Circulating 25-hydroxyvitamin D levels in relation to blood pressure parameters and hypertension in the Shanghai Women’s and Men’s Health Studies

Abstract

Little is known about the association of circulating 25-hydroxyvitamin D (25[OH]D) and blood pressure (BP) parameters, and hypertension in non-Western populations that have not yet been exposed to foods fortified with vitamins and seldom use vitamin D supplements. A cross-sectional analysis of plasma 25(OH)D levels in association with BP measures was performed for 1460 participants (405 men and 1055 women, aged 40–75 years) of two large cohort studies in Shanghai. Multivariable linear and logistic regressions were conducted. Overall, the prevalence of vitamin D deficiency was 55.8% using NHANES/USA criteria and 29.9% using WHO criteria. The median plasma 25(OH)D level was 38.0 nmol/L for men and 33.6 nmol/L for women (P<0.01) among subjects who were not on antihypertensive drugs. Among men, BP parameters (systolic BP, diastolic BP, and MAP) were significantly and inversely associated with higher quintiles of 25(OH)D compared with the lowest quintile (P_trend <0.05 for all). Vitamin D non-deficient status (WHO criteria) was inversely associated with hypertension (OR_adj =0.29, 95% CI: 0.10–0.82). An inverse association was also found between hypertension and the highest quintile of 25(OH)D (OR_adj =0.16, 95% CI: 0.04–0.65 for ≥50.6 nmol/L; P_trend =0.02). Among women, no significant associations were found for BP parameters and hypertension. The present study shows that vitamin D deficiency is common among adults in urban China. Circulating 25(OH)D levels were inversely related to levels of individual BP parameters and hypertension among middle-aged and elderly men but not in women. More research is needed to investigate the potential gender differential associations.

Introduction

Experimental data indicate that vitamin D is involved in various physiological functions in humans, including cellular inflammation, estrogen biosynthesis, regulating calcium homeostasis and the renin–angiotensin system, and vascular muscle function.^{1, 2} Vitamin D deficiency has been associated with increased risk for numerous aging-related chronic diseases, particularly hypertension, cardiovascular disease (CVD), and osteoporosis.^{3, 4} However, the mechanisms by which vitamin D influences these medical conditions remain unclear. Sunlight exposure is a major determinant of vitamin D status, because vitamin D is naturally present in very few foods.³ The circulating 25-hydroxyvitamin D (25[OH]D) concentration is the most reliable biomarker of vitamin D status, but differs in humans by geographic location, status of food fortification, vitamin supplementation, and the varying intensity of ultraviolet (UV) light to which they are exposed.^{3, 5} Moreover, skin pigmentation, age, sex, lifestyle factors, diet, vitamin D supplementation, and genetic factors influence vitamin D status.^6–8

Some epidemiological studies,^9–11 including recent systematic review and a meta-analysis of 3 cohorts¹² conducted in Western countries, have shown that lower levels of 25(OH)D may increase the risk of high blood pressure or hypertension; however, other studies¹³ including randomized clinical trials^{14, 15} found no such association or found a positive association between vitamin D level and blood pressure (BP) that depends on seasonality^{15, 16} or genetic factor.⁸ Studies that comprehensively examine the association of vitamin D status with BP parameters (systolic BP, diastolic BP, pulse pressure [PP], and mean arterial pressure [MAP]) and clinical categories of BP (normal, prehypertenstion, and hypertension) in both men and women are scarce. In addition, to our knowledge, no such study has been conducted in the Chinese population. Unlike Western populations, the Chinese population has not yet been exposed to foods fortified with vitamin D and seldom uses vitamin D supplements or sunscreen products, all of which may affect vitamin D status.⁵ Thus, it remains unknown whether findings on the association between circulating vitamin D levels and BP found in Western populations can be directly generalized to Chinese populations.

This report describes a comprehensive evaluation of the effect of circulating 25(OH)D levels on individual BP parameters (systolic BP, diastolic BP, PP, and MAP) and on the prevalence of prehypertension and hypertension using data from two longitudinal studies of over 135 000 middle-aged and elderly Chinese men and women. In recent decades, China has experienced a rapid increase in cardiovascular morbidity and mortality, presumably driven by elevated BP or hypertension.^{17, 18} The findings of the current study could help with the development of effective lifestyle and dietary interventions to decrease the risk of CVD morbidity and mortality in both men and women.

