Calcium for high blood pressure. Calcium Supplements and Blood Pressure: Understanding the Complex Relationship

31.01.197009.06.2021 by alexxlab

How do calcium supplements affect blood pressure. What is the role of parathyroid hormone in blood pressure regulation. Can calcium intake influence hypertension risk. How does calcium homeostasis impact cardiovascular health.

Содержание

The Intricate Link Between Calcium Intake and Blood Pressure

The relationship between calcium intake and blood pressure has been a subject of extensive research in recent years. Numerous studies have demonstrated an inverse association between calcium consumption and blood pressure levels, suggesting that adequate calcium intake may play a crucial role in maintaining healthy blood pressure.

This relationship is not merely coincidental; it is rooted in complex physiological mechanisms involving calcium homeostasis and the endocrine system. At the center of this intricate connection lies the parathyroid gland and its primary hormone, parathyroid hormone (PTH).

Key Factors in the Calcium-Blood Pressure Relationship

Calcium intake levels
Parathyroid hormone (PTH) secretion
Parathyroid hypertensive factor (PHF)
Calcium homeostasis
Vascular function

Understanding these factors and their interactions is crucial for unraveling the complex relationship between calcium and blood pressure regulation.

The Role of Parathyroid Hormone in Blood Pressure Regulation

Parathyroid hormone (PTH) plays a pivotal role in the regulation of blood pressure, acting as a key mediator in the relationship between calcium intake and hypertension risk. Research has consistently shown that plasma PTH levels are inversely related to calcium intake, while being directly associated with blood pressure.

How does PTH influence blood pressure?

PTH exerts its effects on blood pressure through multiple mechanisms:

Calcium homeostasis regulation
Vascular smooth muscle cell contraction
Renin-angiotensin-aldosterone system activation
Sympathetic nervous system stimulation

These mechanisms collectively contribute to the observed relationship between PTH levels and blood pressure measurements in both normotensive and hypertensive individuals.

Calcium Intake and Its Impact on PTH Levels

The inverse relationship between calcium intake and plasma PTH levels has been extensively studied across various age groups and physiological states. Both acute and long-term studies in humans and animals have consistently demonstrated this association.

What happens when calcium intake is low?

When calcium intake is insufficient, the body responds by increasing PTH secretion to maintain calcium homeostasis. This compensatory mechanism can lead to elevated PTH levels, which in turn may contribute to increased blood pressure.

Conversely, adequate calcium intake helps maintain optimal PTH levels, potentially reducing the risk of hypertension and associated cardiovascular complications.

Evidence from Human Studies: PTH, Blood Pressure, and Calcium Intake

Numerous studies have investigated the relationship between PTH levels, blood pressure measurements, and calcium intake in human subjects. These studies provide valuable insights into the complex interplay between these factors.

Key Findings from Human Studies

Higher PTH levels are associated with elevated blood pressure
Lower calcium intake correlates with increased PTH levels
Calcium supplementation may help reduce blood pressure in some individuals

One notable study by Takagi et al. (1991) demonstrated that calcium supplementation (1 g/day) led to a reduction in blood pressure compared to a diet with lower calcium intake (500 mg/day) in hypertensive subjects aged 65-86 years.

The Parathyroid Hypertensive Factor: An Emerging Player

In addition to PTH, researchers have identified another potential mediator in the calcium-blood pressure relationship: the parathyroid hypertensive factor (PHF). Although not fully characterized, PHF has shown promise in explaining some of the observed effects of calcium intake on blood pressure.

What is known about PHF?

While research on PHF is still in its early stages, several key points have emerged:

PHF is produced by the parathyroid glands
It may contribute to blood pressure elevation
Its production may be influenced by calcium intake
Further research is needed to fully understand its role and mechanisms

The discovery of PHF adds another layer of complexity to the calcium-blood pressure relationship, highlighting the need for continued research in this area.

Calcium Intake and Cardiovascular Health: Beyond Blood Pressure

The impact of calcium intake on cardiovascular health extends beyond its effects on blood pressure. Several studies have shown that PTH levels can independently predict cardiovascular disease risk and mortality.

