Nutrition in Hypertension
Sodium
Sodium is almost certainly the most extensively studied nutrient influencing blood pressure. Evidence from a variety of sources, including epidemiologic studies as well as intervention trials, indicates rather conclusively at this point that sodium contributes to blood pressure elevations on a population and individual basis. Such a conclusion is supported by results of the INTERSALT study, which examined the association between sodium intake and blood pressure in multiple cohorts around the globe. Generally, each incremental increase in sodium intake of 100 mEq per day increases mean systolic blood pressure in a population by 3 to 6 mm Hg.
Although there is decisive evidence that sodium contributes to blood pressure elevation, the causal role of sodium in hypertension is less well established. Studies suggest that roughly 50% of hypertensives in the United States are responsive to sodium, demonstrating blood pressure variation with change in sodium intake; this figure was previously set at about 10%. A substantial but smaller percentage of normotensives are salt sensitive. Salt sensitivity is more prevalent among African Americans than among others; up to 75% of hypertensive African Americans are responsive to dietary
sodium.
The efficacy of sodium restriction in the management of hypertension has been demonstrated in the context of clinical trials, but establishing real-world effectiveness is a greater challenge. Adherence to a low-sodium diet is difficult for most patients, and such diets inevitably introduce other changes that may account in part for blood pressure reduction. Cook et al. asserted that the effect of salt restriction on blood pressure has generally been underestimated. In 2005, the Center for Science in the Public Interest (CPSI) filed suite against the US Food and Drug Administration for according sodium the designation “generally recognized as safe” (GRAS). CSPI alleges that excess dietary salt is responsible for as many as 150,000 premature deaths each year in the United States. Kumanyika has suggested that to achieve recommended sodium intake levels in the United States with any consistency will require appreciable changes to the food supply.
Despite the uncertainties, recommendations for sodium restriction below prevailing levels in the United States can be made with considerable confidence. Intake in the United States generally exceeds the recommended limit of 2,400 mg per day. Ancestral intake, which may indicate optimal levels, was approximately 700 to 800 mg per day, less than one-fourth the average intake today. Advocacy of a health-promoting diet will result in sodium restriction by reducing the intake of fast foods and other highly processed foods.
Patients should be advised of the importance of reading food labels. The sodium content of many commercial breakfast cereals is comparable to that of potato chips and pretzels, although the taste of salt in such products is masked by the sugar. In attempting to limit sodium intake, many patients will report not using a salt shaker. However, the salt added to food during preparation is less readily tasted than the salt shaken on just as the food is eaten. Therefore, selection and preparation of relatively low-salt foods and continued, albeit controlled, use of a salt shaker may be a preferred approach. As with other dietary changes, salt restriction becomes less objectionable as it becomes familiar. Whereas the salt content of many processed foods goes unnoticed by most consumers, those acclimated to a lower-sodium diet begin to taste salt more readily and to prefer lower intake levels. Acclimation to a high-salt diet has the opposite effect.
Salt Substitutes
So-called salt substitutes, which replace some of the sodium with potassium or calcium, may serve as a useful aid to patients struggling to acclimate to a salt-restricted diet. There is some evidence suggesting that the preference for dietary salt may vary with factors other than taste perception, so acceptance of salt substitutes is variable. Clinical trial outcomes suggest a favorable influence on blood pressure of salt substitution.
Potassium
Diets rich in potassium tend to be relatively low in sodium, and vice versa, making the study of isolated dietary potassium difficult. Nonetheless, there is convincing evidence that potassium supplementation has a blood pressure–lowering effect. The evidence is decisive that total dietary modification that results in increased potassium intake, and particularly a potassium intake that exceeds sodium intake, lowers blood pressure. The average intake of sodium in the United States is up to 4,000 mg per day, while average daily intake of potassium is approximately 2,500 to 3,400 mg. Our prehistoric ancestors are estimated to have consumed approximately 750 to 800 mg per day of sodium and nearly 10,500 mg of potassium. As potassium is abundant in a variety of fruits and vegetables, high intake of potassium generally is associated with other dietary changes that may independently lower blood pressure. In the INTERSALT study, blood pressure rose with age in all populations consuming more sodium than potassium, but not in those consuming more potassium than sodium.
