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Dairy Council Digest Archives
A New Look At Dietary Patterns and Hypertension
Dietary Approaches to Stop Hypertension (DASH) Trials
The DASH Trial. The first multicenter DASH trial sponsored by the National Heart, Lung, and Blood Institute (NHLBI) was published in 1997 (7). In this trial, 459 adults with a systolic blood pressure of less than 160mm Hg and diastolic pressure of 80-95mm Hg were randomly assigned to one of the following three dietary plans for eight weeks: a control diet typical of what most Americans eat (i.e., low in fruits, vegetables, and dairy products and containing 36% of calories from fat); a diet similar to what Americans consume but higher in fruits and vegetables; or a combination diet high in lowfat dairy foods, fruits and vegetables, and reduced in total fat (26% of calories), saturated fat and cholesterol (i.e., the so-called DASH diet) (7). The DASH combination diet also includes whole grains, poultry, fish, and nuts and only small amounts of red meat, sweets, and sugar-containing beverages (7). Sodium intake (approximately 3g/day), body weight, and physical activity remained constant throughout the study. An in-depth description of the DASH feeding trial can be found in a supplement to the Journal of the American Dietetic Association (11).
The DASH eating pattern, which is rich in lowfat dairy foods, fruits, and vegetables, substantially lowers blood pressure without weight loss or reduced sodium intake. A combination of the DASH diet and sodium reduction can lower blood pressure even further.
At the end of the eight weeks, the DASH combination diet, which included lowfat dairy foods, fruits and vegetables, produced the largest reductions in blood pressure (7). Overall, this diet reduced systolic blood pressure by 5.5mm Hg and diastolic blood pressure by 3.0mm Hg compared to the control diet. The fruits and vegetables diet also reduced systolic blood pressure (by 2.8mm Hg) and diastolic blood pressure (by 1.1mm Hg), although by a lesser amount than that achieved with the combination diet (7). Participants following the combination diet (at the 2,000 calorie level) consumed almost three servings of dairy foods a day and 8 to 10 servings/day of fruits and vegetables.
The reduction in blood pressure on the DASH combination diet occurred quickly (i.e., within two weeks), remained lower as long as the participants stayed on the diet, and rivaled that achieved with antihypertensive medications (7). The potential savings in healthcare costs attributed to the DASH diet can be expected to be substantial considering that 70% of subjects with high blood pressure responded to the DASH diet such that they no longer needed pharmacological management of their hypertension (12). Also, over 78% of the cases of systolic hypertension were eliminated by the DASH diet (12). Researchers estimate that if Americans eating a typical Western diet adopt the DASH dietary pattern, heart disease could be reduced by 15% and stroke by 27%(7).
The DASH-Sodium Trial. To answer questions raised by the DASH trial (7), a follow-up multicenter, randomized controlled trial of 412 adults with high or normal blood pressure was initiated (8). The participants, more than half of whom were women and more than half of whom were African American, were randomly assigned to either a typical American diet (control) or the DASH diet. Within each of these diets, participants ate foods with high (~3.3g), intermediate (~2.4g), or low (~1.5g) sodium for 30 days each in random order.
The DASH diet significantly lowered systolic blood pressure regardless of the level of sodium consumed (8). This finding confirms and extends the findings of the previous DASH trial that demonstrated that the first step in blood pressure control is improving the overall diet by increasing intake of lowfat dairy foods, fruits, and vegetables (8). With both the control and the DASH diet, reducing sodium intake lowered blood pressure in a step-wise fashion (8). The DASH diet at the highest sodium level reduced systolic blood pressure by 5.9mm Hg compared to the control diet at the same sodium level. For participants consuming the DASH diet, reducing sodium intake to the intermediate level lowered systolic blood pressure by an additional 1.3mm Hg, and reducing sodium to the lowest level resulted in an additional 1.7mm Hg drop in systolic blood pressure (8). Compared to the high sodium control diet, the low sodium DASH diet led to greater reductions in systolic and diastolic blood pressure than either the DASH diet alone or low sodium alone (8). However, the effect was not strictly additive. The greatest reduction in blood pressure, which was found with the DASH diet at the lowest sodium intake compared to the control diet at high sodium intake, was similar to that achieved with blood pressure-lowering drugs (4,8).
