High-sensitivity c-reactive protein and gamma-glutamyl transferase levels are synergistically associated with metabolic syndrome in community-dwelling persons

Participants were recruited at the time of their annual health examination in a rural town located in Ehime prefecture, Japan. Participants were recruited at the time of their annual health examination in a rural town with a total population of 11,136 (as of April 2002) and located in Ehime prefecture, Japan, in 2002. Among the 9,133 adults (4,395 male) aged 19 to 90 years in this population, a random sample of 3,164 (34.6%) subjects was recruited. Other characteristics such as smoking and alcohol habits, and medication, were investigated by individual interviews that were conducted using a structured questionnaire. The final study sample included 1,919 eligible persons. All procedures were approved by the Ethics Committee of Ehime University School of Medicine and each subject gave informed consent to participate.

Information on demographic characteristics and risk factors was collected using the clinical files. Body mass index was calculated by dividing weight (in kilograms) by the square of the height (in meters). We measured blood pressure with an appropriate-sized cuff on the right upper arm of the subjects in a sedentary position using an automatic oscillometric blood pressure recorder (BP-103i; Colin, Aichi, Japan) while they were seated after having rested for at least 5 min. Smoking status was defined as the number of cigarette packs per day multiplied by the number of years smoked (pack year), and the participants were classified into never smokers, past smokers, light smokers (<30 pack year) and heavy smokers (≥30 pack year). The daily alcohol consumption was measured using the Japanese liquor unit in which a unit corresponds to 22.9 g of ethanol, and the participants were classified into never drinkers, occasional drinkers (<1 unit/day), light drinkers (1-1.9 unit/day), and heavy drinkers (≥2 unit/day). Total cholesterol (T-C), triglycerides (TG), HDL-C, fasting blood glucose (FBG), creatinine (enzymatic method), uric acid, immuno-reactive insulin (IRI), plasma high molecular weight (HMW) adiponectin (FUJIREBIO, Tokyo, Japan), hsCRP, and GGT were measured during fasting. Plasma hsCRP concentration was measured using a Behring BN II nephelometer (Dade Behring Inc., Marburg, Germany) and this inter- and intraassay coefficient variations was 3.2 and 6.7%, respectively. Serum GGT concentration was assayed with an automatic analyzer (TBA-c6000, TOSHIBA, Tokyo) and this intraassay-coefficient of variation was 0.87 to 2.11%. Low-density lipoprotein cholesterol (LDL-C) level was calculated by the Friedewald formula. Participants with TG levels ≥400 mg/dL were excluded. Estimated GFR was calculated using the following equation: eGFR = 194×Cr^-1.094×Age^-0.287×0.739 (if female) [18]. Participants with an eGFR <30 mL/min/1.73 m² were excluded. Homeostasis of model assessment of insulin resistance (HOMA-IR) was calculated from FBG and IRI levels using the following formula: {FBG (mg/dL) × IRI (mU/mL)}/405 [19]. Insulin resistance was defined as a HOMA-IR ≥2.6.

We applied condition-specific cutoff points for MetS based on the modified criteria of the National Cholesterol Education Program's Adult Treatment Panel (NCEP-ATP) III report [20]. Metabolic syndrome was defined as subjects with at least three or more of the following five conditions: 1) obesity: BMI ≥25.0 kg/m² according to the guidelines of the Japanese Society for the Study of Obesity (waist circumference was not available in this study) [21, 22]; 2) raised BP with systolic blood pressure (SBP) ≥130 mmHg and/or diastolic blood pressure (DBP) ≥85 mmHg, and/or current treatment for hypertension; 3) Hypertriglyceridemia with a TG level ≥1.69 mmol/L (150 mg/dL); 4) low HDL cholesterolemia with a HDL-C <1.04 mmol/L (40 mg/dL) in men and 1.30 mmol/L (50 mg/dL) in women; and 5) IFG with a FBG level ≥6.1 mmol/L (110 mg/dL) or current treatment for diabetes mellitus.

Data are presented as the mean ± standard deviation (SD) unless otherwise specified, and in the cases of parameters with non-normal distributions (TG, IRI, FBG, HOMA-IR, and GGT) the data are shown as median (interquartile range) values. In all analyses, parameters with non-normal distributions were used after log-transformation. As several background differences between men and women were demonstrated by previous studies [2, 16, 22], statistical analysis was performed according to sex using PASW Statistics 17.0 (Statistical Package for Social Science Japan, Inc., Tokyo, Japan). The differences among groups categorized by sex and presence of MetS were analyzed by Student's t-test for continuous variables or the χ² -test for categorical variables. Correlations between various characteristics and HOMA-IR were determined using Pearson's correlation. Subjects were divided into three groups based on tertiles of serum GGT and hsCRP within sex and then combined to avoid the gender differences. Multiple logistic regression analysis was used to evaluate the contribution of confounding factors for MetS and each component of MetS. The synergistic effect of CRP and GGT was evaluated using a general linear model adjusted for the following parameters: age, smoking status, alcohol consumption, uric acid, and estimated glomerular filtration rate. In addition, we demonstrated whether the ORs for MetS and insulin resistance dose-dependently increased in relation to hsCRP and GGT in subgroups of confounding factors which effected on MetS and insulin resistance (e.g., age, alcohol consumption, uric acid, medication, HMW adiponectin). A p-value < 0.05 was considered significant.

