Vitamin D in Neurological Diseases

Vitamin D may have multiple effects on the nervous system and its deficiency can represent a possible risk factor for the development of many neurological diseases. Recent studies are also trying to clarify the different effects of vitamin D supplementation over the course of progressive neurological diseases. In this narrative review, we summarise vitamin D chemistry, metabolism, mechanisms of action, and the recommended daily intake. The role of vitamin D on gene transcription and the immune response is also reviewed. Finally, we discuss the scientific evidence that links low 25-hydroxyvitamin D concentrations to the onset and progression of severe neurological diseases, such as multiple sclerosis, Parkinson’s disease, Alzheimer’s disease, migraine, diabetic neuropathy and amyotrophic lateral sclerosis. Completed and ongoing clinical trials on vitamin D supplementation in neurological diseases are listed.


Introduction
The multiple and complex effects of vitamin D on human health are more and more clear. Historically, the best-known effects have been those on bone, albeit many studies highlighted the so-called "non-calcemic actions" of vitamin D [1]. Increasing scientific attention has been paid to the association between vitamin D and neurological diseases [2,3]. There is a significant concern about the possibility that having vitamin D insufficiency could negatively influence the development of neurodegenerative and neuroinflammatory diseases [4]. From this perspective, vitamin D receptors are expressed in both neurons and glial cells in numerous important brain areas including substantia nigra, hippocampus, hypothalamus, thalamus, and subcortical grey nuclei [5]. In these regions, vitamin D seems to have a role in the differentiation and maturation of neurons, in the regulation of growth factors synthesis, including neural growth factor, and glial cell line-derived growth factor, and in the synthesis of different neurotransmitters including acetylcholine, dopamine, and gamma-aminobutyric acid [5].
This narrative review aims to summarise the biochemical and metabolic effects of vitamin D and the scientific evidence that links vitamin D insufficiency to the onset and progression of the main neurological diseases, such as multiple sclerosis (MS), Parkinson's disease (PD), Alzheimer's disease (AD), migraine, amyotrophic lateral sclerosis (ALS) and diabetic neuropathy.

Methods of Literature Search
To select the relevant literature for this narrative review the authors firstly searched PubMed, Embase and google scholar databases using the following string: "Vitamin D" OR "25-hydroxyvitamin D", OR "vitamin D2", OR "vitamin D3", OR "ergosterol", OR "cholecalciferol" OR "calcitriol "AND ("neurology" OR "nervous system" OR "brain" OR

The Role of Vitamin D: Gene Transcription and Immune Response
Nutrigenomics represents the subject that studies how the quality of diet, food intake, and lifestyle, influence the expression of the genome [52]. At a genomic level, all the 1,25(OH) 2 D actions are mediated by the VDR. The VDR is made up of three domains, the N-terminal DNA binding domain with two zinc fingers that bind to the grooves of the DNA at discrete sites named vitamin D response elements (VDREs), the C-terminal ligand binding domain, and the hinge region connecting these two domains together. VDR binds to its VDRE, causing the recruitment of coregulatory complexes, that can be both gene and cell-specific. VDR binding sites can be anywhere in the genome, often many thousands of base pairs away from the gene being regulated [53]. VDR regulates also nongenomic activities in bone cells, intestinal enterocytes, kidneys, colonocytes, and skin.
1,25(OH) 2 D directly influences the epigenome and transcriptome at thousands of loci within the human genome [54]. The ability of Vitamin D to modify gene transcription was initially highlighted with the discovery of VDR in the intestine [55]. and other tissues, including parathyroid glands, kidney, and bone [56], and by its similarity with other steroid hormone receptors [57].
VDR forms heterodimers with the retinoid X receptor alpha (RXR) to control the transcription of target genes [58], and this is important to achieve an adequate DNAbinding affinity [59,60]. There are many situations in which VDR acts independently of RXR and it uses other nuclear proteins as alternative cooperative binding partners on genomic DNA [61,62], or indirectly binds to DNA through other transcription factors [63].
