Multiple sclerosis (MS) is an inflammatory progressive disorder of the central nervous system, which affects women more than men [1]. The annual incidence of the disease has increased, and environmental and genetic factors are considered important in disease development [2-4].

Smoking, Epstein-Barr virus infection, habitat latitude, and vitamin D status are among the environmental factors associated with disease risk [5,6].

Previous studies have demonstrated that a longer duration of sunlight exposure, decreasing latitude of residence, and a vitamin-rich diet are associated with a lower risk of developing MS [7-9].

Vitamin D, which is a modulator of calcium and phosphorus, plays an important role in bone formation and maintenance as well as having anti-inflammatory, antiproliferation, and immune intonation functions [10]. The regulation of gene expression in immune cells is thought to explain vitamin D’s immune effects on cells [11].

Experimental studies have shown that vitamin D administration could slow down experimental autoimmune encephalomyelitis progression [12,13]. In humans, lower vitamin D status has been reported at the time of relapse in MS patients as compared with the relapse-free period [14-16]. Some previous studies showed that vitamin D supplements were not effective in reducing MS-related relapses [17,18], whereas others revealed that vitamin D supplements were helpful in reducing relapse rates and MRI lesions in MS patients [19,20].

A systematic review and meta-analysis conducted previously showed that vitamin D supplementation was not beneficial to control MS relapses (odds ratio 0.98, 95% CI 0.45-2.16) by including 5 randomized clinical trials (RCTs) [21].

The main goal of this systematic review and meta-analysis will be to evaluate the effectiveness of vitamin D supplementation on clinical and para-clinical outcomes in patients with multiple sclerosis

We will include RCTs, being single-blinded or double-blinded or open-label trials in which relapse rate was one of the main outcomes after vitamin D supplement therapy. The articles that had been published in the English language will be included.

Cohort studies, case-control studies, and any other types of studies will be excluded.

We will include studies with adult participants (aged more than 18 years) with the relapsing-remitting (RR) form of the disease (as we want to assess the effect of the intervention on relapse rate).

The control group should be patients who received placebo (color, shape, and smell similarity with the vitamin D supplements).

We considered interventions as vitamin D supplements (any doses) whereas controls should have received placebos.

The primary outcome assessment will be carried out by a report of relapse, which is characterized by attacks of new or increasing neurologic symptoms (such as double vision, blurred vision, numbness, the lack of balance, memory loss, muscle cramping secondary to spasticity, bladder, bowel, and sexual dysfunction, bilateral facial weakness or trigeminal neuralgia, nystagmus, and intention tremor, and heat intolerance). The secondary outcomes will be assessed by comparing the number of gadolinium-enhanced plaques before and after the treatment, level of cytokines (tumor necrosis factor and interleukins) before and after the treatment, and progression of disability measured with the Expanded Disability Status Scale (EDSS).

We will search PubMed, Scopus, EMBASE, CINAHL, Web of Science, Ovid, ProQuest, American College of Physicians Journal Club database, Health Technology

Assessment Database (The Cochrane Collaboration), and National Health System Economic Evaluation Database (The Cochrane Collaboration) and gray literature including reference of included studies and conference abstracts.

A search strategy was developed, and it will be used to search in all databases. The search keywords are as following:

In total, 2 independent researchers will independently assess the articles. All obtained articles will be screened by title and abstract, and selected articles will be considered as eligible, not eligible, or may be eligible.

Articles that are considered as not eligible by both researchers will be excluded and others will be searched for by obtaining the full text. Eligible and may be eligible articles will be assessed independently, and in the case of a disagreement, a session for solving the disagreement will be held. The researchers will extract the data from papers and in the case of incomplete/unclear information, surveys will be sent to the authors.

Data regarding the name of authors, year of publication, number of patients, journal title, date, demographic data, method of intervention, dose of vitamin D supplement, duration of the study, frequency of relapses during the study, mean vitamin D levels at baseline and at the end of the study, type of control, number of baseline and final gadolinium enhanced plaques in MRI, and mean levels of cytokines and EDSS at baseline and the end of the study will be extracted by 2 independent researchers and will be recorded.

On the basis of the Cochrane Collaboration Risk of Bias assessment tool, 2 researchers will assess the risk of bias in each study [22]. Thus, each article will be categorized into low risk, high risk, and unclear risk. In the case of a disagreement, a discussion to reach an agreement will be held.

When adequate studies are included, the meta-analysis will be performed by considering the relapse, the number of gadolinium-enhanced plaques, and the level of cytokines and disability (which is assessed by means of EDSS) as the main outcome. For all of the main outcomes, mean differences and standardized mean differences will be applied. In the case of missing SD or SE, it will be calculated directly using the data where possible. If adequate studies are retrieved, a subgroup analysis will be carried out.

A subgroup analysis will be carried out according to sex, age, and EDSS.