populations with inadequate nutritional status, screen eligible individuals for nutritional prevention in primary care, or for supplementation in preventive trials of dementia.

Nutrition, Physical Activity, and Other Lifestyle Factors in the Prevention of Cognitive Decline and Dementia

Multiple factors combined are currently recognized as contributors to cognitive decline. The main independent risk factor for cognitive impairment and dementia is advanced age followed by other determinants such as genetic, socioeconomic, and environmental factors, including nutrition and physical activity. In the next decades, a rise in dementia cases is expected due largely to the aging of the world population. There are no hitherto effective pharmaceutical therapies to treat age-associated cognitive impairment and dementia, which underscores the crucial role of prevention. A relationship among diet, physical activity, and other lifestyle factors with cognitive function has been intensively studied with mounting evidence supporting the role of these determinants in the development of cognitive decline and dementia, which is a chief cause of disability globally. Several dietary patterns, foods, and nutrients have been investigated in this regard, with some encouraging and other disappointing results. This review presents the current evidence for the effects of dietary patterns, dietary components, some supplements, physical activity, sleep patterns, and social engagement on the prevention or delay of the onset of age-related cognitive decline and dementia.

Vitamin D and Blood Parameters

Background: Vitamin D has a steroid- and an anabolic-resembling chemical structure. Vitamin D is essential for many processes in the human body after hydroxylation. Aims of the Study: To investigate the impact of 25-hydroxy-vitamin D plasma concentrations on the blood parameters number of erythrocytes, hematocrit, mean corpuscular hemoglobin and mean corpuscular volume. Methods: Serial assessments were done in 290 patients with multiple sclerosis and repeated after a mean interval of 245 days. A recommendation for vitamin D supplementation was given in case of a concentration lower than 20 ng/mL combined with a prescription of a formulation containing vitamin D but not vitamin K. Results: There was a fall of vitamin D in 119 subjects and a rise in 164, while no change appeared in 7 participants. When vitamin D values went down between both assessments moments, the computed increase of mean corpuscular haemoglobin was significantly lower compared with the rise of mean corpuscular haemoglobin associated with a vitamin D elevation. When vitamin D declined, the computed fall of mean corpuscular volume fall was significantly lower compared with the decrease of mean corpuscular volume, when vitamin D rose. Positive correlations were found between differences of vitamin D and mean corpuscular haemoglobin, respectively mean corpuscular volume. Inverse relations appeared between disparities of vitamin D and erythrocytes, respectively haematocrit. Conclusions: The elevation of vitamin D plasma levels provides enhanced preconditions for a better tissue oxygenation on a cellular level.

Vitamin D and white matter hyperintensities: results of the population-based Heinz Nixdorf Recall Study and 1000BRAINS

BACKGROUND AND PURPOSE

Cross-sectional studies showed an inverse association between serum 25-hydroxyvitamin D (25OHD) and white matter hyperintensities (WMHs) whereas the few longitudinal studies did not. The association between baseline 25OHD and WMHs at 10-year follow-up in the Heinz Nixdorf Recall Study plus 1000BRAINS was investigated.

METHODS

Data of 505 participants (49% women, 56.2 ± 6.6 years) with 25OHD at baseline (2000-2003) and WMH volume and grade of WMHs using the Fazekas classification at 10-year follow-up were analysed. The association between deseasonalized 25OHD and the base-10 logarithm of WMH volume was evaluated by multiple linear regression, adjusted for age, sex, education, smoking, alcohol consumption, sports, diabetes mellitus, systolic blood pressure and total cholesterol. β-estimators were transformed back (10^β ). Using multiple logistic regression, odds ratios (ORs) and 95% confidence intervals (95% CI) were calculated to evaluate the association between deseasonalized 25OHD and Fazekas grades (0, absence and 1, punctate foci vs. 2, beginning and 3, large confluence).

RESULTS

Mean 25OHD was 17.0 ± 8.2 ng/ml, and mean deseasonalized 25OHD was 16.9 ± 7.5 ng/ml. Mean WMH volume was 16.6 ± 17.4 ml, range 1-132 ml. Most grade 2-3 WMHs were found to be periventricular (39% of the participants), parietal (32%) and frontal (31%) (temporal 6%, occipital 3%). The linear regression showed an inverse association between 25OHD and WMH volume. On average, a 25OHD increase of 1 ng/ml was associated with a reduced WMH volume by a factor of 0.99 (95% CI 0.98; 1.00) (fully adjusted). There was also some indication for an inverse association between 25OHD and extent of periventricular (OR 0.98 [95% CI 0.96; 1.01]), frontal (0.99 [0.97; 1.02]) and parietal (0.98 [0.95; 1.00]) WMHs according to the Fazekas classification.

CONCLUSIONS

Lower 25OHD may be a risk factor for the occurrence of WMHs.

