Systematic Review

Citation: Abuelazm, M.; Muhammad, S.; Gamal, M.; Labieb, F.; Amin, M.A.; Abdelazeem, B.; Brašic´, J.R. The Effect of Vitamin D Supplementation on the Severity of Symptoms and the Quality of Life in Irritable Bowel Syndrome Patients: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Nutrients 2022, 14, 2618. https:// doi.org/10.3390/nu14132618

Academic Editor: Bruce W. Hollis

Received: 19 May 2022 Accepted: 10 June 2022 Published: 24 June 2022

Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

1. 1 Faculty of Medicine, Tanta University, Tanta 31527, Egypt; [email protected] (M.A.); [email protected] (M.G.)
2. 2 Department of Internal Medicine, Gulab Devi Hospital, Lahore 54000, Pakistan; [email protected]
3. 3 Faculty of Medicine, Beni-Suef University, Beni-Suef 62511, Egypt; [email protected]
4. 4 Faculty of Medicine, Cairo University, Cairo 12613, Egypt; [email protected]
5. 5 Department of Internal Medicine, McLaren Health Care, Flint, MI 48532, USA; [email protected]
6. 6 Department of Internal Medicine, Michigan State University, East Lansing, MI 48823, USA
7. 7 Section of High Resolution Brain Positron Emission Tomography Imaging, Division of Nuclear Medicine and Molecular Imaging, The Russell H. Morgan Department of Radiology and Radiological Science, The Johns Hopkins University School of Medicine, Baltimore, MD 21287, USA

* Correspondence: [email protected]; Tel.: +1-443-610-0709

Abstract: Irritable bowel syndrome (IBS), a gastrointestinal disorder affecting 7–12% of the population, is characterized by abdominal pain, bloating, and alternating bowel patterns. Data on risk and protective influences have yielded conflicting evidence on the effects of alternative interventions, such as vitamin D. This review focuses on the effects of vitamin D on IBS. A systematic review and meta-analysis considered all articles published until 4 April 2022. The search for randomized controlled trials assessing vitamin D efficacy in IBS with outcomes, primary (Irritable Bowel Severity Scoring System (IBS-SSS)) and secondary (IBS quality of life (IBS-QoL) and serum level of calcifediol (25(OH)D)), was performed on six databases, Google Scholar, Web of Science, SCOPUS, EMBASE, PubMed (MEDLINE), and Cochrane Central Register of Controlled Trials. We included six trials with 616 patients. The pooled analysis found no difference between vitamin D and placebo in improving IBS-SSS (MD: −45.82 with 95% CI [−93.62, 1.98], p = 0.06). However, the pooled analysis favored vitamin D over placebo in improving the IBS-Qol (MD: 6.19 with 95% CI [0.35, 12.03], p = 0.04) and serum 25(OH)D (MD: 25.2 with 95% CI [18.41, 31.98], p = 0.00001). Therefore, further clinical trials are required to reach clinically applicable and generalizable findings.

Keywords: alternative intervention; calciferol; confidence interval; flow chart; gastrointestinal disorder; heterogeneity; mean difference; placebo; protocol; random

Irritable bowel syndrome (IBS), a common gastrointestinal condition affecting 7–12% of the general population [1], is characterized by fluctuating severity of symptoms, including abdominal discomfort, pain, bloating, and alternating bowel habits. Although the pathogenesis is unclear, an association with psychological stress has long been identified, ultimately causing disturbed social functions and poor quality of life [2]. Despite the significant burden of the disease, no treatment has proven to be entirely effective [3].

Given the lack of established therapies, a large proportion of the patients do not show improvement on conventional treatments [4]. Therefore, around one-third of the patients opt for alternative therapies [5]. Since vitamin D deficiency is a common condition [6–9]

Nutrients 2022, 14, 2618. https://doi.org/10.3390/nu14132618 https://www.mdpi.com/journal/nutrients

associated with cardiovascular disease (CVD) [10], cancer [11], insulin resistance [12–14], and other chronic diseases [15], investigators have sought to identify possible associations of vitamin D deficiency with IBS. Therefore, studies have shown an improvement of IBS symptoms with the administration of vitamin D [16,17], an agent playing a critical role in calcium and phosphorous metabolism and homeostasis as well as anti-inflammatory and immunomodulatory activities [18].

