Association between Metabolic Syndrome Components and Serum High-Sensitivity C-Reactive Protein or Interleukin-6 Levels among Congolese Adults ()
1. Introduction
Over the last two decades, obesity and metabolic syndrome (MetS) have become a growing problem for public health and clinical practice, given their increased prevalence due to the rise of sedentary lifestyles and unhealthy eating habits [1] [2] . There are several definitions of MetS, but most of them describe it as a clustering of individual cardiometabolic risk factors [3] [4] [5] . Generally recognized diagnostic criteria include the concurrence of more than two of the following: 1) increased abdominal circumference (waistline of 94 cm or more for men and 80 cm for women), 2) low plasma levels of high-densisity lipoprotein cholesterol (less than 40 mg/dL (men) or under 50 mg/dL (women)), 3) increased values of plasma triglycerides (above 150 mg/dL), 4) elevated blood pressure (130/85 mmHg or higher), and 5) elevated glucose levels (fasting blood glucose level greater than 100 mg/dL), defined by National Cholesterol Education Program Adult Treatment Panel III [6] . It is estimated that around 20 to 25 percent of the world’s adult population suffer from MetS and are twice as likely to die from, and three-times as likely to have a heart attack or a stroke compared with individuals without MetS [7] .
Since studies indicate that beside the five traditional risk factors of MetS, a number of emerging risk factors including genetic predisposition, prothrombotic state and inflammation have been found to be implicated in the etiopathogenicity of obesity, type 2 diabetes and MetS [8] . There is increasing evidence that chronic systemic low-grade inflammation is linked obesity and the development of a MetS [9] [10] [11] . In particular, two systemic inflammatory biomarkers are thought to contribute to driving these cardiometabolic diseases: serum C-reactive protein (CRP) and interleukin-6 [12] [13] [14] .
CRP is an acute phase reactant produced mainly by hepatocytes but also by perivascular adipose tissue. Its production is regulated via IL6-Janus Kinase/Signal Transducers and Activators of transcription (IL-6-JAK/STAT) pathway.
Circulating levels of CRP and IL-6 have consistently been reported elevated in obese subjects and in patients with type 2 diabetes mellitus (T2DM) in observational epidemiology studies [15] [16] [17] . Moreover, individual components of the MetS definition were reported to be associated with hs-CRP and IL-6, to such an extent that it was suggested that elevation of both inflammatory biomarkers should be included in the definition of the MetS, the hypothesis being that CRP and IL-6 serum concentrations were linked to the MetS through its individual components.
Therefore, the purpose of this study was to address the association between MetS components and serum hs-CRP or IL-6 levels among Congolese adults. We also calculated the odds ratio of the individual components of MetS based on systemic levels of these two inflammatory biomarkers.
2. Material and Methods
The study was conducted in Brazzaville, capital of the Republic of Congo. Participants (T2DM patients and obese controls) were recruited respectively at the Medical Center Diabcare and the Mère-Enfant Blanche Gomes Hospital from July to December 2021. The participants were residents of any district of Brazzaville. Only individuals aged more than or equal to 30 years old and having given written consent were enrolled. Anthropometric data (including age, sex, height, weight, and waist circumference) and blood samples were collected to determine serum levels of glucose, routine lipids (CT, TG, HDL-C, and LDL-C derived from Fried Ewald’s equation) and hs-CRP, using a Cobas c111 analyser (Roche Diagnostics GmbH, Mannheim, Germany). For each participant, blood pressure (BP) was measured twice at the right arm after a rest of at least 30 minutes in seated position using an automatic Sphygmomanometer Omron® HEM-705 (Tokyo, Japan). Serum interleukin-6 and insulin quantification were performed using commercially pre-coated ELISA kits (SL1001Hu & SL0933Hu, Sunlong Biotech Co., Ltd., Hangzhou, China), with assays ranges of 2 to 80 ng/L and 0.3 to 20 mU/L, respectively.
