Among all dietary factors that have been examined with respect to essential hypertension, ingestion of sodium, mainly in the form of salt, has the most consistent linkage (Law, 1997). intersalt, the largest multi-culture study conducted examining the relation between sodium excretion and blood pressure, found that a relation was evident in each of its 52 international sites (Stamler et al., 1991; Elliott et al., 1996). The association between sodium excretion and blood pressure was strongest in the older age groups, demonstrating perhaps the effect of chronic exposure to high salt intakes. In another line of research, sodium restriction has been shown to be reliably associated with reductions in blood pressure (Cutler, Follman, and Alexander, 1997), providing further confirmation of the strong association between sodium intake and blood pressure status.
Despite the consistent findings relating sodium intake to blood pressure status, there is evidence to suggest that blood pressure elevations associated with sodium consumption occur in only some individuals. Very simply, some persons display substantial blood pressure elevations following ingestion of sodium while others exhibit minimal alterations in blood pressure given the same amount of sodium (Sullivan, 1991). These observations have led toward investigations of these so-called salt-sensitive hypertensive patients that parallel laboratory observations of the Dahl salt-sensitive rats. Like the DS and DR rats, salt-sensitive essential hypertensive humans exhibit no difference in blood pressure from salt-resistant hypertensives, unless exposed to sodium (Sullivan, 1991). In a review of the literature on salt sensitivity, Sullivan (1991) reported that altered vascular resistance and suppressed plasma renin activity as well as aldosterone were characteristics of salt sensitivity among both humans and animals. Examinations of differences across various demographic variables on salt sensitivity have revealed no significant differences in salt sensitivity associated with gender or body weight (Chrysant et al., 1997). Although there do not appear to be significant differences in sodium sensitivity associated with some demographic variables associated with essential hypertension, there is strong evidence that elevated sodium sensitivity is more prevalent among hypertensive patients (Weinberger et al., 1986).
Given the premise that individual differences in salt sensitivity exist, it could be hypothesized that salt sensitivity affects acute cardiovascular reactions to stress as well as subsequent risk for essential hypertension. Accordingly, salt-sensitive persons who have consumed sodium would be hypothesized to exhibit exaggerated blood pressure responses to stress in contrast to salt-resistant persons or salt-sensitive individuals consuming salt-restricted diets. Let's examine the evidence for this hypothesis.
Falkner, Onesti, and Angelakos (1981) conducted one of the first studies to examine this hypothesis using normotensive adolescents with and without a family history of essential hypertension. In this study, elevated heart rate and blood pressures during a mental arithmetic challenge were observed among individuals with a family history of hypertension, but only during a salt-loading condition; that is, differences between adolescents with and without a family history of hypertension were not significant during the no-salt control condition. Ambrosioni and colleagues corroborated these results using direct measures of intracellular electrolytes and sodium transport (Ambro-sioni et al., 1981; 1982). In these studies, increased intracellular sodium in lymphocytes was observed in both hypertensive patients and nor-motensive offspring of hypertensive patients, and was related to exaggerated blood pressure responses to stress and exercise. Furthermore, restriction of salt among borderline hypertensives was shown to be associated with reductions in blood pressure response to stress and exercise (Ambrosioni et al., 1982). In this regard, sodium intake was clearly linked to the magnitude of blood pressure responses to both mental and physical stressors.
The effects of sodium homeostasis among essential hypertensive patients as well as their normotensive offspring have also been examined by measuring sodium excretion (Light et al., 1983; Light and Turner, 1992). Presumably, if salt sensitivity is associated with increased intracellular sodium concentrations, this would indicate that sodium is being retained in response to stress and that less sodium would be evident in urine output. In a study examining this hypothesis, Light et al. (1983) compared mental stress reactions of young adults at high and low risk for developing hypertension based upon presence of borderline resting blood pressures or parental history of hypertension. No relation between sodium excretion and heart rate response to stress was observed among low-risk participants; however, a highly significant inverse correlation was observed between sodium excretion and heart rate response to stress among the high-risk group. The highest heart rate responders to stress exhibited the most sodium retention. Furthermore, this reduction in sodium excretion was maintained during a post-stress recovery hour, suggesting that the fluid retention persisted long after exposure to stress was terminated. In a related study, Light and Turner (1992) confirmed that reduction in sodium excretion was associated with enhanced blood pressure, heart rate, and cardiac output responses to mental stress; additionally, the reduced sodium excretion response to stress was more likely to be observed among black than white participants. More recent reports have corroborated these findings relating sodium sensitivity and blood pressure response to mental stress (Deter et al., 1997), but have also suggested that the presence of certain psychological characteristics, like anxiety, emotional irritability, or anger, may be associated with increased sodium sensitivity (Deter et al., 2001). Furthermore, demographic variables have also been shown to impact the relation between salt sensitivity and cardiovascular response to stress, with some studies showing a stronger relation with age (Overlack et al., 1995) and among blacks (Falkner and Kushner, 1990).
