Vitamin D and Parkinson’s Disease

Vitamin D is a fat-soluble secosteroid, traditionally considered a key regulator of bone metabolism, calcium and phosphorous homeostasis. Its action is made possible through the binding to the vitamin D receptor (VDR), after which it directly and indirectly modulates the expression of thousands of genes. Vitamin D is important for brain development, mature brain activity and associated with many neurological diseases, including Parkinson’s disease (PD). High frequency of vitamin D deficiency in patients with Parkinson’s disease compared to control population was noted nearly twenty years ago. This finding is of interest given vitamin D’s neuroprotective effect, exerted by the action of neurotrophic factors, regulation of nerve growth or through protection against cytotoxicity. Vitamin D deficiency seems to be related to disease severity and disease progression, evaluated by Unified Parkinson’s Disease Rating Scale (UPDRS) and Hoehn and Yahr (H&Y) scale, but not with age of PD onset and duration of disease. Additionally, fall risk has been associated with lower vitamin D levels in PD. However, while the association between vitamin D and motor-symptoms seems to be possible, results of studies investigating the association with non-motor symptoms are conflicting. In addition, very little evidence exists regarding the possibility to use vitamin D supplementation to reduce clinical manifestations and disability in patients with PD. However, considering the positive balance between potential benefits against its limited risks, vitamin D supplementation for PD patients will probably be considered in the near future, if further confirmed in clinical studies.


Introduction
As consequence of the growth of global population and improvement of the average lifespan, prevalence of neurological disorders is increasing. The scientific community is focused on the research of treatment and prevention of brain aging. Many mechanisms are involved in degeneration: inflammation, oxidative stress, mitochondrial dysfunction, lysosomal depletion, metal dysregulation, impaired RNA homeostasis, misfolding and aggregation of specific proteins, such alpha-synuclein, amyloid β (Aβ), hyperphosphorylated tau [1]. Among others, lower serum concentrations of vitamin D (or 25-hydroxyvitamin D) seems to be associated with psychiatric disorders such as depression, bipolar disorder and schizophrenia, as well as neurological disorders, including neurodegenerative disorders such as dementia and Parkinson's disease (PD) [2]. Consequently, it has been postulated that maintaining adequate vitamin D serum concentration might avoid disease onset and

Neuroprotective Effect of Vitamin D in Parkinson Disease
The role of vitamin D in Parkinson's disease has been widely studied. Lower 25(OH)D levels might be responsible for dopaminergic neuronal death contributing to PD development, due to the lack of its protective function [35]. The specific action of vitamin D protecting against PD is not clear. However, many mechanisms have been correlated with a neuroprotective effect against excitotoxic insults, as showed in Figure 1: 1,25(OH) 2 D 3 stimulates the release of neurotrophin and the synthesis of Ca 2+ -binding proteins such as parvalbumin, it inhibits the synthesis of inducible nitric oxide synthase (iNOS), macrophage colony-stimulating factor (M-CSF) and tumor necrosis factor α (TNF-α), and it induces downregulation of LVSCC and upregulation of γ-glutamyl transpeptidase activity [36][37][38][39][40][41]. Additionally, a lower concentration of vitamin D correlates with high levels of C-reactive protein (CRP), a marker of inflammation [42]. Overall vitamin D role appears fundamental in the prevention of brain aging, considering also its function in the production of growth factors, including nerve growth factor (NGF), ciliary neurotrophic factor (CNTF), glial cell-derived neurotrophic factor (GDNF), glial cell-line-derived neurotrophic factor (GDNF), brain-derived neurotrophic factor (BDNF), and neurotrophin 3 (NT3) [9,[43][44][45].
