Study Characteristics
After completing the search in international databases, 782 studies were retrieved. After removal of duplicates, screening by titles, abstracts, and full texts was performed considering inclusion and exclusion criteria; 22 studies remained to meta-analyze the association between H. pylori infection and insulin resistance and metabolic syndrome (Figure 1). The characteristics of the studies included in this meta-analysis were reported in Table 1.
For this meta-analysis, 18 case-control studies[32-49] were included, of which 9 studies [32, 34, 38, 39, 41, 42, 46, 48, 49] determined the association of H. pylori infection with the occurrence of metabolic syndrome and 9 studies[33, 35, 36, 40, 43-45, 47] determined the association of H. pylori infection with the occurrence of insulin resistance in patients with diabetes. Besides, 4 cohort studies[19, 50-52] were analyzed, of which 2 studies[50, 52] examined the presence of H. pylori infection in connection with metabolic syndrome and 2 studies[19, 51] on insulin resistance (Table 1).
Association of H. pylori infection with metabolic syndrome (a combination of case-control studies)
9 case-control studies were evaluated the association of H. pylori infection with the occurrence of metabolic syndrome. After combining the results of these case-control studies, the pooled estimate of odds ratio was 1.19 with a 95% confidence interval of 1.05 to 1.35. The rate of heterogeneity was significantly lower and equal to zero percent with a significant level of 0.98 (Figure 2a).
The Eggers test and funnel plot showed that the publication bias did not occur (B = 0.54; SE = 0.40; P-value = 0.181) (Figure 2b). Besides, sensitivity analysis showed that if each study was omitted, the pooled odds ratio and its 95% confidence interval would be in line with the overall estimates, except for the study by Refeali et al.[46], which, if omitted, increased the pooled odds ratio to 1.29 with wider 95% confidence interval of 1.02 to 1.64 (Figure 2c).
In Table 2, subgroup analysis was performed based on the diagnostic method of H. pylori. The results showed that regarding the diagnostic method of anti-H. pylori antibody, the pooled odds ratio was 1.26 (95%CI: 1.01, 1.70) and according to the C urea breath test (UBT), it was 1.17 (95%CI: 1.02, 1.35; p value: 0.910).
Association of H. pylori infection with insulin resistance (a combination of case-control studies)
10 case-control studies evaluated the association between H. pylori infection and the occurrence of insulin resistance in diabetic patients. After combining the results of these case-control studies, the pooled odds ratio was 1.54 with a 95% confidence interval of 1.19 to 1.98. (Figure 3a).
The results of the Eggers test and funnel plot showed no publication bias (B = -0.17; SE = 0.61; P-value = 0.791) (Figure 3b). In addition, the sensitivity analysis showed that if any of the studies were omitted, the pooled odds ratio would be in line with the overall pooled odds ratio result. Only if the study of Allam et al. [44]were removed, the pooled odds ratio would be equal to 1.70 with a 95% confidence interval of 1.33 to 2.16 (Figure 3c).
In Table 2, subgroup analysis was performed based on the diagnostic method of H. pylori, types of diabetes, gender of population, and continent. The results showed that regarding the diagnostic method of anti-H. pylori antibody, the pooled odds ratio for the occurrence of insulin resistance in diabetic patients with H. pylori infection was equal to 1.63 (95%CI: 1.25, 2.12) and according to the diagnostic method of the rapid urease test (RUT) & histology, it was equal to 1.33 (95%CI: 0.56, 3.16; p value: 0.661). Subgroup analysis based on the type of diabetes showed that the incidence chance of insulin resistance in diabetes mellitus patients with H. pylori infection was 1.19 (95%CI: 1.00, 1.78), and in type II diabetic patients with H. pylori infection, it was 1.80 (95%CI: 1.34, 2.42; p value: 0.192). The subgroup analysis based on the continent of study showed the odds ratio of the presence of H. pylori infection and the occurrence of insulin resistance in Asian population was 1.67 (95%CI: 1.23, 2.26), and the corresponding estimates in America and Africa were 1.47 (95%CI: 0.89, 2.44) and 1.00 (95%CI: 0.26, 3.77; p value: 0.63), respectively.
Association of H. pylori infection with insulin resistance and metabolic syndrome (a combination of cohort studies)
Finally, cohort studies were evaluated. Two cohort studies determined the association between H. pylori infection and the incidence of insulin resistance; in the study by Hsieh et al.[51], the risk ratio of H. pylori infection and insulin resistance was 1.30 (95%CI: 1.11, 1.52) and in the other study by Jeon et al.[19], the corresponding risk ratio was 2.69 (95%CI: 0.61, 11.84). Also, two cohort studies evaluated the association of H. pylori with metabolic syndrome; in the study by Sayilar et al.[52], the odds ratio was 3.61 (95%CI: 0.83, 15.77) and in the other study by Shin et al.[50], the risk ratio based on histological diagnosis for H. pylori was 1.26 (95%CI: 0.69, 2.31) and the risk ratio based on serological diagnosis for H. pylori was 1.12 (95%CI: 0.60, 2.11). After combining the results of these cohort studies concerning metabolic syndrome, the pooled risk ratio was 1.31 with a 95% confidence interval of 1.13 to 1.51. The heterogeneity rate was zero percent (Figures 4a, 4b, and 4c).
Quality Assessment
In the present meta-analysis, the quality of 18 case-control studies and 4 cohort studies was evaluated using JBI critical appraisal tools and the results of which were presented in Tables 3 and 4, respectively. The quality assessment checklist of the case-control studies showed that most of these studies had a high-quality score. Except for the study of Chen et al.[47], which received a score of 6, the rest of the studies had a high-quality score of more than 6 (Table 3). The quality assessment of cohort studies showed high scores for quality of all studies (Table 4).