MetS was defined as the presence of ≥3 out of 5 modified National Cholesterol Education Program Adult Treatment Panel III (NCEP-ATPIII) criteria: 1) enlarged waist, as proxy for central obesity (waist circumference (WC) ≥94 cm and ≥80 cm for Sub-Saharan African men and women, respectively); 2) fasting hypertriglyceridemia ≥150 mg/dL or use of drugs treatment for elevated triglycerides; 3) low high-density lipoprotein cholesterol ≤40 mg/dL and ≤50 mg/dL for men and women, respectively; 4) elevated blood pressure ≥130/85mmHg or use of antihypertensive drugs; and 5) fasting plasma glucose ≥100 mg/dL or use of glucose-lowering drugs and/or established T2D.
The insulin resistance index was assessed by homoeostasis model assessment (HOMA-IR), and calculated as: [fasting glycaemia (mmol/L) × fasting insulinemia (mU/L)/22.5].
The Ethical Committee of Health Research Sciences approved the study. Before enrolment, written informed consent was obtained for each participant, and the confidentiality of the data was ensured.
The data were entered into Microsoft Excel 2016 and analyzed using R software (version 4.2.1). The normality of all the data was explored by using Shapiro-Wilk normality test and the skewed distributions were natural logarithm (ln)-transformed before analysis. One-way ANOVA and Dunnett’s T3 post hoc test were used for the comparison of continuous variables in different groups. Spearman’s test was used for correlation analysis. A stepwise multivariate logistic regression analysis was applied to test the odds ratio and 95% confidence intervals of serum hs-CRP and IL-6 levels for MetS. Receiver-operating characteristic (ROC) curve analysis was employed to identify the optimal cut-off values of Hs-CRP and IL-6 to diagnose MetS.
3. Results
As shown in Table 1, 357 study participants (108 males and 249 females) of ≥30 years old were enrolled, among whom 114 (31.9%) had a MetS and 243 (68.1%) were without a MetS according to NCEP-ATPIII criteria. Participants with MetS had significantly higher fasting insulin, CT, LDL-c levels, and HOMA-IR, than participants without MetS (p < 0.05). Serum IL-6 levels were significantly higher in participants with MetS than in those without (p = 0.002). No statistical significance difference was found between participants with MetS and participants without MetS concerning serum hs-CRP levels (p = 0.092). The characteristics of the study participants by sex are summarised in the Table 2. Results showed that fasting insulin, HOMA-IR, and LDL-C were significantly higher in participants with MetS than in those without in both sexes. hs-CRP and IL-6 were significantly higher only in females with MetS than those without MetS and not in males.
The correlations between individual components of the MetS definition criteria and serum hs-CRP and IL-6 levels are shown in Table 3. In all participants, there was a strong and significant positive correlation between fasting TG and hs-CRP (r = 0.149, p = 0.007) and between fasting TG and IL-6 (r = 0.116, p = 0.037). There was also a positive and significant correlation between FBG and IL-6 (r = 0.208, p = 0.000), between SBP and IL-6 (r = 0.143, p = 0.010), and an inverse significant correlation between HDL-C and IL-6 (r = −0.119, p = 0.034). An inverse non-significant correlation was also observed between WC and hs-CRP (r = −0.087, p = 0.115) or IL-6 (r = −0.055, p = 0.327). In males, hs-CRP levels were positively correlated with fasting TG (0.316; p = 0.000) and negatively correlated with HDL-C (r = −0.290, p = 0.0022). IL-6 levels were positively correlated with FBG (0.202; p = 0.035) and negatively correlated with HDL-C (r = −0.249, p = 0.009). In females, only IL-6 levels were positively correlated with FBG (0.214; p = 0.000) and SBP (r = 0.227, p = 0.000). No significant correlation was observed in females between hs-CRP levels and any of the five components of the MetS definition criteria.
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Table 1. Clinical, biochemical and inflammatory parameters of the study participant.
Notes: Data are Median (interquartile range). Abbreviations: High-density lipoprotein (HDL); Low-density lipoprotein (LDL); Homeostasis model assessment of insulin resistance (HOMA-IR); high-sensitivity C-reactive protein (hs-CRP); Triglycerides (TG); p-value (p).
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Table 2. Clinical, biochemical and inflammatory parameters between males and females.
