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The Association Between Vitamin D Status, Vitamin D Supplementation, Sunlight Exposure, and Parkinson’s Disease: A Systematic Review and Meta-Analysis

Background This literature review and meta-analysis aimed to determine the association between deficiency of vitamin D, or 25-hydroxyvitamin D, and Parkinson’s disease, and whether vitamin D from supplements and sunlight improves the symptoms of Parkinson’s disease. Material/Methods A literature review and meta-analysis were performed according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement. Systematic literature review was performed using databases that included the Web of Science, PubMed, the Cochrane Library, and Embase. The Jadad scale (the Oxford quality scoring system) and the Newcastle-Ottawa scale (NOS) were used to evaluate the quality of the studies. Results Eight studies were included in the meta-analysis. Both 25-hydroxyvitamin D insufficiency (<30 ng/mL) (OR, 1.77; 95% CI, 1.29–2.43; P<0.001) and deficiency (<20 ng/mL) (OR, 2.55; 95% CI, 1.98–3.27; P<0.001) were significantly associated with an increased risk of Parkinson’s disease when compared with normal controls Sunlight exposure (≥15 min/week) was significantly associated with a reduced risk of Parkinson’s disease (OR, 0.02; 95% CI, 0.00–0.10; P<0.001). The use of vitamin D supplements was effective in increasing 25-hydroxyvitamin D levels (SMD, 1.79; 95% CI, 1.40–2.18; P<0.001), but had no significant effect on motor function (MD, −1.82; 95% CI, −5.10–1.45; P=0.275) in patients with Parkinson’s disease. Conclusions Insufficiency and deficiency of 25-hydroxyvitamin D and reduced exposure to sunlight were significantly associated with an increased risk of Parkinson’s disease. However, vitamin D supplements resulted in no significant benefits in improving motor function for patients with Parkinson’s disease.


Background
Parkinson's disease is a complex neurodegenerative disease that involves the loss of dopaminergic neurons in the substantia nigra [1]. Parkinson's disease is characterized clinically by motor dysfunction, including a resting tremor, bradykinesia and rigidity, and by non-motor symptoms, including sleep disturbance, cognitive impairment, olfactory disorders, and fatigue [1]. Elderly individuals who are more than 60 years of age are most likely to develop Parkinson's disease [2]. For people who are more than 40 years of age, the incidence of Parkinson's disease is 37.55 and 61.21 per 100,000 personyears, for women and the men, respectively [2]. Worldwide, due to the increase in life expectancy, it has been predicted that the number of patients with Parkinson's disease who are more than 50 years of age will double by 2030 [3]. Both genetic and environmental factors have been reported to contribute to the development of Parkinson's disease, including specific genetic mutations [4], gender [5], exposure to pesticides [6], and the use of calcium channel blockers [7].
Vitamin D, or 25-hydroxyvitamin D, is derived naturally from two main sources, the diet and from the effects of ultraviolet B (UVB) (wavelength, 290-315 nm) from sunlight, on the conversion from the steroid precursor, 7-dehydrocholesterol, in the skin. Therefore, lack of exposure to sunlight can result in low levels of vitamin D [8]. Currently, vitamin D has become regarded not as a vitamin, but as a hormone [9]. Apart from being involved in the metabolism of calcium and phosphorus, vitamin D also has a role in the inflammatory response [10], glucose and lipid metabolism [11,12], and in cardiac and vascular regulation [13]. Vitamin D performs its biological functions by binding to vitamin D receptors (VDRs) [14], which are members of the steroid hormone receptor superfamily. VDRs are widely expressed in many tissues [9], including the kidney, bone, the intestine, muscle, the pancreas, and the central nervous system. Vitamin D deficiency is associated with a variety of pathological changes in many organ systems [15], and has been shown to be associated with an increased for several chronic diseases, including multiple sclerosis [16][17][18][19], cardiovascular disease [20], cancer [21], type 2 diabetes [22], Alzheimer's disease [23] and Parkinson's disease [24]. Vitamin D deficiency has been proposed to act as a suppressor of gene expression [25], one of the key genes that is suppressed in vitamin D deficiency is the tyrosine hydroxylase (TH) gene, which plays an essential role in regulation of dopamine biosynthesis [26] and the expression of neurotrophic factors [9].
Studies on the association between vitamin D deficiency and Parkinson's disease have shown conflicting findings. One cohort study [27] reported that there was insufficient evidence to support the hypothesis that vitamin D deficiency had a role in the pathogenesis of Parkinson's disease, or on the integrity of the dopaminergic system. However, other studies [9,28] have reported a high prevalence of vitamin D deficiency in patients with Parkinson's disease, with an inverse relationship between serum levels of 25-hydroxyvitamin D and the severity of Parkinson's disease [29][30][31]. Vitamin D supplements have been shown to reduce the rate of deterioration of motor function, as determined by both the Hoehn and Yahr scale and the Unified Parkinson's Disease Rating Scale (UPDRS) [32]. Preclinical animal experiments have also shown that vitamin D treatment was beneficial in reducing neuroinflammation and dopaminergic neurodegeneration [33]. However, controversy remains regarding the relationship between vitamin D status and the risk of Parkinson's disease. One study in Finland showed that increased vitamin D levels could reduce the risk of developing Parkinson's disease [24]. A further prospective cohort study that included a 17-year follow-up period showed that there was no association between vitamin D levels and the risk of developing Parkinson's disease [34].
Most previously published studies have focused on the prevalence of vitamin D deficiency or insufficiency in patients with Parkinson's disease, but published cohort studies, case-controlled studies, or randomized controlled trials (RCT) are scarce. Findings on the association between vitamin D status and the risk of Parkinson's disease are inconsistent, and it is unclear whether the degree of vitamin D deficiency affects the risk of Parkinson's disease. Also, although vitamin D deficiency is more common in patients with Parkinson's disease, it is unknown whether vitamin D supplements and exposure to sunlight can be regarded as an effective therapy to reduce the symptoms of Parkinson's disease. Therefore, this literature review and meta-analysis aimed to determine the association between deficiency of vitamin D, or 25-hydroxyvitamin D, and Parkinson's disease, and whether vitamin D from supplements and sunlight improves the symptoms of Parkinson's disease. The normal serum reference range for total 25-hydroxyvitamin D is 20-100 ng/mL, and in this study, vitamin D insufficiency was defined as a serum level <30 ng/mL, and vitamin D deficiency was defined as a serum level <20 ng/mL.