Subjects and methods

Participants

The participants of this cross-sectional study came from two large, population-based, prospective cohort studies conducted in Shanghai, China. The Shanghai Women’s Health Study (SWHS) recruited 74 942 women aged 40–70 years between 1997 and 2000 (response rate: 92.7%), while the Shanghai Men’s Health Study (SMHS) recruited 61 582 men aged 40–74 years between 2002 and 2006 (response rate: 74.1%). The SWHS and SMHS followed virtually identical recruitment and follow-up protocols and used similar survey instruments; detailed study protocols have been published elsewhere.^{19, 20} Briefly, permanent residents of seven (for the SWHS) and eight (for the SMHS) urban districts of Shanghai who met the age eligibility criteria were recruited into these studies through in-home visits by trained interviewers. Participants are being followed via biennial questionnaires that gather information on diet, lifestyle, and medical events. At baseline, 56 832 women and 49 169 men contributed blood samples that were immediately stored at −80°C until laboratory analysis could be performed. This study was conducted according to the guidelines laid down in the Declaration of Helsinki and all procedures involving human subjects/patients were approved by the Institutional Review Boards of Vanderbilt University and Shanghai Cancer Institute. Written informed consent was obtained from all participants prior to interview. The current analysis includes 1055 SWHS participants and 405 SMHS participants, who were included in the Cohort Consortium Vitamin D Pooling Project (VDPP) of Rarer Cancers (endometrial, esophageal, gastric, kidney, non-Hodgkin lymphoma, ovarian, and pancreatic cancers) as controls. The VDPP included 10 prospective cohort studies in United States, Finland, and China (i.e., the SMHS and the SWHS), with stored blood samples and was aimed to investigate the association between circulating 25(OH)D levels and rarer cancers using a nested case-control study approach.²¹

Blood pressure measurement and ascertainment of BP categories

Blood pressure was measured for women at the first follow-up survey (2000–2002) and for men at the baseline (2002–2006) survey. Blood pressure measurements (systolic and diastolic BP) were taken uniformly according to standard protocols. Participants were asked to sit and relax. After at least a 5-min rest, heart rate was measured by manual pulse palpation over a period of 30 seconds, and BP was measured using the auscultatory method with a conventional mercury sphygmomanometer. Two BP measurements were taken at 30 second intervals, and an average of these two BP readings (in mmHg) was used in this analysis. The aneroid devices were calibrated every 6 months by staff from the Shanghai Municipal Bureau of Quality and Technical Supervision during the recruitment period.

History of hypertension was defined as having physician-diagnosed hypertension and/or using antihypertensive medication based on self-reported information before BP measurement and blood sampling. Subjects were categorized hierarchically into: normal BP (systolic BP <120 and diastolic BP <80), prehypertension (not normal BP and 120–139 and/or 80–89), and hypertension (systolic BP ≥140 or diastolic BP ≥90) according to the criteria recommended by the Joint National Committee (JNC7) on Prevention, Detection, Evaluation and Treatment of High Blood Pressure.²² The JNC7 criteria for BP categories is the most widely used classification for assessing high blood pressure in China. Subjects on anti-hypertension medication were classified into hypertension group. PP was defined as the difference between systolic BP and diastolic BP. MAP was calculated as diastolic BP plus one-third PP.

Measurement of plasma 25(OH)D

This study included samples that were selected specifically for 25(OH)D measurement as part of the VDPP.¹⁸ The measurement of 25(OH)D in 125μL samples of plasma was performed by Heartland Assays, Inc. (Ames, IA), which is the central laboratory used by the VDPP, using the “LIAISON® 25-OH Vitamin D TOTAL” chemiluminescent immunoassay (CLIA).²¹ Blinded quality control samples were included within each sample batch to assess assay reproducibility. Both the median inter- and intra-assay coefficients of variation (CV) for 25(OH)D measurements were 11.0% and 13.6%, respectively, as described in detail elsewhere.²¹ Circulating 25(OH)D is extremely stable and its measurements are not affected by multiple freeze-thaw cycles.

Ascertainment of covariates

The exposure assessments for both cohorts were essentially identical, except for reproductive and medical information, which was gender-specific. At baseline, information about socio-demographic characteristics, reproductive history, medical history, dietary and lifestyle habits (smoking habits and alcohol and tea consumption), weight history, physical activity, family history of chronic disease, and occupational history was collected, and weight, height, waist circumference, and hip circumference measurements were taken according to a standard protocol. Usual dietary intake was assessed at the in-person interview using a validated food frequency (FFQ) questionnaire and intake of nutrients (g/day or mg/day) and total energy (kcal/day) was estimated using the Chinese Food Composition Tables.^{23, 24}

