Dietary considerations for hypertension.

The Dietary Approaches to Stop Hypertension (DASH) diet emphasizes fruits and vegetables (five to nine servings per day), low-fat dairy products (two to four servings per day), whole grains, poultry, fish, and nuts. Intake of saturated and trans fats, red meat, sweets, and sugar-containing beverages is limited. The DASH diet provides an abundance of naturally occurring potassium, magnesium, and calcium. A large study of the DASH diet revealed that it reduced systolic and diastolic blood pressure by 11.4 and 5.5 mm Hg, respectively, in patients with stage 1 hypertension. Black Americans experienced greater blood pressure reductions than did nonblack Americans.¹⁵ This may be due to the fact that black Americans are more likely to be deficient in magnesium and potassium than whites; both of these nutrients are important in the maintenance of a healthy blood pressure, and this diet provides large amounts of both.

It is well known that some individuals are salt-sensitive and that the restriction of salt in the diet can help reduce blood pressure in some individuals. The DASH diet provides roughly 7.5 g/day of sodium, whereas the standard American diet averages 10 to 15 g. Although research on sodium restriction and hypertension has yielded conflicting results, depending on the population studied, most researchers agree that cutting back on salt is not likely to cause harm and may reduce blood pressure. Meta-analyses of randomized trials have shown that, on average, a reduction in sodium intake of 1.8 g/day is associated with systolic and diastolic blood pressure reductions of approximately 4 and 2 mm Hg, respectively, among individuals with hypertension.¹⁶

Research suggests that hypertensive individuals should limit their daily intake of sodium to 6 g/day. Considering that 1 tsp of salt contains 2.3 g of sodium, this is no easy goal. (The DASH diet is slightly more liberal.) The U.S. Surgeon General has recommended that a trial of sodium restriction be considered for anyone who (1) has a family history of hypertension, (2) is African-American, (3) is older than 45 years, (4) is overweight, or (5) has borderline hypertension.

Sodium restriction has been shown to prevent the development of hypertension in high-risk patients, and it may reduce the amount of medication required in older hypertensive patients. The Trials of Hypertension Prevention showed that sodium reduction, alone or in concert with weight loss, prevented hypertension by approximately 20%.¹⁷ In the Trials of Nonpharmacologic Interventions in the Elderly, a reduction in salt intake, with or without weight loss, effectively reduced blood pressure and the need for antihypertensive medication in older patients.¹⁸

A 30-day randomized, controlled study of 412 adults with untreated systolic blood pressure of 120 to 160 mm Hg and diastolic blood pressure of 80 to 95 mm Hg showed that a combination of the DASH diet with reduced sodium intake was more effective than the DASH diet or sodium restriction alone.¹⁹ The study also revealed that reduction of salt intake decreased blood pressure in men and women, blacks and whites, the young and old, and people with normal or high blood pressure.

Low- and high-fat diets.

The battle between the proponents of high- and low-fat diets continues to rage across America. Most research has been focused on the beneficial effects of eating a diet low in saturated fat and, more recently, trans fatty acids. Saturated fat is the main fat found in animal products (e.g., beef, pork, dairy, poultry) and in coconut and palm oils. Contrary to popular belief, studies have shown that eating shellfish²⁰ and eggs²¹ does not significantly affect the LDL cholesterol level. Although shellfish and eggs are high in cholesterol, they are low in saturated fat. Recently the public has added the words “trans fat” to its dietary vocabulary. Vegetable oils contain one or more double bonds between carbon atoms. When hydrogen is added to vegetable oils to cause the fat to become solid at room temperature (e.g., margarine), the hydrogens are added in the trans position (on the opposite sides of the longitudinal axis of the double bond). Trans fats increase levels of LDL cholesterol and TGs.²² Many physicians recommend that cholesterol intake be limited to 300 mg/day and that consumption of saturated/trans fat be limited to 10% of total daily calorie intake.

Despite the data demonstrating that low-fat diets are beneficial to the cardiovascular system, low-carbohydrate, high-protein, high-fat diets have become increasingly popular in the pursuit of weight loss, cholesterol reduction, and treatment of metabolic syndrome. Despite this popularity and numerous positive anecdotal reports, only limited data have been published until recently. In one year-long multicenter controlled trial, 63 obese men and women were randomly assigned to consume a low-carbohydrate, high-protein, high-fat diet or a low-calorie, high-carbohydrate, low-fat diet. Subjects consuming the low-carbohydrate diet lost more weight than did subjects receiving the low-fat diet at 3 and 6 months, but the difference at 12 months was not significant.