What does the research reveal about calcium and cardiovascular risk?

The Osteoporotic Fractures in Men (MrOS) study, a prospective multicenter investigation involving 1490 men over 65 years old, provided compelling evidence of the link between PTH levels and cardiovascular outcomes. The study, which followed participants for 7.3 years, revealed that:

Higher PTH concentrations were associated with an increased risk of cardiovascular mortality (adjusted hazard ratio per SD = 1.21, 95% CI = 1.00–1.45)
Elevated PTH levels also correlated with an increased risk of all-cause mortality (adjusted HR per SD = 1.15, 95% CI = 1.03–1.29)

These findings underscore the importance of maintaining optimal calcium intake and PTH levels for overall cardiovascular health.

Calcium Supplementation: Considerations and Potential Interactions

Given the established relationship between calcium intake and blood pressure, calcium supplementation has been proposed as a potential strategy for managing hypertension. However, it’s essential to consider the potential interactions between calcium supplements and blood pressure medications.

Are there any concerns with calcium supplementation?

While calcium supplements can be beneficial for some individuals, there are several factors to consider:

Potential interactions with antihypertensive medications
Optimal dosage and timing of supplementation
Individual variations in calcium metabolism and absorption
The importance of balancing calcium intake with other nutrients, such as vitamin D

It’s crucial for individuals considering calcium supplementation to consult with their healthcare provider to determine the most appropriate approach based on their specific needs and medical history.

Future Directions in Calcium and Blood Pressure Research

While significant progress has been made in understanding the relationship between calcium intake, PTH levels, and blood pressure, many questions remain unanswered. Future research efforts are needed to further elucidate the mechanisms involved and develop targeted interventions.

What are the key areas for future investigation?

Several promising avenues for future research include:

Further characterization of the parathyroid hypertensive factor (PHF) and its role in blood pressure regulation
Investigation of genetic factors influencing calcium metabolism and blood pressure responses
Development of personalized approaches to calcium supplementation based on individual risk factors and physiological characteristics
Exploration of the long-term cardiovascular effects of various calcium intake levels and supplementation strategies
Examination of potential synergistic effects between calcium and other nutrients in blood pressure regulation

By addressing these areas, researchers can gain a more comprehensive understanding of the complex interplay between calcium intake, parathyroid function, and cardiovascular health.

In conclusion, the relationship between calcium intake and blood pressure is multifaceted, involving intricate physiological mechanisms centered around parathyroid function and calcium homeostasis. While current evidence supports the potential benefits of adequate calcium intake for blood pressure regulation and cardiovascular health, further research is needed to fully elucidate the underlying mechanisms and develop targeted interventions. As our understanding of this complex relationship continues to evolve, it is essential for healthcare providers and individuals to stay informed about the latest findings and recommendations regarding calcium intake and its impact on blood pressure and overall cardiovascular health.

Mechanisms Involved in the Relationship between Low Calcium Intake and High Blood Pressure

Calcium intake has a role in blood pressure regulation and the parathyroid glands play a role in calcium homeostasis, thus the link between calcium intake, parathyroid function, and blood pressure seems to be intuitively valid. However, few studies have evaluated the pathways linking calcium intake, parathyroid function, and blood pressure altogether. The following section accounts for the physiological bases of this relationship, focusing on the effects of the parathyroid hormone (PTH) and the not completely purified and characterized parathyroid hypertensive factor (PHF).

2.1.1. Mechanisms Mediated by Parathyroid Hormone (PTH)

Several studies have reported that calcium intake is inversely associated with blood pressure, both in humans [3,11,12,21,22,23] and in animals [2,5,6]. The inverse relationship between calcium intake and plasma PTH levels has also been widely studied at different ages and physiological stages in both humans and animals, in acute and long term studies [24,25,26,27]. Similarly, the direct relationship between plasma PTH and blood pressure both in healthy subjects and animals is also well documented [25,28,29,30,31,32,33]. The relationship between the reported parathyroid hormone serum levels and blood pressure measurements in normotensive and hypertensive subjects is shown in . Studies showing a correlation between blood pressure values and quartiles and quintiles of parathyroid hormone levels in human studies are shown in . Some studies in humans have shown low calcium intake, increased PTH levels, and increased blood pressure in the same subjects, although no description of mechanisms involved in this relationship were mentioned [34,35,36] ().

