History and Design of the DASH Study

Currently, hypertension is thought to affect roughly 50 million people in the U.S. and approximately 1 billion worldwide. According to the National Heart, Lung and Blood Institute (NHLBI), citing data from 2002, “The relationship between BP and risk of cardiovascular disease (CVD) events is continuous, consistent, and independent of other risk factors. The higher the BP, the greater is the chance of heart attack, heart failure, stroke, and kidney disease. For individuals 40–70 years of age, each increment of 20 mm Hg in systolic BP (SBP) or 10 mm Hg in diastolic BP (DBP) doubles the risk of CVD across the entire BP range from 115/75 to 185/115 mm Hg.”.

The prevalence of hypertension led the U.S. [National Institute of Health] (NIH) to propose funding to further research the role of key nutrients in the diet. In 1992 the NHLBI directed five of the most well-respected medical research centers in different cities across the U.S. to conduct the largest and most detailed research study to date. The DASH study involved teams of physicians, nurses, nutritionists, statisticians and research coordinators working in a cooperative venture in which participants were selected and studied in each of the these five research facilities. The chosen facilities and locales for this multi-center study were (1) Johns Hopkins University in Baltimore, Maryland, (2) Duke University Medical Center in Durham, North Carolina, (3) Kaiser Permanente Center for Health Research in Portland, Oregon, (4) Harvard School of Public Health in Boston, Massachusetts and (5) Pennington Biomedical Research Center in Baton Rouge, Louisiana.

The DASH trials were designed and carried out as a multi-center, randomized, outpatient feeding study with the purpose of testing the effects of dietary patterns on blood pressure. The standardized multi-center protocol was one of the unique features of the DASH diet. Another unique feature of the DASH diet and design was the foods and menu were chosen based on conventionally consumed food items which would be easily adopted by the general public if results were positive. The initial DASH study was begun in August 1993 and ended in July 1997. Contemporary epidemiological research had concluded that dietary patterns with high intakes of certain minerals and fiber were associated with low blood pressures. The nutritional conceptualization of the DASH meal plans was based in part on this research.

Two experimental diets were selected for the DASH study and compared with each other, and with a third: the control diet. The control diet was characteristically low in potassium, calcium, magnesium and fiber. Moreover, the control diet also featured a fat and protein profile consistent with current or contemporary dietary regimens (a “typical American diet” ). The first experimental diet was an idealized ‘good’ diet consisting of fruits and vegetables but otherwise similar to the control diet (a “fruits and vegetables diet” ), with the exception of fewer snacks and sweets. Magnesium and Potassium levels were close to the 75th percentile of U.S. consumption in the fruits-and-vegetables diet, which also featured a high fiber profile. The second experimental diet combined elements of the previous two (control and fruits-and-vegetables)—this diet has been called ‘the DASH Diet” . The Dash diet (or combination diet) was rich in potassium, magnesium and calcium—a nutrient profile roughly equivalent with the 75th percentile of U.S. consumption. The DASH diet was also high in fruits, vegetables & low fat dairy foods, and also rich in fiber and protein (18%). The combination or ‘DASH’ diet was also high in whole grains, poultry, fish and nuts while being low in fat and red meat content, sweets and sugar-containing beverages.

The DASH diet was also focused on providing liberal amounts of key nutrients thought to play a part in lowering blood pressure, based on past medical studies. The DASH diet was designed to be a whole-food diet, low in processed or refined sugars and high in complex carbohydrates. One of the unique features of the DASH diet design is that dietary patterns rather than single nutrients were being tested. However, it was thought that the richness of complex-carbohydrates would help to minimize the often abrupt increases in blood sugar and insulin levels associated with hardening of the arteries, mood swings and weight gain. Equally important, the DASH diet features a high quotient of anti-oxidant rich foods thought to retard or prevent chronic health problems including cancer, heart disease and stroke.

8,813 people were screened for the study, out of which were ultimately chosen 459 participants whose demographic characteristics most closely resembled the target population and study requirements. The sample population consisted of healthy men and women with an average age of 46, with systolic blood pressures of less than 160 mm Hg and diastolic blood pressures within 80 to 95 mm Hg. African-American and other minority groups were planned to comprise 67% of the study sample, with 49% of the sample being female. Indeed, due to the exceptional burden of high blood pressure in minority populations, especially among African-Americans, a major goal of the trial was to recruit enough ethnic minorities to constitute two thirds of the target sample.

Participants ate one of the three aforementioned dietary patterns in 3 separate phases of the trial, including (1) Screening, (2), Run-in and (3) Intervention. In the screening phase, participants were screened for eligibility based on the combined results of blood pressure readings and a Stanford 7-Day Physical Activity Recall questionnaire. In the 3 week run-in phase, each subject was given the control diet for 3 weeks, had their blood pressure measurements taken on each of five separate days, gave one 24-hour urine sample and completed a questionnaire on symptoms. At this point the subjects were each randomly assigned to one of the three diets outlined above, to begin at the start of the 4th week. The intervention phase followed next; this was an 8-week period in which the subjects followed the diet they had each been randomly assigned to. Blood pressures and urine samples were collected again during this time together with symptom & physical activity recall questionnaires. The first group of study subjects began the run-in phase of the trial in September 1994 while the fifth and final group began in January 1996. Each of the three diets contained the same 3 grams (3,000 mg) of sodium. Participants were also given two packets of salt, each containing 2,000 mg of sodium, for discretionary use. Alcohol was limited to no more than two beverages per day, and caffeine intake was limited to no more than three caffeinated beverages.