Association of Helicobacter pylori infection with diabetes mellitus: A Meta-analysis of Case Control Studies

Background Results of previous studies were showed that the association between H. pylori infection and the risk of diabetes is still controversies. Therefore, this systematic review and meta-analysis study was designed and implemented aimed to determine the association between H. pylori infection and the risk of diabetes.Methods All case control articles were searched in international databases, including Medline (PubMed), Web of sciences, Scopus, EMBASE, and CINHAL. Search was done from January 1990 to March 2019 without language limitations. Also, logarithm and standard error logarithm odds ratio (OR) were used for meta-analysis.Results A total of 41 studies were included in this meta-analysis. The range of association with odds ratio in case control studies which published between 1990 to 2019 was 0.21 to 6.08. The pooled estimate of the association between H. pylori infection with diabetes was 1.27 (95% CI 1.11 to 1.45, P = 0.0001, I 2 = 86.6%). The effect of H. pylori infection on diabetes mellitus, type 1 and type 2 diabetes was 1.17 (95% CI 0.94 to 1.45), 1.19 (95% CI 0.98 to 1.45), and 1.43 (95% CI 1.11 to 1.85) respectively. Subgroup analysis by the geographical regions showed in Asian population risk of the effect of H. pylori infection on diabetes was higher than other population, but in the American, this was a protective relationship.Conclusion In conclusion, this systematic review & meta-analysis study suggested that H. pylori infection was associated with the risk of diabetes as compared to non- diabetes individual.


Abstract
Background Results of previous studies were showed that the association between H. pylori infection and the risk of diabetes is still controversies. Therefore, this systematic review and meta-analysis study was designed and implemented aimed to determine the association between H. pylori infection and the risk of diabetes.Methods All case control articles were searched in international databases, including Medline (PubMed), Web of sciences, Scopus, EMBASE, and CINHAL. Search was done from January 1990 to March 2019 without language limitations. Also, logarithm and standard error logarithm odds ratio (OR) were used for metaanalysis.Results A total of 41 studies were included in this meta-analysis. The range of association with odds ratio in case control studies which published between 1990 to 2019 was 0.21 to 6.08. The pooled estimate of the association between H. pylori infection with diabetes was 1.27 (95% CI 1.11 to 1.45, P = 0.0001, I 2 = 86.6%). The effect of H. pylori infection on diabetes mellitus, type 1 and type 2 diabetes was 1.17 (95% CI 0.94 to 1.45), 1.19 (95% CI 0.98 to 1.45), and 1.43 (95% CI 1.11 to 1.85) respectively. Subgroup analysis by the geographical regions showed in Asian population risk of the effect of H. pylori infection on diabetes was higher than other population, but in the American, this was a protective relationship.Conclusion In conclusion, this systematic review & meta-analysis study suggested that H. pylori infection was associated with the risk of diabetes as compared to non-diabetes individual.

Background
Helicobacter pylori (H. pylori) is a gram-negative spiral bacterium which is found abundantly in the stomach. The H. pylori infection is one of the most common chronic infections in the world, so that more than 50% of the world's population are infected with this infection (1,2). It is now known that H. pylori is responsible for most cases of peptic ulcer disease. Also, the different studies highlighted that it is associated with other important gastrointestinal diseases such as chronic gastritis, gastric adenocarcinoma, and MALT lymphoma which are recognized as a major public health concern in the world (3,4). In addition to the role of H. pylori in gastrointestinal disorders, some researches have suggested the potential role of this bacterium in the development of non-gastrointestinal disorders such as cardiovascular diseases and metabolic syndrome especially diabetes (5)(6)(7). Diabetes is the most common metabolic disease in the world and responsible for about 4 million deaths per year. The global prevalence of diabetes was 4.6% equivalent to 285 million in adults for 2010, which this number has reached 371 million in 2012, and is expected to reach 552 million by 2030(8-10).
As mentioned above, one of the factors that may affect incidence of diabetes is H. pylori. The relationship between H.pylori infection and diabetes was introduced in 1989 (11). It has been suggested that the H. pylori may be contributed to the incidence of cardiovascular disease and diabetes through elevations in inflammatory cytokines levels such as C-reactive protein (CRP) and interleukin-6 (11-13). In general, various studies have investigated the role of H. pylori in the pathogenesis of diabetes and its complications, but the results are inconsistent with each other.
For example, some case-control studies have reported higher prevalence of H. pylori in patients with diabetes (14,15). Also, several cross-sectional studies have shown a significant statistical association between H. pylori and diabetes (3,15). However, some studies in this regard have shown that there is no significant association between diabetes and prevalence H. pylori infection (2,16,17). Therefore, the association between H. pylori infection and the risk of diabetes is still controversies. Hence, this systematic review and meta-analysis study was designed and implemented aimed to determine the association between H. pylori infection and the risk of diabetes.