Table 1

Parathyroid hormone (PTH) serum levels and blood pressure (BP) values in normotensive and hypertensive subjects.

Reference (First Author)	Method	Country and Participants	PTH (pmol/L)	BP (SBP − DBP mmHg)
Young 1990 [28]	Cross-sectional	USA, 115 subjects, ≈45 years	NT = 4. 5 ± 2.2	NT = 120(±11) − 80(±8)
Young 1990 [28]	Cross-sectional	USA, 115 subjects, ≈45 years	HT = 5.0 ± 2.4	HT = 138(±8) − 95(±5)
Brickman 1990 [29]	Cross-sectional	USA, 38 men, ≈56 years	NT = 20.8 ± 1.1	NT = 123(±2.8) − 78(±1.3)
Brickman 1990 [29]	Cross-sectional	USA, 38 men, ≈56 years	HT = 28.4 + 3.5	HT = 150(±3.9) − 97(±0.9)
Morfis 1997 [30]	Cross-sectional	Australia, 123 subjects, 63–88 years	NT = 2. 7 ± 1.1	NT = 125(±12) − 71(±7)
Morfis 1997 [30]	Cross-sectional	Australia, 123 subjects, 63–88 years	HT = 2.9 ± 1.3	HT =135(±14) − 73(±10)
Park 2015 [32]	Cross-sectional	Korea, 1664 postmenopausal women, >50 years	NT = 63.7 ± 23.4	NT = 117.5(±12.4) − 73.3(±8.1)
Park 2015 [32]	Cross-sectional	Korea, 1664 postmenopausal women, >50 years	HT = 68.3 ± 23.6	HT = 149.4(±11.4) − 86.0(±10.1)

Table 2

Blood pressure (BP) values by quartiles or quintiles of parathyroid hormone (PTH) levels in human studies.

Reference (First Author)	Method	Country and Participants	PTH (pmol/L)	BP (SBP − DBP mmHg)
Snijder 2007 [31]	Cross-sectional	The Netherlands, 1205 subjects, participants, 55–85 years	Q1: <2.45	150.1(±26.1) − 82.5(±13.0)
			Q2: 2.45–3. 13	151.7(±24.8) − 82.6(±13.4)
			Q3: 3.14–4.25	154.7(±24.6) – 84.3(±13.6)
			Q4: >4.25	156.2(±27.6) − 83.9(±13.0)
Chan 2011 [25]	Cross-sectional	China, 939 men, >65 years	Q1: <3.1	135.8(±1.7) − 76.5(±0.8)
			Q2: 3.2–4.1	139.9(±1.6) − 76. 5(±0.8)
			Q3: 4.2–5.5	141.4(±1.7) − 76.5(±0.8)
			Q4: >5.5	143.6(±1.8) − 79.9(±0.8)
Yao 2016 [33]	Cohort study	USA, 7504 subjects, 45–64 years	Q1: 3.2–28.8	112(±13) − 68.0(±8.2)
			Q2: 28.9–34.9	113(±12) − 68.4(±8.3)
			Q3: 35.0–41. 5	114(±12) − 69.4(±8.5)
			Q4: 41.6–50.1	115(±12) − 69.9(±8.1)
			Q5: 50.2–162.6	115(±12) − 70.5(±8.2)

Table 3

Parathyroid hormone (PTH) and blood pressure (BP) and dietary calcium intake in human studies.

