Exploration of the Association between Dietary Fiber Intake and Hypertension among U.S. Adults Using 2017 American College of Cardiology/American Heart Association Blood Pressure Guidelines: NHANES 2007–2014

This study aimed to explore the association between dietary fiber intake and hypertension risk using 2017 American College of Cardiology/American Heart Association Blood Pressure Guidelines. Data from the National Health and Nutrition Examination Survey 2007–2014 were used in this study. Dietary fiber data were obtained through two 24-h dietary recall interviews. Hypertension was defined as systolic blood pressure (SBP) ≥ 130 mmHg or diastolic blood pressure (DBP) ≥ 80 mmHg or treatment with hypertensive medications. Logistic regression models and restricted cubic spline models were applied to evaluate the associations between dietary intakes of total, cereal, vegetable, and fruit fiber and hypertension. A total of 18,433 participants aged 18 years or older were included in the analyses. After adjustment for age, gender, body mass index (BMI), race, educational level, smoking status, family income, and total daily energy intake, compared with the lowest tertile, the odds ratios (95% confidence intervals) of hypertension for the highest tertile intakes of total, cereal, vegetable, and fruit fiber were 0.62 (0.52–0.75), 0.80 (0.67–0.96), 0.82 (0.69–0.98), and 0.86 (0.71–1.04), respectively. Dose-response analyses revealed that the risk of hypertension was associated with total fiber intake in a nonlinear trend, while the relationships were linear for cereal and vegetable fiber intakes. Our results suggested that the intakes of total, cereal, and vegetable fiber, but not fruit fiber, were associated with a decreased risk of hypertension in U.S. adults.


Introduction
Hypertension, defined by the American College of Cardiology (ACC) and American Heart Association (AHA) as blood pressure above 130/80 mmHg [1] in 2017, is a public health issue. The number of people with hypertension increased from 600 million in 1980 to 1 billion in 2008 and is expected to reach 1.5 billion by 2025, accounting for almost one-third of the world's population [2]. The prevalence of hypertension is 46% in African adults aged 25 and above [2], and approximately 31% of U.S. adults aged ≥18 years have hypertension [3]. Complications of hypertension cause 9.4 million deaths each year worldwide, and at least 45% of deaths due to heart disease and 51% of deaths due to stroke are attributed to hypertension. Hypertension is considered the major risk factor for the

Study Population
NHANES aims to assess the health and nutritional status of the U.S. population and adopts a complex multi-stage probabilistic sampling design to select representative samples of the civilian non-institutional U.S. population. NHANES participants were first interviewed in their homes and then completed the health examination in a mobile examination center (MEC) [24]. The NHANES database is a publicly available dataset for use by researchers around the world; data are released in 2-year cycles and can be downloaded from the NHANES website [25]. A total of 40,617 individuals participated in NHANES during 2007-2014. We selected 24,732 individuals who were 18 years of age or older. Among them, we excluded participants with incomplete BP readings (n = 1888), incomplete or unreliable 24-h recall data (n = 3874), and missing weight data (n = 164). We also excluded pregnant (n = 194) or lactating (n = 108) females. Moreover, individuals were omitted who had extreme total energy intakes of <500 or >5000 kcal/day for females, and <500 or >8000 kcal/day for males (n = 71). Ultimately, this study contained a total of 18,433 participants aged 18 years or older (9015 men and 9418 women) ( Figure 1). As NHANES is a publicly available dataset, the present study was exempt from approval by an institutional review board. All participants provided informed consents both before the interview and examination stages.

Blood Pressure Measurements
According to the American Heart Association and the NHANES procedures [1,26], all blood BP determinations (systolic and diastolic) were measured in the MEC. After sitting quietly for five minutes, certified examiners measured each participant's seated blood pressure three times using a mercury sphygmomanometer. A fourth attempt was made if a BP measurement was interrupted or incomplete. We calculated means of systolic blood pressure (SBP) and diastolic blood pressure (DBP).