Worldwide, a 30–50% prevalence for vitamin D deficiency has been estimated across different demographic groups [19]. Since vitamin D is essential in maintaining mucosal surfaces like the intestinal mucosal barrier [20], vitamin D deficiency presents with gastrointestinal symptoms led by mucosal damage [20,21]. A comparative case-control study demonstrated a significant relationship between vitamin D deficiency and IBS with vitamin D deficiency in 82% of the IBS patients compared to 32% of healthy participants [22]. Additionally, several studies report vitamin D deficiency in people with IBS [23,24]. Furthermore, various psychiatric disorders such as depression and anxiety are associated with vitamin D deficiency, which in turn shows an association with IBS [25,26].

Accordingly, the small number of randomized controlled trials (RCTs) to evaluate the effect of vitamin D in IBS patients yield contradictory findings [27–32]. Vitamin D supplementation randomized controlled trials (RCTs) demonstrated improved IBS severity of symptoms (IBS-SSS) in some RCTs [27,28,30,32], but not in others [29,31], and improved IBS quality of life (IBS-QoL) in some RCTs (27–29), but not in another [31]. Therefore, in order to resolve these controversies, we conducted a systematic review and meta-analysis to evaluate the effect of vitamin D supplementations on the severity of symptoms and the quality of life in IBS patients.

1. 2.1. Protocol Registration
2. 2.2. Data Sources & Search Strategy
3. 2.3. Eligibility Criteria

We included RCTs with the following PICO criteria: Population (P): irritable bowel syndrome (IBS) patients regardless of disease type, grade, and serum vitamin D baseline level; Intervention (I): vitamin D supplementations regardless of dose and treatment duration; Control (C): placebo; Outcomes (O): primary outcome: IBS severity scoring system (IBS-SSS) [35]. Our secondary outcomes are quality of life assessed by a selfreported specific IBS-related quality of life questionnaire (IBS-Qol) [36] and serum level of calcifediol (25(OH)D) (https://pubchem.ncbi.nlm.nih.gov/substance/53789608) (accessed on 1 June 2022).

The IBS-SSS is a questionnaire validated for application for IBS patients to determine the burden of the disease; during 10 days, IBS-SSS assesses the following (abdominal pain severity, abdominal pain frequency, abdominal distention or tightness severity, bowel habits dissatisfaction, interference with life in general). Each item is scored on a scale from 0 to 100 and with a range of (0–500). Mild, moderate, and severe cases are indicated by scores of 75–175, 175–300, and >300, respectively, and a score reduction of 50 or more is considered clinically significant [35]. The (IBS-Qol) questionnaire included dysphoria, interference with activity, body image, health worry, food avoidance, social reaction, sexual, and other

issues among eight sub-scales and 34 items with a score range from 0 to 100; the higher the score the better Qol [36].

The exclusion criteria involved pilot studies, animal studies, in vitro studies (tissue and culture studies), observational studies (case-control, cross-sectional, case series, and report), press articles, editorial letters, conference abstracts, registered protocols, and book chapters.

Table 1. Search terms and results in different databases.

Database Search Terms Search Field Search Results

(“Vitamin D” OR “Cholecalciferol” OR “Hydroxycholecalciferols” OR “Ergocalciferols” OR “25 Hydroxyvitamin D” OR “Dihydrotachysterol” OR “25(OH)D” OR “25-hydroxyvitamin D” OR

calcifediol OR calciferol OR “Vitamin D”) AND (“Colonic Diseases, Functional”[Mesh] OR IBS OR irritable bowel syndrome OR”functional abdominal pain” OR “functional gastrointestinal” OR FGID OR “irritable colon” OR Colitis, Mucous OR Colitides, Mucous OR Mucous Colitides OR Mucous Colitis)

PubMed

All Field 137

((Vitamin D) OR (Cholecalciferol) OR (Hydroxycholecalciferols) OR (Ergocalciferols) OR (25 Hydroxyvitamin D) (Word variations have been searched)) AND ((irritable bowel syndrome) OR (IBS) OR (functional abdominal pain) OR (Functional Colonic Diseases) OR (irritable colon) (Word variations have been searched))

Cochrane

All Field 127

(“Vitamin D” OR “Cholecalciferol” OR “Hydroxycholecalciferols” OR “Ergocalciferols” OR “25 Hydroxyvitamin D” OR “Dihydrotachysterol” OR “25(OH)D” OR “25-hydroxyvitamin D” OR

calcifediol OR calciferol OR “Vitamin D”) AND (“Colonic Diseases, Functional”[Mesh] OR IBS OR irritable bowel syndrome OR “functional abdominal pain” OR “functional gastrointestinal” OR FGID OR “irritable colon” OR Colitis, Mucous OR Colitides, Mucous OR Mucous Colitides OR Mucous Colitis)