Neuroimaging changes associated with vitamin D Deficiency - a narrative review

ABSTRACTEmerging evidence suggests the role of vitamin D in brain health and its implication in the pathogenesis of cognitive impairment. The Aim of this review is to summarize current evidence on neuroimaging changes seen in vitamin D deficient individuals. Cross-sectional and longitudinal studies have consistently found an association between low serum 25 hydroxyvitamin D and cognitive impairment. Furthermore, investigating the association between serum 25 hydroxyvitamin D and neuroimaging abnormalities could provide an insight into the potential mechanisms underlying the association with cognitive impairment. Animal studies have demonstrated structural changes in the cerebral cortex and hippocampus of vitamin D deficient mice. Neuroimaging studies of the brain have shown increased white matter hyperintensities in periventricular, cortical, and juxtacortical areas and grey matter atrophy of the hippocampus, anterior cingulate cortex, and left calcarine sulcus in elderly with vitamin D deficiency.

Sociodemographic and Health-Related Factors Associated with Severity of Cognitive Impairment in Elderly Patients Hospital-ized in a Geriatric Clinic

Identification of risk factors for cognitive impairment is crucial for providing proper care and treatment. The aim of the study was to investigate the relationship between sociodemographic and health-related factors and the severity of cognitive impairment in elderly patients. In this retrospective study, we assessed the medical documentation of 323 patients aged 60+ years hospitalized in a geriatric clinic of university hospital. The patients were classified into five groups of cognitive impairment severity based on the Mini Mental State Examination and Clock Drawing Test. Kruskal-Wallis and Chi square tests and multivariate ordinal logistic regression were used to assess relationships involved. Cognitive impairment was identified in 84.2% of subjects. The following factors were indicative for higher level of cognitive disorders: primary and vocational education, older age, presence of vascular brain injury, and inability of walking independently. On the other hand, the factors associated with lower severity of cognitive impairment were co-morbid anxiety disorders, ischemic heart disease, and a higher BMI index. Dementia is one of the leading causes of disability and mortality in the elderly. Enhancing knowledge about the risk factors that worsen cognition is particularly relevant for accelerating the diagnosis of dementia and improving patient care.

Racial Differences in Dietary Relations to Cognitive Decline and Alzheimer's Disease Risk: Do We Know Enough?

The elderly population in the US is increasing and projected to be 44% minority by 2060. African Americans and Hispanics are at increased risk of cognitive impairment and Alzheimer’s disease compared to non-Hispanic whites. These conditions are associated with many other adverse health outcomes, lower quality of life, and substantial economic burden. In the past few decades, diet has been identified as an important modifiable risk factor for cognitive decline and Alzheimer’s disease. Some studies report poor diet quality among African American and Hispanic older adult populations compared to their white counterparts. We have a limited understanding of how diet affects brain health in different racial-ethnic groups. One primary reason for our lack of knowledge is that most cohort studies are of majority non-Hispanic white participants. Moreover, those that do include minority participants do not publish their findings stratified by racial-ethnic groups, and likely have a less accurate measurement of dietary intake among minority groups. In this review, we summarize the current, albeit limited, literature on racial/ethnic differences in dietary relations to dementia outcomes. We will also discuss methodological issues in conducting nutrition studies in diverse cultures, and suggestions for future research directions. Overcoming the gaps will make it possible to make dietary recommendations for Alzheimer’s prevention that are more relevant for different racial/ethnic groups and set us on a faster track to reduce health disparities.

Lower Serum Calcium as a Potentially Associated Factor for Conversion of Mild Cognitive Impairment to Early Alzheimer's Disease in the Japanese Alzheimer's Disease Neuroimaging Initiative

BACKGROUND

Effect of serum calcium level to the incidence of mild cognitive impairment (MCI) conversion to early Alzheimer's disease (AD) remains uncertain.

OBJECTIVE

To investigate association between baseline serum calcium and the MCI conversion in the Japanese Alzheimer's Disease Neuroimaging Initiative (J-ADNI) study cohort.

METHODS

In this sub-analysis of J-ADNI study, we reviewed data from MCI participants at baseline regarding their conversion to early AD during the 3 years of observation period and assessed the associated factors including serum calcium level. In addition, we compared our results from the J-ADNI study with the corresponding results from the North American (NA)-ADNI.

RESULTS

Of 234 eligible MCI participants from the J-ADNI cohort, 121 (51.7%) converted to AD during the first 36 months of observation. Using univariate analysis, being female, having shorter years of education, and lower serum calcium level were correlated with increased risk of MCI-to-AD conversion exclusively in J-ADNI cohort. The lower corrected serum calcium level remained as one of conversion-associated factors in the J-ADNI cohort even after adjustment for multiple confounding variables, although this was not observed in the NA-ADNI cohort.