Statistical analysis

The level of plasma 25(OH)D (independent variable) was not normally distributed (slightly right skewed), whereas BP (dependent variable) was normally distributed. Log-transformation was conducted to approach normal distribution for plasma 25(OH). Demographic characteristics and established risk factors for hypertension in relation to plasma 25(OH)D levels (median, 25^th, and 75^th percentiles) were evaluated for men and women by using the Wilcoxon rank-sum test for categorical variables and Spearman’s rank correlation test (correlation coefficient) for continuous variables. Plasma 25(OH)D levels were evaluated in quintile distributions, with the lowest quintile defined as the reference category. In addition, 25(OH)D level was categorized using the following clinically-relevant common cut-off points: deficient (<37.5 nmol/L), insufficient (37.5–74.9 nmol/L), and sufficient, (≥75.0 nmol/L) according the Third National Health and Nutrition Examination Survey (NHANES-III, USA).^{11, 25} We further examined the deficient (<27.5 nmol/L) and non-deficient (≥27.5 nmol/L) categories set out by the World Health Organization (WHO).²⁶ Vitamin D deficient status was used as the reference group throughout the analyses. We also examined the association of BP categories with 25(OH)D level as a continuous variable (overall in log-transformed 25(OH)D and every 5 unit increase). Blood pressure (systolic BP, diastolic BP, PP, and MAP) differences in 25(OH)D levels were explored to estimate the mean difference (non-standardized β coefficient) and its 95% confidence intervals (CIs) in multivariate linear regression models. Subjects on antihypertensive medication were excluded in the analysis of BP measurements. We also estimated standardized regression coefficients of BP measures with 25(OH)D and found similar association patterns. We chose to present the results of non-standardized regression analysis because they are clinically more relevant than results generated from standardized regression analysis.

Models were adjusted for age, body mass index (BMI), family history of hypertension, education, occupation, alcohol consumption, smoking, physical activity (both self-reported and expressed in metabolic equivalents [METs]), intake of total calories, eggs and vitamin D, and season of blood collection. These covariates were chosen based on their associations with circulating 25(OH)D and as established risk factors for hypertension. Dietary intakes of calcium and sodium were considered to be potential confounder, but were not included in the final models as they did not appreciably alter the mean BP differences. Highly correlated variables, such as systolic BP, diastolic BP, PP MAP, age, and menopausal status, were not included in the same model. We performed gender-specific analyses and analyses of men and women combined with additional adjustment for gender.

We evaluated the associations of 25(OH)D levels with BP categories (among normotensive, prehypertensive and hypertensive subjects) using polytomous logistic regression to estimate odds ratios (ORs) and 95% CIs stratified by gender and for all subjects combined. Models were adjusted for the variables mentioned above. Dose-response trends were analyzed by treating ordinal scores of the categorical variables as a continuous variable in the model. Tests for interaction were performed by introducing a multiplicative interaction term into the main effect models. All statistical tests were based on 2-tailed probability and a significance level set at α <0.05 and performed using SAS statistical software, version 9.2 (SAS Institute, Cary, NC).

Results

The median age of the population was 61 years, women were younger than men (59 y vs. 66 y, P <0.001). Overall, the median 25(OH)D level was 34.7.0 nmol/L, which differed considerably between men and women (38.0 and 33.6, respectively, P<0.001) who were not on antihypertensive medication (data not shown). The circulating 25(OH)D levels (median, 25^th, and 75th percentile) in relation to each category of the selected demographic and established risk factors for hypertension and correlation coefficients of 25(OH)D levels with age, physical activity (METs), BMI and dietary intakes in all subjects by gender are shown in Table 1. Lower 25(OH)D levels were found among women with low levels of education (P=0.07) or less-skilled occupations (P<0.01) compared with women with higher levels of education or occupation. Both men and women who ever smoked were more likely to have lower 25(OH)D levels. Interestingly, men who consumed alcohol regularly had higher 25(OH)D levels. 25(OH)D levels differed by the season in which the blood samples had been collected. In general, high levels of plasma 25(OH)D were found during the period from June to November, while lower levels were found from December to May in both men and women. Physical activity and intake of eggs or dietary vitamin D were positively correlated with 25(OH)D level among women. There was a positive correlation between age and self-reported regular physical activity and 25(OH)D levels among men.

Table 1.

Circulating 25(OH)D levels (nmol/L) in relation to selected demographic and established risk factors for hypertension among men and women in the Shanghai Women’s Health Study (SWHS) and Shanghai Men’s Health Study (SMHS), China