1,25(OH) 2 D appears to have numerous epigenetic effects too [69]. Important genes involved in the vitamin D signalling system, including VDR, CYP2R1, CYP27B1, and CYP24A1 have large CpG islands in their promoter regions and this makes them sensitive to silencing by methylation [70]. Moreover, VDR interacts with coactivator and corepressor proteins, which in turn can influence chromatin modifiers, such as histone acetyltransferases (HATs), histone deacetylases (HDACs), histone methyltransferases (HMTs), and chromatin remodelers [70]. Furthermore, certain VDR ligands have DNA demethylating effects [70].
The recently highlighted ability of vitamin D to modulate innate and adaptive immunity [78] seems very interesting from a possible therapeutic perspective in several pathologies [65]. Immune cells can convert 25(OH)D into 1,25(OH) 2 D [79][80][81][82][83] and upregulate the CYP27B1 enzyme [84]. The effects of 1,25(OH) 2 D on immune cells depend on the state of cellular activation, as demonstrated in T cells, where VDR expression reaches its maximum 48 hours after activation [85] or in monocytes with a decrease of VDR expression after differentiation into macrophages and dendritic cells. The effects of vitamin D on immune cells are summarized in Figure 1.
The recently highlighted ability of vitamin D to modulate innate and adaptive immunity [78] seems very interesting from a possible therapeutic perspective in several pathologies [65]. Immune cells can convert 25(OH)D into 1,25(OH)2D [79][80][81][82][83] and upregulate the CYP27B1 enzyme [84]. The effects of 1,25(OH)2D on immune cells depend on the state of cellular activation, as demonstrated in T cells, where VDR expression reaches its maximum 48 hours after activation [85] or in monocytes with a decrease of VDR expression after differentiation into macrophages and dendritic cells. The effects of vitamin D on immune cells are summarized in Figure 1.  Firstly, the differentiation of monocytes to macrophages has been shown to be promoted by vitamin D [86]. Vitamin D has also been demonstrated to increase the antimicrobial activity of these cells against pathogens, including Mycobacterium tuberculosis as well as their chemotaxis and phagocytic capacity [87]. In these cells, the 1,25(OH) 2 D-VDR-RXR complex seems to directly activate the transcription of antimicrobial peptides such as defensin β2 (DEFB2) and cathelicidin antimicrobial peptide (CAMP) [88][89][90]. Vitamin D induces the secretion of IL-10 and reduces the secretion of many pro-inflammatory factors, including IL-1b, IL-6, TNF-α, RANKL, and COX-2 [91] in macrophages. These inflammatory cytokines are downregulated through the upregulation of mitogen-activated protein kinase (MAPK) phosphatase(MKP)-1 and through the subsequent inhibition of LPS-induced p38 activation [92] or through inhibition of COX-2 expression by targeting thioesterase superfamily member 4 [93].
Vitamin D also increases the antimicrobial activities of neutrophils by upregulating the expression of cathelicidin, α, and β-defensins [100] and inducing the formation of extracellular traps (NETs, trap and kill pathogens) [101]. Moreover in neutrophils vitamin D has been documented to reduce the expression of Trappin-2/elafin/skin-derived antileucoproteinase [102] as well as a reduced migration capacity [102].
The effects of vitamin D are also important for T and B cells. In B cells vitamin D inhibits memory and plasma cell generation and promotes the apoptosis of immunoglobulinproducing B cells [103,104]. Furthermore, 1,25(OH) 2 D can directly bind the IL-10 promoter region and upregulate IL-10 production and downregulate CD86 and CD74, suggesting in the first case a lower stimulation of the T cells [105] and in the second case a modulation of MHC-II [106].
Similar data are reported for CD8+ T cells, with downregulation of the production of IFN-γ and TNF-α [117].