After 3 months, no significant differences were found between the groups with regard to total or LDL cholesterol concentration. The increase in HDL cholesterol and decrease in TG concentrations were greater among subjects consuming the low-carbohydrate diet than among those receiving the conventional diet throughout most of the study. This may be of particular interest for women; low HDL and high TG levels are considered particularly serious risk factors. Both diets significantly decreased diastolic blood pressure and insulin response to oral glucose load. The authors concluded that the low-carbohydrate diet was associated with a greater improvement in some risk factors for CHD but noted that adherence was poor and attrition high in both groups.²³

One concern about the long-term use of low-carbohydrate, high-fat diets is the lack of fiber and other key nutrients found in whole grains, fruits, and vegetables. Whole-grain products provide the body with complex carbohydrates, fiber, and other essential nutrients. Populations that consume a diet high in grains and fiber have been shown to have a decreased risk of cardiovascular disease. It is clear that complex carbohydrates (e.g., bread, cereal, pasta) should be chosen over simple carbohydrates (e.g., soda, candy). Soluble fibers such as pectin, oat, and psyllium have been shown to reduce LDL and total cholesterol levels.

There is no question that more research must be conducted to determine which diet is best for individuals at risk for CHD. Indeed, some individuals may be more constitutionally predisposed to a high-protein, low-carbohydrate diet, whereas others do better with a low-fat, high–complex carbohydrate approach. But until longer and larger studies have been conducted to determine the long-term safety and efficacy of low-carbohydrate, high-protein, high-fat diets, it seems wise to keep total fat intake to 30% to 35% of the daily diet, with the bulk accounted for by monounsaturated fats (e.g., olive oil).

The Mediterranean diet and monounsaturated fats.

Researchers have noted that people in countries bordering the Mediterranean Sea exhibit strikingly lower rates of heart disease, obesity, and cancer than their counterparts in the United States and other European countries. Original research on the Mediterranean diet stems from the study of villagers on the Greek island of Crete, whose diet emphasizes green vegetables, fruits, crusty breads, fish, and very little meat or cheese. Villagers typically drink wine with dinner. People living on Crete get more than 30% of their calories from fat, most of it from olive oil. Although this population has a relatively high-fat diet, the fat is mainly monounsaturated. Today the Mediterranean diet is actually a composite of the food habits of several countries and regions, including Spain, southern France, Italy, Greece, Crete, and parts of the Middle East. Unfortunately, many Americans believe they are consuming a Mediterranean diet when they eat huge portions of pasta with heavy cream sauce accompanied by a basket of white bread loaded with butter.

Studies have confirmed that diets low in saturated fat and rich in monounsaturated fats have a beneficial effect on endothelial function and lipid status. Monounsaturated fatty acids reduce the level of LDL cholesterol, but not HDL cholesterol, when they are used to replace saturated fat in the diet. A high intake of the polyunsaturated fatty acids found in other vegetable oils has been shown to reduce the HDL cholesterol level, about a 1% reduction for every 2% of total calories in which polyunsaturated fatty acids replace saturated or monounsaturated fatty acids.²⁴

Soy protein and isoflavones.

Soy provides a beneficial source of fiber and is naturally low in saturated fat and cholesterol. Epidemiologic data suggest an inverse relationship between the consumption of soy and risk of cardiovascular disease. Most clinical trials of soy have involved postmenopausal women. Research results vary, in part because different types of soy products have been used in these trials. Some trials of soy isoflavones have shown a decrease in the concentrations of total and LDL cholesterol plus an increase in the concentration of HDL cholesterol, whereas others have failed to show any beneficial effect on the lipid profile.²⁵

The best documented evidence is the effect of soy protein on plasma lipids and lipoprotein concentrations, with reductions of approximately 10% in LDL cholesterol and small increases in HDL cholesterol concentrations. In addition to the beneficial effect on lipids, dietary soy protein appears to enhance flow-mediated arterial dilation in postmenopausal women.²⁶ A 1995 meta-analysis of 38 clinical trials involving soy showed that the consumption of soy protein in place of animal protein significantly decreases concentrations of total and LDL cholesterol and TGs, with no change in HDL cholesterol.²⁷ Although soy isoflavones contribute to the overall beneficial effects of soy, at this time it appears that it is primarily soy protein that plays the most significant role in lipid reduction. The U.S. Food and Drug Administration found the evidence compelling enough to formally approve a health claim that allows the labels of foods containing at least 6.25 g of soy protein per serving (assuming four servings, or 25 g/day of soy protein) to state that the food reduces the risk of heart disease (Figure 14-1).