Risk factors	Men (n=405)		Women (n=1055)
	Median (25th, 75th) percentile of 25(OH)D	P value*	Median (25th, 75th) percentile of 25(OH)D	P value
Education
≤Primary	40.4 (31.8, 53.9)		32.7 (23.5, 44.5)
Middle school	41.9 (29.0, 55.4)		34.2 (24.0, 43.5)
High school	37.3 (27.5, 49.8)		34.5 (25.3, 48.1)
≥College	37.3 (29.0, 55.2)	0.25	36.2 (27.3, 48.4)	0.07
Occupation
Professional	39.1 (30.4, 54.9)		34.8 (26.3, 49.0)
Clerical	36.5 (26.8, 55.4)		36.9 (25.9, 46.8)
Manual workers	40.0 (29.6, 50.8)	0.77	32.2 (23.5, 42.4)	<0.01
Smoking status
Never	41.4 (31.7, 54.5)		34.2 (25.0, 45.6)
Ever	37.5 (27.2, 50.9)	0.05	28.8 (20.6, 39.0)	<0.01
Regular alcohol consumption
Never	38.3 (29.0, 51.0)		33.8 (24.7, 45.3)
Ever	42.4 (29.0, 57.5)	0.26	37.6 (26.8, 45.1)	0.63
Regular exercise
Never	36.9 (28.0, 50.2)		33.7 (24.2, 45.9)
Ever	40.7 (29.6, 57.1)	0.06	34.1 (25.2, 44.7)	0.76
Family history of hypertension
Never	41.8 (30.7, 55.4)		33.4 (24.1, 44.5)
Ever	36.9 (27.5, 49.9)		34.3 (25.8, 45.9)
Unknown	35.9 (27.9, 50.7)	0.12	32.4 (22.9, 45.9)	0.47
Season of blood draw
Winter-spring (December-May)	33.0 (24.0, 43.6)		29.6 (22.0, 38.1)
Summer-autumn (June-November)	45.3 (34.7, 60.1)	<0.01	40.3 (29.7, 51.8)	<0.01
	Correlation coefficient	P value**	Correlation coefficient	P value
Age	0.12	0.02	−0.03	0.30
Total physical activity (MET h/wk)	0.06	0.21	0.07	0.02
BMI (kg/m2)	−0.05	0.31	−0.04	0.22
Total energy (kcal/day)	0.06	0.24	0.02	0.62
Total fat (g/day)	0.07	0.15	0.04	0.15
Eggs (g/day)	0.03	0.52	0.13	<0.01
Dietary vitamin D intake (mg/day)	0.08	0.13	0.09	<0.01
Sodium (mg/day)	0.03	0.50	0.02	0.44
Calcium (mg/day)	−0.01	0.99	0.04	0.24
Potassium (mg/day)	0.01	0.89	0.003	0.92

^*Wilcoxon rank-sum test for categorical variables

^**Spearman’s rank correlation test, for continuous variables

Analyses restricted to non-users of hypertensive drugs showed that the association between plasma 25(OH)D and BP measurement vary by gender (Table 2). Among men, all BP parameters were lower among those with higher quintiles of 25(OH)D (P_trend <0.05 for all, except for PP). Similar results were observed when 25(OH)D level was analyzed as a continuous variable. Among women, only PP tended to be lower among those with higher quintiles of 25(OH)D level (β= −2.19; 95% CI: −4.96, 0.57 for the highest quintile of 25(OH)D level), however, the results were not significant (P_trend =0.16). A similar non-significant association was observed when 25(OH)D was analyzed as a continuous variable. There was a significant interaction between gender and 25(OH)D levels (both categorical and continues variables) on all BP measurements (P_interaction <0.05 for all, except for PP, data not shown). In the analyses of men and women combined, there was no statistically significant association between circulating 25(OH)D and systolic BP, diastolic BP, PP or MAP regardless the levels of circulating 25(OH)D were analyzed categorically or continuously.

Table 2.

Mean difference (β) and 95 % confidence intervals (CI) individual blood pressure measurements in relation to circulating 25(OH)D levels by gender among non-users of antihypertensive drugs in the SWHS and SMHS