Methods
This systematic review and Meta-analysis was performed according to the Preferred reporting items for systematic reviews and meta-analyses (PRISMA) and Strengthening the Reporting of Observationally Studies in Epidemiology (STROBE) guidelines for reviews of analytical observational studies (case-control) (18-20).

Search Terms and Complex Search Syntax
All original published articles were searched in international databases, including Medline (PubMed), Web of sciences, Scopus, EMBASE, Cochrane, Ovid and CINHAL.
Search was done from January 1990 to March 2019 without language limitations.
The keywords were Diabetes, Diabetes Mellitus (type 1 and 2), Insulin Dependent, IDDM, NIDDM, Noninsulin Dependent, Insulin Sensitivity, Helicobacter pylori, Campylobacter pylori, and H Pylori. The primary search results were reviewed, and some of the articles were eliminated after reviewing their title and an abstract.
Inclusion and exclusion criteria were set by 2 researchers separately (YM and RR) ( Figure 1).

Eligibility Criteria:
A published study had to meet the following inclusion criteria: (1) case-control, nested case control studies, (2) human population, (3) study population were patients with diabetes, and (4) Helicobacter pylori infection was independent variable. Case reports, reviews, animal studies, and cohort studies were removed from the tabulation. The authors resolved all disputes during the collection, compilation, and analysis of data.

Data Extraction
Two review authors (YM and RR) independently extracted and entered study data. A structured checklist was used for the extraction of information on the 1) name of first author, 2) date of publication, 3) country, 4) study subjects, 5) age of patients, 6) sample size, 7) type of diabetes, 8) mean of HbA1C, 9) duration of diabetes, 10) measurement of association, 11) controlled variables, 12) and method of bacteria detection. Additional information on the study results was extracted with respect to the type of instruments. A data extraction form was created based on our group discussion and piloted according to 10 different types of studies. Then, it was modified and used by the data extractor. All process from systematic search to final data extraction were followed independently by two research experts (Kappa statistic for agreement for quality assessment; 0.75). Any disagreement was assessed by both and if a consensus was not reached, a third author (LS) would evaluate the study. The qualities of all studies were assessed by Modified Newcastle-Ottawa Scale for Case Control studies (21).

Statistical Analysis
Logarithm and standard error logarithm odds ratio (OR) were used for the metaanalysis. DerSimonian and Laird method was used to compute the pooled estimate of odds ratio (OR) with confidence interval (CI 95%) using random models (22).
Because the test for heterogeneity was statistically significant in some analyses, the random effects models were used to estimate OR. In this study, w Cochran's Q test and I2 statistic were used to evaluate statistical heterogeneity between studies (23). In addition, a meta-regression and subgroup analysis was performed to assess the source of heterogeneity between studies. Moreover, publication bias was assessed by funnel plot and Egger test (24, 25). Statistical analysis was performed using STATA 14.0 (Stata Corp, College Station, TX, USA), and statistical significance was set at p < 0.05.

Study Characteristics
The first step of search in electronic databases yielded 2027 publications and 200 studies identified through other sources. In the final step, after removing the duplicates, reviewing by title, abstract and full text and considering the inclusion and exclusion criteria, 41 studies were selected for the meta-analysis of pooled association between H. pylori infection and the risk of diabetes (Figure 1). Studies characteristics of each study included in the meta-analysis are reported in Table 1.
The total sample size in the 41 studies that reported the association between H. pylori infection and the risk of diabetes in case and control was 4445 and 5416, respectively. Also, 11 studies reported association between H. pylori infection with DM. Other primary studies reported association between H. pylori infection with type 1 and 2 diabetes. A total of 41 studies were included in this meta-analysis, of which 20 were conduct in European, 12 were in Asian, 7 studies done in African and 2 in American ( Table 2). The range of association with odds ratio in case control studies which published between 1990 to 2019 was 0.21 to 6.08. Of the 41 studies, 18 showed statistically significant between H. pylori infection and the risk of diabetes.

