Vitamin D and Its Association with H. pylori Prevalence and Eradication: A Comprehensive Review

Taking into account previous data that sustain a relationship between vitamin D deficiency and higher H. pylori infection positivity rates, this review aims to assess the influence of vitamin D deficiency and/or insufficiency upon the prevalence of H. pylori infection and its eradication success. Three major databases were searched for articles that analyzed a relationship between vitamin D status and H. pylori infection. The literature search retrieved a total of 37 reports, after the article selection process. Hypovitaminosis D emerged as a potential risk factor for H. pylori infection, given the higher prevalence of vitamin D deficiency and/or insufficiency among H. pylori-positive subjects. Furthermore, the same type of micronutrient deficiency has been directly linked to H. pylori eradication failure. An inverse linear relationship between vitamin D status and gastric cancer risk exists, but the additional involvement of H. pylori in this correlation is still in question. The potential benefit of oral supplements in enhancing the success of classical therapeutic regimens of H. pylori still requires future research. Future population-based studies from larger geographical areas are warranted to address this subject in more depth.


Introduction
Vitamin D represents one of the essential micronutrients in the human body, the main function of which is to ensure calcium hemostasis and bone mineralization [1]. Available from both exogenous-namely dietary-and endogenous-synthetized after exposure to sunlight-sources, vitamin D is initially present in an inactivated form, either as ergocalciferol or as cholecalciferol [2]. The synthesis of the active form of vitamin D, calcitriol, requires two hydroxylations, which take place in the liver and kidney [3,4]. Calcitriol binds to vitamin D receptors (VDRs), which are found throughout the entire human body and present numerous binding sites as well. Hence, vitamin D regulates multiple biologic processes [3]. The omnipresence of VDRs seems to be one of the key elements through which vitamin D metabolites take part in immune defense. The attachment of pathogenassociated molecular patterns (PAMPs) to toll-like receptors (TLRs), which are activated inside the macrophages as a result of lipopolysacharide (LPS) recognition, triggers an increase in the number of VDRs and consequently enhances the conversion of circulating 25-hydroxyvitamin D into its active form, calcitriol [5,6]. Vitamin D is also involved in T-cell antigen receptor maturation and mediates T-cell immune response [7]. Moreover, 25hydroxyvitamin D sufficiency is mandatory for normal macrophage function and adequate The search retrieved a total of 465 records, which further translated into 319 result after the exclusion of duplicates. The article selection process has been represented i more detail in Figure 1, which complies to the PRISMA 2020 statement and its revise flow diagrams [24]. Four non-English language articles and one article without an availa ble abstract were initially excluded. After exclusion of experimental studies, meta-ana yses, review type articles, one case report and one editorial, the final selection pool con sisted of 37 articles, the primary or secondary objective of which was to assess a correlatio between vitamin D and H. pylori infection.

Vitamin D Status: A Possible Risk Factor for H. pylori Infection?
Among various dietary habits, decreased vitamin D intake has emerged as a possibl concern related to H. pylori infection [25]. Vitamin D deficiency, namely serum levels be low 20 ng/mL, poses an increased risk for H. pylori infection according to two studies con ducted on Turkish and Lebanese adult populations [26,27]. Moreover, an inverse linea correlation was established between vitamin D sufficiency and H. pylori infection preva lence, which suggested that adequate vitamin D levels might offer protection against H pylori [26]. Furthermore, one study conducted in Bahrain highlighted that, for each un decrease in serum vitamin D level, the risk of H. pylori infection increases by 1.1 [25]. In study conducted on an older population with sarcopenia, an inverse relationship betwee vitamin D deficiency and H. pylori was proposed, suggesting that H. pylori might actuall be the risk factor for vitamin D level decrease [28]. However, Bashir et al. proved on limited population sample that an 8-week supplementation regimen of high oral vitami D3 doses can significantly decrease H. pylori colonization of the gastric mucosa. Therefore vitamin D intake is probably influencing the prevalence of this particular infection [29]. Shafrir et al. found an inverse association between vitamin D serum levels and H. pylo infection and concluded that those individuals with vitamin D values below 20 ng/mL stan the best chances of acquiring this particular bacterial infection [23]. However, Han et al. re ported significantly lower serum vitamin D levels among their study group infected wit