Vitamin D and Neurological Diseases: Current Evidence
This section discusses the available evidence in relation to the role of vitamin D deficiency in the pathogenesis of the main neurological diseases. Therapeutic implications are also discussed, when available. According to recent opinions, vitamin D status should not be considered as the level of serum 25(OH)D alone, but as a lifestyle marker in general. In fact, low vitamin D levels may reflect dietary habits, levels of physical exercise and can be influenced by many health factors including body mass index. Therefore, serum 25(OH)D levels should not be considered only as an indicator of vitamin D status, but also as a marker of good health [122]. However, when discussing current evidence, it is clear that most trials do not clarify whether lifestyle indicators were taken into consideration, often making it difficult to interpret the results from the point of view of etiological clarification [122].

Alzheimer's Disease
AD represents the most prevalent type of neurodegenerative dementia. It usually starts with memory impairment and can progress to severely compromise the ability to carry out daily activities [123]. From a pathological perspective, AD is characterized by extracellular amyloid beta (Aβ) deposition, pathologic intracellular tau protein tangles, and neuronal loss [124].
Historically, many factors [125][126][127][128][129][130][131][132][133][134][135][136] have been called upon to explain the process of neurodegeneration associated with AD including the amyloid hypothesis, tau propagation hypothesis, mitochondrial dysfunction, and inflammation. Accumulating evidence is highlighting the role of vitamin D deficiency in AD. The rationale for the role of vitamin D in normal cognition is based on the specific functions of this vitamin. 1,25(OH) 2 D enhances the amyloid plaque's phagocytosis and clearance by the immune cells, especially macrophages [137][138][139]. Moreover, transforming growth factor-beta-1 (TGF-beta), which represents a key regulator of the amyloid precursor protein (APP) expression (the APP promoter binding beta site is responsive toTGF-beta) [140], causes activation of SMAD proteins acting as coactivators or transcription factors in the nucleus [141]. Smad3, one of the SMAD proteins downstream in the TGF-beta signalling pathway, is known to be a specific coactivator for ligand-induced VDR transactivation, by forming a complex with a member of the steroid receptor coactivator-1 protein family in the nucleus [141]. Thus, Smad3 may mediate cross-talk between vitamin D and TGF-beta signalling pathways.
Finally, vitamin D has an important role in modulating the inflammatory response, intracellular oxidative stresses, and mitochondrial respiratory function [142]. These effects may be important in AD pathogenesis [143,144].
Several studies aimed to explore the association between vitamin D deficiency and the occurrence of AD, but they are burdened with many drawbacks, including the differences among the methods used for Vitamin D assessment, the high variability of the cut-offs used to define Vitamin D deficiency and insufficiency, and the diagnostic criteria used for cognitive impairment and dementia. With this in mind, the majority of the studies found that subjects with low 25(OH)D serum levels have a higher risk of developing AD [145][146][147][148][149][150][151][152][153], with genetically increased 25(OH)D levels found associated with reduced AD risk in individuals aged 60 years and over [154], albeit this still represents a matter of discussion, as it has not been confirmed by other studies [155,156] and some meta-analyses [157][158][159].
There is less uncertainty when the effects of vitamin D supplementation in AD patients are considered, with the vast majority of the studies finding a lack of benefit [160][161][162][163][164][165][166][167][168] and only a few of them showing promising results [169,170]. There is also evidence on a worsening effect of Vitamin D supplementation on AD progression [171]. Nonetheless, vitamin D supplementation represents a therapeutic strategy that warrants further investigation in AD patients.

Parkinson's Disease
With a worldwide incidence estimate ranging from 5 to more than 35 new cases per 100,000 individuals yearly, PD represents the second-most common neurodegenerative disorder [172]. Although PD is well characterized from a neuropathological point of view, with the specific coexistence of the loss of pigmented dopaminergic neurons in the substantia nigra and the deposition of α-synuclein in neurons, the pathophysiological mechanisms are still not completely clarified, involving intracellular homeostasis of α-synuclein, mitochondrial dysfunction and neuroinflammation [172]. The emerging role of vitamin D in the cellular mechanisms of proliferation, differentiation, and immunoregulation, has led to the hypothesis of its contribution to PD [173].