WOS

All Field 196

(TITLE-ABSKEY ((vitamin AND d) OR (cholecalciferol) OR (hydroxycholecalciferols) OR (ergocalciferols) OR (25 hydroxyvitamin AND d) OR (dihydrotachysterol) OR (25(OH)D) OR (25-hydroxyvitamin AND d) OR (calcifediol) OR (calciferol) OR (vitamin AND d)) AND TITLE-ABS-KEY ((functional AND colonic AND diseases) OR (irritable AND bowel AND syndrome) OR (ibs) OR (functional AND abdominal AND pain) OR (functional AND gastrointestinal) OR (fgid) OR (mucous AND colitides) OR (mucous AND colitis)) AND (LIMIT-TO (DOCTYPE, “ar”))

Title, Abstract, Keywords

879

SCOPUS

(“vitamin deficiency”/exp OR “vitamin deficiency” OR cholecalciferol OR hydroxycholecalciferols OR ergocalciferols OR (25 AND hydroxyvitamin AND d) OR dihydrotachysterol OR (25 AND oh AND d) OR (“25 hydroxyvitamin” AND d) OR calcifediol OR calciferol OR (vitamin AND d)) AND (“irritable colon”/exp OR “irritable colon” OR ibs OR (functional AND abdominal AND pain) OR (functional AND gastrointestinal) OR fgid OR (irritable AND colon) OR (mucous AND colitis) OR (mucous AND colitides)) AND “randomized controlled trial”/de

All Field 88

EMBASE

(“Vitamin D” OR Cholecalciferol OR Hydroxycholecalciferol OR Ergocalciferol OR 25 “Hydroxyvitamin D”) AND (“irritable bowel syndrome” OR IBS OR “functional abdominal pain” OR “Functional Colonic Diseases” OR “irritable colon”)

Google Scholar

All Field 1430 Ext. (first 200 only)

1. 2.4. Study Selection

Using Covidence online software [37], two reviewers (M.G. and F.L.) independently assessed the titles and abstracts of the retrieved records and then the full-text articles for the previous eligibility criteria. A third reviewer (M.A.) resolved any conflicts. The selection process is demonstrated in a PRISMA flow chart [33] (Figure 1).

Figure 1. PRISMA flow chart of the screening process [33].

1. 2.5. Data Extraction
2. 2.6. Risk of Bias and Quality Assessment

1. 2.5. Data Extraction
2. 2.6. Risk of Bias and Quality Assessment

Two reviewers (M.A.A. and F.L.) independently assessed the included studies for the risk of bias (ROB) using The Cochrane Collaboration’s tool for assessing the risk of bias in randomized trials [39]), based on the following domains: random sequence generation (selection bias), allocation concealment (selection bias), blinding of participants and personnel (performance bias), blinding of outcome assessment (detection bias), incomplete outcome data (attrition bias), selective reporting (reporting bias), and other potential sources of bias.

Two reviewers (M.A.A. and F.L.) independently assessed the included studies for the risk of bias (ROB) using The Cochrane Collaboration’s tool for assessing the risk of bias in

bias),

Conflicts were resolved by discussion. Two reviewers (M.A.A. and F.L.) used the Grading of Recommendations Assessment, Development and Evaluation (GRADE) Working Group recommendation [40–42] for quality of evidence assessment. We considered inconsistency, imprecision, indirectness, publication bias, and risk of bias. Our conclusions on the quality of evidence were justified, recorded, and included in the results reporting for each outcome. A third reviewer (M.A.) resolved any conflicts.

1. 2.7. Statistical Analysis

The statistical analysis was conducted using RevMan v5.3 software [43]. The effects of vitamin D supplementations on the continuous outcome variables were estimated by comparing the pooled MD and SD of changes before and after the treatment in the treatment group with those in the control group. The pooled MD was synthesized using the Mantel–Haenszel method. All data were presented with the p-value and corresponding 95% confidence interval (CI), and a p-value < 0.05 was considered statistically significant. We assessed heterogeneity by visual inspection of the forest plots and evaluated by I-square and chi-square tests. The chi-square test determines whether there is significant heterogeneity. In contrast, the I-square evaluates the magnitude of heterogeneity. According to the Cochrane Handbook (chapter nine) [34], an alpha level below 0.1 is considered to be a significant heterogeneity (for the chi-square test), and the I-square test is interpreted as follows: (0–40%: might not be important; 30–60%: may represent moderate heterogeneity; 50–90%: may represent substantial heterogeneity). We used the random effects model in case of significant heterogeneity and the fixed effects model otherwise.