Vitamin D Status: A Possible Risk Factor for H. pylori Infection?
Among various dietary habits, decreased vitamin D intake has emerged as a possible concern related to H. pylori infection [25]. Vitamin D deficiency, namely serum levels below 20 ng/mL, poses an increased risk for H. pylori infection according to two studies conducted on Turkish and Lebanese adult populations [26,27]. Moreover, an inverse linear correlation was established between vitamin D sufficiency and H. pylori infection prevalence, which suggested that adequate vitamin D levels might offer protection against H. pylori [26]. Furthermore, one study conducted in Bahrain highlighted that, for each unit decrease in serum vitamin D level, the risk of H. pylori infection increases by 1.1 [25]. In a study conducted on an older population with sarcopenia, an inverse relationship between vitamin D deficiency and H. pylori was proposed, suggesting that H. pylori might actually be the risk factor for vitamin D level decrease [28]. However, Bashir et al. proved on a limited population sample that an 8-week supplementation regimen of high oral vitamin D3 doses can significantly decrease H. pylori colonization of the gastric mucosa. Therefore, vitamin D intake is probably influencing the prevalence of this particular infection [29].
Shafrir et al. found an inverse association between vitamin D serum levels and H. pylori infection and concluded that those individuals with vitamin D values below 20 ng/mL stand the best chances of acquiring this particular bacterial infection [23]. However, Han et al. reported significantly lower serum vitamin D levels among their study group infected with H. pylori, despite including a significant number of patients with levels under 20 ng/mL among the control group as well [30]. Hence, a certain lower limit of serum vitamin D that increases the risk of H. pylori infection was put into question. Other authors proposed that vitamin D insufficiency, rather than deficiency, is the major risk factor for H. pylori infection [31]. In contrast, one study conducted in Iraq on obese women identified significant differences in terms of H. pylori serum antibody positivity only in patients with vitamin D deficiency, while the presence/absence rate of antibodies directed against the same bacterial infection was not influenced by vitamin D insufficiency [32].
Several studies failed to identify an association between serum vitamin D levels and H. pylori infection. One study that analyzed the impact of concomitant vitamin D deficiency and H. pylori infection upon the prevalence of metabolic syndrome concluded that vitamin D levels were similar among the enrolled individuals, independent of H. pylori infectious status. The authors reported that those subjects with both H. pylori infection and vitamin D deficiency were the most susceptible to developing metabolic syndrome [33]. Furthermore, among the study participants, who were obese and undergoing bariatric surgery, preoperative nutritional deficiencies such as hypoproteinemia, hypoalbuminemia, hypocalcemia, hypomagnesemia, zinc, copper, folate, or vitamin B12 or vitamin D deficits were encountered independently of H. pylori infectious status, confirmed through analysis of gastric mucosa specimens [34]. Contradictorily, another study conducted on obese individuals revealed that vitamin D levels might be higher in the study group infected with H. pylori. However, these differences did not reach statistical significance [35].
H. pylori co-infection in the setting of other infectious diseases or chronic conditions seems to be additionally associated with vitamin D deficiency. One study conducted on 800 dengue fever patients, divided into two equal groups based on H. pylori co-infection status, revealed that those individuals who were positive for the infection presented significantly lower vitamin D serum levels than dengue fever controls [36]. In patients with end-stage renal failure undergoing hemodialysis, a linear positive relationship between vitamin D levels and H. pylori immunoglobulin G (IgG) serum antibodies was discovered [37]. Moreover, Bener et al. revealed that, in patients with type 2 diabetes mellitus and high serum titers of H. pylori specific antibodies, vitamin D levels were significantly lower than in healthy controls [38]. Similarly, in patients with type 1 diabetes and a positive H. pylori stool antigen test, significantly lower values of vitamin D levels were reported [39].
In children, results have been scarce, contradictory, and did not match those obtained in adult populations. Agin et al. found an association between lower vitamin D levels and peptic ulcer in children, but concluded that vitamin D levels did not significantly impact H. pylori infection rates within their pediatric study group [40]. Moreover, Urganci et al. concluded that, in children with chronic gastritis, parameters of bone metabolism, such as serum vitamin D, magnesium, calcium, phosphorus or parathormone levels, are similar, regardless of gastro-duodenal H. pylori colonization [41]. One study suggested that vitamin D deficiency might represent a risk factor for H. pylori infection in infants and toddlers. This study assessed H. pylori infection based on serum antibody positivity, which cannot distinguish present from past H. pylori infection [42]. Another study, in which a histology based-diagnosis of H. pylori was performed, revealed that vitamin D levels are significantly higher in children with with non-H. pylori gastro-duodenal conditions, as opposed to infected individuals. Moreover, the same study identified an inverse association between vitamin D levels and CagA positive H. pylori strains, which are known to increase gastric cancer risk even further [43]. This finding was not confirmed within the general population of an adult study subgroup who took part in the National Health and Nutrition Examination Survey (NHANES) III, which described a lack of correlation between vitamin D status and CagA seropositivity. However, when separately analyzing a population sample of non-Hispanic whites, the authors identified the same vitamin D deficiency-Cag A seropositivity association as in the aforementioned pediatric study [44]. The studies described within this chapter, which analyzed the relationship between vitamin D serum levels and H. pylori, have been depicted in Table 1.