25(OH)D, nmol/L (quintiles)	Systolic BP		Diastolic BP		PP		MAP
	mmHg	β (95% CI)*	mmHg	β (95% CI)*	mmHg	β (95% CI)*	mmHg	β (95% CI)*
Men (n=260)
<23.5 (lowest)	132.2	0.00 (referent)	84.1	0.00 (referent)	48.1	0.00 (referent)	100.2	0.00 (referent)
23.5–31.3	127.5	−5.36 (−13.05, 2.32)	81.9	−.52 (−.07, 3.04)	45.6	−.85 (−.35, 1.65)	97.1	−.80 (−.98, 2.38)
31.4–38.7	128.8	−.62 (−3.39, 2.14)	79.7	−.29 (−.89, −.69)	49.1	−.33 (−.89, 5.22)	96.0	−.40 (−0.63, −.17)
38.8–50.5	126.7	−.65 (−3.26, 1.95)	81.6	−.45 (−.96, 2.06)	45.0	−.20 (−.65, 2.24)	96.6	−.52 (−.65, 1.61)
≥50.6 (highest)	124.0	−.22 (−6.90, −.54)	77.7	−.23 (−.78, −.67)	46.3	−.99 (−.49, 1.51)	93.1	−.56 (−1.73, −.38)
P trend		0.04		0.04		0.24		0.02
Continuous variable†	127.2	−.85 (−.78, −.93)	80.5	−.79 (−.05, −.54)	46.7	−.06 (−.92, 0.80)	96.1	−.48 (−.07, −.89)
With every 5 unit increase	127.2	−.34 (−.78, 0.09)	80.5	−.20 (−.45, 0.05)	46.7	−.14 (−.46, 0.17)	96.1	−.25 (−.54, 0.04)
Women (n=816)
<23.5 (lowest)	123.0	0.00 (referent)	76.6	0.00 (referent)	46.4	0.00 (referent)	92.1	0.00 (referent)
23.5–31.3	122.8	−.01 (−.53, 3.47)	77.4	0.94 (−.96, 2.84)	45.4	−.97 (−.39, 1.44)	92.6	0.62 (−.66, 2.90)
31.4–38.7	121.6	−.63 (−.31, 2.05)	77.2	0.42 (−.58, 2.42)	44.4	−.05 (−.59, 0.49)	92.0	−.26 (−.66, 2.13)
38.8–50.5	125.0	1.98 (−.74, 5.70)	79.7	2.92 (0.90, 4.94)	45.4	−.94 (−.50, 1.63)	94.8	2.61 (0.19, 5.03)
≥50.6 (highest)	121.9	−.90 (−.92, 3.11)	77.9	1.28 (−.90, 3.46)	43.9	−.19 (−.96, 0.57)	92.6	0.56 (−.05, 3.17)
P trend		0.94		0.06		0.16		0.27
Continuous variable†	122.9	−.08 (−.74, 2.59)	77.7	1.35 (−.08, 2.79)	45.2	−.43 (−.28, 0.43)	92.8	0.88 (−.85, 2.60)
With every 5 unit increase	122.9	−.02 (−.39, 0.35)	77.7	0.18 (−.02, 0.38)	45.2	−.20 (−.46, 0.06)	92.8	0.11 (−.13, 0.35)
Men and women combined** (n=1076)
<23.5 (lowest)	124.5	0.00 (referent)	77.9	0.00 (referent)	46.7	0.00 (referent)	93.4	0.00 (referent)
23.5–31.3	123.8	−.86 (−.05, 2.32)	78.3	0.44 (−.34, 2.22)	45.4	−.30 (−.50, 0.90)	93.5	0.02 (−.00, 2.11)
31.4–38.7	123.4	−.76 (−.04, 1.52)	77.8	−.45 (−.29, 1.39)	45.6	−.32 (−.58, 0.95)	93.0	−.89 (−.05, 1.28)
38.8–50.5	125.5	0.69 (−.62, 4.01)	80.2	2.03 (0.17, 3.89)	45.3	−.33 (−.63, −.96)	95.3	1.58 (−.61, 3.77)
≥50.6 (highest)	122.6	−.42 (−.90, 1.06)	77.9	−.11 (−.06, 1.84)	44.7	−.26 (−.67, −.12)	92.8	−.88 (−.18, 1.42)
P trend		0.42		0.53		0.09		0.96
Continuous variable†	124.0	−.11 (−.62, 1.41)	78.4	0.15 (−.26, 1.57)	45.5	−.26 (−.00, 0.48)	93.6	−.27 (−.93, 1.40)
With every 5 unit increase	124.0	−.13 (−.47, 0.21)	78.4	0.02 (−.17, 0.21)	45.5	−.15 (−.39, 0.08)	93.6	−.03 (−.25, 0.20)

^*Derived from linear regression models adjusting for age (continuous), education (categorical), occupation, BMI (continuous), alcohol use (yes/no), smoking (yes/no), regular physical activity (yes/no), total physical activity (MET, h/wk; continuous), family history of hypertension (yes/no/unknown), intake of total calories, eggs and vitamin D (continuous), and season of blood draw

^**Additionally adjusted for gender

^†Log-transformed 25(OH)D

Note: Menopausal status and age were highly correlated (r=0.81, P <0.01), so the two variables were not included in the same model for women PP (pulse pressure) = systolic BP − diastolic BP; MAP (mean arterial pressure) = diastolic BP + 1/3 PP

In our study population, the prevalence of vitamin D deficiency (<37.5 nmol/L) was 55.8%, while the prevalence of vitamin D sufficiency was only 3.9% (≥75.0 nmol/L) using the NHANES-III, USA criteria. When we applied the WHO’s criteria for vitamin D status, the rate for vitamin D deficiency (<27.5 nmol/L) was 29.9% (data not shown). Prevalence of hypertension (37.5%, systolic BP ≥140 or diastolic BP ≥90 mmHg and/or use of antihypertensive medication) was common in this population and was higher among men than women (47.6% vs. 33.6%, P <0.01). In the analyses of men and women combined, both prehypertension and hypertension were tended to be inversely associated with vitamin D sufficiency or with the highest quintile of 25(OH)D level (Table 3). This inverse association between vitamin D status [i.e., 25(OH)D level] and prehypertension (OR=0.57; 95% CI: 0.26–1.22 for insufficient status and 0.46, 0.14–1.56 for sufficient status, P_trend =0.11) and hypertension (OR=0.56; 95% CI: 0.26–1.21 for insufficient status and 0.42, 0.12–1.43 for sufficient status, P_trend =0.09) was more pronounced among men than women. A similar association was found between hypertension and non-deficient vitamin D status (25(OH)D ≥27.5 nmol/L, WHO criteria) among men (OR=0.29, 95% CI: 0.10–0.82). The highest quintile of 25(OH)D (OR=0.16, 95% CI: 0.04–0.65 for ≥50.6 nmol/L; P_trend =0.02), as well as a continuous variable of 25(OH)D (OR=0.41, 95% CI: 0.18–0.92) and every 5 nmol/L increase in 25(OH)D (OR=0.94, 95% CI: 0.88–1.02), were consistently inversely associated with hypertension among men. Among women, no significant associations were observed between vitamin D status or circulating 25(OH)D level and prehypertension or hypertension. The interaction between gender and 25(OH)D level or vitamin D status on prehypertension and hypertension were statistically significant (P_interaction <0.05 for all, data not shown).