Definition of Hypertension
First, participants with self-reported use of BP-lowering medication to reduce BP were considered as hypertensive irrespective of the BP value [27]. Second, following the 2017 high blood pressure clinical practice guideline, other participants were divided into non-hypertension group (SBP < 130 mmHg and DBP < 80 mmHg, and hypertension group (SBP ≥ 130 mmHg and/or DBP ≥ 80 mmHg) [1].

Blood Pressure Measurements
According to the American Heart Association and the NHANES procedures [1,26], all blood BP determinations (systolic and diastolic) were measured in the MEC. After sitting quietly for five minutes, certified examiners measured each participant's seated blood pressure three times using a mercury sphygmomanometer. A fourth attempt was made if a BP measurement was interrupted or incomplete. We calculated means of systolic blood pressure (SBP) and diastolic blood pressure (DBP).

Definition of Hypertension
First, participants with self-reported use of BP-lowering medication to reduce BP were considered as hypertensive irrespective of the BP value [27]. Second, following the 2017 high blood pressure clinical practice guideline, other participants were divided into non-hypertension group (SBP < 130 mmHg and DBP < 80 mmHg, and hypertension group (SBP ≥ 130 mmHg and/or DBP ≥ 80 mmHg) [1].

Dietary Fiber Intake
Dietary fiber intake was assessed by two 24-h dietary recall interviews conducted by trained dietitians. The first dietary interview was collected in the MEC and the second interview was collected by telephone 3 to 10 days later. Nutrient intakes were calculated according to the U.S. Department of Agriculture's Dietary Research Food and Nutrition Database for Dietary Studies [28]. Dietary fiber from different types of food was determined according to the food code. In this study, the dietary fiber intakes from both dietary recall interviews were averaged and adjusted to the body weight. Dietary fiber intakes (g/kg/day) were divided into tertiles. It should be noted that the total dietary fiber included fiber from supplements.

Other Potential Factors for Hypertension
In addition to dietary fiber intake, we investigated the influence of potential confounding factors., which included: age (18-39 years, 40-59 years, and ≥60 years), gender (male and female), race (Mexican American, other Hispanic, non-Hispanic White, non-Hispanic Black, and other race), educational level (below high school, high school, and above high school), annual household income (<$20,000, $20,000 to <$50,000, $50,000 to <$75,000, and ≥$75,000), body mass index (BMI) (normal: <25 kg/m 2 ; overweight: 25 to <30 kg/m 2 ; obese: ≥30 kg/m 2 ), total energy intake and smoking status (never, never smoked or smoked <100 cigarettes in life; current, smoked ≥100 cigarettes in life and currently smoking; former, smoked ≥100 cigarettes in life and currently no longer smoking). Total energy intake was calculated by aggregating daily energy intake from diet and dietary supplements.

