Background

Helicobacter pylori infection affects ~ 50% of the world’s population and has been recognized as one of the most common chronic infections in human. [1] The overall prevalence is high in developing countries. H. pylori infection cause upper gastrointestinal diseases including gastritis, peptic ulcer disease and also increase the risk of gastric cancer. Interestingly, several studies suggest that H. pylori infection may influence the gut microbiome [2,3,4]. Further, diverse extragastric diseases have been linked to H. pylori infection, including dyslipidemia [5], type 2 diabetes [6], insulin resistance [7] and metabolic syndrome [8]. The correlation of H. pylori infection and bilirubin levels has not been reported. Nevertheless, H. pylori infection appears to play an important role in the development of metabolic disorders in which require further investigations.

In the current study, we aimed to investigate additional metabolic parameters and their clinical impact with regard to H. pylori infection in a Chinese population.

Methods

Study design and population

We performed a case-control study by selecting subjects who were overtly positive for H. pylori as cases and overtly negative controls matched by sex and age. We screened subjects aged 18–79 years who were receiving annual health examinations including 13C –urea breath test (UBT) in Beijing Tongren Hospital from March 2016 to May 2017. Subjects with 13C-UBT values≥10‰ or ≤ 1‰ were defined as overtly positive (cases) or overtly negative (controls) for H. pylori, respectively. Subjects with 13C-UBT results between 1‰ and 10‰ were excluded from the study. Subjects in the H. pylori positive group were matched 1:1 with age and sex to H. pylori negative individuals. After the primary assessment of the baseline characters for all subjects, we further excluded participants with liver and gall bladder diseases (hepatitis, jaundice, cholecystitis, biliary calculus), abnormal liver function (alanine aminotransferase (ALT) or aspartate aminotransferase (AST) > 1.5 times upper normal limit, or bilirubin > twice upper normal limit), abnormal kidney function (Cr > upper normal limit) to better eliminate the potential biases caused by diseases.

Anthropometric and laboratory measurements

Each subject had anthropometric measurements. Presence of systematic or previous diseases, such as diabetes mellitus (DM), hypertension, hepatitis, jaundice, cholecystitis or biliary calculus were noted. Body mass index (BMI) was measured as weight (kg) divided by height (meters) squared (kg/m2). Waist circumference (WC) was measured at the level of the umbilicus in cm. Blood pressure (BP) was measured three times when participants were seated, and the average of the last two measurements was recorded. Blood samples were collected after an overnight fasting for the determination of plasma glucose, glycosylated hemoglobin A1c (HbA1c), total cholesterol (TC), triglycerides (TG), high density lipoprotein cholesterol (HDL-C), low density lipoprotein cholesterol (LDL-C), direct bilirubin, total bilirubin, total bile acids, alkaline phosphatase (ALP), ALT, AST [9], γ-glutamyl transpeptidase (γ-GT), blood urea nitrogen [10], serum creatinine (SCr) and uric acids (UA) concentrations.

Statistical analysis

Data are presented as the mean ± SD. The baseline characteristics of subjects were compared using the chi-squared test for categorical variables and the Student t-test for continuous variables. Distribution of discrete/qualitative variables was compared by trend chi-square test. Binary logistic regression analysis was used to estimate crude and adjusted odds ratios (ORs) (95% CIs) to assess the risk of bilirubin change associated with H. pylori infection. When data were not normally distributed, the correlations of bilirubin and cholesterol were determined by the Spearman correlation coefficient analysis. Calculations were performed using SPSS 24.0 statistical software (SPSS Inc., Chicago, IL, USA), and significance was established at a two-tailed P < 0.05.

Results

Clinical characteristics of the study population

A total of 1982 subjects participated in the health examination including 13C -UBT, physical examination and blood tests. Based on selection criteria, 623 subjects were initially defined as cases, 1010 were defined as controls, and 349 were excluded from the study. The primary baseline characters of the two groups are shown in Additional file 1: Table S1. Unexpectedly, our results unveiled an intriguing association between bilirubin decrease and H. pylori infection (direct bilirubin 2.50 ± 0.99 vs. 2.37 ± 0.90 μmol/L, P = 0.015, total bilirubin 14.49 ± 5.66 vs. 13.87 ± 5.32 μmol/L, P = 0.049). We doubted if other disorders may influence the bilirubin levels and caused the inauthentic results, hence, we excluded participants with diseases which could affect bilirubin levels pathologically. According to the specified criteria, 29 patients were excluded from the study. Of the remaining 1953 subjects, 617 (31.6%) overtly positive for H. pylori were matched 1:1 with age and sex to 988 (50.6%) overtly negative for H. pylori, 348 subjects with 13C-UBT results between 1‰ and 10‰ were excluded from the study. Thus, 617 overtly positive H. pylori individuals (case group) and 617 sex and age- matched overtly negative H. pylori subjects (control group) comprised the case-control study (Fig. 1).