The findings of the DASH-Sodium trial support intake of the DASH diet in combination with reduced dietary sodium to prevent and treat hypertension (8). However, the researchers acknowledge that the long-term health benefits of this dietary pattern have yet to be demonstrated and will depend on people's willingness to consistently choose lower-sodium foods and the food industry's commitment to increase the availability of lower-sodium foods (8). One author (4) states that reducing sodium intake below the currently recommended level (2.4g/day) (13) will require "aggressive action by the government in terms of legislation or labeling requirements." A so-called Premier Study, a behavioral intervention trial, is currently underway to investigate the ability of free-living adults to adopt and maintain the DASH combination diet with other effective lifestyle interventions (e.g., weight loss, sodium reduction, increased physical activity, limited alcohol intake) (14).
The DASH eating pattern, which is rich in lowfat dairy foods, fruits, and vegetables, substantially lowers blood pressure without weight loss or reduced sodium intake. A combination of the DASH diet and sodium reduction can lower blood pressure even further.
Who Benefits From the DASH Dietary Pattern? All participants in the DASH trials experienced a reduction in blood pressure leading the researchers to suggest that the findings from these trials could apply to most people in the U.S. (7,8). However, the blood pressure-lowering effect of the DASH dietary pattern was greater for some population groups than for others (7,8).
The DASH diet lowered blood pressure more in persons with hypertension than in those without hypertension (7,8,15). In subjects without hypertension, the DASH combination diet lowered systolic and diastolic blood pressures by 3.5mm Hg and 2.1mm Hg, respectively, compared to the control diet (7). However, in hypertensive subjects, systolic and diastolic blood pressures were reduced by 11.4mm Hg and 5.5mm Hg, respectively, compared to the control diet (7). A recent subgroup analysis of 133 hypertensive participants in the DASH trial supports the benefits of this dietary pattern for individuals with hypertension (12). The effectiveness of the DASH eating pattern for hypertensives was also observed in the DASH-Sodium trial (8). Compared to the control diet with a high sodium level, the DASH diet with a low sodium level decreased systolic blood pressure by 7.1mm Hg in nonhypertensive participants, whereas systolic blood pressure was reduced by 11.5mm Hg in participants with hypertension (8).
The DASH dietary pattern is particularly effective for minorities such as African Americans who are at higher risk of developing hypertension and its complications than are their white counterparts (8,15-18). According to a recent subgroup analysis of the DASH trial, the DASH combination diet lowered systolic blood pressure significantly more in African Americans (6.8mm Hg) than in whites (3.0mm Hg) (15). Similar findings are reported in the DASH-Sodium trial (8). The increased effectiveness of the DASH combination diet for African Americans supports other findings suggesting racial differences in blood pressure responses to diet (17,18). African Americans' high risk for hypertension may be explained in part by deficiencies of multiple nutrients (19). A study of 180 minority adolescents at risk of developing high blood pressure found that blood pressure was lower in the teens who had higher intakes of potassium, calcium, magnesium, and vitamins (19). The researchers suggested that "diets rich in a combination of nutrients derived from fruits, vegetables, and lowfat dairy products could contribute to primary prevention of hypertension when instituted at an early age" (19).
Consistency with Other Studies. The beneficial effect of a dietary intake pattern similar to the DASH diet was recognized in the early 1980s when investigators analyzed data from the first National Health and Nutrition Examination Survey (9). In another investigation involving 560 adults with hypertension, dyslipidemia, or diabetes at 10 medical centers in the U.S., intake of a pre-packaged meal plan that was accompanied by improvements in dietary levels of calcium, potassium, and magnesium approximating intakes achieved in the DASH trial improved cardiovascular risk factors, including blood pressure (20). The researchers concluded that intake of nutritionally balanced meals that meet the recommendations of national health organizations can improve blood pressure status (20).
The blood pressure-lowering effect of the DASH diet is applicable to most people in the U.S. However, this eating pattern is particularly effective for individuals with hypertension and for African Americans who are at high risk for hypertension.
The DASH Diet Reduces Blood Homocysteine Levels. An added benefit of the DASH combination diet is its ability to lower blood levels of homocysteine (21). Abnormally high blood levels of this amino acid, which is synthesized when the body metabolizes protein, are associated with increased risk for heart disease and stroke. According to a recent randomized, controlled feeding trial of 118 adults, intake of a low fat diet rich in fruits, vegetables, and lowfat dairy products significantly reduced blood levels of homocysteine when compared to a typical Western diet (21). Because inadequate intakes of folate, vitamin B12, and vitamin B6 are linked to increased homocysteine levels in the bloodstream, the ability of the DASH diet to lower blood homocysteine levels may be explained by increased intake of these nutrients found in fruits, vegetables, and dairy products. Based on the reduction in homocysteine levels, the researchers estimate that intake of the DASH diet could lower atherosclerotic cardiovascular disease by 7 to 9%(21), in addition to the 15 to 27% reduction in heart disease estimated by the DASH-induced decreased in blood pressure (7).
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