Table 3.

Odds ratios (95% confidence intervals)^* for the association between clinical categories of blood pressure and circulating 25(OH)D by gender in the SWHS and SMHS

25(OH)D, nmol/L	Men (n=405)		Women (n=1055)		Combined (n=1460)**
	Prehypertension n=155 (38.3%)	Hypertension n=193 (47.6%)	Prehypertension n=452 (42.8%)	Hypertension n=354 (33.6%)	Prehypertension n=607 (41.6%)	Hypertension n=547 (37.5%)
NHANES/USA criteria1
Deficient (<37.5)	1.00 (referent)	1.00 (referent)	1.00 (referent)	1.00 (referent)	1.00 (referent)	1.00 (referent)
Insufficient (37.5–74.9)	0.57 (0.26–1.22)	0.56 (0.26–1.21)	1.11 (0.77–1.60)	1.09 (0.73–1.63)	1.00 (0.73–1.37)	1.00 (0.71–1.40)
Sufficient (≥75.0)	0.46 (0.14–1.56)	0.42 (0.12–1.43)	1.51 (0.49–4.60)	1.07 (0.31–3.72)	0.94 (0.44–2.03)	0.86 (0.38–1.95)
P trend	0.11	0.09	0.42	0.71	0.96	0.84
WHO criteria2
Deficient (<27.5)	1.00 (referent)	1.00 (referent)	1.00 (referent)	1.00 (referent)	1.00 (referent)	1.00 (referent)
Non-deficient (≥27.5)	0.42 (0.15–1.20)	0.29 (0.10–0.82)	1.29 (0.90–1.84)	1.02 (0.69–1.52)	1.15 (0.83–1.60)	0.93 (0.66–1.32)
By quintile
<23.5 (lowest)	1.00 (referent)	1.00 (referent)	1.00 (referent)	1.00 (referent)	1.00 (referent)	1.00 (referent)
23.5–31.3	0.54 (0.12–2.39)	0.25 (0.06–1.08)	1.08 (0.66–1.76)	0.95 (0.55–1.63)	1.04 (0.66–1.63)	0.82 (0.51–1.33)
31.4–38.7	0.71 (0.15–3.31)	0.35 (0.08–1.59)	1.24 (0.75–2.05)	0.91 (0.52–1.60)	1.25 (0.79–1.99)	0.93 (0.57–1.53)
38.8–50.5	0.50 (0.12–2.13)	0.30 (0.07–1.25)	1.41 (0.83–2.41)	1.14 (0.63–2.06)	1.29 (0.80–2.08)	1.07 (0.64–1.77)
≥50.6 (highest)	0.33 (0.08–1.35)	0.16 (0.04–0.65)	1.06 (0.61–1.85)	0.97 (0.53–1.79)	0.91 (0.56–1.48)	0.77 (0.46–1.29)
P trend	0.09	0.02	0.48	0.86	0.92	0.65
Continuous variable†	0.54 (0.25–1.21)	0.41 (0.18–0.92)	1.16 (0.77–1.74)	1.14 (0.73–1.80)	1.00 (0.71–1.42)	0.95 (0.66–1.38)
With every 5 unit increase	0.95 (0.88–1.03)	0.94 (0.88–1.02)	1.02 (0.97–1.08)	1.02 (0.96–1.09)	1.00 (0.95–1.05)	1.00 (0.95–1.05)

^*Derived from polytomous logistic regression models adjusting for age (continuous), education (categorical), occupation, BMI continuous), alcohol use (yes/no), smoking (yes/no), regular physical activity (yes/no), total physical activity (MET, h/wk; continuous), family history of hypertension (yes/no), intake of total calories, eggs, and vitamin D (all continuous), and season of blood draw

^**Additionally adjusted for gender

^†Log-transformed 25(OH)D

¹National Health and Nutrition Examination Survey III, USA

²Guidelines on Food Fortification with Micronutrients, WHO/FAO

Note: Subjects with normal BP (<120/80) and non-users of antihypertensive drugs) were the referent for both prehypertension (120–139/80–89 and non-users of antihypertensive drugs) and hypertension (≥140/90 and/or users of antihypertensive drugs) categories

Discussion

In this population-based study, we found a high prevalence of vitamin D deficiency (55.8% using the NHANES III, USA criteria; 29.9% using the WHO criteria) among middle-aged and elderly Chinese men and women in Shanghai. This rate is comparable to the rate found among Chinese people of the same age range in Beijing.²⁷ Overall, we found that higher levels of circulating 25(OH)D were not associated with BP measures. However, in gender-specific analyses, we observed significant inverse associations between 25(OH)D level and BP parameters including systolic BP, diastolic BP and MAP among men. Consistently, vitamin D non-deficiency or higher quintiles of 25(OH)D levels were inversely associated with hypertension among men. There was no strong evidence for associations between circulating 25(OH)D and BP parameters and hypertension among women.