Literature search
This systematic review and meta-analysis were performed according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines (http://www.prismastatement.org). Variations or synonyms of keywords were also used to ensure that a comprehensive search was undertaken. Reference lists from review articles were also viewed to ensure that no potentially eligible studies were omitted. The detailed search strategy for the PubMed database is presented in Table 1.

Publication selection
Two reviewers independently reviewed the titles and abstracts of the retrieved publications. The full text of the publications was reviewed for studies where the inclusion eligibility could not be determined from the titles and abstracts alone. Inclusion criteria were case-controlled studies, cohort studies, or randomized clinical trials (RCTs) that included patients with a diagnosis of Parkinson's disease and serum vitamin D (25-hydroxyvitamin D) measurement to determine whether there was insufficiency or deficiency. Inclusion criteria also required details of the use of vitamin D supplements and sunlight exposure, the relative risk (RR) or odds ratio (OR), and changes in serum vitamin D levels. The use of the Unified Parkinson's Disease Rating Scale (UPDRS) (Part III) over time was an inclusion criteria or could be calculated from reported data.
Published studies were excluded if they were cross-sectional studies, case reports, review articles, animal studies, editorials, or letters, or if they had incomplete data. In the latter case, there were attempts to contact the authors to supply missing data. Only studies selected by both reviewers were included in the meta-analysis. Any disputes regarding publication inclusion criteria were resolved by discussion or referral to a third reviewer.
Data extraction and quality assessment of the study methods using the Jadad scale (the Oxford quality scoring system) and the Newcastle-Ottawa scale (NOS) Data from the included studies were independently extracted by two reviewers, with the use of a data extraction checklist. Confirmation of data extraction was performed and any inconsistent data were verified by a third reviewer. Extracted publication data included the first author, the year of publication, the region, the study type, the characteristics of the study participants, including age, gender, body mass index (BMI), Hoehn and Yahr stage, and baseline levels of 25-hydroxyvitamin D. Exposure measurement, effect size, adjusted factors for cohort or case-controlled studies, and treatment methods, time of follow-up for RCTs were also recorded. Any disagreements were resolved by discussion until a consensus was reached.
The Jadad scale (the Oxford quality scoring system) [35] was used to evaluate the quality of the study methods used for RCTs, and the Newcastle-Ottawa scale (NOS) [36] was used to evaluate the study quality for cohort or case-controlled studies. The requirements of the Jadad scoring system included the generation of a random sequence, allocation concealment, participants, investigators and assessors who were blinded to the study, the integrity of data, selective reporting, and evaluation of sources of bias. The maximum score using the Jadad scale was 7 points, and studies with a Jadad score of >4 points were considered to be of high quality. The NOS was mainly used to assess the methodological quality of non-RCTs, including cohort and case-controlled studies. The maximum score using the NOS was 9 points, and studies with a NOS score of >6 points were considered to be of high quality.