Figure 14-1 A variety of foods can be incorporated into a diet to help prevent heart disease. A, Oily fish, such as salmon, contain the omega-3 fatty acids that tend to lower the risk of heart disease. B, Carrots are key sources of carotenoids, which are useful for smokers who are prone to heart disease. C, Foods containing soy, such as soy milk and tofu, provide plant estrogens that reduce the risk of heart disease.

Fish and the omega-3 fatty acids.

A growing body of evidence derived from epidemiologic data and clinical trials has consistently demonstrated that fish oil reduces TG concentrations, as well as the risk of a variety of cardiovascular events. The main beneficial components of fish oil are eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These fatty acids have a positive effect on vascular function, improving endothelial function through stimulation of nitric oxide, altering vascular tone by way of actions on selective ion channels, and maintaining vascular integrity.²⁸ Vascular integrity is maintained by making smooth muscle cells less responsive to proliferation.²⁹ EPA has several antithrombotic actions, including the inhibition of platelet-activating factor,³⁰ prostaglandin I₂, and thromboxane A₂, compounds involved in platelet aggregation and vasoconstriction.³¹

In concordance with the findings of basic science and animal research, clinical trials such as the Diet and Reinfarction Trial and the Indian Experiment of Infarct Survival have demonstrated reductions in cardiac death rates and the incidence of cardiac symptoms among patients taking fish oil.³² The authors of a meta-analysis of 11 trials (comprising 15,806 patients) published between 1966 and 1999 concluded that omega-3 fatty acid–enriched diets reduce the risk of nonfatal MI, fatal MI, and sudden death among patients with CHD.³³ A recent Cochrane systematic review showed that fish oil supplementation reduced TG levels among patients with type 2 diabetes but also increased levels of LDL cholesterol, especially in individuals taking high doses. No beneficial or adverse effects on glycemic control were noted.³⁴ The American Heart Association now recommends the consumption of at least two servings of fish, especially salmon, per week.

The amount of omega-3 fatty acids needed to decrease serum TG levels, approximately 1 g/day, is easily obtained through the addition of fish to the diet. Administering as little as 0.21 g of EPA and 0.12 g of DHA per day in fish oil supplements has been shown to significantly decrease serum TG concentrations in subjects with hyperlipidemia.³⁵ Patients with primarily hypertriglyceridemia would appear to benefit from fish oil supplementation, however, those with increased LDL cholesterol and TG may be better off with combination therapy. The use of dietary interventions with lipid-lowering medications may be superior to treatment alone in patients with mixed forms of hyperlipidemia. A randomized, controlled 10-month crossover trial of 120 previously untreated hypercholesterolemic men ranging in age from 35 to 64 years showed that a modified Mediterranean-type diet rich in omega-3 fatty acids effectively potentiated the cholesterol-lowering effect of simvastatin, while counteracting the drug’s insulin-increasing effect. Unlike simvastatin therapy alone, it did not decrease serum levels of β-carotene and coenzyme Q10 (CoQ10).³⁶

Dyslipidemia often develops in patients who have undergone organ transplantation because of the long-term administration of corticosteroids and cyclosporine. Dietary interventions should be recommended and supported. However, if diet is not sufficient, combined treatment with low-dose pravastatin and fish oil has been shown to be more effective than pravastatin alone in improving the lipid profile after renal transplantation.³⁷

A word of caution: Mercury levels in fish can range from 10 to 1000 ppb. Mercury, which finds its way into rivers, lakes, and oceans from coal-burning power plants and other industrial sources, is known to cause learning disabilities and developmental delays. Health care providers should recommend that patients eat fish high in omega-3 fatty acids that are least likely to contain methyl mercury—salmon, for instance. One 3-oz serving of salmon contains approximately 1.2 to 1.5 g of omega-3 fatty acids. Fish high in mercury, such as king mackerel and shark, should be avoided, especially by children and pregnant women. Mercury-free fish oil capsules are available for those who do not care for fish or are concerned about the presence of methyl mercury (and other pollutants). A recent study of 20 fish oil supplements taken from store shelves revealed that all were free of any detectable level of mercury (<1.5 ppb).³⁸ Fish oil supplements contain both DHA and EPA.

Alcohol.

