Dietary Factors That Lower Blood Pressure

Weight Loss

On average, as weight increases, so does blood pressure. The importance of this relationship is reinforced by the high and increasing prevalence of overweight and obesity throughout the world. With rare exception, clinical trials have documented that weight loss lowers blood pressure. Importantly, reductions in blood pressure occur before and without attainment of a desirable body weight. In one meta-analysis that aggregated results across 25 trials, mean systolic and diastolic blood pressure reductions from an average weight loss of 5.1 kg were 4.4 and 3.6 mmHg, respectively. Greater weight loss leads to greater blood pressure reduction.

Additional trials have documented that modest weight loss can prevent hypertension by approximately 20% among overweight, prehypertensive individuals and can facilitate medication step-down and drug withdrawal. Lifestyle intervention trials have uniformly achieved short-term weight loss, primary through a reduction in total caloric intake. In some instances, substantial weight loss has also been sustained over 3 or more years.

In aggregate, available evidence strongly supports weight reduction, ideally attainment of a body mass index less than 25 kg/m2, as an effective approach to prevent and treat hypertension. Weight reduction can also prevent diabetes and control lipids. Hence, the beneficial effects of weight reduction in preventing cardiovascular-renal disease should be substantial. Finally, in view of the well-recognized challenges of maintaining weight loss, efforts to prevent weight gain among those with a normal body weight are critical.

Reduced Salt Intake

On average, as dietary salt (sodium chloride) intake rises, so does blood pressure.1 To date, more than 50 randomized trials have tested the effects of salt on blood pressure, including several dose-response trials. Approximately 10 meta-analyses have aggregated data across these trials. In a recent meta-analysis that focused on moderate reductions in salt intake, a reduced sodium intake of 1.8g/day (77mmol/day) led to average systolic/diastolic blood pressure reductions of 5.2/3.7mmHg in hypertensives and 1.3/1.1 mmHg in nonhypertensives.

One of the most important dose-response trials is the DASH-Sodium trial, which tested the effects of three different salt intakes separately in two distinct diets—the DASH (Dietary Approaches to Stop Hypertension) diet and a control diet more typical of what Americans eat. As displayed in Figure 3, the rise in blood pressure with higher salt intake was evident in both diets. Of note, the blood pressure response to salt intake was nonlinear. Specifically, decreasing salt intake caused a greater lowering of blood pressure when the starting sodium intake was less than 2.3 g/ day (100mmol/day) than when it was above this level.

The blood pressure response to changes in salt intake is heterogeneous. Despite the use of the terms 'salt sensitive' and 'salt resistant' to classify individuals in research studies, the change in blood pressure in response to a change in salt intake is not binary. Instead, the change in blood pressure from a reduced salt intake has a continuous distribution, with individuals having greater or lesser degrees of blood pressure reduction. Genetic factors influence the response to salt reduction. Concomitant diet also modifies the effects of salt on blood pressure. The rise in blood pressure for a given increase in salt intake is blunted in the setting of either the DASH diet or a high potassium intake (Figure 3). In general, the effects of salt on blood pressure tend to be greater in blacks, middle-aged and older people, and individuals with hypertension, diabetics, or chronic kidney disease. Although it is possible to identify groups that tend to be salt sensitive, it is impossible, given currently available diagnostic tools, to identify individuals who are salt sensitive.

1In view of the format of published data and of dietary recommendations, data are presented as g/day (mmol/day) of sodium rather than g/day of salt.

ST 125 % 120 115 110

Figure 3 Mean systolic blood pressure (SBP) change in the DASH-Sodium trial from salt reduction in two diets and from the DASH diet at three salt levels. (Adapted with permission from Sacks FM, Svetkey LP, Vollmer WM et al. (2001) A clinical trial of the effects on blood pressure of reduced dietary sodium and the DASH dietary pattern (The DASH-Sodium Trial). New England Journal of Medicine 344: 3-10.)