Reference (First Author)	Method	Country and Participants	Ca Intake (mg/day)		PTH (pmol/L)	BP (SBP − DBP mmHg)
Takagi 1991 [34]	Clinical trial	Japan, 9 HT, 65–86 years, CaSup (1 g) vs. diet Ca 500 mg, 8 weeks	1 g/day (CaSup)		27	−13.6 mmHg to −5.0 mmHg
Takagi 1991 [34]	Clinical trial		500 mg/day (diet)		33	−1.5 mmHg to +1.0 mmHg
Jorde 2000 [36]	Cohort study	Norway, 1113 subjects, 30–79 years	592.1(±459.6)		4.5(±1.2)	143.4(±19.9) − 84.3(±10.4)
Jorde 2000 [36]	Cohort study	Norway, 1113 subjects, 30–79 years	400. 3(±227.3)		9.1(±2.4)	153.9(±27.1) − 89.7(±14.1)
Kamycheva 2004 [35]	Cross-sectional	Norway, 3570 subjects, >24 years	♂	499(±259)	Q1 <1.9	136.9(±17.7)
				476(±257)	Q2 1.9–2.6	140.1(±19.6)
				443(±233)	Q3 2. 61–3.5	142.0(±20.3)
				430(±243)	Q4 >3.5	145.2(±20.3)
			♀	478(±277)	Q1 <1.8	133.2(±18.5)
				428(±227)	Q2 1.8–2.4	135.5(±21.5)
				431(±226)	Q3 2.41–3.3	141. 9(±22.4)
				408(±217)	Q4 >3.3	146.5(±23.2)

Several studies have shown that PTH levels can independently predict cardiovascular disease and mortality [35,37,38,39]. The prospective and multicenter Osteoporotic Fractures in Men (MrOS) study, including 1490 men older than 65 years and followed up for 7.3 years, shows that concentrations of PTH were associated with an increased risk of cardiovascular mortality (adjusted hazard ratio SD = 1.21, 95% CI = 1.00–1.45) and from all causes (adjusted HR per SD = 1.15, 95% CI = 1.03–1.29) [37]. The Multi-Ethnic Study of Atherosclerosis (MESA) cohort study in which 3002 men and women, aged 59 ± 9.7 years, without cardiovascular antecedents were followed up for 9.0 years found that higher PTH serum concentrations were associated with a greater risk of hypertension, even after adjusting for potential confounders (HR = 1. 27, 95% CI = 1.01–1.59) [39]. The population-based, cross-sectional Tromsø study including 3570 men and women aged 25–79 years found a positive relationship between serum PTH and SBP with the highest quartile of serum PTH found to be an independent predictor of coronary heart disease in both sexes (OR = 1.70, 95% CI = 1.08–2.70 for males, OR = 1.73, 95% CI = 1.04–2.88 for females, p < 0.05).

PTH is an 84-amino-acid polypeptide (≈9.5 kilodalton (kDa)) secreted by the chief cells of the parathyroid gland. PTH acts on the cell membrane of its target tissues through a G-protein-coupled receptor, the parathyroid hormone receptor type 1 (PTHr-1). Expression of PTH receptors has been reported in many tissues, including vascular smooth muscle and endothelial cells [40]. The PTHr-1 couples to several signaling pathways, namely: the Gαs/adenylate cyclase (AC)/cAMP/protein kinase A (PKA), the Gαq/phospholipase C (PLC)β/inositol trisphosphate (IP3)/intracellular Ca/protein kinase C (PKC), the Gα12/13 phospholipase D/RhoA pathway, and the mitogen-activated protein kinase (extracellular signal regulated kinase [ERK1/2]) signaling cascade [41,42].

Several mechanisms have been proposed to explain the effect of PTH on blood pressure: (a) an increase in cytosolic free calcium concentration ([Ca²⁺]_i) through the PTH receptor (PTHr-1) in vascular smooth muscle, (b) increase calcitriol concentration, and (c) a cross-talk with the renin–angiotensin–aldosterone system (RAAS). The last two will be described in its corresponding sections.

