DASH Diet - History and Design

History and Design

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. (NIH) to propose funding to further research the role of dietary patterns on blood pressure. In 1992 the NHLBI worked with 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 used a rigorous design called a randomized controlled trial (RCT), and it 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) Brigham and Women's Hospital in Boston, Massachusetts and (5) Pennington Biomedical Research Center in Baton Rouge, Louisiana.

Two DASH trials were designed and carried out as multi-center, randomized, outpatient feeding studies with the purpose of testing the effects of dietary patterns on blood pressure. The standardized multi-center protocol is an approach used in many large-scale multi-center studies funded by the NHLBI. A unique feature of the DASH diet was that the foods and menu were chosen based on conventionally consumed food items so it could be more 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 low in potassium, calcium, magnesium and fiber and featured a fat and protein profile so that the pattern was consistent with (a “typical American diet at the time”). The first experimental diet was higher in 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 was high in fruits-and-vegetables and in low-fat dairy products, as well as lower in overall fat and saturated fat, with higher fiber and higher protein compared with the control diet—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 combination or “DASH” diet was also high in whole grains, poultry, fish and nuts while being lower in red meat content, sweets and sugar-containing beverages.

The DASH diet was designed to provide liberal amounts of key nutrients thought to play a part in lowering blood pressure, based on past epidemiologic studies. One of the unique features of the DASH study was that dietary patterns rather than single nutrients were being tested. The DASH diet also features a high quotient of anti-oxidant rich foods thought by some 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. 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, subjects who were compliant with the feeding program during the screening phase 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 were provided the diet to which they had been randomly assigned. 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, selected because that was the approximate average intake in the nation at the time. Participants were also given two packets of salt, each containing 200 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.