Vitamin D contributes in intraneuronal calcium (Ca 2+ ) homoeostasis and cytosolic Ca 2+ glial concentration, acting on the regulation of L-type voltage sensitive Ca 2+ channel (LVSCC), modifying neuronal function and upregulating the synthesis of parvalbumin and calbindin. High concentrations of Ca 2+ have toxic effects causing an elevation of ROS levels and mitochondrial dysfunction and inducing neuronal cell death. Vitamin D leads to decrease in excitotoxicity injury triggered by cytoplasmic Ca 2+ , especially when there is a sudden increase in calcium level [36]. Vitamin D has also antioxidant effects, reducing the formation of free radicals and production of reactive oxygen species (ROS), due to the capacity to reduce the synthesis of nitric oxide synthase, reducing activity of NFkB Figure 1. Principal neurotrophic and neuroprotective effects of Vitamin D on CNS. nerve growth factor (NGF), glial cell-derived neurotrophic factor (GDNF), transforming growth factor (TGF)-b2, ciliary neurotrophic factor (CNTF), neurotrophin 4 (NT4), neurotrophin 3 (NT3), brain-derived neurotrophic factor (BDNF), proto-oncogene tyrosine-protein kinase receptor Ret (C-Ret), p75 neurotrophin receptors (p75 NTR), L-type voltage sensitive Ca2+ channel (LVSCC), nuclear factor kappa-light-chainenhancer of activated B cells (NFkB), macrophage colony-stimulating factor (M-CSF), tumor necrosis factor α (TNF-α), inducible nitric oxide synthase (iNOS), increased activity (↑), decreased activity(↓).
Vitamin D contributes in intraneuronal calcium (Ca 2+ ) homoeostasis and cytosolic Ca 2+ glial concentration, acting on the regulation of L-type voltage sensitive Ca 2+ channel (LVSCC), modifying neuronal function and upregulating the synthesis of parvalbumin and calbindin. High concentrations of Ca 2+ have toxic effects causing an elevation of ROS levels and mitochondrial dysfunction and inducing neuronal cell death. Vitamin D leads to decrease in excitotoxicity injury triggered by cytoplasmic Ca 2+ , especially when there is a sudden increase in calcium level [36]. Vitamin D has also antioxidant effects, reducing the formation of free radicals and production of reactive oxygen species (ROS), due to the capacity to reduce the synthesis of nitric oxide synthase, reducing activity of NFkB (nuclear factor kappa-light-chain-enhancer of activated B cells), and enhancing the activity of the gamma glutamyl transpeptidase [49].
In addition, vitamin D is a renin-angiotensin system regulator (RAS), and its altered function could lead to sympathetic dysfunction [50].
Considering all these significant actions, a neuroprotective effect of vitamin D is likely and consequently might reduce progression towards neurodegenerative process.
As a consequence, inadequate vitamin D status could lead to a loss of dopaminergic neurons in the brain, and therefore could contribute to development of PD [51].
Reduced concentration of vitamin D in PD patients, compared to those of sex-and age-matched healthy controls, has been described [52,53].
Many studies investigated the influence of a reduced 25-hydroxyvitamin D level, hypothesizing that a low concentration of vitamin D may be a risk factor, and defined an increasing risk of PD for vitamin D deficiency (<50 nmol/L) compared to insufficiency (<75 nmol/L) [51,54,55]. In a cross-sectional study it was demonstrated that serum 25(OH)D concentrations are decreased in PD patients compared to patients with Alzheimer's disease, but it also showed a decreased levels compared to age-and sex-matched healthy controls [56]. These results could be explained by the longer clinical history and higher immobility in PD patients, compared to AD patients, causing a reduction to sunlight exposure and consequently lack in skin synthesis [57].
Though reduced mobility and therefore sunlight exposure in patients with PD are plausible explanations for reduced 25(OH)D levels, concentration of 25(OH)D are significantly lower in PD patients with adequate sunlight exposure, compared to healthy controls [58]. Ding et al. detected both lower 25(OH)D 3 and lower total 25(OH)D serum concentrations in PD patients compared with controls [55]. In a post hoc analysis, it has been shown a higher incidence of PD in subjects with the lowest quartile of 25(OH)D serum concentration, compared to the highest quartile of 25(OH)D concentration [51].
Further studies analyzed the association of sunlight exposure (>15 min/week) with decreased risk of PD [59]. It was hypothesized that persistent insufficient levels of vitamin D could play a role in the pathogenesis in neurodegenerative disease, including PD [58,60]. In a case-control study vitamin D dispensation, outdoor activity and consequent sunlight exposure were inversely associated with PD [61].
However, for many authors, there is not sufficient evidence to confirm a function of vitamin D in PD pathogenesis [62]. Shrestha et al., in a prospective study, found no significant association between serum 25(OH)D and PD risk [63].