In addition to lowering blood pressure, clinical trials have documented that a reduced salt intake can prevent hypertension by approximately 20% (with or without concomitant weight loss) and can lower blood pressure in the setting of antihypertensive medication. Evidence from observational studies suggests that a reduced salt intake can blunt the age-related rise in systolic blood pressure (Figure 4) and can potentially prevent atherosclerotic cardiovascular events and heart failure. A reduced salt intake may also reduce the risk of left ventricular hypertrophy, osteoporosis, and gastric cancer.

50 100 150 200

Median sodium excretion (mmol/24 h)

Figure 4 Slope of systolic blood pressure increase with age plotted by median sodium excretion in 52 communities worldwide: results from INTERSALT. (Adapted with permission from Rose G, Stamler J, Stamler R et al. (1988) INTERSALT: An international study of electrolyte excretion and blood pressure. Results for 24 hour urinary sodium and potassium excretion. British Medical Journal 297: 319-328.)

50 100 150 200

Median sodium excretion (mmol/24 h)

Figure 4 Slope of systolic blood pressure increase with age plotted by median sodium excretion in 52 communities worldwide: results from INTERSALT. (Adapted with permission from Rose G, Stamler J, Stamler R et al. (1988) INTERSALT: An international study of electrolyte excretion and blood pressure. Results for 24 hour urinary sodium and potassium excretion. British Medical Journal 297: 319-328.)

Still, the effects of salt on health have been debated. Some have argued that the increases in plasma renin activity and perhaps insulin resistance that occur as a result of a reduced salt intake mitigate the beneficial effects of salt reduction on blood pressure. However, in contrast to blood pressure, the clinical relevance of increased plasma renin activity is uncertain, especially because antihypertensive medications that raise plasma renin levels actually lower cardiovascular disease risk. It has also been argued that a reduced salt intake has little or no effect on blood pressure in many individuals and that other aspects of diet (e.g., increased potassium intake or adoption of a mineral-rich diet) mitigate the harmful effects of salt on blood pressure. Although one cannot guarantee that all individuals will achieve a lower blood pressure from salt reduction, the fraction of individuals who will benefit is substantial.

In view of the progressive dose-response relationship between salt intake and blood pressure, it is difficult to set specific levels for dietary recommendations. Recently, an Institute of Medicine committee set 1.5g/day (65 mmol/day) of sodium as an adequate intake level and 2.3g/day (100 mmol/day) as an upper limit. Western-type diets that provide 1.5g/day (65 mmol/day) have been shown to provide adequate levels of other nutrients. This level of salt intake also allows for excess sweat salt loss among unacclimatized individuals who become physically active or who become exposed to high temperatures. Numerous policymaking organizations have recommended an upper limit of 2.3 g/day (100 mmol/day) for sodium intake.

In most Western counties, average intake of sodium is high, greatly exceeding 2.3g/day (100 mmol/day). In the United States, the median intake of sodium from foods, not including salt added at the table, varies by age and, according to a recent survey, ranges from 3.1 to 4.7g/day (135 to 204 mmol/day) in adult men and 2.3 to 3.1g/day (100 to 135 mmol/day) in adult women. Worldwide, there is greater variation in sodium intake, ranging from an estimated mean intake of 0.02g/day (1.0 mmol/day) in Yanomamo Indians to more than 10.3 g/day (450 mmol/day) in northern Japanese.

In aggregate, available data strongly support current populationwide recommendations to lower salt intake. To reduce salt intake, consumers should choose foods low in salt and limit the amount of salt added to food. However, even motivated individuals find it difficult to reduce salt intake because more than 75% of consumed salt comes from processed foods (Figure 5). Hence, any meaningful strategy to reduce salt intake must involve the efforts of food manufacturers, who should reduce the amount of salt added during food processing.

Food processing 77%

Food processing 77%

Inherent in food 12%

at the table

during cooking 5%

Figure 5 Sources of dietary sodium. (Data from Mattes RD and Donnelly D (1991) Relative contributions of dietary sodium sources Journal of the American College of Nutrition 10: 383-393.)