A possible mechanism through which vitamin D affects BP levels may be the involvement of calcitriol or 1,25 dihydroxy vitamin D (1,25(OH)2D) in the renin-angiotensin system (RAS).^{2, 28} Tomaschitz et al, found inverse associations between circulating 25(OH)D and 1,25(OH)2D and circulating renin and angiotensin levels.²⁸ It has been suggested that 1,25 (OH)2D suppresses renin gene transcription by activating the vitamin D receptor (VDR), which binds the cyclic AMP-response element binding (CREB) protein and blocks the renin gene promoter activity, thereby resulting in a decrease in renin production.^{2, 29} An elevated renin level was found to be related to increased BP via increasing angiotensin II, which causes arterial constriction and increased extracellular fluid volume.⁴ A positive association between hypertension and vitamin D deficiency,³⁰ particularly among those with higher parathyroid hormone(PTH) level has been previously reported.²⁸ Young et al found that hypertensive men had a 36% higher serum PTH level and a slightly lower serum 1,25(OH)2D level than normotensive men. However, they did not observe this relationship in women.³¹ Thus, the modifying effect of gender on vitamin D-related changes in BP parameters and hypertension in our study may be explained either by the hypothesis that estrogen regulates vitamin D activity through the autocrine/paracrine system or due to the gender-dependent response of parathyroid hormone (PTH) to vitamin D regulation.^{1, 32}

In general, the results of our study are supported by previous epidemiological findings that suggest a beneficial effect related to sufficient levels of circulating vitamin D on elevated individual BP parameters and hypertension in the US population.^{9, 10} In the NHANES III (1988–1994) and NHANES 2001–2006 cross-sectional studies, an inverse association between serum 25(OH)D levels and systolic BP was observed among non-Hispanic blacks and Mexican Americans with no history of hypertension.^{10, 15} In the recent Insulin Resistance Atherosclerosis Family Study (IRASFS), higher 25(OH)D levels were associated with a decrease in both systolic BP and diastolic BP among Hispanic and African American populations.³³ In a study of among a young Lebanese population (mean age of 23.9±2.9 y), serum 25(OH)D was inversely associated with SBP in men, but not in women similar to the findings of the current study. It has been suggested that vitamin D and estrogen levels influence each other and play a role in maintaining calcium homeostasis.³⁴ Correction of estrogen deficiency in postmenopausal women was associated with increased 1,25(OH)2D production and calcium absorption.³⁵

A recent review and meta-analysis of observational and intervention studies found a significant association between incident hypertension and lower 25(OH)D level in meta-analysis of 3 cohorts.¹² In contrast, a study in Spain found a significant positive association between serum 25(OH)D levels and systolic and diastolic BP in Caucasian men (mean age of 36 years). In men, this association was present only in those with the BB genotype of the BsmI polymorphism in the VDR gene.⁸ A similar tendency was found for PP in their follow-up study of Caucasian men with the BsmI genotype. No such association was found in women.³⁶ These studies suggest that the combined effect of BsmI VDR gene polymorphisms and circulating 25(OH)D on BP may be altered by estrogen.^{8, 36} In addition, Argiles et al. observed a positive association between systolic and diastolic BP and the seasonal variations of circulating 25(OH)D among hemodialysis patients (mean age: 56 years).¹⁶ However, the Longitudinal Aging Study Amsterdam (LASA) study, which included 1,205 older Caucasian men and women (≥ 65 years),¹³ and a small population-based, cross-sectional study of men³⁷ found that neither systolic BP nor diastolic BP were associated with circulating 25(OH)D level. Only two studies have previously evaluated the association between plasma 25(OH)D and blood pressure in a Chinese population and found a contradicting results.^{27, 38} Lu et al found an inverse association between plasma 25(OH)D and diastolic BP in the combined analyses of 1443 men and 1819 women aged 50–70 years from Beijing and Shanghai in China. In contrast, Chan et al found no association between serum 25(OH)D and systolic BP or diastolic BP among men in Hong Kong.³⁸ This inconsistent evidence may be due to differences in vitamin D status in these two study populations.³⁸ More studies are needed to evaluate the gender influence of the association between circulating vitamin D levels and blood pressure.