Regarding the association between 25(OH)D concentrations and the risk of developing Parkinson's disease no definitive results are available and the literature presents controversial conclusions. The first study was conducted by Knekt and colleagues, based on the Mini-Finland Health survey, and carried out in 40 areas of Finland with the recruitment of 3173 participants representing Finnish adults aged 30 years and over. The subjects were free from PD and did not use antipsychotic medication at baseline and during a 29-year follow-up 50 PD cases were identified. In this cohort, low serum 25(OH)D concentrations predicted an elevated risk of incidence of this neurodegenerative disease [174]. Lately, two large studies failed to confirm the correlation between 25(OH)D concentrations and Parkinson's disease risk. In particular, Shrestha and colleagues, using data from the Atherosclerosis Risk in Communities (ARIC) Study, did not identify this association assessing 12,762 participants [175]. Similarly, in the Parkinson's Associated Risk Syndrome (PARS) study, Fullard et al. found no data to support the hypothesis that chronic vitamin D insufficiency contributes to the pathogenesis of PD [176].
A possible explanation for the conflicting conclusions in the aforementioned studies could be found in the geographical and behavioural differences of the investigated populations. A different way to investigate the association between vitamin D and PD risk is to study VDR polymorphisms. In this context, a comprehensive meta-analysis by Wang et al. demonstrated that SNP FokI is associated with a decreased risk of PD in Asian populations but not in Caucasian populations [177].
One of the most consistent findings in the literature is the association between serum 25(OH)D concentrations and motor symptom severity in PD, assessed by Unified Parkinson's Disease Rating Scale (UPDRS) and the Hoehn and Yahr stage (H&Y). Compared to what was previously reported for PD incidence, there is more agreement in the literature regarding this association. Several authors documented an inverse association between 25(OH)D concentrations and motor symptom severity [178][179][180][181]. Although a possible bias could be the so-called "reverse causation" (i.e., lower sun exposure in patients with more severe motor symptoms) data in the literature documented the non-occurrence of a decrease in 25(OH)D concentrations during the progression of PD [182].
Few data are available about the association between vitamin D and non-motor symptoms in PD. Overall, the published research documented a worsening of these symptoms (sleepiness, olfactory dysfunction, cognitive decline) in association with low 25(OH)D [183][184][185]. In most cases, however, the published results would need to be confirmed by further studies.
Although falls may be attributed to a wide range of etiologies, it is interesting to explore the effectiveness of vitamin D supplementation in preventing falls.
Vitamin D influence muscle strength and vitamin D insufficiency has been associated with a worse physical performance, increased bone resorption, decreased bone mineral density and therefore with an increased risk of fracture [186]. However, the studies exploring the effectiveness of vitamin D supplementation in preventing falls have heterogeneous results and, as highlighted in a recent meta-analysis, further research is needed to determine the relationship between 25(OH)D concentration and falls [186]. A metanalysis conducted by Bischoff-Ferrari et al. showed that vitamin D reduced the risk of falls among healthy ambulatory or institutionalized older individuals by 22% [187]. A Cochrane review suggested that vitamin D did not appear to reduce falls [188]. On the contrary, a recent metaanalysis [186] showed that daily doses of 700 IU to 2000 IU of supplemental vitamin D are associated with a lower risk of falling among ambulatory and institutionalized older adults, but this benefit appears small and might depend on additional calcium supplementation.