Fig. 1
figure 1

Design and inclusion flow chart of the case-control study. 13C-UBT: 13C-urea breath tests; HP: H. pylori

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The demographic and biochemical parameters of the cases and the controls are shown in Table 1. The case group had significantly less favourable lipid profiles than controls: LDL-C 2.98 ± 0.76 vs. 2.89 ± 0.75 mmol/L (P = 0.033), HDL-C 1.39 ± 0.37 vs. 1.44 ± 0.41 mmol/L (P = 0.044). Unsurprisingly, the case group also had lower bilirubin levels compared with the control group: direct bilirubin 2.34 ± 0.38 vs. 2.47 ± 0.90 μmol/L (P = 0.008), total bilirubin 13.65 ± 4.84 vs. 14.28 ± 5.01 μmol/L (P = 0.026). There was no significant difference in other demographic and clinical characteristics.

Table 1 Comparison of various clinical characteristics classified by H. pylori infection
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Association between serum bilirubin levels and H. pylori infection

To further investigate the correlation between H. pylori infection and bilirubin levels, subjects from both groups were assigned to 4 grades based on quartiles of serum bilirubin concentrations and the proportions of each grade in case and control groups are shown in Fig. 2. In the H. pylori positive group, the proportion of subjects with direct bilirubin levels in the highest quart (> 2.8 μmol/L) was smaller than that of the H. pylori negative group (21.1% vs 28.4%) (Trend χ2 = 4.119, P = 0.042). The same trend was also observed in subjects assigned by total bilirubin level (Trend χ2 = 6.256, P = 0.012). This suggests that H. pylori infection is associated with both direct and total bilirubin reduction.

Fig. 2
figure 2

The association between serum bilirubin levels and H. pylori infection. a Proportion of subjects according to the quartiles of direct bilirubin. The percentages of each grade from higher to lower direct bilirubin levels were 28.4, 25.4, 22.4, 23.8% respectively in H. pylori overt negative group and 21.1, 27.9, 26.5, 24.5% respectively in H. pylori overt positive group. b Proportion of subjects according to the quartiles of total bilirubin. The percentages of each grade from higher to lower total bilirubin levels were 27.2, 25.1, 24.0, 23.7% respectively in H. pylori overt negative group and 21.8, 22.9, 28.7, 26.6% respectively in H. pylori overt positive group. HP, H. pylori

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Similar trend was observed when subjects were grouped according to quartiles of LDL-C level (Trend χ2 = 4.577, P = 0.032, data not shown), but was not seen when subjects grouped according to quartiles of HDL-C level (Trend χ2 = 1.185, P = 0.288, data not shown) although the mean of HDL-C was significant different in H. pylori overtly positive and negative groups (Table 1).

The risk of bilirubin changes according to the infection of H. pylori

Given the association between increased serum bilirubin levels within the reference range and better health outcomes (or conversely lower bilirubin concentrations with higher morbidities) [11,12,13,14,15,16], we assigned subjects into two groups with direct bilirubin (2.8) and total bilirubin (16.4) in the upper distribution quartile. Bilirubin levels below the upper quartile were defined as “bilirubin not increase”. Logistic regression analysis was performed to determine the independence of the association between H. pylori infection and risk for “bilirubin not increase”. As shown in Table 2, H. pylori infection was associated with higher risk of both direct bilirubin (OR = 1.483, 95% CI =1.143–1.925, P = 0.003) and total bilirubin (OR = 1.336, 95% CI =1.030–1.733, P = 0.029) “not increase” in univariate analysis. After adjustment for age and sex (model 2), further adjustment for BMI, ALT and AST (model 3), and then additional adjustment for HDL-C, LDL-C, TC and TG (model 4), the ORs remained significant.

