To test the effectiveness of dietary patterns in lowering blood pressure.

BACKGROUND:

The strong relationship between diet and blood pressure and the risk of hypertension has been well recognized. Currently, three specific diet-related factors have been recommended by the Joint National Committee on High Blood Pressure as a first-line approach in preventing and treating hypertension: caloric restriction for weight reduction, reduced consumption of alcohol, and lower sodium intake. Many studies, primarily observational, have shown significant associations between blood pressure and other diet-related factors, including micronutrients such as potassium, calcium, and magnesium; macronutrients such as type and amount of dietary fats, particularly polyunsaturated, and protein; and dietary fiber. However, results from individual, randomized, controlled clinical trials testing the effects of these nutrients on blood pressure have been inconsistent and equivocal. On the other hand, both observational and randomized intervention studies have consistently linked vegetarian dietary patterns to lower blood pressure.

To determine which specific dietary component may account for the blood pressure-lowering effect produced by vegetarian diets, numerous studies have examined individual micronutrients, macronutrients, and fiber. A large majority of cross-cultural and observational studies have shown a significant inverse association of blood pressure with the micronutrients, potassium, calcium, and magnesium, while the evidence linking dietary fiber to lower blood pressure comes primarily from studies on vegetarian and other high fiber diets. However, only a few of numerous clinical trials found that potassium or calcium supplements lower blood pressure. Randomized trials of magnesium supplementation generally have not shown a significant reduction in blood pressure. The few intervention studies of fiber supplements likewise generally have not demonstrated significant lowering of blood pressure. Pooled estimates from meta-analyses of potassium and calcium trials have reported small but significant reductions in systolic (potassium and calcium) and diastolic (potassium) blood pressure. All of these results are compatible with small effects of individual dietary components.

Only a few observational and intervention studies have demonstrated a relationship between the macronutrients, dietary fats or protein, and blood pressure. However, recent data analyses shown significant direct associations with saturated fat and dietary cholesterol, and inverse associations with protein intake.

Thus, an overall summary of the literature suggests that there is strong potential for a number of macronutrients and micronutrients to play an important role in reducing blood pressure. Testing dietary patterns rather than specific nutrients is a promising approach because it is not yet known which nutrients might be most effective in lowering blood pressure and because the effect of individual nutrients may be small. Testing dietary patterns would naturally include several nutrients that cumulatively are likely to lower blood pressure. Furthermore, most intervention studies which have not demonstrated decreases in blood pressure from individual nutrients used supplements rather than foods. Therefore, it may be important to test nutrients as components of foods and dietary patterns and not as isolated or processed nutrients.

DESIGN NARRATIVE:

DASH, a randomized, multicenter, controlled trial, compared the effects of three dietary patterns on blood pressure. The study design consisted of a three-week run-in period during which eligible participants were provided a diet that approximated a typical American diet in nutrient content. After run-in, eligible participants were randomized to one of three dietary patterns which lasted for eight weeks: a 'typical' American diet in which the macronutrient and fiber content reflected current United States consumption and was relatively low in calcium, potassium, and magnesium; a diet that was high in fruits and vegetables, and therefore high in potassium, magnesium, and fiber. The macronutrient and calcium content of this diet otherwise resembled that of the typical American diet. The third diet, termed the DASH diet, was high in fruits, vegetables, and low-fat dairy products, and had a favorable macronutrient profile. This diet was low in saturated fat, total fat, and cholesterol, was moderately high in protein, and had a high content of calcium, potassium, magnesium, and fiber. All three diets had about 3 grams of sodium daily--slightly below the average U.S. consumption-- and all included fresh, frozen, canned, and dried foods. None of the diets was vegetarian or used specialty foods containing fat substitutes. Alcohol and caffeine-containing beverages were limited and monitored. Because it was important that good compliance to the diet be assured, food was provided to the free-living study participants. The primary endpoint was change in diastolic blood pressure from baseline to the end of the study. The study compared the changes in diastolic blood pressure among the three dietary patterns. Screening began in June 1994. Randomization began in September 1994 and was expected to take two years. Support for the trial ended in July 1997.

NHLBI funded an ancillary study of DASH entitled, Folic Acid Dose Response. The purpose of the study was to determine the dose-response relationship between dietary folic acid intake and plasma folate and homocysteine. A total of 133 healthy people aged 60 years or older were enrolled. Participants were assigned to either 0, 100, 400, 1000, or 2000 micrograms of folic acid for 6 weeks. Study participants were newly recruited and were not DASH participants.

The study completion date listed in this record was obtained from the "End Date" entered in the old format Protocol Registration and Results System (PRS).

Men and women with higher than optimal diastolic blood pressure or with Stage 1 (mild) hypertension.