Many intervention studies have examined the effect of vitamin D supplementation on BP parameters and provided inconsistent results,¹² and few have evaluated the effect in relation to circulating 25(OH)D levels. A double-blind, placebo-controlled study in Sweden found an inverse association between 25(OH)D and both systolic BP and diastolic BP with the use of vitamin D supplements (cholecalciferol or vitamin D3) among middle-aged individuals with normal BP and normal weight. This study also found a decrease in BP with vitamin D supplement use among patients with mild and moderate hypertension, particularly among those with primary hyperparathyroidism or low plasma renin activity.^{39, 40} Similarly, another intervention study from Germany found a decrease in systolic BP after 8 weeks of vitamin D and calcium supplementation in elderly women with insufficient levels of 25(OH)D (<50.0 nmol/L).⁴¹ In contrast, in a meta-analysis of 10 randomized trials, vitamin D supplementation was not significantly associated with both systolic and diastolic BP.¹²

Longitudinal observational studies have also investigated the relationship between circulating 25(OH)D and hypertension, and most of these studies have found an inverse association between hypertension and 25(OH)D level in both women and men of European ancestry.^{9, 12} In the Health Professionals’ Follow-up Study and Nurses’ Health Study in the US, an increased risk of incident hypertension was independently associated with lower 25(OH)D levels in both men and women.⁹ Similarly, in NHANES-III, lower serum 25(OH)D levels were significantly associated with CVD risk factors, including hypertension, among elderly (aged ≥60 years) non-Hispanic African and Mexican American populations.⁴² However, the LASA study found no association between the risk of hypertension and vitamin D status among elderly individuals.¹³ Several other studies found no effect of circulating 25(OH)D on metabolic syndrome, including hypertension in both women and men.^{43, 44} In contrast, a recent systematic review and meta-analysis of 28 studies including 99 745 participants found a 43% reduction in cardiometabolic disorders, including cardiovascular disease, diabetes, and metabolic syndrome, associated with the highest levels of circulating 25(OH)D in a middle-aged and elderly population.⁴⁵

In intervention studies on the use of ultraviolet light, both UVB and UVA lights have been shown to decrease BP. One study observed a significant decrease in systolic BP and normalized 25(OH)D level with the use of full body UVB light (three times weekly for 8 minutes, which is equivalent to a 12- to 24-minute exposure to sunlight) in women (aged 26–66 years) with untreated mild hypertension and low plasma 25(OH)D) levels (<50.0 nmol/L).⁴⁶ Another study found that systolic BP significantly decreased with the use of UVB/UVA lights in both Caucasian and African American women without or with hypertension.⁴⁷ Similarly, a recent experimental study of healthy volunteers showed that BP parameters (systolic BP, diastolic BP, and MAP) were significantly lower after exposure to UVA light of up to 60 minutes.⁴⁸ Although the results from these intervention studies are not directly comparable to our findings, they clearly provided additional support for the association between the levels of BP parameters and circulating 25(OH)D.

Our study has several limitations. The association between vitamin D status and risk of high BP could have been obscured by the effect of seasonal variation on the blood samples and demographic, dietary and lifestyle factors, and medical history. However, we carefully adjusted for many of these factors in the statistical analyses. Nevertheless, the possibility of residual confounding remains. The relatively small number of individuals with sufficient levels of vitamin D (≥75 nmol/L 25[OH]D) in this study prevented us from conducting any subgroup analyses. Another limitation is the use of only a single measurement of 25(OH)D, which may have led to an underestimation of the association between vitamin D status and BP levels. However, prior studies have shown that circulating 25(OH)D concentration is relatively stable over time.⁷ Our study is also limited because the clinical categories of BP (normotensive, prehypertensive or hypertensive) were classified by using BP measurements taken in a single home visit and self-reported antihypertensive medication use, which could have lead to misclassification of disease status. However, misclassification in the assessment of hypertension is likely to be independent of 25(OH)D level. Thus, the misclassifications are most likely to be non-differential that will result in an underestimation of the true association. Lastly, we did not collect information on plasma/serum PTH; we were therefore unable to investigate their confounding or modifying effect on the efficiency of 25(OH)D levels.

This study has several notable strengths, including the use of data from population-based cohort studies with high response rates and a relatively large sample size, which minimized selection bias, and the uniqueness of the geographic location, lifestyle, dietary characteristics, and biological samples represented by our participants. Circulating 25(OH)D was measured using blood samples that were collected prior to or during the BP measurement. BP was measured by retired medical professionals at the participants’ home according to a standard protocol. Unlike foods in many Western countries, the Chinese food supply is not fortified with vitamin D. Therefore, plasma 25(OH)D concentrations tend to be consistent. This enabled us to investigate the effect of vitamin D status, which mainly determined by natural exposures, on BP.

In summary, we found that circulating 25(OH)D levels are inversely associated with levels of individual BP parameters and the prevalence of hypertension among middle-aged and elderly Chinese men but not in women. Further studies are needed to confirm this gender-specific association and to identify the underlying mechanisms.