High [Ca²⁺]_i increases vascular reactivity, and therefore peripheral vascular resistance and responsiveness to the sympathetic and the RAAS, which all elevate blood pressure. Calcium channel blockers, such as nifedipine and verapamil, are valuable antihypertensive drugs as they inhibit Ca²⁺ entry to the cell and reduce [Ca²⁺]_i. In the same way, calcium supplementation in subjects with low calcium intake has been described to decrease [Ca²⁺]_i [43,44], hence diminishing blood pressure. It has been shown that PTH increases calcium entry into a variety of mammalian tissues and cell lines, such as cardiomyocytes [45], enterocytes [46], kidney [47], liver [48], peripheral nerves [49], osteosarcoma cells [50], and osteoblastlike cells [51]. Significantly higher [Ca²⁺]_i was also found in human platelets and lymphocytes of hypertensive patients [29,52]. The activation of PTHr-1/Gq/PLC/IP3, PTHr-1/Gα12-13/phospholipase D/RhoA cascades, and of calcium channels are the signaling pathways by which PTH increases [Ca²⁺]_i and blood pressure [47].

A controversial effect is the vasodilator effect of acute PTH infusion, both in vivo and in vitro. In vascular smooth muscle cells, PTHr-1 couples primarily to Gαs, which increases cAMP and decreases [Ca²⁺]_i [53]. Nonetheless, the sustained activation of this cascade shows desensitization to PTH in a time- and concentration-dependent fashion [54,55,56]. The chronic infusion of PTH has been associated with arterial hypertension [57]. Long-standing high levels of PTH, such as in hyperparathyroidism, are frequently related to hypertension, whereas parathyroidectomy is associated with a decrease in [Ca²⁺]_i and blood pressure [58]. A rise of [Ca²⁺]_i through PTHr-1/Gαs/AC/cAMP via opening calcium channels in a cell line derived from fetal rat aorta was also described [59]. Therefore, the desensitization of the cAMP pathway to PTH, as well as the stimulation of other blood pressure mediators considered below, like the RAAS and calcitriol, may explain the long-term pressor effects of PTH.

2.1.2. Parathyroid Hypertensive Factor (PHF)

In the early 1990s, Lewanczuk et al. described that the infusion of plasma from hypertensive rats and from hypertensive subjects on normotensive rats increased the mean arterial pressure of those rats [60,61]. They attributed this effect to the presence of a novel hypertensive factor in the plasma of the hypertensive donors. The same group also reported the parathyroid origin of this factor by transplanting parathyroid glands from hypertensive rats into parathyroidectomized normotensive rats. An increase in blood pressure was shown in the rats after transplantation [60,62,63]. Due to this, the non-isolated substance was called parathyroid hypertensive factor (PHF) [60].

In spontaneously hypertensive rat strains, low calcium intake increases blood pressure that the authors explained was due to an increase of PFH [64]. These authors also proposed that PHF regulates blood pressure by modifying the concentration of [Ca²⁺]_i in vascular smooth muscle [60,65,66]. In isolated vascular smooth muscle cells from rat tail arteries, Shan et al. found that the infusion of semi-purified plasma from hypertensive rats enhanced the opening of the L-type calcium channel, an effect antagonized by the dihydropyridine calcium channel blocker nifedipine [67].

Although the effect of plasma from hypertensive rats has been well-documented, the purification and characterization of PHF is uncomplete. Benishin et al. showed the correlation of a UV spectrum of dialyzed plasma from hypertensive rats with a small (≈3 kDa), trypsin-inactivated and boiled-resistant peptide [68]. Years later, the same group proposed that the PHF structure has both peptide and lysophospholid motifs, critical for its biological activity [69]. Schlüter et al. also isolated a peptide-like vasopressor of low molecular weight (0.6–2.5 kDa) from parathyroid tissue of patients with tertiary hyperparathyroidism. The eluent was shown to be polar, hydrophilic, and protease-sensitive, but not heat-resistant. Schlüter’s group also suggested that further purifications were needed as several substances were shown on the mass spectrometry [70].

Although the existence of PHF may explain the blood pressure changes induced by alterations in a calcium diet, there is still much to know about this mediator. First, the published results have been poorly replicated outside the group that described PHF [71]. On the other hand, after 30 years of being described, the definitive chemical structure of this mediator is still unknown and there have not been reports of new attempts to purify it. Nonetheless, the function of the parathyroid gland seems to play a key role in blood pressure regulation, as was reported both in humans and in normotensive and hypertensive animals.