In light of the above data, although sometimes inconclusive, vitamin D supplementation might have positive effects in PD patients by reducing neuronal damage and limiting the phenomena of neuroinflammation. The results of a randomized placebo-controlled clinical trial, reported by Suzuki and colleagues and aiming to evaluate the relationship between 1200 IU/day of vitamin D administration and PD disease progression for a two years follow-up, documented a better neurological outcome in patients who received vitamin D [189]. However, these results were not confirmed by another study [190]. In conclusion, although there is limited evidence in favour of vitamin D supplementation in PD, the potential beneficial effects and the limited associated risks still suggest considering it. It will be necessary, in the future, for this therapeutic option to be supported by more consistent clinical trial data.

Amyotrophic Lateral Sclerosis
ALS is a heterogeneous multisystem neurodegenerative disorder characterized by the progressive degeneration of both upper and lower motor neurons. With an estimated annual incidence and prevalence in Europe ranging respectively from 2 to 3 cases per 100,000 and 10 to 12 per 100,000 individuals, similarly to other neurodegenerative diseases, ALS presents a complex pathogenetic architecture with a combination of genetic causes, environmental and lifestyle factors, and aging-related dysfunction. The underlying pathogenetic mechanisms include impaired protein homeostasis, aberrant RNA metabolism, cytoskeletal disturbances, axonal transport defects, impaired DNA repair, excitotoxicity, oligodendrocyte degeneration, neuroinflammation, and mitochondrial dysfunction. There is currently no effective treatment available for this neurological disorder, with only two compounds (riluzole and edavarone) approved as disease-modifying drugs. The currently employed therapeutic options focus on the symptomatic management of disease manifestations with a multidisciplinary approach to care, comprising neurologists, pulmonologists, psychologists, nutritionists, physical therapists, and specialized nurses, to improve the patient's quality of life [191,192].
Despite the growing number of published studies, the role of vitamin D in ALS patients is a controversial issue. The lack of definitive results regarding this aspect is due to the frequent low methodological quality that has distinguished the majority of the research carried out so far. Systematic reviews and meta-analyses can help clarify the relationship between vitamin D and this neurodegenerative disease. In particular, Lanznaster and colleagues analysed studies reporting data about the role of vitamin D level as a biomarker for ALS diagnosis, for vitamin D level as a prognostic factor, or regarding the vitamin D supplementation effect on clinical outcomes. The authors considered clinical trials, cohort studies, or case-control studies, including 13 research articles [193]. With regards to the prevalence of vitamin D deficiency in ALS patients, although overall these subjects presented lower 25(OH)D concentrations compared with the normal range, no articles reported a significant difference in 25(OH)D concentrations between ALS patients and controls [194][195][196]. Only Cortese et al. reported a significant difference in an abstract, but these data were not subsequently confirmed in a full article [197]. About the prognosis, the analysis of the overall data from the studies analysed by Lanznaster and collaborators [195,196,198,199] did not document a correlation between vitamin D deficiency and motor dysfunction assessed by ALS Functional Rate Score-Revised (ALSFRS-R). Focusing on survival data, the data are conflicting. Blasco et al. showed a deleterious prognosis effect of higher 25(OH)D concentrations, independently of BMI, suggesting a direct effect, independently from the nutritional status [199]. On the contrary, Camu et al. support a neuroprotective function of vitamin D on motoneurons, reporting a positive association between vitamin D status and life expectancy [200]. In a further article, published by Yang et al., 25(OH)D concentrations were not correlated to the survival of ALS patients [201]. More recently, Juntas-Morales and colleagues performed a prospective study in which vitamin D deficiency was an independent prognostic factor for subjects affected by ALS [202].
In relation to the association between VDR polymorphisms and ALS, it has been reported that ApaI A allele was more frequent in the ALS group than in the control group and may be an ALS risk factor [203]. Furthermore, the BB genotype of the intronic Bsml polymorphism between exons 8 and 9 may be related to the disease [204]. Calcium absorption and lead toxicity caused by an alteration of VDR function have been hypothesised [204].
Regarding the beneficial effects of vitamin D supplementation in ALS patients, there are currently insufficient data to demonstrate its efficacy [196,205,206].

