Background: Despite the vitamin D treatment in patients with multiple sclerosis (MS), there continues to be controversial discrepancy in outcomes according to the current research. Many systematic reviews have evaluated the effect of vitamin D as an adjuvant treatment in patients with MS; however, there is no consensus on the optimum administration time and dosage of vitamin D intake. A meta-analysis for exploring the different administration time and dosage of vitamin D is warranted. Methods: Randomized controlled trials (RCTs) on the effect of different administration time and dosage of vitamin D in patients with MS were recorded within 7 databases. This meta-analysis was performed with 2 clinical outcomes: EDSS (Expanded Disability Status Scale) and relapses during research. Results: The pooled results indicated that receiving different administration time and dosage of vitamin D as an adjuvant treatment had no significant therapeutic effect on MS according to the EDSS scores and relapses during research. Conclusion: According to our meta-analysis, the administration of vitamin D in different dosages (ranging from 2,857 to 14,007 IU/day) and treatment period (ranging from 6 to 24 months) did not affect the clinical outcomes (EDSS and relapses during research) in patients with MS. Additional RCTs should be conducted to explore whether a longer duration and a larger dosage of vitamin D without serious adverse effects might produce therapeutic effects in patients with MS.
Multiple sclerosis (MS) is an immune-mediated inflammatory demyelinating disease characterized by multifocal areas of demyelination, axonal damage, and oligodendrocyte in the central nervous system . Although the etiology of MS is unclear, it is recognized that the causes of MS result from the association of genetic, environmental, and other risk factors that can trigger excessive autoimmune attacks . Many of these risk factors intervene, particularly in MS, between pregnancy and the end of adolescence, in a period corresponding to the maturation of the immune system and thymus . Immunotherapies have been widely recommended in adults with MS. Studies of patients treated with disease-modified therapy (DMT) have reported decreases in their relapse rates and the accumulation of new brain lesions [4, 5].
Vitamin D and its analogs not only involve the bone metabolism and formation but also the pathogenesis of MS known as its immune-modulatory effect [6, 7], including lymphocyte activation and proliferation, T-cell differentiation, and inflammatory cytokine suppression . A lower concentration of vitamin D was observed in the serum of patients with MS compared to that in the healthy population . The low circulating vitamin D in serum accelerated the progress of MS . Pierrot-Deseilligny et al.  found that in patients with relapsing-remitting MS, there is a 68% decrease in relapses with an increase of 20 ng/mL of vitamin D serum level. Furthermore, vitamin D administered either orally or intraperitoneally has proven to alleviate the clinical symptoms and suppress the development of experimental autoimmune encephalomyelitis [12, 13]. Therefore, vitamin D as a potential adjuvant therapy in patients with MS has attracted much attention.
Three randomized controlled trials (RCTs) [14-16] on the therapeutic effect of vitamin D supplementation in patients with MS have been recently completed. However, despite the vitamin D treatment in patients with MS, controversial discrepancy still exists within the laboratory, radiological, and clinical outcomes. Several studies have confirmed that the combined therapy of vitamin D and DMT in patients with MS could reduce the radiological lesions [15-18], the Expanded Disability Status Scale (EDSS) scores , the annual relapse rate (ARR) [17, 19], and the level of inflammatory cytokines , whereas other studies [14, 20-23] have found no difference. Although many systematic reviews [24-27] have evaluated the effect of vitamin D supplementation as an adjuvant therapy for patients with MS, there is currently no consensus regarding the optimum administration time and the dosage of vitamin D intake. It is therefore necessary to perform a meta-analysis that categorizes patients by subgroups according to the different administration time and dosage of vitamin D. To eliminate the extra impact of vitamin D supplementation in controls, we further divided the controls into 2 subgroups depending on whether the patients received the placebo.
Materials and Methods
Data Sources and Search Strategy
This systematic search, which included 7 databases (PubMed, The Cochrane Library, EMBASE, CENTRAL, ClinicalTrials.gov, Web of Science, and the WHO International Clinical Trials Registry Platform), was conducted in May 2020. The search combined MeSH and free terms related to MS and vitamin D (vitamin D3/D2, vitamin D supplementation, calciferol, and cholecalciferol). The search was restricted to human RCTs published in English. Incomplete data were further pursued by consulting the original authors.