Notes: Data are Median and interquartile range (IQR). Abbreviations: Total Cholesterol (TC); Diastolic blood pressure (DBP); Fasting blood glucose (FBG); High-density lipoprotein cholesterol (HDL-C); Homeostasis model assessment of insulin resistance (HOMA-IR); High-sensitivity C-reactive protein (hs-CRP); lnterleukin-6 (IL-6); Low-density lipoprotein cholesterol (LDL-C); Triglycerides (TG); Systolic blood pressure (SBP); Waist circumference (WC).
Results from univariate logistic regression revealed no independent association between FBG and hs-CRP (OR = 0.96, 95% CI: 0.69, 1.39; p = 0.837) nor between fasting TG and IL-6 (OR = 1.19, 95% CI: 0.75, 2.01; p = 0.493). In the multivariate logistic regression, we tested the hypothesis that serum hs-CRP and IL-6 levels would increase alongside all components of the MetS (SBP, DBP, WC, FBG, fasting TG and HDL-C) considered simultaneously. Based on stepwise backward selection method, we observed that four components of the MetS (WC, FBG, fasting TG, and HDL-C) were major predictors of increased hs-CRP and IL-6 levels (Table 4, Table 5). Lastly, we performed a ROC curve analysis to explore the diagnostic values of serum hs-CRP and IL-6 for prevalent MetS. As presented in Figure 1, the area under the curve (AUC) for hs-CRP was 0.675 and for IL-6 0.656. The optimal cut-off value for hs-CRP was ≤5 mg/L, and for IL-6 ≤ 15 pg/mL. For hs-CRP the sensitivity and specificity were 70% and 65%, respectively, and for IL-6 the sensitivity and specificity were 52% and 78%, respectively. Moreover, combining hs-CRP and IL-6 increased the discriminatory power for predicting a MetS, with 0.698 AUC, 61% sensitivity 61% and 72% specificity.
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Table 3. Correlation between MetS components and hs-CRP or IL-6.
Signification codes: *** < 0.001; ** < 0.01; * < 0.05. Abbreviations: Diastolic blood pressure (DBP); Fasting blood glucose (FBG); High-density lipoprotein cholesterol (HDL-C); high-sensitivity C-reactive protein (Hs-CRP); lnterleukin-6 (IL-6); Triglycerides (TG); Systolic blood pressure (SBP); Waist circumference (WC).
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Table 4. Logistic regression analysis of MetS components with serum hs-CRP levels.
Notes: Odds ratio (OR); Confidence interval (CI); p-value (p).
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Table 5. Logistic regression analysis of MetS components with serum IL-6 levels.
Notes: Odds ratio (OR); Confidence interval (CI); p-value (p).
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Figure 1. ROC curves of serum Hs-CRP, IL-6, and the combination of hs-CRP and IL-6.
4. Discussion
This cross-sectional study evaluated for the first time the association between MetS components and serum hs-CRP and IL-6 levels among Congolese adults. It is currently agreed that chronic systemic low-grade inflammation is a driver for incident and prevalent MetS. This subclinical inflammatory state is characterized by increased circulating levels of acute-phase proteins such as C-reactive protein (CRP), fibrinogen, serum amyloid A (SAA) and pro-inflammatory cytokines, such as tumor necrosis factor alpha (TNFα), interleukin (IL)-1β and IL-6 [18] . A recent systematic review performed by Rocha F et al. showed that inflammatory biomarker C-reactive protein (CRP) and its inducer IL-6 are associated with components of MetS and may be used in predicting MetS among adolescents with or without excess weight [19] . However, such association has not been described in older persons, particularly from central sub-Saharan Africa, including population from Congo-Brazzaville. The findings of our study show that circulating hs-CRP and IL-6 levels in subjects with MetS are significantly higher than in those without MetS in an age-dependent manner. These results are in agreement with a study by Christianna et al. among Nigerian adults aged 30 to 70 years [20] , and with another study by Belfki et al. among Tunisian adults aged 35 to 70 years [21] . In contrast, the results of study performed by Alenad et al. among Saudi adults aged 30 to 65 years revealed that circulating IL-6 levels were lower in subjects with MetS than in those without [22] . This discrepancy between findings could be related to differences in sample sizes and ages. IL-6 and CRP levels in elderly persons systematically increase in an age-dependent manner, as confirmed