Multiple Sclerosis
MS is an immune-mediated demyelinating disease of the central nervous system and the most common acquired disabling neurological disease affecting young adults in developed countries [207]. The majority of MS patients experience an initial relapsingremitting course, characterized by episodes of neurological worsening, followed by full or sometimes partial remission. After a variable period, the disease may enter into a secondary progressive phase characterized by gradual worsening of disability [208]. A minority of MS patients, diagnosed with primary progressive MS, experience a progressive disease from the beginning and show a continuous irreversible increase of functional impairment over the years [209].
There is strong evidence supporting the role of vitamin D in the pathogenesis and in influencing the course of MS. The first clue regarding the possible role of vitamin D in the pathogenesis of MS came from the so-called "latitude gradient" [210]. In fact, several epidemiology studies demonstrated that MS frequency increases with increasing latitude [211][212][213][214][215]. Furthermore, the prevalence of MS is lower than expected in those populations living at high latitudes eating high amounts of vitamin D-rich fatty fish [216,217]. However, it should be noted that this "latitude gradient" is progressively disappearing in the last decades and this has been related to an increased trend toward reducing sun exposure even in warmer climates, widespread use of sunscreens, and reduced time spent outdoors [210]. Sun exposure seems to influence the MS risk also during childhood, as the risk of developing MS is higher in those individuals who spent a lower amount of time outdoors during childhood and adolescence in the summer [218]. 25(OH)D concentrations have been demonstrated to be lower in patients who develop MS, before the onset of the disease and this should be viewed as a risk factor rather than a consequence of MS [219,220]. Several studies tried to define the links between vitamin D insufficiency and disease course in patients affected by MS. Some studies found 25(OH)D concentrations correlated with clinical disability [221][222][223][224][225][226] whereas others did not [227,228]. Moreover, it seems that a low vitamin D status at the time of MS diagnosis may be associated with early conversion to secondary progression [229].
Furthermore, there is a general agreement that 25(OH)D concentrations may correlate with disease activity [223,226,[230][231][232], albeit the effects of vitamin D supplementation on the relative risk of relapse in MS are less clear [233]. There is evidence from Mendelian randomisation studies which have shown that 25(OH)D is inversely associated with risk of MS as well as relapse [234][235][236].
Whether 25(OH)D concentrations differ among the different forms of MS is a matter of debate with some authors reporting higher values in relapsing-remitting MS patients than in progressive forms of MS [221,223] and others showing no significant differences [222].
An interesting and well-performed systematic meta-analysis analysed the clinical trials assessing the efficacy of vitamin D supplementation in MS and clinically isolated syndrome (CIS)/optic neuritis. Unfortunately, the analysis is burdened by the enrolment of a small number of patients, and by the high variability of the doses of vitamin D supplementation utilized. The 12 trials identified [237][238][239][240][241][242][243][244][245][246][247][248] demonstrated a significant elevation of serum 25(OH)D in treated patients, with a dose-response effect and with no increased risk of adverse effects [249]. The analysis of these trials showed no significant beneficial effects for vitamin D in these patients, with a possibility of an increased risk of relapse in MS patients on high doses of vitamin D supplementation. The Authors conclude that even if vitamin D supplementation may have a therapeutic role in the treatment of MS, these findings do not lead to the recommendation of vitamin D supplementation in MS, but further placebo-controlled clinical trials are needed.
The results are somehow overlapping when the most recent trials performed after McLaughlin's meta-analysis are considered. The EVIDIMS trial compared the effects of every other day high-(20,400 IU) vs low-dose (400 IU) cholecalciferol supplementation on clinical and imaging markers of disease activity in relapsing-remitting MS and CIS patients [250] The Authors recognized that the sample size of this trial was underpowered but there was no significant difference in terms of clinical and MRI metrics (including lesion development, enhancing lesions, and brain atrophy) between the two groups, after 18 months.