Statistical Analysis
Student's t-tests were used to compare the mean values between participants with and without hypertension. Chi-square tests were used to compare the percentages of categorical variables between individuals with and without hypertension. Binary logistic regression models were used to analyze the association between hypertension and intakes of total, cereal, vegetable, and fruit fiber. In multivariate logistic regressions, model 1 adjusted for age and sex, and model 2 further adjusted for race, educational level, annual household income, BMI, total energy intake, and smoking status. Furthermore, stratified analyses were performed based on age (18 to <45 years, 45 to <65 years, and ≥65 years) and gender (male and female) to evaluate the relationship between total fiber intake and hypertension. The lowest tertile of dietary fiber intake was used as the reference group. The odds ratios (ORs) and 95% confidence intervals (CIs) were calculated from logistic regression analyses. After one percent abnormal values before and after were rejected, the dose-response relationship was assessed by binary logistic regression model with the use of restricted cubic spline function with three knots located at the 5th, 50th, and 95th percentiles of the exposure distribution. To increase the authenticity of the observed association, the restricted cubic spline model adjusted for the same confounding factors as those adjusted in the logistic regression model 2. The p-value for non-linearity was calculated by testing the value of the coefficient of the second spline of zero. All statistical analyses were conducted using Stata 15.0. To conduct a nationally representative estimate, appropriate sampling weights, primary sampling unit, and strata information were considered in this study. All reported probabilities (p-values) were two-sided with p < 0.05 considered as significant. Table 1 summarizes the information of the sample that included a total of 18,433 eligible participants. The prevalence of hypertension was 44.93%. The percentage of participants with hypertension was higher in the over 60 years group. Hypertension was more likely to occur in males and Non-Hispanic Black participants. Compared with non-hypertensive participants, those with hypertension received a lower level of education and income. Those with hypertension were more likely to smoke and be obese. Moreover, those without hypertension consistently consumed more dietary fiber and dietary fiber subtypes than those with hypertension (Table 1). The weighted ORs (95% CIs) of hypertension according to tertiles of total, cereal, vegetable, and fruit fiber for all participants are shown in Table 2. In univariate logistic regression analyses, total and cereal fiber were associated with a decreased risk of hypertension. Compared with the lowest tertile, the ORs of hypertension for the highest tertile intake of vegetable and fruit fiber were 0.86 (0.76-0.97) and 0.82 (0.71-0.95), respectively. After adjustment for age and sex (model 1), the ORs of hypertension indicated that all levels of total, cereal, vegetable, and fruit fiber were associated with a decreased risk of hypertension. After further adjustment for total energy intake, educational level, race, annual household income, smoking status, and body mass index (model 2), total, cereal, and vegetable Nutrients 2018, 10, 1091 6 of 11 fiber intakes were inversely associated with the risk of hypertension. However, the protective effect of fruit fiber intake on hypertension was no longer significant after adjustment for more covariates. The association between total fiber intake and hypertension in stratified analyses is displayed in Table 3. In stratified analyses by age, all levels of total fiber intake were associated with a decreased risk of hypertension in three models for participants aged less than 45 years. For participants aged 45 to 65 years, all levels of total fiber intakes were associated with a decreased risk of hypertension in the unadjusted model and model 1. In multivariate-adjusted model 2, the inverse association between the highest level of total fiber intake and hypertension was significant. For participants aged 65+ years, the inverse association with the risk of hypertension was significant in the highest tertile of total fiber intake in the unadjusted model and model 1, with ORs (95% CIs) of 0.64 (0.47-0.88) and 0.61 (0.45-0.81), respectively. However, the inverse association between total fiber intake and the risk of hypertension was not significant in multivariate adjusted model 2. In stratified analyses by sex, the inverse associations of total fiber intake with hypertension were similar in males and females and significant in the unadjusted model and the age-adjusted model; the ORs (95% CIs) of hypertension in the highest tertile of total fiber were 0.66 (0.52-0.84) and 0.58 (0.45-0.75), respectively, in model 2.

Results
The result of the dose-response relationship analysis between total dietary fiber and hypertension is shown in Figure 2. A nonlinear negative correlation between total dietary fiber intake and hypertension was found (P for nonlinearity < 0.01). With an increase of total fiber intake, there are no further reductions in hypertension risk beyond 0.35 g/kg/day (OR: 0.47; 95% CI: 0.36-0.62).
The dose-response relationships between cereal and vegetable fiber intakes and hypertension are presented in Figure 3. In restricted cubic spline models, cereal and vegetable fiber intakes were linear inversely associated with the risk of hypertension (P for nonlinearity = 0.34 and 0.12, respectively). When the intakes of cereal fiber reach 0.016 g/kg/day (OR: 0.97; 95% CI: 0.94-0.99) and vegetable fiber reach 0.052 g/kg/day (OR: 0.76; 95% CI: 0.63-0.99), both dietary fibers show meaningful protective effects on the hypertension. The does-response relationship of fruit fiber was not carried out because no significant association was observed between dietary intake and BP in logistic regression model 2. Hypertensive weighted prevalence (%) in tertiles. 3 Calculated using binary logistic regression, model 1 adjusted for age and gender, model 2 adjusted for age and gender, total energy intake, race, body mass index (BMI), annual household income, smoking status, and educational level. The corresponding stratified variables were excluded from the adjusted models. The lowest tertile of dietary fiber intake was used as the reference group. Results are survey-weighted. * p < 0.05; ** p < 0.01.
The result of the dose-response relationship analysis between total dietary fiber and hypertension is shown in Figure 2. A nonlinear negative correlation between total dietary fiber intake and hypertension was found (Pfor nonlinearity < 0.01). With an increase of total fiber intake, there are no further reductions in hypertension risk beyond 0.35 g/kg/day (OR: 0.47; 95% CI: 0.36-0.62).