Inclusion and Exclusion Criteria
Studies eligible for inclusion in this meta-analysis were as follows: (1) RCTs involving patients diagnosed with MS (including all subtypes of MS) or clinically isolated syndrome according to the guidelines of the McDonald criteria [28, 29]; (2) patients who received a high-dose vitamin D as the treatment group, and patients who were treated with placebo or a low-dose vitamin D as the control group; both groups of patients received the DMT during the study; and (3) studies that focused on the clinical outcomes (relapses during research and EDSS scores) and the definitions of these clinical outcomes in RCTs consistently met the inclusion criteria [28-30]. The studies with incomplete data that only focused on radiological or laboratory outcomes and that had different definitions of outcomes were excluded from this meta-analysis.
Data Extraction and Quality Assessment
Two reviewers extracted the data and assessed the quality of the included studies independently. Disagreement was resolved through discussion, and consensus was reached using a third reviewer. The risk of bias about the included studies was assessed by consulting the Cochrane Handbook  (including random sequence generation, allocation concealment, blinding of participants and personnel, blinding of outcome assessment, incomplete outcome data, selective reporting, and other bias). The Grading of Recommendations Assessment, Development and Evaluation (GRADE)  was used to evaluate the quality of evidence according to research limitations, inconsistencies, indirectness, imprecision, and publication bias in this meta-analysis. The following information of eligible studies was extracted: authors, publication year, sample size, sex, ages, duration, risk of bias, and the detail of treatment and control groups.
Revman 5.3 was used for statistical analysis. The continuous variable was expressed by mean ± standard deviation and the dichotomous expressed by odds ratio (OR). Heterogeneity was evaluated using the I2 test. If there is no obvious heterogeneity (I2 <50%) among the eligible studies, the fixed-effect model will be used. In contrast, if I2 ≥50%, the random-effect model will be used after excluding the potential influencing factors. Due to the small number of included studies, neither Funnel Plot nor Egger Test could be adopted to assess the publication bias. Statistical significance was defined as p < 0.05.
We initially collected 3,074 relevant studies from PubMed, 2,894 studies from the Cochrane Library, 2,561 studies from EMBASE, 2,329 studies from the Web of Science, 432 studies from the CENTRAL, 113 studies from the ClinicalTrials.gov, and 621 studies from the WHO International Clinical Trials Registry Platform with reference to the inclusion and exclusion criteria. After removing duplications, excluding reviews and nonhuman RCTs, and screening for full texts, 9 eligible studies [15-23] were included in the quantitative synthesis. The EVIDIMS  was excluded for the different definitions used in relapses during the research. Figure 1 shows the flow diagram of search.
Risk of Bias Assessment
After 2 reviewers assessed independently the risk of bias, 4 [17, 21-23] of 9 studies were defined as low risk, 4 studies [15, 16, 19, 20] were defined as unclear risk, and only 1 study  was included in the high-risk group. The results are shown in online suppl. Figure 1 (see www.karger.com/doi/10.1159/000515131 for all online suppl. material).
Quality of Evidence
In the subgroups analysis of the relapses during this research, patients treated with vitamin D >12 months, patients treated with a low dose of vitamin D, and patients treated with vitamin D ≤2,857 IU/day were downgraded by one level of evidence because of the serious imprecision of results. Besides, the total EDSS was downgraded by one level of evidence because of the serious inconsistency. In the subgroups of analyses of EDSS scores, the levels of evidence were low because of the serious imprecision and inconsistency of the results. The results are shown in online suppl. Figure 2.
Among the patients included in the 9 eligible studies, the treatment and the control groups comprised 356 patients and 362 patients, respectively. A total of 305 patients received the placebo, and the remaining 57 patients received a low dose of vitamin D supplementation in the control group. The patients in the treatment group received a high dosage of vitamin D supplementation (range, 2,857–14,007 IU per day). The vitamin D treatment ranged from a duration of 24–96 weeks. The primary characteristics of the patients included in the eligible studies are shown in Table 1.
Number of Patients Who Experienced Relapses during Research
In this meta-analysis, the number of patients who received different administration time (Fig. 2) and different administration dosage (Fig. 3) of vitamin D supplementation in the treatment group had no significant difference in relapse when compared with the controls (Fig. 4). Besides whether the low dose of vitamin D was added or not (placebo) to the treatment of the control patients, the vitamin D supplementation did not show a therapeutic effect on patients with MS according to relapses during this research (Fig. 5).
