Study Design and Participants
Participants aged 30–79 years were enrolled in the population-based cross-sectional study from The China Multi-Ethnic Cohort (CMEC) Study between July 2018 and April 2019, using a multistage, stratified cluster sampling method, in Guizhou Province, China. We recruited initially 7,239 individuals and all people completed a questionnaire, had physical measurements taken, and provided biological samples. After excluding individuals who missed data about diagnosis of MetS (n=644) or missed data of RHR (n=6), 6,589 participants remained in our study sample for observational analyses of the RHR–MetS and its components relationship ultimately.
The study protocol was approved by the Sichuan University Medical Ethical Review Board (K2016038) and the Research Ethics Committee of The Affiliated Hospital of Guizhou Medical University (2018). Written informed consent from all study participants were obtained before taking part in the study.
Data Collection and Measurements
The sociodemographic characteristics, health-related behaviors, and medical history, including sex, age, marital status, residency, educational level, smoking status, alcohol drinking, tea intake, beverage intake, physical activity, taking antiarrhythmic or antihypertensive drugs, and family history of hypertension or diabetes, were collected by face-to-face interviews. Details about the investigation were previously published[21, 22].
Smokers were defined as people who had smoked 100 or more cigarettes during their lifetime. Alcohol drinkers were defined as having consumed alcohol once or more times per week in the past half year. Tea drinkers were defined as people who had consumed tea per week, which lasting more than six months. Beverage drinkers were defined as having consumed beverage weekly and sustained for six months or more. The level of physical activity for each individual was calculated by summing the metabolic equivalent tasks (METs)-hours/day for activities related to occupation, transportation, housework, and leisure-time activities. Taking antiarrhythmic or antihypertensive drugs were defined as the use of antiarrhythmic or antihypertensive drugs in the past two weeks at the time of investigation. Family history of hypertension or diabetes was defined as having at least one first-degree family member with hypertension or diabetes.
Height, weight, and waist circumference (WC) were measured in duplicate using
standard methods. BP level, including systolic blood pressure (SBP) and diastolic blood pressure (DBP), and RHR level were measured in triplicate by using an electronic BP monitor (HEM-770AFuzzy, Omron, Japan) with participants in a seated position after at least a 5-min rest and at 30-sec intervals, and the mean was used for this analysis. Body mass index (BMI) was calculated as weight (kg) divided by height (m) squared.
An overnight fasting blood sample was taken for the measurement of FPG, total cholesterol (TC), TG, HDL-C, and low-density lipoprotein-cholesterol (LDL-C) levels.
Outcomes and Definitions
The Guideline for Prevention and Treatment of Type 2 Diabetes in Chinese (2013 edition) were applied in the diagnosis of MetS and its components, and MetS requires the presence of at least three out of five factors: 1) central obesity (WC ≥ 90 cm for males and ≥ 85 cm for females); 2) elevated BP (SBP ≥ 130 mmHg or DBP ≥ 85 mmHg or drug treatment for hypertension); 3) elevated TG (TG ≥ 1.70 mmol/L); 4) elevated FPG (FPG ≥ 6.1 mmol/L or drug treatment for elevated glucose); 5) decreased HDL-C (HDL-C < 1.04 mmol/L). The information of medication was collected on the basis of a selfreported history.
Participants were grouped into four categories based on RHR level in quartiles (Q1-Q4). All continuous variables were non normal distribution and presented as median (interquartile range), and categorical variables were expressed as number (percentage). Participants characteristics of each group were compared by Kruskal-Wallis H or Chi-square tests. Also, the test for trend was performed with a polynomial linear contrast test in ANOVA test and logistic regression for continuous and categorical variables, respectively.
Logistic regression model was used to calculate adjusted odds ratios (ORs) and 95% confidence intervals (95% CIs). Primarily, RHR level was assessed in quartiles and lowest quartile as the reference group, the test for trend was performed by logistic regression model after the RHR of each group was converted into a continuous variable replaced by the median value of each group. Also, we treated RHR as a continuous variable and calculated the ORs and 95% CIs per 5 beats/min increment in RHR. Model 1 was initially sex- and age-adjusted; Model 2 was further adjusted for marital status, residency, educational level, smoking status, alcohol drinking, tea intake, beverage intake, and physical activity; Model 3 was additionally adjusted for BMI, TC, and LDL-C based on Model 2. In addition, the association between RHR and the clustered metabolic risk based on the abnormal metabolic numbers of MetS components was investigated, participants with 0 abnormal metabolic numbers was considered the reference group. MetS components include central obesity, elevated BP, elevated TG, decreased HDL-C, and elevated FPG. Sensitivity analyses was conducted to test the robustness of the results by excluding participants who took antiarrhythmic or antihypertensive drugs in Model 4 and further excluding participants with family history of hypertension or diabetes in Model 5. Participants were then sub-grouped by sex, age (30–59 and 60–79 years), educational level (middle school or below, and high school or above), smoking status, alcohol drinking, physical activity (tertile, T1-T3), and BMI categories (< 24, 24–27.9, and ≥ 28 kg/m2). To describe the RHR–MetS dose-response relationship, restricted cubic splines model was incorporated in multivariable logistic regression model with adjusted for Model 3.
Analyses involved the use of SPSS 22.0 (SPSS Inc., Chicago, IL, USA) and Stata 12.0 (Stata Corp, College Station, Texas). All reported P values were two-sided, with P < 0.05 considered statistically significant.