We excluded one study at a time and repeated the analysis to perform the sensitivity analysis to assess the impact of each study on the overall study effects size of the outcome. In addition, we performed a sub-group analysis based on the vitamin D dose and diagnostic criteria to test the stability of our results. We did not perform funnel plots to indicate the publication bias because we included less than ten studies [44].

1. 3.1. Search Results and Study Selection
2. 3.2. Characteristics of Included Studies
3. 3.3. Risk of Bias and Quality of Evidence

All the included trials were low risk of selection, performance, and detection biases. Five trials [28–32] exhibited a low risk of attrition bias, but one [27] showed a high risk of attrition bias due to unbalanced and unexplained loss of follow-up between the two arms. Four trials [27,29–31] exhibited a low risk of reporting and other biases, but two trials [28,32] were unclear due to the lack of published protocol (Figure 2). The quality of evidence using the GRADE system [40–42] is demonstrated in Table 3.

Table 2. Characteristics of the included studies.

Vitamin D Placebo Number

Follow-up Duration (Months)

Baseline Serum Vitamin D Mean (SD)

Baseline Serum Vitamin D Mean (SD)

IBS Sub-type

Study ID Country Study Design Total Participants

Female n (%)

Age (Years) Mean (SD)

Female n (%)

Age (Years) Mean (SD)

Dose Number

BS-D IBS-A IBS-C

Abbasnezhad et al., 2016 [27]

Single-center double blinded RCT

50,000 IU fortnightly

41 29 (70.7) 38.45 (9.85) 18.62 (11.23) Zeid et al., 2020 [32]

Iran

85

6 44 28 (63.6) 37.45 (8.11) 19.65 (10.35)

Single-center double blinded RCT

4000 IU daily

40 N/A 38.03 (6.37) N/A Williams et al., 2021 [31]

Egypt

80 N/A 3 40 N/A 37.64 (11.13) N/A

Single-center double blinded RCT

3000 IU daily

67 51 (76.1) 31.1 (10.85) 49.71 (27.05) Sikaroudi et al., 2020 [30]

UK

135 N/A 3 68 55 (80.9) 28.94 (10.03) 48.75 (27.91)

Single-center double blinded RCT

50,000 IU weekly

44 22 (50) 35.61 (8) 17.83 (7.84) Jalili et al., 2019 [29]

Iran

88 IBS-D 2 44 25 (56.8) 35.07 (11.73) 17.68 (7.69)

Multi-center Double blinded RCT

50,000 IU weekly

Iran

116 N/A 1.5 58 58 (100) 52.24 (12.26) N/A

58 58 (100) 40.06 (13.37) N/A

IBS-C IBS-U IBS-M IBS-D

El Amrousy et al., 2018 [28]

Single-center double blinded RCT

2000 IU daily

Egypt

112

6 56 29 (52) 16.4 (1.5) 17.2 (1.3)

56 33 (59) 16.2 (1.1) 17.5 (1.1)

RCT: randomized controlled trial, SD: standard deviation, N/A: not available; IU: international unit.

Figure 2. Quality assessment of risk of bias in the studies in the meta-analysis. (A) The upper panel presents a schematic representation of risks (low = red, unclear = yellow, and high = red) for specific types of biases of each of the studies in the review [27–32]. (B) The lower panel presents risks (low = red, unclear = yellow, and high = red) for the sub-types of biases of the combination of studies included in this review [43].

Table 3. GRADE evidence profile: vitamin D compared to placebo for IBS.

Certainty Assessment No. of Patients Effect Certainty Importance No. of Studies

Study Design

Other Considerations

Relative (95% CI)

Absolute (95% CI)

Risk of Bias Inconsistency Indirectness Imprecision

Vitamin D Placebo

IBS-SSS 6

randomized trials

not serious

very serious a

MD 45.86 lower (93.65 lower to 1.93 higher)

not serious very serious b none 310 306 -

Critical IBS-Qol

Very low

randomized trials

MD 6.19 higher (0.35 higher to 12.03 higher)

serious c very serious a not serious serious d none 226 222 -

4

Important Serum 25(OH)D

Very low

randomized trials

not serious

very strong association

MD 30.03 higher (20.72 higher to 39.34 higher)

very serious a serious e not serious

4

168 164 -

Important

Moderate

CI: confidence interval; MD: mean difference; ⊕: positive evidence of certainty. a I square test is more than 60 percent. b The 95% confidence interval does not exclude the null hypothesis of 0 mean difference. c Abbasnezhad et al. [27] is at a high risk of attrition bias and represents 42 percent of the pooled analysis weight. d The 95% confidence interval (CI) does not exclude the MD of a 0.5 with a very wide CI. e Despite the large effect and significant CI the total number of participants is less than 400.