The characteristics of the study participants in relation to sex are illustrated in Table 1. The study included 822 men, aged 61 ± 14 (range, 20-89) years, and 1,097 women, aged 63 ± 12 (range, 21-88) years. Smoking status, alcohol consumption, history of CVD, DBP, TG, uric acid, FBG, hsCRP, and GGT were higher in men than in women, but age, HDL-C, LDL-C, presence of antilipidemic medication, IRI, HOMA-IR, and HMW adiponectin were higher in women than in men. There was no inter-group difference in BMI, SBP, presence of antihypertensive medication, eGFR, and diabetic medication.

Table 2 shows the risk of MetS and abnormalities of its components in relation to hsCRP and GGT among 822 men and 1,097 women. Of these, 141 men (17.2%) and 170 women (15.5%) had MetS. As shown Table 2, BMI, SBP, DBP, TG, LDL-C, FBG, HOMA-IR, presence of diabetic medication, hsCRP, and GGT showed a higher level in participants with MetS than those without in both genders, but HDL-C and HMW adiponectin showed a lower level in those with MetS. Age, presence of antilipidemic medication, and uric acid were higher only in women with MetS, but eGFR was lower in women with MetS. In men, the HOMA-IR increased significantly in correlation with an increase in BMI, SBP, DBP, presence of antihypertensive medication, TG, LDL-C, presence of antilipidemic medication, uric acid, FBG, presence of diabetic medication, hsCRP, and GGT, but decrease in age, HDL-C, and HMW adiponectin. In women, the HOMA-IR increased significantly in correlation with an increase in age, BMI, SBP, DBP, presence of antihypertensive medication, TG, LDL-C, presence of antilipidemic medication, uric acid, FBG, presence of diabetic medication, hsCRP, and GGT, but decrease in HDL-C, eGFR, and HMW adiponectin.

As shown Table 3, after adjustments for age, smoking status, alcohol consumption, uric acid, and eGFR, the prevalence rate of MetS increased significantly in relation to hsCRP and GGT in both genders. In men, the ORs (95% CI) for MetS across tertiles of hsCRP and GGT were 1.00, 1.69 (1.01-2.80), and 2.13 (1.29-3.52), and 1.00, 3.26 (1.84-5.78) and 6.11 (3.30-11.3), respectively. In women, the ORs (95% CI) for MetS across tertiles of hsCRP and GGT were 1.00, 1.54 (0.92-2.60), and 3.08 (1.88-5.06), and 1.00, 1.70 (1.04-2.79) and 2.67 (1.66-4.30), respectively. In men, the ORs of hsCRP were significantly high for MetS components of obesity, low HDL cholesterolemia, and impaired fasting glucose, and the ORs of GGT were significantly high for obesity, raised blood pressure, hypertriglyceridemia, and impaired fasting glucose. In women, the ORs of hsCRP were significantly high for MetS components of obesity, hypertriglyceridemia, low HDL cholesterolemia, and impaired fasting glucose, and the ORs of GGT were significantly high for obesity, hypertriglyceridemia, and impaired fasting glucose. The ORs for HOMA-IR ≥2.6 also increased significantly in relation to hsCRP and GGT in both genders.

In addiction to their direct associations, we observed a synergistic effect between hsCRP and GGT (Figure 1). In Figure 1 subjects were divided into three groups (tertiles) according to CRP and GGT levels within sex. We assessed the statistical significance of the synergistic relationship using a general linear model with the following confounding factors: age, smoking status, alcohol consumption, uric acid, eGFR (Table 4). The interaction between increased hsCRP and GGT was a significant and independent determinant for MetS and HOMA-IR ≥2.6 in both genders.

Next, to control potential confounding by MetS and insulin resistance, the data were further stratified by age, alcohol consumption, uric acid, HMW adiponectin, and medication (e.g., antihypertensive, antilipidemic, and diabetic medication) (Table 5). The ORs for MetS and HOMA-IR ≥2.6 increased significantly in relation to hsCRP and GGT in almost all the subgroups.