Another electrolyte, potassium, has received considerable attention regarding its role in the etiology of essential hypertension (He and Whel-ton, 1999). In contrast to dietary sources of sodium, most of our dietary sources for potassium are vegetables and fruits, foods that individuals in most industrialized countries do not consume in adequate amounts. Therefore, unlike sodium, if potassium is related to onset of problems with blood pressure regulation, it is due to inadequate consumption. Because of the overconsumption of sodium-rich foods and underconsumption of potassium-rich foods in modern industrialized countries, it is difficult to disentangle the unique contributions to hypertension risk associated with each electrolyte. Analysis of data from the TONE study, however, indicated that risk for high blood pressure was independently associated with increased sodium consumption and decreased potassium consumption (Espeland et al., 2002). Additionally, there is convincing epidemiologic evidence that increased dietary intake of potassium is related to lower blood pressures (He and Whelton, 1999).
Only a few studies have examined the relation between potassium and cardiovascular response to stress (Sudhir et al., 1997; West et al., 1999). Although Sudhir et al. (1997) did not observe any cardiovascular response differences to stress following salt loading, they did report that reductions in potassium were associated with increased blood pressure response to the cold pressor in blacks. West et al. (1999) examined the relation between electrolytes and cardiovascular response to stress by measuring responses to stress after participants consumed each of three different controlled diets (low salt, high salt, high potassium) for 8-10-day periods. Among salt-sensitive participants, both low-salt and high-potassium diets resulted in reductions in resting blood pressures. Change scores, representing standard measures of cardiovascular reactivity, did not differ across groups. Comparable reductions in resting blood pressure were obtained for participants during the low-salt and high-potassium intervention weeks, suggesting that alterations to either electrolyte were beneficial regarding blood pressure status but had no effect on acute cardiovascular reactivity to stress. However, due to the inconsistency of these two studies and general paucity of the data examining cardiovascular response to stress and potassium, there is inadequate evidence supporting a relation between potassium and cardiovascular reactivity to stress at this point in time.
Several studies have explored the relation between calcium intake and blood pressure. As with potassium, there is an inverse relation between calcium intake and blood pressure (lower calcium intake is associated with higher blood pressures; Griffith et al., 1999). Studies of dietary supplementation have generally found that increased calcium intake was related to reductions in blood pressure, although it was often difficult to disentangle the unique effect of calcium from other dietary alterations that occurred in these studies such as lower sodium intake or increased potassium intake. In fact, MacGregor and Cappuccio (1993) hypothesized that the lower calcium observed in studies of hypertensive patients might be a consequence of increased sodium intake and that calcium had no unique predictive relation to onset of essential hypertension. Although studies examining the relation between calcium consumption and cardiovascular response to mental stress are few, calcium supplementation has been shown to reduce blood pressure responses to stress in salt-sensitive SHRs fed high-sodium diets (Scrogin, Hatton, and McCarron, 1991).
Only a few studies have examined the relation between magnesium and blood pressure. Although one study found a relation between lower magnesium intake and increased incidence for essential hypertension in a sample of female nurses (Ascherio et al., 1996), only minimal effects of magnesium supplementation on blood pressure have been observed (Kawano et al., 1998). As with calcium, no research has been conducted examining the relation between magnesium and cardiovascular response to stress in humans.
Omega-3 Polyunsaturated Fatty Acids
Consumption of eicosapentanoic (EPA) and docosahexanoic (DHA) acids, two omega-3 polyunsaturated fatty acids commonly found in fish, has been shown to be associated with reduced risk for coronary heart disease as well as essential hypertension (Mori et al., 2000). Additionally, significant reductions in blood pressure have been observed among hypertensives who increased consumption of omega-3 polyunsaturated acids (Bao et al., 1998). However, no studies to date have examined the relation between consumption of omega-3 polyunsatu-rated fats and cardiovascular reactivity to stress.
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