3.4. Primary Outcome IBS-SSS

3.4. Primary Outcome IBS-SSS

The pooled mean difference showed no difference between vitamin D and placebo (MD: −45.82 with 95% CI [−93.62, 1.98], p = 0.06) (very low-quality evidence). Pooled studies were not homogenous (p = 0.00001, I-square = 95%) (Figure 3A, Table 3). To resolve heterogeneity, we conducted a sensitivity analysis, excluding one study in each scenario. However, heterogeneity was not resolved by sensitivity analysis (Table 4). Sensitivity analysis showed similar effect except after removing the study of Jalili et al., 2019 [29] and Williams et al., 2021 [31]; the pooled mean difference favored vitamin D over placebo ((MD: −59.82 with 95% CI [−111.85, −7.79], p = 0.02)–(MD: -57.82 with 95% CI [−110.94, −4.71], p = 0.03)), respectively (Table 4).

The pooled mean difference showed no difference between vitamin D and placebo (MD: −45.82 with 95% CI [−93.62, 1.98], p = 0.06) (very low-quality evidence). Pooled studies were not homogenous (p = 0.00001, I-square = 95%) (Figure 3A, Table 3). To resolve heterogeneity, we conducted a sensitivity analysis, excluding one study in each scenario. However, heterogeneity was not resolved by sensitivity analysis (Table 4). Sensitivity analysis showed similar effect except after removing the study of Jalili et al., 2019 [29] and Williams et al., 2021 [31]; the pooled mean difference favored vitamin D over placebo ((MD: −59.82 with 95% CI [−111.85, −7.79], p = 0.02)–(MD: -57.82 with 95% CI [−110.94, −4.71], p = 0.03)), respectively (Table 4).

Figure 3. Forest plot of the included outcomes ((A)—IBS-SSS, (B)—IBS-Qol, (C)—serum 25(OH)D). I2: I-squared; CI: confidence interval [27–32,43].

Figure 3. Forest plot of the included outcomes ((A)—IBS-SSS, (B)—IBS-Qol, (C)—serum 25(OH)D). I2: I-squared; CI: confidence interval [27–32,43].

We conducted ag-group analysis based on the following: A- Vitamin D3 dosage, to evaluate the effect of 50.000 IU vitamin D3 dose and dosage less than 50.000 IU vitamin D3 on IBS-SSS; pooled mean difference showed no difference between vitamin D and placebo ((MD:−24.2 with 95% CI [−65.79, 17.39], p = 0.25)–(MD: −67.07 with 95% CI [−164.66, 30.53], p = 0.18)) respectively (Figure 4A). B- IBS diagnostic criteria, pooled mean difference showed no difference between vitamin D and placebo either in IBS patients diagnosed with ROME III criteria (MD: −26.18 with 95% CI [−65.96, 13.61], p = 0.2) or ROME IV criteria (MD: −66.08 with 95% CI [−170.59, 38.42], p = 0.22) (Figure 4B).

Table 4. Sensitivity analysis. No. of王 participants (Vitamin D/Placebo)

Heterogeneity Analysis

No. of王 Trials

Quantitative Data Synthesis

MD 95% CI Z Value p-Value df p-Value I2 (%) IBS-SSS

310/306 6 −45.82 [−93.62, 1.98] 1.88 0.06 5 0.00001 95

[−118.33,

1.3 0.19

266/265 5 −47.23

Table 4. Sensitivity analysis.

No. of participants (Vitamin D/Placebo)

Quantitative Data Synthesis Heterogeneity Analysis MD 95% CI Z Value p-Value df p-Value I2 (%) IBS-SSS

No. of Trials

Outcome

All studies 310/306 6 −45.82 [−93.62, 1.98] 1.88 0.06 5 0.00001 95 Omitting Abbasnezhad et al., 2016 [27]

266/265 5 −47.23 [−118.33, 23.88] 1.3 0.19 4 0.00001 95 Omitting El Amrousy et al., 2018 [28]

254/250 5 42.28 [ 99.7, 15.13] 1.44 0.15 4 0.00001 96 Omitting Jalili et al., 2019 [29]