After treating with different administration time (Fig. 6) and different administration dosage (Fig. 7) of vitamin D supplementation in patients with MS, the pooled results demonstrated that vitamin D supplementation as an adjuvant treatment had no significant therapeutic effect on MS according to the EDSS scores (Fig. 8). Similarly, control patients who received a placebo or a low dose of vitamin D showed no significant reduced EDSS scores when compared with the outcomes of the treatment group (Fig. 9).
This meta-analysis initially displayed that the vitamin D supplementation as an adjuvant treatment ranging from 2,857 to 14,007 IU/day and therapy duration from 6 to 24 months did not affect the clinical outcomes (EDSS and relapses during research) in patients with MS. Furthermore, despite the administration of placebo or a low dose of vitamin D supplementation in the controls, the pooled results demonstrated that receiving vitamin D supplementation had no significant therapeutic effect on patients with MS according to the EDSS scores and their relapses during research.
In previous systematic reviews, McLaughlin et al.  summarized 12 relevant studies and revealed no difference in the 4 outcomes (ARR, EDSS, new gadolinium-enhancing lesions, and new T2 MRI lesions) after vitamin D supplementation treatment in patients with MS. Even worse, a high dose of vitamin D increased the ARR significantly when compared with low-dose supplementation. These results were consistent with the outcomes observed by Zheng et al.  but contrary to what Martínez-Lapiscina et al.  observed in ARR. We speculated that the reason for this discrepancy may be related to the non-RCTs included in the latter study. Our meta-analysis initially reported the effect of different administration time and dosage of vitamin D supplementation on clinical outcomes for patients with MS. Similar to our outcomes, all of these 4 meta-analyses [24-27] indicated no significant association between the EDSS scores and the patients with MS treated with vitamin D supplementation. Effective therapy may need to extend the period and add the dose of vitamin D supplementation during treatment.
Regarding the adverse effects of vitamin D supplementation in the eligible studies, 5 of 9 studies have reported adverse effects to some extent. Hupperts et al.  found that the incidence of treatment-emergent adverse events (TEAEs) was 87.6% in the high dose of vitamin D3 group and 80.2% in the placebo group. The most common TEAEs were headache, nasopharyngitis, influenza, and influenza-like illness. The incidence rates of severe TEAEs were similar between the treatment and the control groups. The severe TEAEs might be related to a potentially higher dose of vitamin D3 intake. Camu et al.  recorded a lower incidence rate (61.9%) than Hupperts et al.  observed in the subjects who suffered at least one adverse event. Nine subjects discontinued treatment because of severe or life-threatening TEAEs. Soilu-Hänninen et al.  detected patients with the following TEAEs: diarrhea and fever, with serious TEAEs of erysipelas in the interferon injection site, elective hip surgery, and elbow fracture. Burton et al.  recorded rare adverse events such as chronic pelviectasis without nephrolithiasis in treatment patients. Additionally, researchers found some overlapped symptoms in progressive MS and vitamin D intoxication [33-35]. Therefore, they indicated that the vitamin D toxicity may mimic progressive MS .
There are several limitations in this meta-analysis. First, the low samples of eligible studies may increase the random error, and we cannot rule out publication bias. Second, because of a different focus on the end points among the included studies, the effect of vitamin D supplementation on laboratory and radiological outcomes warrants further investigation. Third, a subgroup analysis for different lifestyle, ages, sex, country, season, and cosupplementation as well as race and ethnicity was not performed because of the lack of detailed information in the included studies. Fourth, another potential limitation is the heterogeneity and the imprecision of the results in some of the RCTs; therefore, the results should be applied with caution.
This meta-analysis initially showed that the vitamin D supplementation as adjuvant treatment ranged from 2,857 to 14,007 IU/day and treatment duration from 6 to 24 months did not affect the clinical outcomes (EDSS and relapses during research) in patients with MS, according to publications thus far. Therefore, we suggest that the longer duration and larger dosage of vitamin D without adverse effects might be further explored in RCTs.
We thank Prof. Fuqiang Guo for technical help.
Statement of Ethics
Ethical review and approval was not required as the study is based exclusively on published literature.
Conflict of Interest Statement
The authors declare that there are no conflicts of interest.
This study was funded by Key Research and Development Program, Science & Technology Department of Sichuan Province (grant number: 2017SZ0007).
Jie Huang contributed to the conception of the study and revised the manuscript; Xiaofan Yuan and Lei Guo contributed to manuscript preparation, performed the study and wrote the manuscript; Chuan Jiang performed the data analyses; Xu Yang helped perform the analysis with constructive discussions.
Xiaofan Yuan and Lei Guo contributed equally in this study.