Table 2 The risk of bilirubin decreases according to the infection of H. pylori
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Correlation between direct bilirubin and cholesterol

We found that lipid profiles improved with increasing direct bilirubin levels regardless of H. pylori status. LDL-C was negatively correlated with direct bilirubin concentration (R = − 0.260, P < 0.0001) (Fig. 3a), while HDL-C was positively correlated with direct bilirubin level. (R = 0.063, P = 0.028) (Fig. 3b).

Fig. 3
figure 3

Correlation between direct bilirubin and cholesterol. a Correlation between LDL-cholesterol (LDL-C) and direct bilirubin (DBil). b Correlation between HDL-cholesterol (HDL-C) and DBil

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Discussion

With the sensitivity and specificity exceeding 90%, UBT is often considered as the gold standard test in the diagnosis of H. pylori infection [17,18,19]. However, there is a “grey zone” of uncertainty when UBT values range from 2.0 to 5.0‰ [20, 21]. Fortunately, positive and negative UBT results tend to cluster outside this range [22]. We performed this study by selecting subjects at the extreme ends of the range of 13C-UBT values to comprise the study groups. We selected subjects with 13C-UBT values ≤1 ‰ and ≥ 10 ‰ to avoid false-positive and false-negative results.

In this case-control study, we found that subjects with high 13C-UBT values had lower bilirubin concentrations and less favourable lipid profiles compared those with low 13C-UBT values. In addition to being a breakdown product of heme, serum bilirubin is also a powerful antioxidant [23, 24]. High normal concentrations of serum bilirubin correlate with better health outcomes [11,12,13,14,15,16]. Bilirubin concentrations have been reported as being inversely associated with risk for cardiovascular disease [15, 25], metabolic syndrome [16], diabetes [11], inflammatory disease [13] and some cancers [14]. However, little is known of determinants of bilirubin levels within the reference range. To our knowledge, this study indicates for the first time that H. pylori infection may be associated with decreased bilirubin concentrations within the reference range. H. pylori infection has been shown to result in chronic inflammation and influence of bile reflux [9, 26], which may at least in part explain the bilirubin changes, but further research relating to possible mechanisms is required.

Our study also found an association between H. pylori infection and lipid profiles. Serum LDL-C level was significantly higher and HDL-C significantly lower in H. pylori infected subjects. This association was first observed in 1996 in Finnish subjects [27]. Since then, several studies have been performed in different populations. However, the results are still equivocal. Most studies have supported the significant correlation between H. pylori infection and elevated lipids levels [5, 10, 28,29,30,31,32,33]. However Elizalde et al. found that H. pylori infection had no influence on blood lipids in 686 H. pylori-positive patients before and 3 months after eradication therapy with a low treatment rate (53.6%) [9]. It should be noted that cases and controls were not matched for sex and age which are key influence determinants of serum lipids. Furthermore H. pylori infection is associated with a long-term effect on human health [34] and 3 months may not be long enough to observe changes resulting from eradication of H. pylori. In our relatively large study, we selected individuals with extreme 13C-UBT values to better distinguish the differences associated with H. pylori infection and matched the subjects by sex and age. We found that H. pylori infected subjects had significantly higher LDL-C and lower HDL-C levels.

Our study also found that decreased direct bilirubin was correlated with adverse lipid profiles, an important cause of cardiovascular disease and a feature of clusters of metabolic disease risk factors. Recent in vivo and in vitro studies suggest that this may due to bilirubin regulation of the fat burning nuclear receptor, PPAR-α and γ levels and thus inhibited lipid accumulation [35, 36]. Given that H. pylori infection may influence lipid profiles and that we could not prove a causal relationship between bilirubin and cholesterol levels. It is conceivable that the elevated cholesterol levels may be a result of both H. pylori infection and decreased bilirubin concentrations.

Limitations of our study should be acknowledged. First, our study was not a prospective study, so we could not examine the effects of eradication therapy. Comparing bilirubin and lipid levels before and after eradication of H. pylori would enable more definitive conclusions. Second, our study was a cross-sectional study. Despite of statistical significance in lipid profiles between groups, we could not assure a real difference in clinical practice. Larger sample size prospective study is still required in the future.

Conclusion

Our data suggest that H. pylori infection may be an independent risk factor for serum bilirubin reduction and adverse lipid profiles. If confirmed, this would provide further evidence for the importance of diagnosis and eradication of H. pylori infection.