Figure 2.
Examination of the dose-response relationship between total dietary fiber intake and the risk of hypertension by restricted cubic splines model. The lowest level of total fiber intake (0.071 g/kg/day) was used as the reference group. The restricted cubic splines model adjusted for age, gender, total energy intake, race, body mass index (BMI), annual household income, smoking status, and educational level. The solid line and dashed line represent the estimated ORs and the corresponding 95% confidence intervals, respectively. OR, odds ratio.
The dose-response relationships between cereal and vegetable fiber intakes and hypertension are presented in Figure 3. In restricted cubic spline models, cereal and vegetable fiber intakes were linear inversely associated with the risk of hypertension (Pfor nonlinearity = 0.34 and 0.12, respectively). When the intakes of cereal fiber reach 0.016 g/kg/day (OR: 0.97; 95% CI: 0.94-0.99) and vegetable fiber Figure 2. Examination of the dose-response relationship between total dietary fiber intake and the risk of hypertension by restricted cubic splines model. The lowest level of total fiber intake (0.071 g/kg/day) was used as the reference group. The restricted cubic splines model adjusted for age, gender, total energy intake, race, body mass index (BMI), annual household income, smoking status, and educational level. The solid line and dashed line represent the estimated ORs and the corresponding 95% confidence intervals, respectively. OR, odds ratio. reach 0.052 g/kg/day (OR: 0.76; 95% CI: 0.63-0.99), both dietary fibers show meaningful protective effects on the hypertension. The does-response relationship of fruit fiber was not carried out because no significant association was observed between dietary intake and BP in logistic regression model 2. 12; the lowest level of vegetable fiber intake (0.004 g/kg/day) was used as the reference group. The restricted cubic splines model adjusted for age, gender, total energy intake, race, body mass index (BMI), annual household income, smoking status, and educational level. The solid line and dashed line represent the estimated ORs and the corresponding 95% confidence intervals. OR, odds ratio.

Discussion
In this cross-sectional study based on a nationally representative large-scale database and updated classification system for hypertension in 2017 [1], we found that the intakes of total, cereal, and vegetable dietary fiber, but not fruit fiber, were associated with a lower risk of hypertension in the US population aged 18 years and older. In the stratified analysis of age and sex, the total fiber intake was inversely related to hypertension in the 18 to <45 years old group, the 45-64 years old group, and in males and females. After adjustment for age, gender, total energy intake, educational level, race, annual household income, smoking status, and body mass index, the association still had statistical significance.
We also found that a nonlinear relationship was apparent for total fiber intake and hypertension. Higher intake of total fiber up to 0.35 g/kg/day was the threshold at which the line started to plateau. The risk of hypertension was reduced by 53% when the total fiber intake increased from 0.07 g/kg/day to 0.35 g/kg/day (Figure 2). Moreover, cereal and vegetable fiber intakes had linear inverse associations with the risk of hypertension.
Although we used new standards for hypertension [1], our findings of the protective effect of total, cereal, and vegetable dietary fiber on hypertension were consistent with previous studies from different populations [6,[14][15][16][17][18]22,29]. Previous cross-sectional studies demonstrated that higher intakes of dietary fiber were associated with lower risk of hypertension [6,14,17,22]. A Mediterranean cohort study also revealed that cereal dietary fiber was inversely associated with a lower risk of hypertension [15]. Meta-analysis studies showed that increasing fiber intake in the general population may contribute to the prevention of hypertension [16,18].
We did not find a significant association of dietary fiber from fruits on hypertension. However, fruit fiber has been largely recognized as a protective factor in hypertension [29,30]. A cross-sectional

