This cohort study, as a part of KoGES, was conducted primarily to evaluate the association between lifestyle factors and genetic risk factors associated with the incidence of chronic diseases in Korean adults, which began in 2001 [18].

The cohort included 10,030 Korean adults from 40 to 69 years who lived in suburbs near the capital city. Baseline data were collected from 2001 to 2002, and follow-up surveys were conducted every 2 years over an 8-year period (2009–2010). Participants at baseline were asked to complete questionnaires on demographic information, lifestyle, medical history and health conditions; and blood pressure and medication use were assessed biennially.

A total of 5,012 were excluded from this study for the following reasons; 3,271 had hypertension at baseline, 111 had cardiovascular disease or cancer, 707 did not participate in the follow-up surveys, 207 had extraordinary energy intake (< 500 kcal or > 6,000 kcal) [1319], 574 did not complete the food frequency questionnaire (FFQ), and 142 provided insufficient information. As a result, the data from 5,018 individuals (2,439 men and 2,579 women) were used for this study.

This study using secondary data from KoGES was approved by the Institutional Review Boards of the Korea Centers for Disease Control and Prevention and Kyung Hee University (KHSIRB-16-022).

Dietary assessment was done using a 103-item semiquantitative FFQ with validation [20] at baseline (2001–2002) and at the follow-up survey (2005–2006). In the FFQs, the participants were asked about the frequency and portion sizes of their food consumption during the previous year. The frequency of the consumption of each food was classified into nine categories: never/seldom, once/month, 2–3 times/month, 1–2 times/week, 3–4 times/week, 5–6 times/week, once/day, 2 times/day, and 3 times or more/day. The portion sizes of the consumed foods were classified into three categories: 1/2 serving, 1 serving (standard), and ≥ 2 servings. The portion sizes were multiplied by the consumption frequencies for each food. The refined grains were classified into three groups: white rice, noodles, and breads. Noodles included ramen, chopped noodles, jangguk noodles, udon, buckwheat noodles, noodles with black bean sauce, and cold noodles. Breads included white bread, red bean bread, custard cream bread, cake, pizza and hamburger buns. The consumption of white rice was divided into five categories (never, 1 - < 2, 2 - < 3, 3, > 3 servings/day), because white rice is very frequently consumed among Koreans. The consumption of noodles was divided into five categories (never, 0 - < 1, 1 - < 3, 3 - < 5, ≥ 5 servings/week) and bread consumption was divided into four categories (never, 0 - < 1, 1 - < 3, ≥ 3 servings/week). Individual food consumption was expressed as food intake per week or day, and the total intake of each food was summed. Fruit included twelve items; vegetables included twenty items; meats included seven items, including pork, beef, chicken and processed meat (ham, sausage, etc.); and dairy foods included milk, yogurt, and cheese. Nutrient intake was measured using the Korean Food Composition Table [21].

Blood pressure was measured by trained technicians using mercury sphygmomanometers. Each individual underwent the measurement after at least five minutes of resting in a seated position (W.A. Baum Co. Inc., New York, USA). Systolic blood pressure (SBP) and diastolic blood pressure (DBP) were recorded twice, and the average was taken for analysis. Hypertension was diagnosed as SBP ≥ 140 mmHg or DBP ≥ 90 mmHg or the use of antihypertensive medication. SBP and DBP were calculated at Korotkoff phase I and Korotkoff phase V, respectively [22].

Demographic factors, socioeconomic status, and lifestyle factors were examined using structured questionnaires given by trained research staff. Educational level was classified into three groups: ≤ 6 years, 7 - ≤ 12 years, and > 12 years. Monthly household income was classified into four groups: < 1 million Korean Won (KW) (approx. < 890 US$ in 2018), 1 - < 2 million KW (approx. 890 - < 1,780 US$), 2 - < 3 million KW (approx. 1,780 - < 2,670 US$), and ≥ 3 million KW (≥ 2,670 US$). Smoking status was categorized into lifetime non-smoker, former smoker, and current smoker. Alcohol consumption was categorized into lifetime non-drinker, former drinker, and current drinker. Physical activity was evaluated using metabolic equivalent of task (MET) [23]. To obtain METs, the participants reported hours spent sleeping and five types of activities classified according to activity intensity. Height was measured to the nearest 0.1cm using a stadiometer (Aluminum anthropometer, Samhwa Instrument, Seoul, Korea), and body weight was measured to 0.1 kg in light clothes using Inbody 3.0 (Biospace Corp., Seoul, Korea). Body mass index (BMI) was defined as body weight divided by the square of height in meters (kg/m²).

All data analyses were performed with SAS software version 9.4 (SAS Institute, Cary, NC, USA) [24]. Data were presented as means and standard deviations for continuous variables, and were presented as numbers and percentages for categorical variables. Differences between baseline characteristics according to the development of hypertension were examined with Chi-squared tests or Student's t-tests. Comparisons of variables according to noodle intake were examined with Chi-squared tests or generalized linear models. The hazard ratios (HRs) and 95% confidence intervals (CIs) for hypertension risk across noodle intake were calculated with Cox proportional hazard models. Survival analysis was performed separately for men and women. Age was adjusted in Model 1. The covariates in Model 1 plus educational level, household income, smoking status, alcohol intake, physical activity, and BMI were adjusted in Model 2. The covariates included in Model 2 plus nutrient intakes and food intakes were adjusted in Model 3. Variables for adjustment were selected through potential confounders from the published scientific literature and the statistical approach [25].

For individuals who developed hypertension or were censored between baseline and the follow-up survey, noodle consumption was assessed based on the FFQ at baseline. For those who developed hypertension or were censored after the follow-up survey, noodle consumption was assessed based on the average of FFQ at baseline and FFQ at the follow-up.

The assumptions of proportional hazards were assessed using log-log plots [26] and Schoenfeld's residuals [27], and accepted. P values of < 0.05 were considered to be statistically significant for all tests.

The average follow-up period was 72.1 months, and the follow-up rate was 64%. During the follow-up period, 1,377 cases of hypertension (710 men and 667 women) were recorded among 5,018 subjects. The baseline characteristics of the subjects, according to the development of hypertension during follow-up, are presented in Table 1. The subjects who acquired hypertension were older and more likely to be men, to live in a rural area, and to be exercisers compared to subjects who did not acquire hypertension. The subjects who acquired hypertension had lower income, were less likely to be educated, and were more likely to be current smokers than those who did not acquire hypertension. The subjects with hypertension had a higher BMI than those who did not acquire hypertension. Also, the energy intake from carbohydrates was higher, and fruit and dairy intake was lower, in subjects who had acquired hypertension compared with those without hypertension.

Among the men, the frequent noodle consumers (≥ 5 servings/week) were younger and more likely to have a higher household income and be current-drinkers, compared with infrequent noodle consumers or never consumers (all P < 0.05) (Table 2). Among the women, frequent noodle consumers were younger, more likely to live in a rural area, to be current alcohol drinkers, and to be educated compared with infrequent noodle consumers or never consumers (all P < 0.05). Frequent noodle consumers among men and women had a higher intake of total energy, sodium, and energy from protein and fat, but a lower intake of energy from carbohydrates compared with infrequent noodle consumers or never consumers (all P < 0.05) (Table 3).

Among the women, frequent noodle consumers (≥ 5 servings/ week) had a 2.3-fold higher risk of hypertension after adjusting for potential confounders (HR = 2.31, 95% CI = 1.33–4.01, P for trend = 0.0001) (Table 4). However, no significant association between noodle intake and risk of incident hypertension was observed in men. The consumption of other grain products, specifically white rice and breads, was not associated with the risk of developing hypertension.