Data synthesis and statistical analysis
The primary outcomes evaluated in this review and metaanalysis included the association between vitamin D status and the risk of Parkinson's disease and the effects of vitamin D supplements on serum levels of vitamin D (25-hydroxyvitamin D) and motor function for individuals with Parkinson's disease. The secondary outcome was the relationship between sunlight exposure and the risk of Parkinson's disease. The normal serum reference range for total 25-hydroxyvitamin D is 20-100 ng/mL. In this study, vitamin D insufficiency was defined as a serum level <30 ng/mL (<75 nmol/L), and vitamin D deficiency was defined as a serum level <20 ng/mL (<50 nmol/L) [37]. A cutoff value of 15 min/week was used to evaluate the effects of sunlight exposure on the prevention of Parkinson's disease, as previously reported [38].
The RR or OR with a 95% confidence interval (CI) were pooled for dichotomous outcomes. Continuous outcomes measured using the same method were summarized as the weighted mean difference (MD) with 95% CI. The pooled standardized mean difference (SMD) and 95% CI were calculated for outcomes with different measures. The I 2 statistic was used to determine heterogeneity across studies. A fixed-effects pooled analysis method was used to combine the results if no or low heterogeneity existed (I 2 <50%), or a random-effects metaanalysis was performed. Visual interpretation of funnel plots and Egger's test [39] were used to determine publication bias, where appropriate. Sensitivity analysis, based on the quality of the study methods, was performed to test the stability of the studies, where required. Stata version 12.0 statistical software was used to perform the meta-analysis and to create forest plots.

Publication search results
A flow diagram of the study publication screening method is shown in Figure 1. A total of 1,965 publications were initially identified according to the search criteria. Then, the titles and abstracts of 1,651 publications were reviewed after removing duplicate studies, and 1,627 non-relevant publications were removed. For the remaining 24 publications, the full texts were retrieved for further assessment, and 16 publications were excluded. Finally, eight studies [28,31,32,[40][41][42][43][44] were included in the meta-analysis (Figure 1).

Characteristics and quality of the methods used in the included studies
Of eight studies, three studies [28,40,44] originated from the United States, three [31,32,43] studies were from Japan, one study was from Canada [41], and one study was from Egypt [42].
Of the eight studies, there were five case-controlled studies [31,[40][41][42][43], one cohort study [28] and two randomized clinical studies (RCTs) [32,44]. The characteristics of cohort study, case-controlled studies, and RCTs are presented in Tables 2  and 3. All studies were considered to be of high-quality, except for one study [43]. The details of the Newcastle-Ottawa scale (NOS) or the Jadad scale (the Oxford quality scoring system) for each study are shown in Tables 2 and 3.    (Figure 2).

Synthesis of meta-analysis: Sunlight exposure and the risk of Parkinson's disease
Three case-controlled studies involving 295 patients with Parkinson's disease and 200 controls evaluated the association between exposure to sunlight and the risk of Parkinson's disease ( Figure 3) [31,42,43]. There was high heterogeneity across studies (I 2 =78.8%; P=0.003), and a random-effects meta-analysis was performed to combine data. Exposure to sunlight (³15 min/week) was associated with a significantly reduced risk of Parkinson's disease (OR, 0.02; 95% CI, 0.00-0.10; P<0.001). Similar results were obtained for the sensitivity analysis (OR, 0.03; 95% CI, 0.00-0.26; P<0.001) (Figure 3).

Synthesis of meta-analysis: Vitamin D supplements and vitamin D levels in patients with Parkinson's disease
Two RCTs involving 144 patients reported the effects of vitamin D supplement on vitamin D level for individuals with Parkinson's disease [32,44]. There was no heterogeneity between published trials (I 2 =0.0%; P=0.783). In the fixed-model analysis of pooled data, the standardized mean difference (SMD) was 1.79 (95% CI, 1.40-2.18; P<0.001) indicating that vitamin D supplements increased serum levels of vitamin D (Figure 4).

Synthesis of meta-analysis: Vitamin D supplements and motor function in patients with Parkinson's disease
Motor function in patients with Parkinson's disease evaluated by the Unified Parkinson's Disease Rating Scale (UPDRS) (Part III) was reported in two RCTs (three intervention groups) [32,44]. Vitamin D supplementation had no significant effect on motor function in patients with Parkinson's disease (MD,   Figure 5). There was no heterogeneity between studies (I 2 =0.0%, P=0.921) ( Figure 5).

