The main contents include: (1) Questionnaire design: questionnaire is designed according to the research contents and purposes. The standard questionnaires will be used to collect the general information, dietary habits, lifestyle, medical history and family history etc.(2)Anthropometric measurements: blood pressure, height, body weight, waist and hip, pulse will be acquired by medical practitioners who received professional trainings based on World Health Organization (WHO) standards, and passed relevant examinations.(3) Dietary intervention: The dietary intervention comprise a 3-day baseline observation period, a low-salt diet for 7 days (3g of salt or 51.3mmol of sodium per day) and a high-salt diet for 7 days (18g of salt or 307.8mmol of sodium per day). During the baseline period, each subject is given detailed dietary instructions to avoid table salt, cooking salt, and high-salt foods for the 21-day study duration. To ensure compliance of study participants with the intervention program, they are required to have their breakfast, lunch, and dinner at the study kitchen under supervision of the study staff during the entire study period. All foods are cooked without salt. Onsite study staff members add prepackaged salt to the meals of individual subjects as indicated by the study protocol. (4) Blood and urine samples collection: morning fasting blood, nocturnal enuresis, and 24-hour urine specimen will be collected by medical staffs, and blind tubes will be set up according to 5% of the total samples. Blood biochemistries including serum total cholesterol, HDL cholesterol, LDL cholesterol, triglycerides, fasting glucose, creatinine and hsCRP can be measured using automatic biochemical analyzer. The sodium and potassium concentrations in the urine will be measured by flame photometry. The total sodium and potassium excretions in urine in 24h were calculated by multiplying the concentration and 24-h volume of urine. (5) DNA methylation and LncRNA detection: Using a method of modifying a hydrogen sulfite salt and gene chip technology to detect DNA methylation and LncRNA. (6) LncRNA: LncRNA will be measured with gene chip technology after the high salt intervention period. (7) Auxiliary examinations: Auxiliary examinations including carotid intima-media thickness, endothelium-dependent vasodilation (FMD), measurements of brachial-ankle pulse wave velocity (baPWV) and electrocardiographic parameters. The measurement will be done in hospitals by medical practitioners who received professional trainings and passed relevant examinations. (8) Data processing and statistics: the investigators should input the data to the database, and utilize software such as SPSS, STATA, Haploview and FBAT to analyze. (9)Quality control: ① Questionnaire is strictly designed and amended by epidemiologist and clinical experts. ② A standardized "Investigator Handbook" will be stipulated according to the guidelines. ③ Staff members are required to do a rigorous training and pass the exam. ④ The investigation process will be strictly supervised by the principal investigator. ⑤ Data entry use the parallel double entry method. ⑥ During laboratory testing, blank control and blind detection are applied to ensure the quality. ⑦Data analysis is performed by 2-3 postgraduates, inconsistent data need to be double checked.