Considering the possible role of vitamin D in neurodegeneration, the attention of many authors has been turned into the role of vitamin D receptor (VDR). VDR is expressed in the CNS neurons, astrocytes and oligodendrocytes, and above all in substantia nigra, cortex, subcortex, hippocampus, hypothalamus, thalamus, and vessel walls [64,65]. The presence of VDR and 1-alpha-hydroxylase, the enzyme that converts 25(OH)D to its active form 1,25-dihydroxyvitamin D (1,25(OH)D), in substantia nigra underlines the role of vitamin D in PD suggesting that vitamin D hydroxylation and activation is also completed in central nervous system (CNS), and therefore the deficit of vitamin D concentration causes dopaminergic neurons death [62,66].
Further confirmation of the role of vitamin D in PD etiopathogenesis is supported by motor impairment in VDR knockout mice, and genotypes were recognized in the VDR gene that are linked with some characteristics of PD [67,68].
Many studies confirmed the presence of VDR in animal and human brains and that vitamin D administration increased dopamine (DA) production. This would be due to the increased expression of tyrosine hydroxylase (TH) (the rate-limiting synthetic enzyme for dopamine) with the consequent increase in neuronal survival DA neurons, both in vitro and in vivo [8,[69][70][71][72][73].
Vitamin D administration reduces dopaminergic toxicity of 6-hydroxy dopamine [74], and the lack of vitamin D receptor (VDR) induced a significantly impaired motor function in mice [67,75]. Several genetic association studies, supporting the possible role of vitamin D in PD risk, searched for the connection between VDR single nucleotide polysmorphisms (SNPs) and PD [56,[76][77][78]. A genome-wide association study (GWAS) demonstrated the association of VDR polymorphisms with both risk and age at onset of PD [68].
Therefore, as shown in some studies [35,76,77,79], response to vitamin D administration in PD patients depends upon the genotype of VDR.
VDR CC allele (FokI) is associated with PD, and subjects with FokI CC demonstrated no significant response to vitamin D administration with respect to placebo compared with other VDR genotypes [76,77]. Specifically, subjects with FokI TT allele had a significant response to vitamin D administration compared to individuals with VDR FokI CT, who had a moderate response [79]. FokI CC was also associated with milder form of PD [35].
Genetic factors might be a cause of vitamin D deficiency, and the association of PD with single nucleotide polymorphisms (SNPs) in the VDR and vitamin D binding protein (VDBP/GC) have been investigated in PD, such as a possible cerebrospinal fluid (CSF) biomarker in PD [56,[76][77][78]80].
Further support to the hypothesis of a neuroprotective role of vitamin D comes from numerous studies. Among these two cross-sectional studies supports the hypothesis that vitamin D insufficiency is associated with an increased risk of Parkinson's disease [56,81], as well as an association between high serum 25(OH)D levels and a reduced risk of PD has been observed in a longitudinal prospective study by Knekt P et al. [51].
Another supporting factor is that outdoor work seems to be related to a lower risk of developing PD, suggesting that greater sun exposure and the consequent increased vitamin D synthesis in skin represent a protective factor [82,83].

Vitamin D in Relation with Parkinson Disease Symptoms and Disease Progression
While the role of Vitamin D as a protective factor in PD is promising, though still controversial, its role in PD symptoms progression is much more complex.
In an animals model, Kaluef et al. showed that genetic ablation of VDR in mice is associated with specific impairments in motor performance, and this, according to their explanation, could possibly be attributed to the localization of VDR in the brain and spinal cord and suggests that vitamin D plays a crucial role in the functioning of the motor system [75].
Meamar et al. conducted a cross-sectional study to investigate the relationships between serum 25(OH)D 3 levels and serum UA concentrations as well as their interaction with severity of PD. In their population of Iranian PD patients, a negative significant association between serum 25(OH)D 3 and UPDRS score was found, indicating that higher serum levels of 25(OH)D 3 was associated with lower PD severity; this finding was restricted to patients older than 62 years [82].
Therefore, the important role of Vitamin D in humans was demonstrated by the negative correlation between serum 25(OH)D 3 levels and PD severity, determined by UPDRSIII score, conversely the 25(OH)D concentrations seems do not correlate with freezing, postural instability and abnormal postures, all signs of PD disease progression [53,82].