In summary, the bulk of information of this section orients towards an increase of the parathyroid gland activity by low calcium intake leading to an increase of [Ca²⁺]_i in vascular smooth muscle cells and consequently a rise in blood pressure (). In addition to the direct effect of PTH (or PHF) in smooth muscle, we will later discuss the effects of PTH mediated by calcitriol and RAAS. A demonstration of the effect of parathyroid activity in the regulation of blood pressure related to calcium intake is shown in a study of parathyroidectomized rats in comparison with sham operation rats. After 10 weeks on a calcium-free diet, the sham operation rats showed an increase in SBP of 3.44 (SE = 1.95) mmHg, while the parathyroidectomized rats showed a decrease in SBP of −9.67 (SE = 2.05) mmHg. A highly statistical significant difference of 13.11 mmHg was found between the two groups [2].

Daily calcium intake and its relation to blood pressure, blood lipids, and oxidative stress biomarkers in hypertensive and normotensive subjects

Nutr Res Pract. 2012 Oct; 6(5): 421–428.

,¹,² and ³

Mi-Hyun Kim

¹Department of Food and Nutrition, Kangwon National University, Samcheok 245-711, Korea.

So Young Bu

²Division of Food Science, Kyungil University, Gyeongsan 712-701, Korea.

Mi-Kyeong Choi

³Division of Food Science, Kongju National University, 54 Daehak-ro, Yesan, Chungnam 340-742, Korea.

¹Department of Food and Nutrition, Kangwon National University, Samcheok 245-711, Korea.

²Division of Food Science, Kyungil University, Gyeongsan 712-701, Korea.

³Division of Food Science, Kongju National University, 54 Daehak-ro, Yesan, Chungnam 340-742, Korea.

Corresponding author.

Received 2012 May 24; Revised 2012 Jun 29; Accepted 2012 Jul 16.

Copyright ©2012 The Korean Nutrition Society and the Korean Society of Community NutritionThis is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons. org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.This article has been cited by other articles in PMC.

Abstract

Several studies revealed that low calcium intake is related to high prevalence of cardiovascular diseases such as hypertension. The prevalence of hypertension is high in Koreans along with their low dietary calcium consumption. Thus, the aim of this study was to evaluate the status of calcium intake between the hypertension and normotension groups and to investigate the correlation between dietary calcium intake and blood pressure, blood lipid parameters, and blood/urine oxidative stress indices. A total of 166 adult subjects participated in this study and were assigned to one of two study groups: a hypertension group (n = 83) who had 140 mmHg or higher in systolic blood pressure (SBP) or 90 mmHg or higher in diastolic blood pressure (DBP), and an age- and sex-matched normotension group (n = 83, 120 mmHg or less SBP and 80 mmHg or less DBP). The hypertension group consumed 360.5 mg calcium per day, which was lower than that of the normotension group (429.9 mg) but not showing significant difference. In the hypertension group, DBP had a significant negative correlation with plant calcium (P < 0.01) after adjusting for age, gender, body mass index (BMI), and energy intake. In the normotension group, total calcium and animal calcium intake were significantly and positively correlated with serum triglycerides. No significant relationship was found between calcium intake and blood/urine oxidative stress indices in both groups. Overall, these data suggest reconsideration of food sources for calcium consumption in management of the blood pressure or blood lipid profiles in both hypertensive and normotensive subjects.

Keywords: Dietary calcium intake, hypertension, oxidative stress indices, plant calcium

Introduction

The relationship between calcium intake and cardiovascular diseases (CVD) has been explored through numerous molecular, animal, and human studies for several decades. An epidemiological study showed that residents who consumed hard water which was rich in calcium and magnesium had a lower mortality rate caused by circulatory diseases than those who had consumed soft water [1]. This finding suggested that the intake of calcium might be related to cardiovascular diseases. Also, several systematic reviews revealed that calcium supplementation reduced systolic blood pressure (SBP), particularly among hypertensive subjects but not among normotensive subjects [2-6]. A reduction of serum total cholesterol has been reported in hypercholesterolemic subjects given 1 g of calcium per day for 8 weeks [7,8]. More recent studies showed that 1 g/day of calcium supplementation for one year increased the level of HDL-cholesterol and decreased the level of LDL-cholesterol [9,10].

