Zinc, copper, and blood pressure: Human population studies

Background

An estimated 1 billion individuals worldwide have high blood pressure, which is associated with approximately 7.1 million deaths per year [1]. In the United States, at least 50 million individuals are using hypertension treatment [2–3]. Therefore, the prevention and management of hypertension are major public health challenges [2–6]. Although a number of important nutritional and metabolic factors for hypertension are identified, including inadequate intake of fruits, vegetables, and potassium; excess sodium (sodium chloride) intake; excess body weight; inadequate physical activity; and excess alcohol intake [7–9], it is important to recognize that many other factors also play a role in this heterogeneous disorder. One of these factors, in particular, is the influence of essential trace metals, such as copper and zinc, on blood pressure. The involvement of copper and zinc in blood pressure regulation is particularly hinted in the human studies via manipulation of dietary copper and zinc levels. For example, copper deficiency reduces hemoglobin synthesis and leads to anemia [10], and anemia is considered as a contributor to increase cardiac output and blood pressure. Deficiency in zinc intake has been proposed to play a role in blood pressure regulation by altering the taste of salt [10]. Indeed, higher dietary zinc intake results in a better taste acuity for salt in healthy young females [11]. Thus, people with zinc deficiency tend to increase salt intake, which can lead to an increase in blood pressure [10]. It has been posited that an imbalance in the homeostasis of zinc metabolism can lead to high blood pressure [12], while copper deficiency can enhance the vulnerability of the heart and blood vessels [13]. Although these dietary studies imply a relationship between inadequate intake of these metals and high blood pressure, it is important to ask a more direct question whether copper and zinc levels are altered in patients with hypertension. Thus, in this article, we will examine reported correlations between tissue copper and zinc levels and high blood pressure, focusing on recent human population studies. We specifically highlight the challenges in data interpretation and describe gaps in knowledge that should addressed in future investigations. Please note that, in this article, we do not attempt an analysis of the large body of observations now on record regarding the action of these metals in animal models; such a review of zinc metabolism in arterial hypertension has been provided by Tubek [12].

Relationship Between Blood Pressure and Levels of Zinc and Copper

Many studies in humans hint a correlation between zinc and copper levels and hypertension. For example, zinc and the zinc/copper ratio are decreased in hypertensives as compared with normotensives [14,15]. Chiplonkar et al. reported lower erythrocyte membrane zinc in hypertensive compared to normotensive lacto-vegetarians [16]. Suliburska et al. reported lower zinc in hair of obese hypertensives [17]. Olatunbosun found significantly increased serum copper levels in hypertensive patients [18]. In contrast, Taneja and colleagues reported increased serum zinc, and decreased levels of copper, in hypertensives, while urinary levels of both zinc and copper were increased in hypertensives [19]. Ghayour-Mobarhan et al. reported that serum copper is higher in hypertensives [20]. However, de la Sierra A et al. reported that there is no correlation between the degree of endothelial dysfunction and serum copper or zinc levels [21].

It is critical to understand what underlies the discrepancies regarding trends of copper and zinc levels in these studies. First, we should notice that the study design is different between each study. Table 1 compares the differences in design of studies reviewed in this article. As we can see, the differences between each study include (but are not limited to) age, gender ratio, medication status, smoking and alcohol history, and hypertension evaluation procedures. Second, these discrepant results also suggest that the involvement of copper and zinc in hypertension appears far more complicated than is currently understood. Thus, using a global population approach to understand the relationship between trace zinc, copper and hypertension may not be optimal. In this article, we posit that instead of studying the whole hypertensive population, it is more important to investigate the specific pathogenic subsets of patients with hypertension, as indicated in the following sections.

