In this case-control study, the required data was extracted from the Prospective Epidemiologic Research Studies in Iran (PERSIAN) in Sabzevar, Iran. The Persian Cohort study Protocol was published elsewhere [21]. A total of 5,000 residents of Sabzevar city, Iran were initially selected by convenient sampling. The inclusion criteria were people with Iranian nationality, households located within the study area, aged 35 to 70 years, consenting to the participation in the study, and recent diagnosis of heart diseases, ischemia, infarction, and high blood pressure for the case group. On the other hand, People who did not want to continue the participation in the study, people who had communication problems and were unable to answer the study questions, those with mental illnesses (characterized by a clinically significant disturbance in an individual’s cognition, emotional regulation, or behaviour [22]), mental retardation, hearing and intellectual impairments or visual loss, and any other disorder that could interfere with the research process were all excluded from both groups. Other exclusion criteria included incomplete medical records, the diagnosis of some serious underlying diseases like cancer, diagnosis of heart diseases more than two months before entering the study, and the use of steroid supplements, barbiturates, and carbamazepine. Finally, 759 people were excluded and a total of 4241 people participated in the study. The case group included 1535 patients (977 patients with hypertension, 435 patients with myocardial ischemia, 70 patients with myocardial infarction, and 53 patients with stroke) and the control group included 2706 healthy adults.

Data were initially collected using valid and reliable questionnaires designed to gather information from participants in PERSIAN cohort, with the approval of the pertinent officials at the PERSIAN Cohort Center in Sabzevar, Iran. The participants were invited to the cohort site and the required data was collected by trained personnels. The collected information was as bellow:

A valid and reliable PERSIAN cohort general questionnaire [21] was used to collect information on demographic characteristics, physical activity level, occupation (yes/no), smoking (yes/no), and drinking alcohol (yes/no), medical history (e.g. use of certain specific medications such as statins, heart diseases, diabetes, and hypertension), and family history of heart attack, stroke, and diabetes. Physical activity was measured in four levels: Level 1 activities defined as sedentary work mostly done while sitting (e.g., driving); Level 2 activities defined as standing or occasional walking (e.g., teaching); Level 3 activities defined as mainly indoor activities causing a mild increase in heart rate and sweating (e.g., housekeeping); and Level 4 activities defined as those causing a significant increase in heart rate and sweating usually performed outdoors (e.g., farming). The level of physical activity was represented as the metabolic equivalent of task (MET).

Three times measurements of systolic (SBP) and diastolic (DBP) blood pressure were performed with a validated sphygmomanometer (Omron M10-IT model, Omron Healthcare, Kyoto, Japan), and the average of the last two measurements was considered for each participant. The measurements were made on the participant’s dominant arm in a seated position after at least 5 min of rest with a cuff of appropriate size, as determined by measurement of the upper arm circumference and following the recommendations of the European Society of Hypertension [23].

The presence of at least two of the following criteria was used to diagnose acute MI: (1) typical chest pain lasting more than 30 min, (2) ST elevation > 0.1 mV in at least two adjacent electrocardiograph leads and (3) an increase in the serum level of cardiac markers (including creatine kinase (CK), creatine kinase-myoglobin binding (CK-MB), CK-MB mass (CK-MBm),

or troponin (cTn) [24]. Stroke was defined based on the definition proposed by the World Health Organization. Accordingly, a rapid-onset focal neurological disorder persisting at least 24 h with probable vascular origin was defined as a stroke case. Ischemic heart disease (IHD) included unstable angina, MI, and sudden cardiac death [25].

Participants’ weight and height were measured with light clothes and without shoes. Weight was measured in kilograms to the nearest 0.1 kg using a mechanical column scale (Seca 755, Germany) and height was measured in centimeters (cm) to the nearest 0.1 cm using a stadiometer (Seca 204, Germany). The BMI of participants was calculated as weight (in kilograms) divided by the square of height (in meters). According to the World Health Organization (WHO), individuals with BMIs of lower than 18.5, 18.5 to 24.9, 25 to 29.9, and 30 kg/m2 and higher were classified as underweight, normal weight, overweight, and obese, respectively [26].

The data of participant’s habitual food intake was obtained through a reliable and valid semi-quantitative food frequency questionnaire (FFQ), comprising 168 food items usually consumed by Iranians [27]. The frequency of food consumption over the past year was examined through a face-to-face interview. Household measures were taken into account for portion sizes and then were converted to grams. The food composition table (FCT) of the United States Department of Agriculture (USDA) was used to evaluate the amount of energy and nutrients. The Iranian FCT was considered for local foods that were not existed in the FCT. Data collected from FFQ were converted to grams of nutrients using Nutritionist IV software (First Databank Division, the Hearst Corporation, modified for Iranian foods). Based on FFQ, participants were asked about their frequency of habitual egg consumption during the last 12 months.

Descriptive statistics, such as standard deviation, median, and mean for quantitative variables, and number and percentage for qualitative variables were used to describe the subject’s sociodemographic and anthropometric markers. Chi-squared test and independent t-test methods were used for qualitative and quantitative data, respectively. Multivarible loggestic regression was used to assess the association between egg consumption and CVDs including HTN, CI, MI, Stroke, and all CVDs after adjusting the potential confounding factors which were selected based on the results of previous studies. The odds ratio (OR) with a 95% confidence interval (CI) was presented for the occurrence of different types of CVDs. Data analysis was accomplished in SPSS software version 21 (SPSS Inc, Chicago, USA) and a probability level of P < 0.05 was regarded as statistically significant for all analyses.