CONCLUSION

Our findings suggest that lower serum calcium may be associated with an increased risk of MCI conversion to AD in Japanese cohorts. The reason for this correlation remains unclear and further external validation using other Asian cohorts is needed. It would be interesting for future AD studies to obtain serum calcium levels and other related factors, such as vitamin D levels, culture-specific dietary or medication information.

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.

Genetic, environmental and biomarker considerations delineating the regulatory effects of vitamin D on central nervous system function

Studies show that vitamin D (vit-D) (25(OH)D), the bioactive metabolite (1,25(OH)2D3) and vit-D receptors (vit-D receptor; protein disulphide isomerase, family A member 3) are expressed throughout the brain, particularly in regions pivotal to learning and memory. This has led to the paradigm that avoiding vit-D deficiency is important to preserve cognitive function. However, presently, it is not clear if the common clinical measure of serum 25(OH)D serves as a robust surrogate marker for central nervous system (CNS) homeostasis or function. Indeed, recent studies report CNS biosynthesis of endogenous 25(OH)D, the CNS expression of the CYP group of enzymes which catalyse conversion to 1,25(OH)2D3 and thereafter, deactivation. Moreover, in the periphery, there is significant ethnic/genetic heterogeneity in vit-D conversion to 1,25(OH)2D3 and there is a paucity of studies which have actually investigated vit-D kinetics across the cerebrovasculature. Compared with peripheral organs, the CNS also has differential expression of receptors that trigger cellular response to 1,25(OH)2D3 metabolites. To holistically consider the putative association of peripheral (blood) abundance of 25(OH)D on cognitive function, herein, we have reviewed population and genetic studies, pre-clinical and clinical intervention studies and moreover have considered potential confounders of vit-D analysis.

25-OH Vitamin D Levels and Cognitive Performance: Longitudinal Assessment in a Healthy Aging Cohort

Background: Declining serum levels of 25-hydroxyvitamin D [25(OH)D, a biomarker of vitamin D status] with aging is a well-recognized phenomenon. However, scarce information is available on the relation between 25(OH)D levels and cognitive performance over time in older individuals. Our purpose was to evaluate, longitudinally, the association of 25(OH)D with cognitive function in a healthy older adults’ cohort.

Methods: Sixty-four individuals over 55 years-old with no cognitive impairment, clustered as healthy “Poor” and “Good” cognitive performers, were followed for an average of 18 months. Seasonal-adjusted 25(OH)D serum levels (measured by high-performance liquid chromatography-tandem mass spectrometry) were related, longitudinally, with cognitive (memory and general/executive) composite scores.

Results: Overall seasonal-adjusted median serum 25(OH)D level was of 47 nmol/l [interquartile range (IQR), 38–60 nmol/l]. A negative correlation between baseline 25(OH)D and the general/executive composite score was found in the “Poor” cognitive performers (r^s = −0.52, p = 0.006), an association lost after adjusting 25(OH)D levels for the season. No effect was found in both groups between seasonal-adjusted 25(OH)D levels and the variation of both memory and general/executive composites during follow-up when adjusted for age, gender and education level.

Conclusion: In this healthy older population with no cognitive impairment, lower serum levels of 25(OH)D were not longitudinally associated with poorer cognitive scores.

Vitamin D deficiency as a risk factor for dementia and Alzheimer's disease: an updated meta-analysis

BACKGROUND

We aimed to comprehensively explore the associations between serum 25(OH)D deficiency and risk of dementia and Alzheimer's disease(AD).

METHODS

We systematically searched Pubmed, the Cochrane Library, Embase and the reference lists of pertinent review articles for relevant articles published from database inception up until January 2019. Pooled hazard ratios (HRs) and 95% confidence intervals (CIs) were calculated with random effects models using the Stata 12.0 statistical software package.

RESULTS

Twelve prospective cohort studies and four cross-sectional studies were included in this meta-analysis. The pooled HRs of dementia and AD, respectively, were 1.32 (95%CI: 1.16, 1.52) and 1.34 (95%CI: 1.13, 1.60) for vitamin D deficiency (< 20 ng/ml). In the subgroup analyses, the pooled HRs of dementia and AD, respectively, were 1.48 (95%CI: 1.19, 1.85) and 1.51 (95%CI: 1.04, 2.18) for moderate vitamin D deficiency (10-20 ng/ml) and 1.20 (95%CI: 0.99, 1.44) and 1.36 (95%CI: 1.01, 1.84) for severe vitamin D deficiency (< 10 ng/ml).

CONCLUSION

There are significant associations between vitamin D deficiency and both dementia and AD. There are stronger associations between severe vitamin D deficiency (< 10 ng/ml) and both dementia and AD compared to moderate vitamin D deficiency (10-20 ng/ml).