Another clinical trial has recruited from March 2012 through April 2019 at 16 neurology clinics in the United States. 172 patients were assigned to low-dose (600 IU/day) versus high-dose (5000 IU/day) regimen of vitamin D3 as an add-on therapy to glatiramer acetate (Copaxone). Both therapeutic regimens were generally well tolerated. No significant difference was shown in terms of the proportion of subjects that experienced a relapse nor in terms of annualised relapse rate [251].
Interestingly, CHOLINE [248] and SOLAR [247] randomized controlled trials showed positive effects of vitamin D supplementation on secondary endpoints. More precisely, vitamin D supplementation resulted in a lower number of enlarging or new T2 lesions, new T1 lesions, as well as lower hypointense T1 lesion volume and disability progression.
On these bases, we can conclude that there is a general agreement that testing serum levels of 25(OH)D in patients with MS is useful due to the higher prevalence of low levels in this population. From this perspective, given the low bone mineral density frequently documented even at very early stages [252] and considering the higher risk of falls and fractures of MS patients [253,254], the correction of the low serum 25(OH)D concentrations should be always performed when documented [255]. There is a current agreement that vitamin D deficiency should be prevented in MS patients, and that levels around 100 nmol/L or somewhat higher should be achieved [255].
Uncertainty on the best dosage to advise still exists, albeit vitamin D3 supplementation from 1000-2000 IU/day [255] to 10,400 IU/day [242] is considered well tolerated and may be prescribed. However, as already highlighted, the higher doses should be reserved for MS patients living in North Europe and North America, whereas lower doses of supplementation may be prescribed to Australian and South European patients [255].
Indeed, it appears that the beneficial UVB effect could also be independent from the actual production of 25(OH)D 3 , and, from this perspective, phototherapy, alternatively or in parallel with vitamin D supplementation, has been proposed as a hypothetical treatment for MS patients [256].

Migraine
Migraine is among the most prevalent neurological conditions in the general population, with a worldwide burden constantly increasing in recent years and a higher impact in the most developed countries [257]. Clinically, migraine is characterized by headaches usually accompanied by nausea, vomiting, osmophobia, photophobia, and phonophobia. Migraine is more prevalent in females [257] and causes significant functional impairment, reduced health-related quality of life [258][259][260], and psychiatric comorbidities [261]. There is a significant gap between migraineurs who need preventive treatment and those who are actually on it [262], and this may be due to several reasons including the reduced tolerability of some medicines, their adverse event profile, and the presence of comorbidity [263][264][265].
A recent study [278] found that vitamin D deficiency was more frequent in patients affected by chronic migraine-medication overuse migraine compared to those with episodic migraine or tension-type headache. This was independent of the season of evaluation and by the patient lifestyle or headache treatment.
However, the sixth survey of the Tromsø Study (Tromsø6) on 11,614 subjects found no significant association between migraine and serum 25(OH)D concentrations, but the risk of non-migraine headache was negatively associated with vitamin D serum levels [279]. Similarly, another study failed to demonstrate any significant differences in the serum 25(OH)D concentrations in migraineurs compared to controls [280] 25(OH)D levels are lower in patients with migraine and tension-type headache, but a straightforward temporal association is yet to be determined (i.e., if vitamin D deficiency can lead to headache or if headache might lead to vitamin D deficiency). In fact, in the literature, there is a lack of longitudinal investigations [281,282].
Vitamin D supplementation has also been tested in clinical trials in migraine patients [283][284][285][286]. In these trials, a variable dosage of vitamin D was used and the follow-up period was variable. However, a significant reduction in the number of headache attacks and improved frequency of the episodes was generally reported [283][284][285][286], but the effects on the intensity of pain were less evident [286].

Diabetic Neuropathy
Diabetic neuropathy represents the main complication of diabetes [287][288][289] and its clinical manifestations can be distinguished by distribution and [290] course. Its prevalence varies according to the duration of the disease with approximately 50% of diabetic patients eventually developing neuropathy [288].