252/248 5 −59.82 [−111.85, −7.79] 2.25 0.02 4 0.00001 95 Omitting Sikaroudi et al., 2020 [30]

266/262 5 43.01 [ 98.52, 12.49] 1.52 0.13 4 0.00001 96 Omitting Williams et al., 2021 [31]

242/239 5 −57.82 [−110.94, −4.71] 2.13 0.03 4 0.00001 95 Omitting Zeid et al., 2020 [32]

270/266 5 24.33 [ 54.85, 6.19] 1.56 0.12 4 0.0003 81

All studies 226/222 4 6.19 [0.35, 12.03] 2.08 0.04 3 0.03 66 Omitting Abbasnezhad et al., 2016 [27]

182/181 3 8.23 [0.15, 16.31] 2 0.05 2 0.13 51 Omitting El Amrousy et al., 2018 [28]

170/166 3 3.26 [2.14, 4.39] 5.67 0.00001 2 0.98 0 Omitting Jalili et al., 2019 [29]

168/164 3 6.41 [−0.37, 13.18] 1.85 0.06 2 0.01 77 Omitting Williams et al., 2021 [31]

158/155 3 7.41 [ 0.96, 15.77] 1.73 0.08 2 0.01 77

All studies 226/222 4 25.20 [18.41, 31.98] 7.28 0.00001 3 0.00001 93 Omitting Abbasnezhad et al., 2016 [27]

182/181 3 22.58 [15.39, 29.76] 6.16 0.00001 2 0.0002 88 Omitting El Amrousy et al., 2018 [28]

170/166 3 28.16 [14.87, 41.46] 4.15 0.0001 2 0.00001 95 Omitting Williams et al., 2021 [31]

158/155 3 22.54 [15.80, 29.29] 6.55 0.00001 2 0.00001 94 Omitting Jalili et al., 2019 [29]

168/164 3 29.33 [20.29, 38.38] 6.36 0.00001 2 0.00001 94

CI: confidence interval; df: degrees of freedom; MD: mean difference.

Nutrients 2022, 14, x FOR PEER REVIEW 12 of 19

Forest plot of the sub-group analysis ((A)—dosage sub-group analysis, (B

1. Figure 4. Forest plot of the sub-group analysis ((A)—dosage sub-group analysis, (B)—diagnostic criteria sub-group analysis) I2: I-squared; CI: confidence interval [27–32,43].
2. Figure 5.Figure 5. Forest plot of IBS-Qol after exclusion El Amrousy et al. [Forest plot of IBS-Qol after exclusion El Amrousy et al. [27]. I2: I-squared; CI: confidence27]. I2: I-squared; CI: confidence interval.

Figure 4. Forest plot of the sub-group analysis ((A)—dosage sub-group analysis, (B)—diagnostic criteria sub-group analysis) I2: I-squared; CI: confidence interval [27–32,43].

3.5. Secondary Outcomes 3.5.1. IBS-Qol

The pooled mean difference favored vitamin D over placebo (MD: 6.19 with 95% CI [0.35, 12.03], p = 0.04) (very low-quality evidence). Pooled studies were not homogenous (p = 0.03, I-square = 66%). (Figure 3B, Table 3) To resolve heterogeneity, we conducted a sensitivity analysis, excluding one study in each scenario. Heterogeneity was best resolved by excluding the study of El Amrousy et al. [27] (p = 0.98, I-square = 0%). The pooled mean difference favored vitamin D over placebo (MD: 3.26 with 95% CI [2.14, 4.39], p = 0.00001) (Figure 5, Table 4).

p (p

p

p

1. 3.5.2. Serum 25(OH)D

The pooled mean difference favored vitamin D over placebo (MD: 25.2 with 95% CI [18.41, 31.98], p = 0.00001) (moderate-quality evidence). Pooled studies were not homogenous (p = 0.00001, I-square = 93%). (Figure 3C, Table 3) To resolve heterogeneity, we conducted a sensitivity analysis by excluding one study in each scenario. However, heterogeneity was not resolved by sensitivity analysis (Table 4).

In our meta-analysis, we evaluated the efficacy of vitamin D supplementation to determine whether it causes any improvements in symptoms of IBS. Our analysis showed that vitamin D supplementation failed to improve IBS symptoms. However, the IBS-QoL score was improved after the treatment with vitamin D.