Discussion
A systematic literature review identified eight studies that included five case-controlled studies, one cohort study, and two randomized clinical trials (RCTs). Meta-analysis was performed to investigate the associations between vitamin D status, as determined by the serum levels of 25-hydroxyvitamin D, sunlight exposure, and the risk of Parkinson's disease. The therapeutic effects of vitamin D supplements on serum levels of vitamin D and on motor function for patients with Parkinson's disease were also investigated in the meta-analysis. To our knowledge, this was the first systematic review and meta-analysis to investigate the relationship between vitamin D supplements and sunlight exposure and the risk of Parkinson's disease. This meta-analysis was performed according to Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement [45]. Keywords and their variations or synonyms were used to search electronic publication databases, and reference lists of eligible studies were also reviewed. Of the eight identified studies that underwent quantitative data synthesis, the Jadad scale (the Oxford quality scoring system) and the Newcastle-Ottawa scale (NOS) were used to evaluate the quality of the methods used in the studies.
Previously published studies have reported that patients with Parkinson's disease had reduced serum levels of vitamin D when compared with normal individuals [46,47]. A prospective cohort study that included 3,173 participants with a 29-year follow-up, showed that vitamin D could protect individuals from Parkinson's disease [24]. Also, increased vitamin D receptors (VDRs) and enzymes involved in the formation of active vitamin D have been reported in the substantia nigra, an area associated with the development of Parkinson's disease [48]. The neuroprotective function of vitamin D may occur due to the regulation of neurotrophic factors, antioxidative mechanisms, immunoregulation, regulation of calcium-binding protein, and modulation of neuronal excitation [24,48,49]. Together, the results of the present meta-analysis and the findings from previous studies provide evidence that supports the beneficial effects of vitamin D in reducing the risk of Parkinson's disease.
However, this meta-analysis showed that the use of vitamin D supplements increased the serum levels of vitamin D (25-hydroxyvitamin D) in patients with Parkinson's disease (SMD, 1.79; 95% CI, 1.40-2.18; P<0.001), but there were no significant effects from the use of vitamin D supplements on motor function (MD, -1.82; 95% CI, -5.10-1.45; P=0.275). In support of these findings, a previously published cross-sectional study from Iran showed that vitamin D status was not associated with Hoehn and Yahr stage and the Unified Parkinson's Disease Rating Scale (UPDRS) score in subjects with Parkinson's disease [50].
The findings of this present study also showed that exposure to sunlight (³15 min/week) had a significant role in preventing   One study [51] also reported that exposure to UVB was associated with a reduced risk of developing Parkinson's disease. UVB is a component of sunlight, and cutaneous synthesis of vitamin D requires UVB radiation [52], and this process has an important role in controlling serum vitamin D levels [53]. Sunlight exposure reduces the risk of Parkinson's disease by increasing serum levels of vitamin D.
Because there is evidence to support that vitamin D has a potential role in the prevention of Parkinson's disease, for individuals with vitamin D insufficiency, especially those who are at high risk of developing Parkinson's disease, it might be important to take vitamin D supplements or to include foods in the diet that are rich in vitamin D, including fish, liver, and nuts, to maintain D levels within the normal range. Spending more time outdoors to increase the duration of sunlight exposure, might be a better option. However, in patients with established Parkinson's disease, currently, there is no evidence that vitamin D should be recommended for symptom control. Therefore, with regard to the effects of vitamin D on Parkinson's disease, currently, vitamin D appears to have a more important role in the prevention rather the treatment of Parkinson's disease This study had several limitations. Only eight studies were identified, which included a small number of study participants and short-term follow-up. In the study design, although comprehensive keywords and search strategies were used to identify relevant published studies, there was still a possibility that some studies were missed, particularly studies with negative results, which are more likely not to be published. There was also the possibility of study bias, as publications were limited to those in the English language, and the possibility of publication bias was not explored due to the limited number of published studies. This analysis only investigated the influence of vitamin D supplements on serum concentrations of vitamin D and on motor function for patients with Parkinson's disease, an overall evaluation on effects of vitamin D supplement was not made due to limited or unavailable data. Only two RCTs [32,44] reported the effects of vitamin D supplements, and this and other limitations may result in non-representative results. Future large-scale, multi-center, controlled, randomized clinical studies are required to provide evidence on the association between vitamin D and Parkinson's disease, which include long-term follow-up, particularly for the effects of vitamin D on motor function and other rehabilitation outcomes.

Conclusions
Vitamin D levels below the normal range are associated with an increased risk of Parkinson's disease, which increases with decreasing levels of vitamin D. Also, sunlight exposure (³15 min/week) is beneficial in reducing the risk of developing Parkinson's disease. Vitamin D supplementation is beneficial for increasing vitamin D levels in patients with Parkinson's disease but has no effects on motor function. Therefore, maintenance of normal vitamin D levels from a combination of diet, sunlight, and supplements are low-cost approaches to prevent Parkinson's disease, but vitamin D supplementation might not be a suitable addition to the treatment regime and patient rehabilitation in patients with established Parkinson's disease. Due to the few included studies in this meta-analysis, highquality multicenter studies are still needed to confirm the conclusions and to explore the effectiveness of vitamin D supplements on non-motor symptoms and quality of life for patients with Parkinson's disease.