Diabetic neuropathy is burdened by significant morbidity and is associated with the occurrence of diabetic foot ulcers, with possible infections, including osteomyelitis, Charcot neuroarthropathy, and amputations [291].
The most common form of diabetic neuropathy is distal symmetrical polyneuropathy, which is characterized by slowly progressive distal hypoesthesia due to the degeneration of sensory axons, followed, in the most advanced stages by distal paresis due to the late involvement of motor axons [289]. The typical sensory involvement has been classically described as "stocking-glove" hypoesthesia. The impact of diabetic polyneuropathy on the disease course can be very disabling with a significant reduction of the quality of life of the affected patients [292].
Among others, vitamin D deficiency has been recognized as a risk factor for the development of diabetic neuropathy [293,294], after the evidence that hypovitaminosis D is linked to the occurrence and severity of sensory polyneuropathy in these patients [294][295][296][297]. A recent meta-analysis has confirmed that vitamin D deficiency is highly prevalent among diabetic patients with neuropathy [298].
The mechanisms by which vitamin D deficiency could predispose diabetic patients to the onset of neuropathy are manifold and still largely remain speculative. They include atherosclerosis, worsening of insulin resistance, increased inflammation, and oxidative injury to blood vessels eventually leading to nerve ischaemia [294,299,300]. Furthermore, vitamin D has been demonstrated to have multiple trophic effects on the peripheral nervous system, including myelination, axonal homogeneity of peripheral nerves, and neuronal-cell differentiation [301].
Vitamin D deficiency has been involved in the pathogenesis of small-fiber neuropathy, particularly affecting nociceptor fibers [302], but also parasympathetic fibers, especially in younger patients affected by type 2 diabetes [303].
In relation to the supplementation of vitamin D, the results of the completed clinical trials seem quite encouraging [298]. Supplementation has been found to be beneficial for neuropathic pain as well as for the other symptoms of neuropathy [304][305][306][307][308][309][310]. A very interesting recent study has demonstrated that treatment with cholecalciferol 40,000 IU/week leads to a clinically significant improvement as well as improvement of cutaneous microcirculation and to a decrease in serum levels of IL-6 and an increase of serum IL-10 after 24 weeks of vitamin D supplementation [311].
It should be kept in mind that vitamin D deficiency also favours bone loss by stimulating parathyroid hormone secretion, with an increased risk of fracture, and that vitamin D supplementation of at least 800 to 2000 IU of vitamin D per day is currently generally recommended in any case for diabetic patients to reach serum 25(OH)D levels of 75 nmol/L since these levels prevent bone mineralization defects [312]. Table 1 summarizes the ongoing clinical trials exploring the effect of vitamin D supplementation in neurological diseases. Table 2 summarizes the results of the main completed clinical trials exploring the effects of vitamin D supplementation in neurological diseases. Recruiting III Vitamin D + multi-vitamin for 3 weeks. At the end of 3 weeks they will complete an online or paper questionnaire and blood work will be done.

Conclusions and Future Directions
Our review shows that a role of vitamin D in neurological diseases is biologically plausible. The effects of vitamin D on the course of different neurological diseases are only partly understood, and current and future clinical trials may clarify its possible therapeutic applications and the best dose required for each condition. Future trials may also clarify the role of routine vitamin D supplementation in the general population to reduce the risk of developing neurological diseases.  Conflicts of Interest: D.P., G.P., C.M., S.L., S.S. declare no conflict of interest. N.D.S. is a consultant for Biogen, Merck, Novartis, Sanofi-Genzyme, Roche, and Teva and is on the speakers' bureaus of Biogen, Merck, Novartis, Roche, Sanofi-Genzyme, and Teva. He has received travel funds from Merck, Novartis, Roche, Sanofi-Genzyme, and Teva and has grants pending from FISM. He is a co-founder of Siena-Imaging.