Convincing evidence indicates that moderate consumption of alcohol (one or two drinks per day) reduces insulin resistance, decreases blood pressure, and increases the level of HDL cholesterol.³⁹ The authors of one metaanalysis found strong and consistent evidence linking moderate alcohol intake with increased HDL cholesterol and apolipoprotein A₁ levels and decreased concentration of fibrinogen. The authors calculated an overall predicted 24.7% reduction in risk of CHD associated with alcohol intake of 30 g/day in accordance with changes in these markers.⁴⁰ The findings of more recent research suggest that alcohol consumption is associated with a decreased probability of an increased level of CRP.⁴¹

Exactly what form of alcohol consumption offers the best protection against cardiovascular disease remains a matter of debate. Some researchers believe that any type of ethanol is beneficial,⁴² whereas others contend that red wine specifically offers the most benefit.⁴³ The findings of one recent review of the clinical and experimental evidence suggest that red wine may offer greater cardiovascular protection than other types of alcoholic beverages.⁴⁴ This protection is thought to be conferred by the antioxidant, vasorelaxant, and antithrombotic properties of the polyphenolic compounds present in wine.⁴⁵

We are well aware of the many problems associated with alcohol consumption—fetal alcohol syndrome, alcoholism, hypertension, and increased TG level, to name but a few. Research has shown that individuals who consume more than three drinks per day are more likely to experience harm than benefit.⁴⁶ The risk of breast cancer is increased among women who consume two or more servings of alcohol per day, causing some specialists to recommend that women limit themselves to an average of one daily serving of alcohol.⁴⁷ Individuals who enjoy having a glass of wine, a beer, or a mixed drink should be encouraged to consider it a part of a healthy lifestyle, but those who abstain from alcohol should not be encouraged to start drinking. It is important to note that many of the antioxidant compounds found in wine can also be found in purple grape juice.⁴⁸

Exercise.

Obesity is an urgent and growing health problem in the United States. The Office of the U.S. Surgeon General reports that the risks of overweight or obesity may soon cause as much disease and death as does cigarette smoking. The percent of women 55 to 75 years of age who are overweight, according to the National Institutes of Health criterion of a body mass index of 25 kg/m², is estimated to be between 53.9% and 55.8%.⁵² Implementation of strategies to achieve optimal health, not necessarily a “normal” body weight, is probably the most effective approach for health care providers to take. It has been shown repeatedly that losing just 10 to 20 lb can significantly decrease cardiovascular risk in obese individuals (see Chapter 2).

Hypertension.

Hypertension is two to three times more prevalent among overweight people than in lean individuals. Approximately 60% of people with hypertension are overweight. Clinical trials have demonstrated that a reduction in body weight also causes a reduction in blood pressure. A review of 11 weight-loss studies showed that the average reductions in systolic and diastolic blood pressure per kilogram of weight loss were 1.6 and 1.1 mm Hg, respectively.⁵³ In addition, a recent study demonstrated that weight loss not only reduces blood pressure but also decreases left ventricular wall thickness.

Blood pressure is reduced when excess body weight is lost, even if ideal body weight is not achieved. It is postulated that obese people have some degree of tissue resistance to insulin, resulting in hyperglycemia. As a result, more insulin is released into the bloodstream. Insulin directly stimulates the release of the adrenal catecholamine hormones, which increase vascular resistance and cause the kidneys to retain sodium. Sodium retention increases intravascular volume and ultimately results in higher blood pressure. High levels of insulin also thicken arterial smooth muscle, which leads to an increase in blood pressure. As excess weight is shed, much of the cellular resistance to insulin is lost, and catecholamine levels fall.

Lipids.

Obesity is often associated with increased levels of TGs and low levels of HDL cholesterol.²⁴ It is thought that an increase in the TG level leads to increased catabolism of TG-rich HDL cholesterol, resulting in lower levels of HDL cholesterol.⁵⁴ Weight loss in most obese individuals increases the level of HDL cholesterol and decreases the TG level.⁵⁵ However, it is important to note that low-fat diets often reduce HDL cholesterol levels. For this reason, saturated fat should be replaced with monounsaturated fat in any weight loss program designed to increase HDL cholesterol levels.⁵⁶

Heart health.

In prospective studies, endothelial dysfunction has been associated with an increased incidence of cardiovascular events. The most effective nonpharmacologic approach for preventing endothelial dysfunction is aerobic physical activity, which can reduce oxidative stress associated with aging. Physical activity also improves endothelial dysfunction in patients with cardiovascular risk factors such as essential hypertension.⁵⁷

Approximately 25% of women report engaging in no regular sustained physical activity, and this number is even higher among those 55 years and older. Unfortunately, health care providers often fail to adequately discuss and encourage patients to increase their daily activity. Data from the Nurses’ Health Study indicate that simply walking 3 or more hours per week at a brisk pace can reduce the incidence of coronary events by 30% to 40%.⁵⁹ Exercise reduces concentrations of cholesterol and TGs, decreases blood pressure, and reduces the risk of stroke while helping maintain a healthy weight and fitness level. Regular exercise helps reduce abdominal fat, the type most associated with the development of diabetes and high blood pressure.

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