Other authors demonstrated an inverse association between higher UPDRS scores and Hoehn and Yahr (HY) staging, and low levels of 25(OH)D 3 and total 25(OH)D levels, concluding that decreased concentration of vitamin D are related to increased disease severity but not with disease duration and age of PD patients [55,83,84].
These results were confirmed in the DATATOP study, asserting that 25(OH)D levels did not decrease during progression of PD [85]. Suzuki et al. described a slower PD progression, evaluated by means of the H&Y scale, in the activities of daily living by UPDRS Part II, and the quality of life by Parkinson's Disease Questionnaire (PDQ39), in the group of PD patients receiving vitamin D reintegration, compared to PD patients who received a placebo [34].
In a genome-wide association (GWA) study, Gezen-Ak et al. indicated a relationship between low serum levels of 25(OH)D and PD, identifying several polymorphisms in vitamin D receptor (VDR) genes that correlate with different degrees of severity in Parkinson's in relation to serum levels of 25(OH)D [86].
Vitamin D deficiency might have an impact in progression of PD and its clinical motor and non-motor manifestations.
Sleeman et al. concluded that the worst PD progression, referring to motor impairment severity evaluated by UPDRS part III, was related to lower serum 25(OH)D to baseline [83].
Moghaddasi et al. found that low 25(OH)D levels were associated with severe postural instability, freezing gait, and postural abnormalities [53].
Among motor symptoms, Peterson et al. investigated the relationship between vitamin D levels and balance control. Considering that postural instability represents a common symptom in PD, and that in heathy subjects the automatic postural responses are symmetric while PD patient have asymmetrical reaction, the authors found a correlation between lower vitamin D concentrations and the rate of asymmetrical postural responses [87]. Automatic postural response impairment might be caused by decreased muscular strength and alteration in latency of postural responses, involving cortex, spinal cord, brainstem, cerebellum and basal ganglia, and VDR is localized in all these structures [88,89].
Some authors observed an increased risk of fracture in PD patients than general population [90]. This could be dependent on a higher risk of falls, motor impairment, balance difficulties, postural instability and by lower levels of calcium and of vitamin D compared to healthy controls [83,91], possibly explained by less sunlight exposure, immobility and insufficient intake of calcium and vitamin D with the diet [59].
Among non-motor symptoms, depression and cognitive impairment seem to be influenced by 25(OH)D 3 . Better scores in neuropsychiatric testing, especially verbal fluency and verbal memory, are associated with higher 25(OH)D 3 serum levels [92], and 25(OH)D concentration was correlated with depression and anxiety scores [93]. Gatto et al. assessed the role of VDR polymorphisms in cognitive decline in patients with PD and especially in subject with FokI polymorphism [94].
Fewer data are available on other non-motor symptoms. Lower levels of 25(OH)D correlated with increased sleepiness, evaluated by Epworth Sleep Scale (ESS) [95] while 25(OH)D 3 levels are correlated with the severity of olfactory dysfunction in PD patients [96].
To date, very limited evidence exists on the association between non-motor symptoms and vitamin D deficiency.

Vitamin D Supplementation in Parkinson Disease
Many attempts have been made in order to address a crucial unmet demand in the management of Parkinson's disease: the discovery of a drug potentially able to slow down, stop or reverse the process of neurodegeneration.
Considering that 1,25(OH)2D3 passes through the blood-brain barrier, hypothesizing that its systemic administration could reduce neuronal injury, some studies investigated the use of Vitamin D as a treatment to reduce some PD symptoms.
We already argued that the stimulation of neurotrophin production by 1,25(OH) 2 D 3 improves cell survival and reduces toxicity mediated by reactive oxygen species. In a study conducted in rats, 1,25-dihydroxyvitamin D (D3) pretreatment protected against 6-OHDA damage of the substantia nigra (SN) reducing hypokinesia [72]. Treatment with 1,25(OH) 2 D 3 seems to increase DA release in striatum, and it could be mediated by enhanced GDNF and glutathione levels in rats' central nervous system, and specifically in mesencephalic dopaminergic neurons [71,99,100].