Vitamin D and Its Influence on H. pylori Eradication
Vitamin D deficiency has been regarded as a risk factor for H. pylori eradication failure [20,45]. The studies investigating this topic have been summarized in Table 2. Differences in the eradication rates of H. pylori positive patients have been linked to vitamin D sufficiency, which translated into serum levels exceeding at least 20 ng/mL. Significantly lower mean serum vitamin D levels have been reported among patients with therapeutic failure [45]. Through their study, Shatla et al. also emphasized that vitamin D sufficiency is one of the key factors for H. pylori eradication success [20]. This theory was tested using a classical clarithromycin-based triple eradication therapy of 14 days duration [20]. Magsi et al. also reported an even higher, ascending discrepancy in vitamin D levels between H. pylori infected patients that responded to the same therapeutic regimen and those who did not [46]. Similar findings were reported by Almani et al. in their study, in which the identical, classical therapeutic scheme failed to eradicate H. pylori infection in more than 80% of patients with vitamin D levels below 30 ng/mL [47]. However, even when considering a modified, novel therapeutic approach consisting of bismuth potassium citrate, proton pump inhibitors (PPIs), amoxicillin, tetracycline, furazolidone, or levofloxacin, vitamin D levels under 20 ng/mL qualified as an independent risk factor for H. pylori eradication failure [48]. Therefore, the therapeutic hypothesis of adding vitamin D3 in conjunction with classical eradication schemes emerged. El Shahawy et al. are the first to support the addition of oral vitamin D 3 supplements to a classical regimen consisting of PPPIs, amoxicillin, and clarithromycin, and to report superior eradication rates of H. pylori [49].  Legend: H. pylori-Helicobacter pylori, PPI-proton pump inhibitor, RCT-randomized controlled trial. A lower vitamin D serum level of 10 ng/mL was proposed as referential for its impact on H. pylori eradication. In the study of Yildirim et al., over 80% of the patients in whom H. pylori eradication attempt was unsuccessful had serum vitamin D levels below 10 ng/mL [21]. Another study confirmed the very low H. pylori eradication rates in patients with vitamin D levels under 10 ng/mL and the best therapeutic success rates for populations with vitamin D serum values ranging between 30 and 34.9 ng/mL [23]. This 10 ng/mL threshold might represent the lowest necessary vitamin D serum level for H. pylori eradication success [30].
As previously described, lower vitamin D levels have been found in patients with diabetes and H. pylori infection/positive serum antibodies [38,39]. However, Huang et al. were the only ones who evaluated how vitamin D deficiency interferes with H. pylori therapeutic success in patients with type 2 diabetes mellitus. Vitamin D levels under 20 ng/mL significantly negatively impacted H. pylori eradication, whereas levels ranging between 20 and 30 ng/mL did not influence therapeutic success. Furthermore, the authors acknowledged that vitamin D deficit might also have enhanced dyslipidemia in their study cohort, which, together with prolonged evolution of the glycemic imbalance, could have also contributed to eradication failure [50].
In children, data on the role of vitamin D in H. pylori eradication rates are currently limited to a few studies. Zhang et al. found the lowest vitamin D serum values among a pediatric population with H. pylori recurrence, despite adequate treatment. Within their study, significantly higher values of the same parameter were found in those patients in whom bacterial eradication was successful and in healthy controls, but each of the analyzed study groups had a low percentage of vitamin D-deficient and -insufficient individuals [51]. Sorokman et al. found that vitamin D levels of over 20 ng/mL are more likely to ensure eradication success. Their study provided no information related to vitamin D supplementation amount and status, which is a routine approach at pediatric ages [43]. In fact, literature data regarding vitamin D intake/supplementation and H. pylori eradication success are still very limited. Only one population-based study has managed so far to identify a correlation between a higher vitamin D intake, positively correlated with a higher dietary fish intake, and efficacy of H. pylori eradication [52].