Inherent in food 12%

at the table

during cooking 5%

Figure 5 Sources of dietary sodium. (Data from Mattes RD and Donnelly D (1991) Relative contributions of dietary sodium sources Journal of the American College of Nutrition 10: 383-393.)

Increased Potassium intake

High levels of potassium intake are associated with reduced blood pressure. Observational data have been reasonably consistent in documenting this inverse relationship, whereas data from individual trials have been less consistent. However, three meta-analyses of these trials have each documented a significant inverse relationship between potassium intake and blood pressure in nonhypertensive and hypertensive individuals. In one meta-analysis, average net systolic/ diastolic blood pressure reductions from increased potassium intake were 4.4/2.4 mmHg. Available studies have documented greater blood pressure reductions from potassium in African Americans compared to non-African Americans. A high potassium intake has been shown to blunt the rise in blood pressure in response to increased salt intake. Potassium has greater blood pressure lowering in the context of a higher salt intake and lesser blood pressure reduction in the setting of a lower salt intake. Conversely, the blood pressure reduction from a reduced salt intake is greatest when potassium intake is low. These data are consistent with subadditive effects of reduced salt intake and increased potassium intake on blood pressure.

Most trials that tested the effects of potassium on blood pressure used pill supplements, typically potassium chloride. However, in foods, the conjugate anions associated with potassium are mainly citrate and other bicarbonate precursors. The latter is important because other potential benefits of foods rich in potassium (i.e., reduced risk of kidney stones and reduced bone turnover) likely result from effects of the conjugate anion. Because a high dietary intake of potassium can be achieved through diet rather than pills and because potassium derived from foods also comes with a variety of other nutrients, the preferred strategy to increase potassium intake is to consume foods, such as fruits and vegetables, rather than supplements.

On the basis of available data, an Institute of Medicine committee set an Adequate Intake for potassium of 4.7g/day (120 mmol/day) for adults. This level of dietary intake should maintain lower blood pressure levels, reduce the adverse effects of salt on blood pressure, reduce the risk of kidney stones, and possibly decrease bone loss. Currently, dietary intake of potassium is considerably lower than this level. In recent surveys, the median intake of potassium by adults in the United States was approximately 2.9-3.2 g/day (74-82 mmol/ day) for men and 2.1-2.3 g/day (54-59 mmol/day) for women. Because African Americans have a relatively low intake of potassium and a high prevalence of elevated blood pressure and salt sensitivity, this subgroup of the population would especially benefit from an increased potassium intake.

In the generally healthy population with normal kidney function, a potassium intake from foods higher than 4.7 g/day (120 mmol/day) poses no potential for increased risk because excess potassium is readily excreted in the urine. However, in individuals whose urinary potassium excretion is impaired, a potassium intake of less than 4.7 g/day (120 mmol/day) is appropriate because of adverse cardiac effects (arrhythmias) from hyperkalemia. Common drugs that impair potassium excretion are angiotensin converting enzyme inhibitors, angio-tensin receptor blockers, and potassium-sparing diuretics. Medical conditions associated with impaired potassium excretion include diabetes, chronic renal insufficiency, end stage renal disease, severe heart failure, and adrenal insufficiency. Elderly individuals are at increased risk of hyper-kalemia because they often have one or more of these conditions or take one or more of the medications that impair potassium excretion.

Moderation of Alcohol Intake

The relationship between alcohol intake and blood pressure is direct and progressive, particularly at an alcohol intake above approximately two drinks per day (~1 oz. or ^28 g of ethanol per day). A meta-analysis of 15 trials reported that decreased consumption of alcohol (median reduction in self-reported alcohol consumption of 76%) lowered systolic and diastolic blood pressure by 3.3 and 2.0 mmHg, respectively. In nonhypertensives and hypertensives, blood pressure reductions were similar. In aggregate, evidence supports moderation of alcohol intake (among those who drink) as an effective approach to lower blood pressure. It is recommended that alcohol consumption be limited to no more than 1 oz. (30 ml) of ethanol (e.g., 24 oz. (720 ml) beer, 10 oz. (300 ml) wine, or 2oz. (60 ml) 100-proof whiskey) per day in most men and to no more than 0.5 oz. (15 ml) ethanol per day in women and lighter weight people.