Among studies conducted in humans, Suzuki et al. evaluated, in a randomized placebo-controlled clinical trial, the relationship between 1200 IU/day of vitamin D administration and disease progression for two years follow-up. Patients with Parkinson's disease receiving placebo had a worse neurological outcome, determined by Hoehn and Yahr scale (H&Y) and Unified Parkinson's Disease Rating Scale (UPDRS), and worse quality of life evaluated by PDQ-39 compared to patients who received vitamin D supplements [34]. Luthra et al. conducted a cohort study in early PD patients followed for three years and divided into three groups: supplement multivitamin (MVI), vitamin D administration ≥400 IU/day, and vitamin D + MVI. The authors did not observe differences in disease progression within the three groups [101].

Conclusions
In this review, we summarized the current literature concerning the possible role of vitamin D in numerous physiological functions, from the modulation of the immunological response to the regulation of brain development and aging (Table 1). We discussed how serum levels of 25(OH)D are possibly related to PD symptoms and clinical progression, and about its possible use in order to improve clinical manifestation in patients with PD. 1,25OH2D3 play a role in regulation on CNS immune response, by modification of astrocytes response to an inflammatory stimulus Garcion et al. [41] 1996 1,25D3 could be an effector controlling detoxification processes in the brain.

Study Type Authors Year Main Conclusion
De Viragh et al. [38] 1989 Vitamin D influences the concentration of calcium-binding-proteins in the periphery and brain Moghaddasi et al. [53] 2013 Non-PD patients were detected lower 25OHD level and it was significantly associated with FOG, postural instability and abnormal postures in vivo Calvello et al. [69] 2017 Vitamin D exhibits substantial neuroprotective effects in this PD animal model, by attenuating pro-inflammatory and up-regulating anti-inflammatory processes Cass et al. [70] 2014 Calcitriol can upregulate GDNF and dopaminergic release in striatum, increasing DA levels in the substantia nigra Cass et al. [71] 2012 In animals treated with 6-OHDA followed by calcitriol there was significantly greater potassium and amphetamine evoked overflow of DA from the lesioned striatum compared to that from the control animals Cui et al. [16] 2010 the DVD-deficient embryos had a significant reduction in factors crucial in specifying dopaminergic phenotype, such as Nurr1 and p57Kip2 Smith et al. [74] 2006 Long-treatment with calcitriol can provide partial protection for dopaminergic neurons against the effects of 6-OHDA Burne et al. [67] 2005 VDR mice knockout have motor impairments but seemingly no compromission in cognition Kalueff et al. [75] 2004 VDR genetic ablation produces severe motor impairment Eyles et al. [18] 2003 Rats born to vitamin D3-deficient mothers had alterations in the brain at birth: lateral ventricles were enlarged, the cortex was thinner Wang et al. [72] 2001 D3 pretreatment reduces the hypokinesia and DA neuronal toxicity induced by 6-OHDA Prüfer et al. [104] 1999 The widespread distribution of vitamin D3 receptor suggests multiple functions of 1,25OHD3 in the CNS.
Fahmy et al. [52] 2020 Serum 25OHD3 was lower in PD patients and was negatively correlated with age and age at onset of disease, but not with disease duration and PD severity. Serum 25OHD3 was not found to be predictor for severity of PD  [98] 2016 Vitamin D status may play a role in the pathogenesis of delayed gastric emptying in drug-naive PD.
Wang et al. [54] 2015 Association between vitamin D levels and PD is not simply due to lack of sunlight exposure PD patients with impaired mobility Jang et al. [97] 2015 Low vitamin D status is associated with OH in patients with PD  In summary, as low serum 25(OH)D levels might be correlated with an increased risk of developing PD, higher 25(OH)D levels seems to be associated with better motor symptoms, especially with improved balance control. It is not yet clear if vitamin D is related to the severity of symptoms of PD and with clinical progression; therefore, its It is not yet proven if vitamin D reintegration could be an appropriate support to pharmacological and rehabilitative therapy in PD patients. However, though insufficient evidence is available to introduce vitamin D as supportive therapy in PD patients, considering its limited risks, we are confident enough to insinuate, as a dietary intervention, that vitamin D supplementation would act at three different levels: (1) improve public health considering its possible role in brain development and its influence in pathogenesis of many neurological disorders, including PD; (2) slowing down the worsening of some PD symptoms; (3) finally, considering the increased risk of falls during disease progression, reduce the risk of fracture in PD patients.