Our analysis found no difference between vitamin D and placebo in improving IBSSSS. However, four RCTs [27,28,30,32], two in Iran and two in Egypt, showed an improvement in IBS symptoms, while two RCTs [29,31] showed no preference for vitamin D over the placebo. Our dosage sub-group analysis found no difference between high dosages (50,000 IU) and lower dosages (less than 50,000 IU). This surprising finding indicates no dose-response relationship between vitamin D and reducing IBS-SSS. However, the treatment duration of the RCTs that used both dosages varies, which may significantly impact our findings. Regarding IBS-Qol, our pooled analysis favored vitamin D over placebo. Our results are consistent with three RCTs [27–29] while only one study by Williams et al. [31] showed no improvement in IBS-QoL.

Moreover, pooled analysis favored vitamin D over placebo regarding increasing the serum level of 25(OH)D. In all the included studies, the serum level of vitamin D was higher in the vitamin D group compared to the placebo. Williams et al. [31] showed no difference in QoL between the vitamin D group and placebo. The contradictory results of Williams et al. [31] regarding IBS-SSS and IBS-Qol can be attributed to a few critical differences in the study design that set it apart from the rest of the included RCTs. First, Williams et al. [31] used community-based sampling rather than outpatient and institutionbased selection in other RCTs. Second, Williams et al. [31] had the largest population size compared to the rest of the studies. Finally, the regional difference might have also played a role as all the vitamin D supporting studies were based in the Middle East, compared to the UK-based RCT by Williams et al. [31]. Moreover, Jalili et al. [29] failed to show any preference for vitamin D administration in improving IBS-SSS. This can be explained by the short duration of vitamin D treatment [29]. Therefore, after excluding Williams et al. [31] or Jalili et al. [29], our pooled analysis advocated using vitamin D over the placebo.

The reason behind the effect of vitamin D on symptoms and quality of life in IBS patients is still to be investigated. To clarify, IBS is known to have complex pathophysiology, and both peripheral and central factors have been suggested to play critical roles [45]. Vitamin D can impact the gastrointestinal system’s health due to its immunomodulatory characteristics [3]. In the pathogenesis of IBS, immunological and inflammatory roles are of particular importance as literature has emphasized the activation of the inflammatory mediator to have a crucial part in the IBS development [46]. The upregulated mast cells, Tcells, and other pro-inflammatory cytokines are among the few known key players [2,45,47]. The increased inflammation upregulates the neural activity in the intestine leading to visceral hypersensitivity and worsened feeling of abdominal pain [47]. Given the antiinflammatory effects of vitamin D, its usage can improve intestinal inflammation [48]. Therefore, decreased inflammation can improve the sensory nervous system in the gut, causing normalization of gut functionality and managing IBS symptoms [3]. This can be explained by the presence of vitamin D receptors in the nervous system, where they play a role in the synthesis, maintenance, and upregulation of neurotransmitters levels [49,50]. Accordingly, the administration of vitamin D can improve IBS symptoms, which is contradictory to our findings.

Another leading etiology of IBS is psychological factors. Anxiety and depression have long been implicated in the development of IBS, and studies have shown a direct connection between depression and the associated bowel-related symptoms [2]. Many patients with IBS reported the presence of some psychological symptoms before their IBS started [4]. Furthermore, psychological stress can cause alteration in bowel patterns, and the treatment of stress and anxiety show a remarkable improvement in IBS symptoms [2]. In addition, literature shows that patients with depression are typically vitamin D deficient; therefore, vitamin D administration can improve their depressive symptoms [49,51]. Hence, vitamin D supplementation also applies in managing this etiology and improving the quality of life in accordance with our findings.

Regarding the safety and tolerability of vitamin D in IBS, vitamin D is absorbed through the enterocyte’s apical membrane, and 25(OH)D levels increase slowly and reach the peak within 7 to 14 days [52]. Vitamin D supplementation in healthy individuals should be administrated with caution. The safe therapeutic limit of vitamin D intake is 4000 IU daily to prevent side effects [53]. Exogenous vitamin D can elevate the risk of hypercalcemia and hypercalciuria. It may also cause mineral deposition in soft tissues. Severe side effects may arise when long-term dosage exceeds ten-fold the endorsed quantity. Toxicity causing vomiting, lethargy, confusion, and arrhythmia have been reported in the literature [54]. Despite using very high intermittent doses of vitamin D (50,000 IU) in three of the included RCTs [27,29,30], no adverse effects were noted. The most likely reason behind this seems to be the short duration of treatment. The three RCTs [28,31,32] (two Egyptian and one UK-based) kept the vitamin D dosage within the suggested limits. Patients show excellent tolerance when the recommended dosage is used [52]. In addition, vitamin D supplements are available over the counter with an easily affordable price tag.