Whole Dietary Patterns

Vegetarian diets Vegetarian diets have been associated with low blood pressure. In observational studies, vegetarians also experience a markedly lower, age-related rise in blood pressure. Aspects of a vegetarian lifestyle that might affect blood pressure include nondietary factors (e.g., physical activity), established dietary risk factors (e.g., salt, potassium, weight, and alcohol), and other aspects of a vegetarian diet (e.g., high fiber and no meat). To a very limited extent, observational studies have controlled for the well-established determinants of blood pressure. Hence, it is unclear whether blood pressure reductions result from established dietary risk factors that affect blood pressure or from other aspects of a vegetarian diet.

The DASH diet The DASH trial tested whether modification of whole dietary patterns might affect blood pressure. In this trial, participants were randomized to eat one of three diets: (i) a control diet, (ii) a diet rich in 'fruits and vegetables' but otherwise similar to control, or (iii) the DASH diet. The DASH diet emphasizes fruits, vegetables, and low-fat dairy products; includes whole grains, poultry, fish, and nuts; and is reduced in fats, red meat, sweets, and sugar-containing beverages. Accordingly, it is rich in potassium, magnesium, calcium, and fiber and reduced in total fat, saturated fat, and cholesterol; it is also slightly increased in protein.

Among all participants, the DASH diet significantly lowered mean systolic blood pressure by 5.5 mmHg and mean diastolic blood pressure by 3.0mmHg. The fruits and vegetables diet also reduced blood pressure but to a lesser extent— approximately half of the effect of the DASH diet. The effect was relatively rapid; the full effect was apparent after 2 weeks (Figure 6). In subgroup analyses, the DASH diet significantly lowered blood pressure in all major subgroups (men, women, African Americans, non-African Americans, hypertensives, and nonhypertensives). However, the effects of the DASH diet were especially prominent in African Americans, who experienced net systolic/diastolic blood pressure reductions of 6.9/3.7 mmHg, and hypertensive individuals, who experienced net blood pressure reductions of 11.6/5.3 mmHg.

Results from the DASH trial have important clinical and public health implications. The effect of the DASH diet in hypertensive individuals was similar in magnitude to that of drug monotherapy. From a public

-o- Control Fruit / Veg Dash

-o- Control Fruit / Veg Dash

3 4 Weeks

Figure 6 Blood pressure by week during the DASH feeding study in three diets (control diet, fruits and vegetables diet, and DASH diet). (Adapted with permission from Appel LJ, Moore TJ, Obarzanek E et al. (1997) The effect of dietary patterns on blood pressure: Results from the Dietary Approaches to Stop Hypertension (DASH) clinical trial. New England Journal of Medicine 336: 1117-1124.)

3 4 Weeks

Figure 6 Blood pressure by week during the DASH feeding study in three diets (control diet, fruits and vegetables diet, and DASH diet). (Adapted with permission from Appel LJ, Moore TJ, Obarzanek E et al. (1997) The effect of dietary patterns on blood pressure: Results from the Dietary Approaches to Stop Hypertension (DASH) clinical trial. New England Journal of Medicine 336: 1117-1124.)

health perspective, the DASH diet could potentially shift the population distribution of blood pressure downward, thereby reducing the risk of blood pressure-related cardiovascular disease (Figure 2).

Fish Oil Supplementation

High-dose, omega-3 polyunsaturated fatty acid (commonly termed 'fish oil') supplements can lower blood pressure in hypertensive individuals. In a meta-analysis of trials, average systolic and diasto-lic blood pressure reductions in hypertensive individuals were 5.5 and 3.5 mmHg, respectively. The effect of fish oil appears to be dose dependent, with blood pressure reductions only occurring at relatively high doses, namely 3 g/day or more. In nonhypertensive individuals, blood pressure reductions were nonsignificant and small. Side effects, including belching and a fishy taste, are common. In view of the side effect profile and the high dose required to lower blood pressure, fish oil supplements are not routinely recommended.

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