25-hydroxyvitamin D status and its impact on cognitive functions in postmenopausal woman

Aim: The purpose of the study was to analyze the cognitive functions in postmenopausal women carriers of Apolipoprotein E gene polymorphisms (APOE) with different status of vitamin D levels. Material/Methods: 170 ambulatory individuals aged 50 years or older were evaluated. A computerized battery of Central Nervous System Vital Signs (CNS VS) was used for diagnostic cognitive functions. APOE genotype was performed by multiplex PCR. Serum 25(OH)D and estradiol levels were measured using the 25OHD EIA assay and Estradiol ELISA Kits. Results: Considerably worse scores in global cognitive performance index (NCI) were obtained by women with severe deficiency of 25(OH)D (p <0.001). The cognitive effects of very low 25(OH)D levels were apparent in memory, executive functioning, complex attention, and cognitive flexibility. The genotype APOE ε3/ε4 or ε4/ε4 were most common (19.6%; 15.2%, respectively) in women with 25(OH)D severe deficiency which had the weakest average results in terms of NCI value. Conclusions: The severe deficiency of 25(OH)D vitamin was related with a greater likelihood of cognitive impairment and risk of cognitive decline in postmenopausal women with no dementia.

Mid-life serum Vitamin D concentrations were associated with incident dementia but not late-life neuropsychological performance in the Atherosclerosis Risk in Communities (ARIC) Study

BACKGROUND

Activated Vitamin D has anti-inflammatory properties and adequate 25-hydroxyvitamin D [25(OH)D] concentrations may be important for neurocognitive function and protection against neurologic injury. We examined whether mid-life 25(OH) D concentrations were associated with later-life performance on neuropsychological testing, functional ability, depressive symptoms, and incident dementia.

METHODS

We studied 13,039 white and black ARIC participants who had serum 25(OH) D measured mid-life at visit 2 (1990-1992). Over the next ~ 20 years through visit 5 (2011-2013), participants underwent 3 additional in-person visits, annual telephone calls, and hospitalization surveillance. An extensive battery of neuropsychological outcomes were assessed at visit 5 using standardized protocols. Incident dementia was ascertained through a formal algorithm that included data from in-person cognitive testing, telephone interviews, hospital discharge codes, and death certificate codes. Diagnoses of dementia were adjudicated by expert clinician committee. For the primary cognitive analyses, we imputed for missing covariates and outcomes and used linear regression to evaluate non-concurrent cross-sectional associations of mid-life 25(OH) D (visit 2) with late-life neuropsychological outcomes (visit 5). We also used Cox regression models to examine associations of mid-life 25(OH) D and incident dementia.

RESULTS

In mid-life, the mean (SD) age of participants was 57 (6) years, 57% were women, and 24% black. Mean (SD) 25(OH) D was 24.3 (8.6) ng/mL; 33% had deficient (< 20 ng/mL), 44% intermediate (20- < 30 ng/mL), and 23% sufficient (≥30 ng/mL) 25(OH) D concentrations. Association between mid-life 25(OH) D and late-life performance on neuropsychological testing were mostly null. There was no significant association with functional ability or depressive symptoms. Results were similar in a sensitivity analysis using complete-case data (no imputation). However, after a median follow-up of 20 years, low 25(OH) D concentrations were associated with increased risk for incident dementia (p = 0.01 for trend across categories), with HR of 1.26 (95% CI 1.06, 1.49) for participants with deficient 25(OH) D, compared to sufficient concentrations.

CONCLUSION

In this community cohort, mid-life serum 25(OH) D concentrations were associated with incident dementia but not with performance on neuropsychological testing, functional ability, or depressive symptoms, 20 years later. Whether serum 25(OH) D concentrations are causally related to dementia or confounded by poorer health status remains uncertain.

TRIAL REGISTRATION

Registered on clinicaltrials.gov NCT00005131 .

Non-Skeletal Activities of Vitamin D: From Physiology to Brain Pathology

Vitamin D is a secosteroid hormone regulating the expression of almost 900 genes, and it is involved in the regulation of calcium and phosphate metabolism, immune response, and brain development. Low blood vitamin D levels have been reported in patients affected by various diseases. Despite a large amount of literature data, there is uncertainty surrounding the role of vitamin D as a serum biomarker in Alzheimer’s disease (AD) and Parkinson’s disease (PD). Indeed, the lack of internationally recognized 25(OH)D₃ reference measurement procedures and standard materials in the past led to unstandardized serum total 25(OH)D₃ results among research and clinical care laboratories. Thus, most of the literature studies reported unstandardized data, which are of little use and make it difficult to draw conclusions of the role of vitamin D in AD and PD. This review summarizes the extra-skeletal actions of vitamin D, focusing its role in immunomodulation and brain function, and reports the issue of lacking standardized literature data concerning the usefulness of vitamin D as a biomarker in AD and PD.