Renin-Angiotensin System, Metal, and Blood Pressure

One important contributor for the pathogenesis of essential hypertension is the renin-angiotensin system (RAS) [22]. Generally, a decrease in circulating blood volume leads to lowered blood pressure. These circumstances trigger the kidneys to release renin. Mediated by angiotensin converting enzyme (ACE), renin transforms angiotensinogen into angiotensin II. Angiotensin II acts on multiple target organs throughout the body including the brain, promoting the generation of reactive oxygen species [23], vasoconstriction, the adrenal release of aldosterone, and the activation of sympathetic nerve discharge, ultimately increasing circulating volume and blood pressure [24]. Therefore, the levels of renin, ACE, and aldosterone in blood serve as an index of RAS activation. However, whether RAS activation is correlated with trace metal levels in the circulation, cardiovascular tissues and/or anatomic regions of brain is unknown. Tubek [25] studied the correlation between zinc metabolism and the RAS in patients with essential hypertension. This study included 38 patients, 24 men and 14 women, between 16–59 years of age. In women, plasma renin and ACE were negatively correlated to total zinc efflux from lymphocytes; ACE and serum aldosterone were negatively correlated to oubain-dependent zinc efflux from lymphocytes, and all three RAS parameters were positively correlated to lymphocyte zinc levels. In men, the only correlation observed was a positive correlation between aldosterone and serum zinc. With regard to blood pressure, women showed a negative correlation to lymphocyte zinc, serum zinc, oubain-dependent zinc efflux from lymphocytes, and total zinc efflux from lymphocytes. Men had a negative correlation to lymphocyte zinc as well. These results indicate that there are notable gender differences in the relationship between zinc metabolism and the RAS. Also, this study shows a connection between these specific parameters and blood pressure in patients with primary hypertension. The author posited that women with mild hypertension, but not men, exhibit an association between zinc regulation and RAS activation. Determination of the underlying mechanisms for these gender differences and the reasons for the observed correlations will require more focused investigations.

Obesity, Trace Metals, and Blood Pressure

Leptin is predominantly produced by adipocytes, and serum leptin concentration is correlated with obesity [26]. Olusi et al. investigated associations among serum leptin, zinc, copper, and zinc/copper ratio in healthy individuals (n=570; 223 males vs. 347 females; aged 15 yr and older) in the normal Arab population [27]. Interestingly, this study showed that serum leptin was positively associated with serum copper, but negatively associated with the zinc/copper ratio, while there was no significant association between serum leptin and zinc [27]. Because leptin can also induce sodium retention and systemic vasoconstriction leading to an elevation in blood pressure [28], it is considered to be a contributing factor in essential hypertension. Canatan et al. [14] investigated plasma levels of leptin, zinc and copper in primary essential hypertensives (n=35; 18 female vs. 17 male) and healthy normotensives (n=50; 28 female vs. 22 male). The authors reported a negative correlation between leptin and zinc as well as leptin and the zinc/copper ratio; in obese patients with primary hypertension, leptin levels were increased, zinc and zinc/copper ratio were decreased. However, copper alone did not show significant correlation with leptin in this study [14]. Because obesity contributes to increased blood pressure in most patients with essential hypertension, which appears to be mediated in part by increased levels of leptin [29], the population study by Canaton et al. suggests an interesting link between copper, zinc, and leptin in obesity. The role of trace metals in regulating leptin function and metabolism, and the mechanistic basis for the observed correlations, are deserving of further investigation.

Environmental Metal Pollution and Blood Pressure

Environmental factors, such as diet, physical activity, and water and air pollution, affect the development of cardiovascular disease [30]. Iron and copper are two common elements of particulate matter contributing to air pollution. Air pollution is a mixture of gases, liquids, and particulate matter, and there is increasing concern regarding its potential deleterious effects on human health. After being released into the atmosphere (troposphere), pollutants are carried back in rainwater to further generalize environmental pollution. Tubek et al. calculated the yearly average number of hospitalizations caused by certain diseases in the region of Opole Voivodship, Poland [31]. Using rainwater as a monitor for environmental pollution, they investigated the correlation between chemical elements in rainwater and the hospitalization frequency of certain diseases. This study hinted a mild correlation between zinc and cadmium levels and hospitalizations for hypertension. The authors also posited that absorption of zinc through the lungs is a contributing factor, because ACE is highly localized to lung and requires zinc for its activity [31].