In a recent systematic review and meta-analysis, Chong et al. [55] reported that vitamin D significantly improved IBS-SSS but found no difference between vitamin D and placebo in improving IBS-Qol, which is contradictory to our findings. On the one hand, the difference in IBS-SSS outcome can be attributed to the fact that Chong et al. [55] included Tazzyman et al. [56] which is a pilot study that used IBS visual analogue score (VAS-IBS) and considered underpowered to provide significant findings, hence excluded from our analysis, and Jalili et al. [57] who compared soy isoflavones and/or vitamin D. However, vitamin D was administrated along with placebo to substitute soy isoflavones which can significantly impact the findings. To clarify, patients that receive a placebo have significantly improved findings compared to baseline because IBS patients’ mental status is significantly affected by the placebo given their expectations and desire to receive therapy are more crucial than the drug’s composition [30,58]. Hence, this was excluded from our analysis. On the other hand, the difference in IBS-Qol outcome can be attributed to an error in the analysis by Chong et al. [55]. To clarify, they miss-extracted the data of Williams et al. [31] by changing the values of IBS-Qol outcome numbers from negative to positive changing the direction of the effect of the Williams et al. [31] study and their pooled analysis. We clarified the methodological flaws of Chong et al. [55] in our recently published editorial [59]. In another recent review by Haung et al. [60], vitamin D was effective in improving IBS-SSS which is also contradictory to our findings. This difference is attributed to the narrow and defective searching technique excluding three includable studies [29,31,32] especially Williams et al. [31] and Jalili et al., 2019 [29] which showed no effect of vitamin D on IBS-SSS. To conclude, our review introduces more robust findings on the effect of vitamin D in IBS patients.

1. 4.1. Strengths

The major strength of our study is that we strictly followed the PRISMA statement [33] and the Cochrane Handbook of Systematic reviews and meta-analysis [34]. Moreover, we assessed the quality of evidence following GRADE guidelines [40–42] and prospectively registered and published our review protocol. Furthermore, we conducted a thor-

ough analysis including sub-group analysis and sensitivity analysis to test the stability of our results.

1. 4.2. Limitations
2. 4.3. Implications for Future Research

Future large RCTs are required to assess the following: First, the long-term sustained effect of vitamin D supplementations in IBS patients beyond 24 weeks. Second, future trials should assess the effect of vitamin D in different grades and severity of IBS because each sub-type is a complicated disorder with a different clinical presentation and IBS treatment is generally dependent on its sub-type. Third, future trials should assess vitamin D levels in the baseline and report their results for the vitamin D deficient patients separately because vitamin D deficiency is a major confounding variable, given the significant difference in IBS-Qol between the vitamin D deficient and replete patients, and the improvement in IBSQol can be attributed to IBS symptoms severity improvement [27]. Fourth, we recommend the usage of a uniform dosage of vitamin D rather than impractical higher dosages to reach optimal and clinically practical vitamin D doses. Finally, baseline assessment of psychiatric, dietary, and physical factors is recommended due to its great impact on IBS pathogenesis.

Our systematic review and meta-analysis found uncertain evidence on the efficacy of vitamin D in improving the severity of symptoms and the quality of life in irritable bowel syndrome patients. However, we found significant statistical improvement in the IBS-Qol. Therefore, treatment with 25(OH)D may be beneficial for people with IBS [61]. Hence, the clinical usability of this meta-analysis is limited, and high-quality RCTs with larger populations for optimal reviews [62] are required before the clinical application of vitamin D in IBS can be endorsed.

Author Contributions: Conceptualization, M.A. and B.A.; methodology, M.A., B.A., F.L., M.G. and M.A.A.; software, M.A. and B.A.; validation, M.A., B.A., F.L., M.G., M.A.A., S.M. and J.R.B.; formal analysis, M.A. and B.A.; investigation, F.L., M.G. and M.A.A.; data curation, F.L., M.G. and M.A.A.; writing—original draft preparation, M.A. and S.M.; writing—review and editing, M.A., B.A., F.L., M.G., M.A.A., S.M. and J.R.B.; supervision, B.A. and J.R.B.; project administration, B.A. All authors have read and agreed to the published version of the manuscript.

Funding: This research received no external funding. Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: The authors thank Welch Medical Library of the Johns Hopkins University School of Medicine, Baltimore, Maryland, USA, for providing access to publications. Conflicts of Interest: The authors declare no conflict of interest.