Subgroup analysis
Based on the random effect model, the effect of H. pylori infection on diabetes mellitus, type 1 and type 2 diabetes was 1.17 (95% CI 0.94 to 1.45), 1.19 (95% CI 0.98 to 1.45), and 1.43 (95% CI 1.11 to 1.85) respectively. Effect size of H. pylori infection on type 2 diabetes was higher than type 1 and diabetes mellitus ( Table   2).
Subgroup analysis by the geographical regions showed in Asian population risk of the effect of H. pylori infection on diabetes was higher than other population, but in the American, this was a protective relationship. In addition, the relationship between H. pylori and the risk of diabetes according to age showed that risk in individual with 30 to 60 year was 1.34 (95% CI 1.09, 1.65), in 10 to 30 years, and upper 60 years was 1.34 (95% CI 1.05, 1.62) and 1.03 (95% CI 0.93, 3.23), respectively ( Table 2).
Based on methods of detecting H. pylori infection, the effect of H. pylori infection on diabetes mellitus in detect by rapid urease test was higher than other methods, but this effect not significant ( Table 2).

Discussion
The purpose of this systematic review and meta-analysis study was to determine the association between H. pylori infection and the risk of diabetes. In the present study, the range of OR for case-control studies included in meta-analysis was 0.21 to 6.08. The results showed a significant statistical association between H. pylori infection and the risk of diabetes (overall OR: 1.27; 95% CI: 1.11 -1.45). The results of subgroup analysis by type of diabetes revealed a significant association between H. pylori infection and the risk of type 2 diabetes (OR: 1.43; 95% CI: 1.11 -1.85), however, there was no significant relationship between H. pylori and the risk of type 1 diabetes (OR: 1.19; 95% CI: 0.98-1.45) and diabetes mellitus (OR: 1.17; 95% CI: 0.94 -1.45). Subgroup analysis by the geographical regions showed a significant direct relationship between H. pylori and the risk of diabetes in Asian, Europe and Africa but in the American, this was a protective relationship. In addition, in subgroup analysis, the relationship between H. pylori and the risk of diabetes was different according to age, level of HbA1C, duration of diabetes and methods for H. pylori detection. This suggests that these factors could be an important source of heterogeneity in the studies included in the meta-analysis.
Our meta-analysis suggests that H. pylori infection can increase the risk of diabetes by up to 27%. These findings are consistent with the results of several metaanalysis studies that have been done in this field. The meta-analysis study by Jun- However, some studies did support significant association between H. Pylori infection and the risk of diabetes (16, 28).
The Several mechanisms have been proposed for the relationship between H. pylori infection and risk of diabetes. Inflammatory cytokine may lead to induce phosphorylation of serine residues on the insulin receptor substrate and subsequently this phenomenon may impair the interaction between the substrate and the insulin receptors due to impaired insulin function (7,29). Also, Lipopolysaccharides from gram-negative bacteria such as H. pylori may activate Toll-like receptors and subsequently insulin resistance occurs (30). All of these events can lead to reduced blood sugar control and consequently diabetes mellitus.
In addition, the presence of bacterial infections can lead to microvascular failure and eventually incidence of atherosclerosis (31).
In subgroup analysis of geographical regions by the type of continent, we explored a significant direct relationship between H. pylori and the risk of diabetes in Asian, Europe and Africa but in the American, this was a protective relationship, but there was still high heterogeneity within these subgroups. It was consistent with study of

Strengths and Limitations
This study also has several limitation and strengths. The first strength of this study is deal with heterogeneity through a subgroups analysis based on Type of diabetes, geographical regions, age, and level of HbA1C, duration of diabetes and methods for H. pylori detection. Another strength was considerable number of studies included in the meta-analysis (41 study) that would be possible to investigate the exact effect of the publication bias on the results. Also, this study has several limitations.
Firstly, missing potential studies e.g. limiting full-text review to English language articles may be lead to some degree of selection bias. Secondly, all studies included in meta-analysis were case-control, hence, the design and implementation of cohort studies are essential for detailed assessment of the association between H. pylori infection and diabetes. Thirdly, personal judgments may be effect on search of articles, data extraction and assessment of included articles in meta-analysis.

Conclusions
In conclusion, this systematic review & meta-analysis study suggested that H. pylori infection was associated with the risk of diabetes as compared to non-diabetes individual. However, in subgroup analysis by type of diabetes, this association was only significant for type 2 diabetes.

Consent for Publication
Not applicable.

Availability of Data and Material
Input data for the analyses are available from the corresponding author on request.

Competing Interests
The authors declare that they have no competing interests.

Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.     Funnel plot of association between Helicobacter pylori and Risk of diabetes