VDR and H. pylori Infection
H. pylori induces increased expression of VDRs, which have been shown to play an important role in innate immunity and to render antimicrobial activity. One study conducted on adult patients showed that cells of gastric mucosa specimens harvested from H. pylori-infected patients presented a significant elevation in VDR expression, as opposed to healthy controls [53]. Starting from the theory of possible interference of VDRs with H. pylori infection, recent research tried to evaluate the impact of VDR gene's polymorphisms as well. Martins et al. found a differential distribution of BsmI genotypes of the VDR gene within a study population, divided based on H. pylori positivity within gastric mucosa biopsy specimens. The other three polymorphisms studied-namely FokI, ApaI, and TaqI-presented no significant genotype distribution variation among the two study groups [54]. However, in another study, a significant correlation between the Fokl and Apal polymorphisms of the VDR gene and H. pylori infection was found [55].

Vitamin D and Gastric Cancer
Vitamin D intake seems to decrease gastric cancer risk, as reflected through a study conducted on a Vietnamese population. The same study also revealed that vitamin D intake is inversely associated with both positive and negative IgG serum antibody titers directed against H. pylori [56]. Thus, this study concluded that vitamin D supplementation can protect against gastric cancer, but failed to establish a relationship between daily vitamin D oral intake and H. pylori infection [56]. The possible link between hypovitaminosis D and increased cancer risk is further strengthened by Antico et al., who comparatively assessed vitamin D status between four different study groups: one group with atrophic gastritis type A, one group with H. pylori gastritis, a group of patients diagnosed with non-specific lymphocytic gastritis, and a control group of healthy subjects. The study group of patients with atrophic, autoimmune-induced gastritis presented the lowest vitamin D levels, followed by the H. pylori gastritis group, which also exhibited significantly lower values of serum vitamin D than the other two study groups [57]. Contrary to previous findings, other authors claimed that a higher intake of vitamin D accounts for an increase in gastric cancer risk, even in patients without serological traces of H. pylori infection [58].