Chandigarh is a city with high occurrence of hypertension in India. Interestingly, in this city, zinc concentrations are higher in vegetables (reddish, turnip, and carrot) irrigated with underground water (120 mg/kg diet) compared with vegetables irrigated with surface water (40 mg/kg diet), while the copper concentrations are not different. Taneja et al. [19] measured dietary intake of zinc and copper in the hypertensive (n=250) and normotensive (n=250) men in this city. This study revealed a positive correlation between serum zinc and blood pressure, and a negative correlation between copper and blood pressure. There was also a positive correlation between urinary zinc and copper and blood pressure. Thus, increased intake of zinc seems to correlate with the increased prevalence of hypertension in Chandigarh, hinting that a dietary-derived disturbance of metal homeostasis might be an important factor contributing to primary hypertension. Moreover, it is notable that soft water areas have a 10–15% higher rate in cardiovascular disease mortality than the areas with medium hardness in water [32].

Methods Commonly Employed to Assay Zinc and Copper in Human Specimens

Several methods have been applied to assay trace metals in human studies. First, the metal concentration can be measured in plasma [14], serum [19], urine [19], and hair or fingernail specimens [33]. These methods are commonly employed because the samples are easily collected. Tooth has also been used in one study [34]. The turnout rate of trace metals in the blood and urine is rapid; whereas the hair, fingernail and tooth accumulate these elements over a long period. Tang et al. reported metal contents in human serum, hair, and fingernails between aged patients with hypertension and coronary heart disease (diseased group) versus aged healthy controls (healthy group) [33]. The zinc content and Zn/Cu ratio in serum of the diseased group were significantly higher than that of the healthy group. Conversely, the zinc content in the hair and fingernails, and Zn/Cu ratio in the hair, of the diseased group were significantly lower than that of the healthy group. Thus, the source of biological material for copper and zinc content analysis must be taken into account when comparing results from different clinical studies.

However, the limitations of these measurements are that they only provide information about trace metals at the tissue levels. Thus, several complementary methodologies have been devised to measure metal concentrations and efflux in blood cells, such as lymphocytes [25] and erythrocytes [35]. For example, in order to measure zinc efflux from lymphocytes, the cells are first incubated with zinc chloride to increase cellular zinc content. Then, the cells are treated with medium free of zinc chloride to determine time-dependent zinc release, which is used to calculate the efflux rate coefficient (ERC). Alternatively, metal content within the erythrocyte membranes can be measured [16,36], which is considered as a more sensitive measurement of zinc [16].

Another way to study metal metabolism in humans is to record their dietary intake of metal, because dietary patterns play an important role in the pathogenesis of many diseases [37], including hypertension. By conducting a food frequency questionnaire documenting specific quantities of food items consumed throughout the year, an estimate of the average daily intake of nutrients and metals can be determined [16]. Dietary metal intake varies from one population to the next due to the wide variations of diet around the world. Thus, it is not uncommon for certain communities to be exposed to different levels of metals due to different life styles and environmental conditions. For example, the traditional Indian diet, known as the lacto-vegetarian diet, consists of high levels of fiber and minerals and reduced fat content. Recently, Chiplonkar et al. [16] determined whether there was an association between dietary metal intake and hypertension in Indian lacto-vegetarians. The authors compared normotensives (n=115; 30 female vs. 85 male) with hypertensives (n=109; 26 female vs. 83 male). This study showed that copper intake was significantly lower in hypertensive compared with normotensive subjects. In addition, erythrocyte membrane zinc was negatively associated with systolic blood pressure.

Table 2 summarizes the representative methods and reference values for each metabolic parameter discussed above. It should be noted that atomic absorption spectrometry (AAS) is widely applied in human populations, presumably due to its lower cost. However, AAS only can detect one element in a single analysis of biological samples. There are several new techniques with multielemental capability, such as inductively coupled plasma mass spectrometry and synchrotron radiation X-ray fluorescence spectrometry, as recently reviewed [38,39]. Finally, the activity and expression of copper containing enzymes might also provide supplemental information about copper status in humans (see review by Danzeisen et al. [40]).

Suggestions and Future Directions

Many human population studies suggest a relationship between copper, zinc and hypertension, although currently we cannot distinguish between causality and association. It is also not surprising that some results are apparently contradictory. In addition to the differences in experimental design between different population studies, the contradictory results also suggest that the relationship between copper, zinc and hypertension is highly complex; depending on the levels, duration of exposure, geographic location, and other preconditions, either promotion of or protection against hypertension is possible. Therefore, more studies are required to definitively establish their roles. Our following suggestions may be helpful.