In addition, some observational studies reported that low calcium intake was associated with increased risk of CVD. The Iowa Women Health Study found that Caucasian women aged 55-69 years with low calcium intake (< 696 mg/day) had a high mortality rate due to cardiovascular heart disease (RR 1. 58, 95% CI 1.02-2.50) compared to those with high calcium intake (> 1,425 mg/day) [11]. Also, Wang et al. [12] found a higher rate of hypertension (RR 1.12, 95% CI 1.05-1.20) among women aged over 45 years consuming low calcium (< 558 mg/day) compared to those with high calcium consumption (1,679 mg/day). Other cohort studies reported the association between low calcium intake and increased risk of hypertension [13,14]. Meanwhile, several studies reported that calcium intake had an association with oxidative stress and suggested that calcium supplementation might increase the lipid oxidation [15,16]. As the pathogenesis of hypertension is often associated with oxidative stress, the relationships among calcium intake, blood pressure, and oxidative stress status should be investigated.

Traditionally, the Korean diet has been highly dependent on plant based foods, such as cereals, pulses, roots and various vegetables, now known for having many healthy benefits. Despite many healthy benefits of plant oriented foods, limited consumption in dairy and animal foods in Korean diet relative to western diet has been concerned for insufficient calcium. According to the recent Korean National Health and Nutrition Examination Survey (KNHANES) 2010, the mean daily calcium intake of Korean adults aged 19 and older was 535.2 mg, which was 75.6% of the Korean recommended level. Also, 64.5% of these adults consumed calcium under estimated average requirement (EAR) [17]. This national data indicate that the average calcium intake of Korean adults is within the range of high CVD related mortality risk or high prevalence of hypertension as reported by the previous researches [11,12].

Furthermore, the prevalence of hypertension in Korean adults (≥ 20 years) is 28.9% (30.1% for males, 27.7% for females) [17]. This is slightly lower than the prevalence of hypertension for Americans (31.4%-32.1% for males, 32.8% for females) [18]. But considering the significant difference in the prevalence of overweight and obesity between US adults (BMI ≥ 25; males 72.8%, females 63.0%) and Korean adults (BMI ≥ 25; males 36.5%, females 26.4%), the prevalence of hypertension in Korea seems to be very high. Based on these concerns, a study to investigate the calcium intake status among Korean adults with or without hypertension and its relation to blood pressure and CVD risk-related parameters is needed.

Therefore, this study was carried out for the following two aims: first, to evaluate the calcium intake status between the hypertension and normotension groups; and second, to investigate the correlation between dietary calcium intake and blood pressure, blood lipid parameters, and blood/urine oxidative stress indices in the hypertension and normotension groups.

Subjects and Methods

Participants

Initially, Korean adults were recruited for this study. Study participants agreed to provide their personal information regarding the purpose and the procedures of the project. Blood pressure was measured twice in seated posture by using a mercury sphygmomanometer, and the two measurements were averaged for systolic and diastolic pressures. Eighty-three subjects with 140 mmHg or higher in SBP or 90 mmHg or higher in diastolic blood pressure (DBP) were assigned to a hypertension group according to JNC-VII guidelines [19]. Participants with 120 mmHg or less SBP and 80 mmHg or less DBP were assigned to a normotension group. Each group was consisted of a similar percentage of subjects in age and gender (42 hypertensive men vs. 41 normotensive men). This study was approved by the College of Medicine, Chung-Ang University Institutional Review Board (IRB), and written informed consent was obtained from each subjects.

Anthropometric measurements

Height was measured using a standard stadiometer following study protocols, and weight in kilograms was measured on a calibrated Inbody (Biospace, Seoul, Korea) system. Measurements were repeated at least two times for each subject and BMI was calculated as weight in kilograms divided by height in meters, squared.

