DASH diet

The DASH diet (Dietary Approaches to Stop Hypertension) is a dietary pattern promoted by the U.S.-based National Heart, Lung, and Blood Institute (part of the National Institutes of Health, an agency of the United States Department of Health and Human Services) to prevent and control hypertension. The DASH diet is rich in fruits, vegetables, whole grains, and low-fat dairy foods; includes meat, fish, poultry, nuts and beans; and is limited in sugar-sweetened foods and beverages, red meat, and added fats. In addition to its effect on blood pressure, it is designed to be a well-balanced approach to eating for the general public. It is now recommended by the United States Department of Agriculture (USDA) as an ideal eating plan for all Americans.

The DASH diet is based on NIH studies that examined three dietary plans and their results. None of the plans were vegetarian, but the DASH plan incorporated more fruits and vegetables, low fat or nonfat dairy, beans, and nuts than the others studied. The diet reduced systolic blood pressure by 6 mm Hg and diastolic blood pressure by 3 mm Hg in patients with high normal blood pressure, now called "pre-hypertension." Those with hypertension dropped by 11 and 6, respectively. These changes in blood pressure occurred with no changes in body weight. The DASH dietary pattern is adjusted based on daily caloric intake ranging from 1600 to 3100 dietary calories.

Background
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 Institutes of Health] (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 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.

Diet
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.

Study Population
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.

Study Design
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.

Study results
The DASH trial showed that dietary patterns can and do affect blood pressure in the high normal BP to moderately hypertensive adult population (systolic < 180 mm Hg & diastolic of 80 to 95 mm Hg). Respectively, the DASH or “combination” diet lowered blood pressures by an average of 5.5 and 3.0 mm Hg for systolic and diastolic, compared with the control diet. The minority portion of the study sample and the hypertensive portion both showed the largest reductions in blood pressure from the combination diet against the control diet. The hypertensive subjects experienced a drop of 11.4 mm Hg in their systolic and 5.5 mm Hg in their diastolic phases. The fruits-and-vegetables diet was also successful, although it produced more modest reductions compared with the control diet (2.8 mm Hg systolic and 1.1 mm Hg diastolic). In the subjects with and without hypertension, the combination diet effectively reduced blood pressure more than the fruits-and-vegetables diet or the control diet did. The data indicated that reductions in blood pressure occurred within two weeks of subjects’ starting their designated diets, and that the results were generalizable to the target sample of the U.S. population. Side effects were negligible, but the NEJM study reports that some subjects reported constipation as a problem. At the end of the intervention phase, 10.1, 5.4 & 4.0 percent of the subjects reported this problem for the control, fruits-and-vegetables and combination diets, respectively, showing that the fruits and vegetables and combination diets reduce constipation. Apart from only one subject (on the control diet) who was suffering from cholecystitis, other gastrointestinal symptoms had a low rate of incidence.

DASH-Sodium study design
The DASH-Sodium study was conducted following the end of the original DASH study to determine whether the DASH diet could produce even better results if it were low in salt and also to examine the effects of different levels of sodium in people eating the DASH diet. The researchers were interested in determining the effects of sodium reduction when combined with the DASH diet as well as the effects of the DASH diet when at three levels of sodium intake. The DASH-Sodium trial was conducted from September 1997 through November 1999. Like the previous study, it was based on a large sample (412 participants) and was a multi-center, randomized, outpatient feeding study where the subjects were given all their food. The participants were adults with prehypertension or stage 1 hypertension (average systolic of 120 to 159 mm Hg & average diastolic of 80 to 95 mm Hg) and were randomly assigned to one of two diet groups. The two randomized diet groups were the DASH diet and a control diet that mirrored a “typical American diet”, and which was somewhat low in key nutrients such as potassium, magnesium and calcium. The DASH diet was the same as in the previous DASH study. After being assigned to one of these two diets, the participants were given diets that differed by 3 distinct levels of sodium content, corresponding to 3,000 mg, 2,400 mg or 1,500 mg/day (higher, intermediate or lower), in random order, for 30 consecutive days each. During the two week run-in phase, all participants ate the high sodium control diet. The 30 day intervention phase followed, in which subjects ate their assigned diets at each of the aforementioned sodium levels (high, intermediate and low) in random order, in a crossover design. During the 30 day dietary intervention phase, each participant therefore consumed his or her assigned diet (either DASH or control) at all three sodium levels.

DASH-Sodium Study results and conclusions
The primary outcome of the DASH-Sodium study was systolic blood pressure at the end of the 30 day dietary intervention periods. The secondary outcome was diastolic blood pressure. The DASH-Sodium study found that reductions in sodium intake produced significantly lower systolic and diastolic blood pressures in both the control and DASH diets. Study results indicate that the quantity of dietary sodium in the control diet was twice as powerful in its effect on blood pressure as it was in the DASH diet. Importantly, the control diet sodium reductions from intermediate to low correlated with greater changes in systolic blood pressure than those same changes from high to intermediate (change equal to roughly 40 mmol per day, or 1 gram of sodium).

As stated by Sacks, F. et al., reductions in sodium intake by this amount per day correlated with greater decreases in blood pressure when the starting sodium intake level was already at the U.S. recommended dietary allowance, than when the starting level was higher (higher levels are the actual average in the U.S.). These results led researchers to postulate that the adoption of a national lower daily allowance for sodium than the currently held 2,400 mg could be based on the sound scientific results provided by this study. The U.S. Dietary Guidelines for Americans recommend eating a diet of 2300 mg of sodium a day or lower, with a recommendation of 1500 mg/day in adults who have elevated blood pressure; the 1500 mg/day is the low sodium level tested in the DASH-Sodium study. The DASH diet and the control diet at the lower salt levels were both successful in lowering blood pressure, but the largest reductions in blood pressure were obtained by eating a combination of these two (i.e., a lower-salt version of the DASH diet). The effect of this combination at a sodium level of 1,500 mg/day was an average blood pressure reduction of 8.9/4.5 mm Hg (systolic/diastolic). The hypertensive subjects experienced an average reduction of 11.5/5.7 mm Hg. The DASH-sodium results indicate that low sodium levels correlated with the largest reductions in blood pressure for participants at both pre-hypertensive and hypertensive levels, with the hypertensive participants showing the greatest reductions in blood pressure overall.