This cross-sectional study was carried out on 203 individuals without metabolic syndrome and 123 individuals with metabolic syndrome who were registered in the Isfahan Cohort Study. As a prospective cohort study, the Isfahan Cohort Study started in 2002 and will continue until the end of 2012. It used multistage cluster sampling to select people older than 34 years of age from Isfahan, Najafabad, and Arak (3 cities in central Iran). All the demographic and behavioral data, as well as the indices such as blood pressure, body mass index (BMI), and the results of routine blood tests were recorded for all participants in the first year of the study. Afterwards, the occurrence of cardiac infarction, stroke, sudden death, and hospitalization were asked via telephone calls with two-year intervals. The clinical examinations and routine blood tests were reperformed in the fifth and sixth years of the follow-up. Further details were published by Sarraf-Zadegan et al. in 2003.30
The current study used accessible sampling to select 260 individuals with metabolic syndrome and 260 individuals without metabolic syndrome from the participants of the Isfahan Cohort Study. Using the provided telephone numbers in the records of participants, the individuals were invited to take part in the study. Finally, 123 individuals with metabolic syndrome and 203 individuals without the syndrome participated in all steps of the study. They were invited to the Isfahan Cardiovascular Research Center (Isfahan, Iran) at an appropriate time based on their schedule. They then attended interviews and the objectives and methodology of the study were explained for them. After signing a written consent, they were included in the study. The metabolic syndrome was approved according to the protocol of the National Cholesterol Education Program/Adult Treatment Panel (NCEP/ATPIII). Therefore, individuals having three or more of the following criteria were considered to have the metabolic syndrome:
- waist circumference above 102 cm in men and above 88 cm in women;
- blood triglyceride (TG) level ≥ 150 mg/dl;
- fasting blood sugar (FBS) ≥ 110 mg/dl;
- high density lipoprotein (HDL) level ≤ 40 mg/dl in men and ≤ 50 mg/dl in women; and
- systolic blood pressure (SBP) ≥ 135 mmHg or diastolic blood pressure (DBP) ≥ 85 mmHg.34
The members of the control group were individuals without metabolic syndrome who were selected from the same population in the Isfahan Cohort Study. They were included after matching for gender and age. The exclusion criteria were not being able to perform the exercise test, the presence of an absolute contraindication for performing exercise test, for instance myocardial infarction in the recent two days, advanced heart block, acute pulmonary emboli, uncontrolled HTN, acute myocarditis, severe aortic valve stenosis, or uncompensated cardiac failure, pregnancy, and not being willing to participate.4
The participants referred to the Isfahan Cardiovascular Research Center for the examinations after 12 hours of fasting. General physical examination and blood sampling were followed by waist circumference, blood pressure, weight, and height measurements according to international standards.35
TG and total cholesterol (TC) levels were determined using the enzymatic method by an autoanalyzer (Hitachi 902). After evaluating HDL levels using heparin-magnesium precipitation,36
LDL levels were calculated according to Friedewald formula.37
If the TG level was above 400 mg/dl, the LDL level was directly measured using a specific kit. The fasting blood sugar (FBS) level was determined using an enzymatic method (glucose oxidase). The 12-lead electrocardiogram (ECG) was taken by a trained technician according to the World Health Organization's multinational monitoring of trends and determinants in cardiovascular disease (WHO MONICA).38
Exercise test was then carried out according to Bruce or modified Bruce protocol and the HRR value was determined and recorded in each participant's file. The level of hs-CRP was quantitatively determined using specific kits. Since the values were expressed as ≥ zero, we considered levels of 0-6 mg/l as negative and above 40 mg/l as +3.
The data obtained was adjusted for age and then analyzed by generalized linear regression and student's t-test in SPSS15 (SPSS Inc., Chicago, IL, USA). P values below 0.05 were considered to be statistically significant.
In addition, hypercholesterolemia, hypertriglyceridemia, diabetes, and HTN were defined as TC > 200 mg/dl while using cholesterol lowering agents, TG > 200 mg/dl while using TG lowering agents, FBS > 126 mg/dl despite taking anti-diabetic drugs, and SBP > 140 mmHg or DBP > 90 mmHg while taking at least one anti-hypertensive drug, respectively.24