in a recent study by Puzianowska-Kuznicka et al. [23] . We found also significant differences between males and females in serum hs-CRP and IL-6 levels among Congolese adults. Previous studies have reported gender differences in circulating hs-CRP and IL-6 levels, while Al-Daghri et al. [24] and Zafar et al. [25] . According to the present study, there was a significant correlation between hs-CRP and fasting TG in both gender. Additionally, a significant positive correlation between hs-CRP and TG and a negative significant correlation between hs-CRP and HDL-C were found in males, and a significant correlation between hs-CRP with each MetS component in females. In line with our study, Haddad [26] reported a significant positive correlation between hs-CRP and TG in both gender among adults Iraqi. Other studies among adults Iranian and Taiwanese have also shown a significant positive correlation between CRP and TG in all participants [27] [28] . In contrast, our findings of a negative significant correlation between hs-CRP and HDL-C in males and no significant correlation between hs-CRP and any of the MetS components in females do not fit with a previous study by Belfeki et al. [21] among adults Tunisian. Contrary to hs-CRP, a significant correlation between IL-6 and certain MetS components was reported. The same study conducted by Chen et al. [28] in Taiwan also found a significant correlation between IL-6 and some MetS components such as WC, DBP and HDL-C. On the one hand, IL-6 levels were positively correlated with FBG and negatively with HDL-C in males. On the other hand, IL-6 levels were correlated positively with FBG and SBP in females. In a study by Yilmaz et al. [29] from Turkey among obese women with MetS aged between 25 and 45 years old, the only significant correlation was seen for TG. Stepwise multivariate regression analysis reveals that serum hs-CRP and IL-6 levels increased alongside the number of MetS components. These results corroborate several other studies.
Further, a ROC curve analysis was used to evaluate the diagnostic value of serum hs-CRP and IL-6 levels for predicting MetS. The area under the curve (AUC) for hs-CRP alone was 0.675, and for IL-6 alone 0.656. The sensitivity and specificity of hs-CRP were 70% and 65%, respectively, and for IL-6 52% and 78%, respectively. These results show that serum hs-CRP and IL-6 levels may be considered for establishing a diagnosis of MetS. In support of our results, a study performed by Haddad [26] among Iraqi adults found that serum hs-CRP increased alongside the number of MetS components and their ROC curve analysis demonstrated that hs-CRP could best predict MetS at a cut-off value of 6.5 mg/L (AUC 0.946, sensitivity 81.3%, specificity 93.3%). Another study by Sinatora in Brazil et al. [30] also found by ROC curve analysis of IL-6 in postmenopausal women that this biomarker could not predict MetS at a cut-off value of 2.56 pg/mL (AUC 0.57, sensitivity 72.2%, specificity 42%). We found that combining serum hs-CRP and IL-6 delivered improved discriminatory power to predict a MetS, with AUC 0.698, 61% sensitivity%, and 72% specificity. The diagnostic value of the combination of serum hs-CRP and IL-6 for prevalent MetS needs to be confirmed in other large-scale prospective studies. Lifestyle modification is the most effective way to reduce the incidence of cardiovascular complications caused by MetS. Lifestyle modification (weight control, smoking cessation, alcohol drinking in moderation, diet control, exercise and physical activity, control blood pressure and blood sugar based on evidence in management of MetS [31] . This study has several limitations. The first is its cross-sectional design, which does not allow us to elucidate the directional or causal relationships between serum hs-CRP or IL-6 levels and MetS. Second, the diagnostic values of hs-CRP and IL-6 for MetS that we observed requires confirmation in prospective studies with larger sample size across various ethnicities.
5. Conclusion
Congolese adults with a MetS have higher serum hs-CRP and IL-6 levels than those without this condition, and circulating levels of both progressively increase alongside the number of individual components of the MetS definition. Serum hs-CRP and IL-6 could turn out to be non-invasive biomarkers for diagnosing a MetS, with combined hs-CRP and IL-6 having the greater diagnostic accuracy for MetS.