Calcium
There is suggestive evidence that high dietary calcium intake contributes to lowering of blood pressure. In the DASH trial, calcium is considered a potentially important mediator of the hypotensive effects of nonfat dairy products. Meta-analysis suggests that calcium, either in the diet or as a supplement, has a modest antihypertensive effect . However, on the basis of an extensive literature review, the Canadian Hypertension Society has advised against calcium supplementation as a means of either treating or preventing hypertension. The isolated effects of calcium supplementation on blood pressure appear to be modest; a dietary pattern providing abundant calcium may be of greater benefit. A particular benefit of calcium in the management and prevention of pregnancy-induced hypertension has been suggested . In the aggregate, evidence supports a hypotensive benefit of calcium intake at levels advisable on other grounds.
Magnesium
Diets rich in potassium tend to be rich in magnesium and vice versa. Magnesium supplementation may be beneficial in the treatment of hypertension in magnesium-deficient patients. Meta-analysis of clinical trials suggests a modest hypotensive effect of supplemental magnesium . While routine supplementation of magnesium is not advocated on the basis of current evidence, a dietary pattern providing abundant magnesium certainly is.
Fiber
A potential benefit of dietary fiber in the regulation of blood pressure has been reported in both adults and children. At the population level, the isolated effects of dietary fiber on blood pressure are difficult to establish, as dietary patterns associated with high fiber intake tend to exert a favorable influence on blood pressure by other means as well. Clinical trials have suggested a beneficial effect of soluble fiber from oats on blood pressure and indicated that regular intake of oats may reduce the need for medication in hypertensives. A recent meta-analysis revealed a modest hypotensive effect of supplemental dietary fiber independent of other factors. In the aggregate, the evidence is persuasive that increasing dietary fiber intake is likely to exert a favorable influence on blood pressure. Patients should be encouraged to increase fiber intake on general principles, as both soluble and insoluble fiber offer a potential array of health benefits; average intake levels in the United States are well below the recommended levels, and a healthful dietary pattern is naturally high in fiber.
Alcohol
Alcohol contributes to blood pressure elevations when intake exceeds 30 to 45 g of ethanol daily and may contribute at lower intakes in patients with hypertension. Moderate alcohol intake below this level may actually lower blood pressure slightly or may have no effect on blood pressure. The cardiovascular benefits of alcohol may help reduce the risk of myocardial infarction in well-controlled hypertensives. When blood pressure is not well controlled, alcohol intake should be discouraged. Moderation of alcohol intake is among the established interventions for blood pressure control advocated by the National Heart, Lung, and Blood Institute.
Garlic
Garlic is reputed to have antihypertensive effects. Garlic stimulates nitric oxide synthase, providing a mechanism by which it might lower blood pressure. Meta-analysis supports a modest antihypertensive effect of garlic, but the evidence is limited. Reliance on garlic to control blood pressure based on available evidence is not advisable.
Amino Acids
Arginine and taurine may have antihypertensive properties, but evidence to date is limited. Arginine is a precursor in the synthesis of nitric oxide, an endothelium-derived vasodilator; a link between blood pressure and endothelial function is clear, although the direction of causality is not . Limited data suggest a favorable influence of supplemental sustained-release arginine on both blood pressure and endothelial function . Evidence is insufficient at present to justify recommendations of amino acid supplementation in efforts to regulate blood pressure.
Coenzyme Q10
An antihypertensive effect of coenzyme Q10 is claimed, and practitioners of alternative medicine use coenzyme Q10 in the management of hypertension. The evidence for such an effect is limited and not sufficient to justify routine clinical application. Small clinical trials have been promising, however, and further study of the compound for a role in blood pressure control is warranted.
Caffeine
Caffeine is a pressor and acutely raises blood pressure, generally to a modest degree. The effects of caffeine on blood pressure are apparently greater in hypertensives than in normotensives. Evidence is insufficient to warrant population-wide recommendations for caffeine restriction as a means of improving blood pressure. However, caffeine restriction in hypertensives is both reasonable and prudent, even though additional research is needed to provide definitive evidence of benefit.