Discussion
One meta-analysis of 48 studies pointed out that H. pylori infection has been associated with a decrease in serum levels of various vitamins, including vitamin B12, vitamin C, and vitamin D. Higher serum vitamin D and vitamin B12 levels were warranted to improve H. pylori eradication rates, whereas vitamin C supplementation helped achieve the same result [59]. This meta-analysis revealed some shortcomings of the analyzed studies, which included limited geographical research areas-as a significant number of studies were conducted on Turkish populations-as well as controversy surrounding adequate vitamin D supplementation [59]. One other meta-analysis, limited to 10 articles, only focused on the impact of vitamin D status upon H. pylori infection. Despite the heterogenicity of the included studies, the authors concluded that lower vitamin D levels might account for a higher prevalence of H. pylori infection and might negatively influence bacterial eradication [22]. Similar to this meta-analysis, our review included only those studies that analyzed the manner in which vitamin D impacts H. pylori infection occurrence and its subsequent eradication. As seen in Tables 1 and 2, and further detailed in the results section, most of the clinical studies included in this review confirm an inverse relationship between serum vitamin D levels and H. pylori infection and eradication rates. However, despite the important number of relevant studies on this topic, their heterogeneity cannot be neglected and hinders the performance of a meta-analysis. Thus, H. pylori detection methods vary greatly, whereas the data were mostly limited to Asian studies, which might significantly impact the interpretation of results due to the miscellaneous dietary and nutrition patterns across the globe. Moreover, a handful of studies found no significant relationship between vitamin D and H. pylori, and very few of the studies included in this review assessed the importance of vitamin D supplementation [33][34][35]49,52]. The main population benefitting from regular vitamin D supplementation belongs to the pediatric segment. However, the limited number of pediatric studies performed on this subject did not offer any information regarding the regularity, amount, and/or adequacy of the intake of vitamin D supplements within the analyzed study populations [40][41][42][43]51]. Hence, future prospective studies and randomized controlled trials are required to clarify the relationship between vitamin D serum levels, its supplementation, and H. pylori, and to evaluate whether the addition of vitamin D3 to therapeutic regimens influences bacterial eradication. Cofounding factors such as age, interfering in the relationship between vitamin D supplementation and H. pylori prevalence and eradication, are also subject to future debate, given the lack of regular vitamin D intake among adult populations and the possible inverse relationship between H. pylori and vitamin D deficiency, as proposed by Bahs , i et al. [28].
Our review excluded experimental data, due to the significant abundance of clinical data and the limited number of studies that included animal populations. However, some important aspects can be retained from experimental studies as well, which might aid future clinical research. After artificial infection of the stomach with H. pylori in mice, the administration of oral vitamin D3 produced a significant decrease in colonization rates, as well as an upregulation of VDRs [60,61]. The mouse model proved that anti-H. pylori activity of vitamin D3 is further enhanced by the activation of the VDR-Cathelicidin antimicrobial peptide (CAMP) pathway [60,61]. Another in vitro study showed that vitamin D3 supplementation triggers a restoration of lysosomal degradation through the activation of the protein disulfide-isomerase A3 (PDIA3) receptor, which promotes calcium release from lysosomes, lysosomal acidification and, consequently, the elimination of H. pylori through the autolysosomal pathway [62]. Furthermore, the synthetical production of indene compounds derived from vitamin D led to selective antibacterial action against H. pylori [63,64]. More specifically, vitamin D decomposition products, such as vitamin D3 decomposition product 1 (VDP1), detain an anti-H. pylori action through induction of the bacteria's cell membrane structure collapse [65]. A synergistic effect of the addition of 1α, 25-dihydroxyvitamin D3 to the standard quadruple therapy in the eradication of H. pylori has been proposed, after the compound was proven to protect against H.pylori-induced apoptosis of gastric epithelial cells [66]. This theory is further supported by the important tumor suppressor role of the vitamin D3 upregulated protein 1 (VDUP1), which was shown to protect against gastric carcinogenesis [67]. Thus, in light of these recently surfaced experimental data, future population-based studies are required to shine a light upon the role of vitamin D3 supplementation in preventing H. pylori infection and in facilitating its eradication. Pediatric studies could provide more insight into this matter, given that vitamin D supplements are routinely administered in children [68]. The data available so far for pediatric ages did not provide any information regarding the amount of vitamin D supplementation that the enrolled populations were receiving [40][41][42][43]51].
The link between vitamin D intake and gastric cancer risk, while taking into consideration the presence/absence of H. pylori infection, still requires more research. Two review articles clearly underlined an inverse correlation between vitamin D serum levels and gastric cancer risk, morbidity, and mortality, but most of the studies included in these articles did not perform a separate analysis on H. pylori infection status [69,70]. As previously reported in the results section, patients with gastric cancer and atrophic gastritis, infected with H. pylori, might present lower vitamin D serum levels, but research on this matter needs to be expanded [4,57]. Furthermore, the clear mechanisms through which vitamin D offers protection against gastric cancer, possibly through VDR interaction, still require elucidation [69,70].
A few previous reviews also provided some important take-home messages on the role of vitamin D in H. pylori infection. The bacteriolytic action of hydrophobic lipid structures, such as indene compounds, vitamin D, and 3-carbonyl steroids, has been recently highlighted [71]. Among multiple micronutrients that can influence H. pylori's survival, pathogenicity, and eradication, vitamin D distinguished itself as an important regulator of host immune response [72]. Hence, one literature survey (Golpour et al.) proposed potential antibiotic-independent approaches for H. pylori treatment based on vitamin D supplements [73]. However, these review articles had a narrative structure and referred to fewer population-based studies than the present work.

Conclusions
This comprehensive review highlights the impact of vitamin D deficiency and/or insufficiency upon an ascending prevalence of H. pylori infection and its eradication failure. Vitamin D-deficient subjects might be more prone to developing H. pylori infection, but the influence of H. pylori infection upon vitamin D status is still puzzling. The role of vitamin D intake is still in question, as most of the reviewed studies failed to analyze whether the addition of oral supplements influences the performance of current therapeutic regimens. Thus, the screening of vitamin D levels in subjects with H. pylori infection might establish the need for its supplementation in ensuring therapeutic success. Vitamin D deficit might be connected to an increase in gastric cancer risk, but the additional influence of H. pylori in this linear relationship is still unclear. Furthermore, the currently available data are hindered by the enrollment of populations from restricted geographical regions, as well as by the heterogenicity of H. pylori infection detection methods and the reduced number of pediatric populations included so far. Future population-based studies and randomized clinical trials from larger geographical areas are warranted to address this subject in more depth and to analyze the synergistic effect of vitamin D supplementation and classical therapeutic schemes.

Data Availability Statement:
No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest:
The authors declare no conflict of interest.