Discussion
In this cross-sectional study based on a nationally representative large-scale database and updated classification system for hypertension in 2017 [1], we found that the intakes of total, cereal, and vegetable dietary fiber, but not fruit fiber, were associated with a lower risk of hypertension in the US population aged 18 years and older. In the stratified analysis of age and sex, the total fiber intake was inversely related to hypertension in the 18 to <45 years old group, the 45-64 years old group, and in males and females. After adjustment for age, gender, total energy intake, educational level, race, annual household income, smoking status, and body mass index, the association still had statistical significance.
We also found that a nonlinear relationship was apparent for total fiber intake and hypertension. Higher intake of total fiber up to 0.35 g/kg/day was the threshold at which the line started to plateau. The risk of hypertension was reduced by 53% when the total fiber intake increased from 0.07 g/kg/day to 0.35 g/kg/day (Figure 2). Moreover, cereal and vegetable fiber intakes had linear inverse associations with the risk of hypertension.
Although we used new standards for hypertension [1], our findings of the protective effect of total, cereal, and vegetable dietary fiber on hypertension were consistent with previous studies from different populations [6,[14][15][16][17][18]22,29]. Previous cross-sectional studies demonstrated that higher intakes of dietary fiber were associated with lower risk of hypertension [6,14,17,22]. A Mediterranean cohort study also revealed that cereal dietary fiber was inversely associated with a lower risk of hypertension [15]. Meta-analysis studies showed that increasing fiber intake in the general population may contribute to the prevention of hypertension [16,18].
We did not find a significant association of dietary fiber from fruits on hypertension. However, fruit fiber has been largely recognized as a protective factor in hypertension [29,30]. A cross-sectional study showed the lowering effect of fruit fiber intake on blood pressure level based on a sample of Nutrients 2018, 10, 1091 9 of 11 805 men aged 40-69 years who were free from clinical hypertension [30]. Additionally, a prospective study demonstrated that fiber from fruit had a significant inverse association with the risk of hypertension among 30,681 white U.S. males aged 40-75 years [29]. The inconsistent results between our study and the abovementioned studies may be explained by differences in demographic characteristics and methodology. First, both studies were conducted in males aged 40 years old and above. Second, for the above studies, the frequency of BP measurements was twice [30] or based on self-reported BP data [29]. Third, fiber intakes were calculated by semiquantitative food-frequency questionnaire in two studies, whereas our findings were derived from three averaged BP measurements and two more detailed 24-h dietary recall interviews conducted by trained interviewers. In addition, the participants in our study were males and females over 18 years old, so further exploration of the relationship between fruit fiber and hypertension based on age and gender stratification analysis is needed.
Our study has several advantages. First, since the update of the standard for hypertension in 2017, we conducted the first study to explore the relationship between total, cereal, vegetable, and fruit dietary fiber and the risk of hypertension, using the new standard for hypertension. Second, we investigated the dose-response relationship between fiber intake and the risk of hypertension.
However, our study also has some limitations. First, we cannot infer causal interpretations of the relationship between dietary fiber intake and hypertension risks because of the cross-sectional design of the study. Second, although a number of potential confounding factors were controlled, we cannot exclude the possibility of residual confusion caused by unmeasured confounding factors. Third, our sample population contains many people who were previously diagnosed with hypertension or were currently taking antihypertensive medications. The dietary patterns of these individuals may have changed, which may have an impact on the outcome. Fourth, we were unable to estimate separate associations between hypertension risks and soluble or insoluble fiber, which may have differential impacts on hypertension risks. Moreover, the two 24-h diet recalls used in the study may be affected by recall bias and cannot accurately reflect individuals' usual intake. Moreover, we cannot infer the mechanisms behind the reverse association between dietary fiber consumption and the risk of hypertension as an epidemiological study.

Conclusions
In conclusion, the intakes of total, cereal, and vegetable fiber, but not fruit fiber, were associated with lower risk of hypertension in U.S. adults in this cross-sectional study. The risk of hypertension gradually decreased as total dietary fiber intake increased until up to 0.35 g/kg/day. Therefore, it might be advantageous to select fiber-rich foods to prevent and control hypertension.