Dietary intake survey

The dietary intake survey was conducted by a 24-hour recall method for 3 days. Study subjects in each group were interviewed by trained research staff to examine the types and amounts of all foods that the subjects had consumed in the previous 3 days. Food models and photographs were used to assist in estimating serving sizes of foods. Dietary intake of calcium and other nutrients were analyzed by using Can-Pro 3.0 (The Korean Nutrition Society, Seoul, Korea).

Collection of blood and urine samples

The subjects fasted overnight after the dietary intake survey, and then 15 ml of venous blood was collected – half volume in an evacuated blood collection tube and the other half in an EDTA-coated blood collection tube. In the evacuated blood collection tube, serum was separated by centrifugation at × 400 g for 15 minutes. Blood samples collected in the EDTA-coated blood collection tube were stored at -20℃ until a test was performed. Spot urine samples were collected and aliquoted for analysis of multiple indices and kept in a freezer at -20℃.

Analysis of biochemical indices

Serum triglyceride, total cholesterol, glutamic-oxaloacetic transaminase (GOT) and glutamic-pyruvic transaminase (GPT) were analyzed by using a commercial kit based on enzymatic principles, and the content of HDL-cholesterol was analyzed by the colorimetric method after separating LDL and VLDL by using a dextran sulfate-Mg²⁺ sedimentation [20]. Serum LDL-cholesterol was calculated by the Friedewald formula [21] by utilizing triglyceride, total cholesterol, and HDL-cholesterol values.

Glutathione (GSH) in red blood cells was analyzed by using a modification of van Klaveren’s method [22]. After centrifugation of the blood samples at × 2,500 g and 4℃, red blood cells were hemolyzed by adding Millipore grade water. Tricholoroacetic acid (TCA) was added to the hemolyzed sample, and centrifuged at × 10,000 g for 5 minutes. Supernatants were subjected to reaction buffer (0.248 mg/ml NADPH in 143 mM sodium phosphate, 6.3 mM Na₄-EDTA at pH 7.5, 6 mM DTNB) with glutathione disulfide reductase (5 U/ml) for 20 minutes at 35℃ and the absorbance was measured in 412 nm by spectrophotometer (UVIKON, Kontron Inc., Milan, Italy). The amount of GSH in red blood cells was indicated as µmol/g hemoglobin (Hb). Glutathione peroxidase (GPx) activity in the red blood cells was analyzed by using methods by Aydin et al. [23]. Regarding GPx, absorbance was measured in 340 nm for 3 minutes using a spectrophotometer (UVIKON, Kontron Inc. , Milan, Italy) by adding 8.8 mmol/l H₂O₂ after 50 mM Tris buffer (pH 7.6), 1 mM Na₂EDTA, 2 mM NADPH, 4 mM sodium azide, and 1 unit/ml glutathione reductase, which were reacted with the 20 ul hemolyzed sample at 37℃ for 5 minutes. GPx activity in red blood cells was indicated as U/g Hb.

Urinary lipid peroxidation was analyzed according to the manual of Tagesson et al. [24]. Briefly, 50 µl urine samples were mixed with 23 mM thiobarbituric acid reagent, 0.5 M phosphoric acid 300 µl and then heated at 95℃ for one hour and then cooled down. After adding 100 µl methanol, samples were subjected to HPLC (501 Waters, Messachusetts, USA) for thiobarbituric acid reactive substance (TBARS) analysis. Urinary malondialdehyde (MDA) concentration in urine aliquot was indicated as µmol/g creatinine (Cr).

Statistical analysis

Statistical analyses including the mean and standard deviation values of all results were performed using the SAS program (version 9. 1; SAS Institute Inc, Cary, NC, USA). Student’s unpaired t-test was used to analyze the differences between the hypertension group and the normotension group and between men and women. Pearson’s partial correlation coefficient was used to determine an association between variables and to verify significance. All statistical tests were two-sided, and value of P < 0.05 was considered statistically significant.

Results

General characteristics

The general characteristics of the hypertension group and the normotension group are presented in . There were no significant differences in age, height, and weight between the hypertension and normotension groups; however, the hypertension group had significantly higher BMI (P < 0.001), SBP (P < 0.001), and DBP (P < 0.001) than the normotension group.