Researchers from 5 major medical centers (Harvard (Dr. Moore), Duke, Johns Hopkins, Pennington Biomedical Research Center, and the Kaiser Center for Health Research) designed the DASH eating pattern and proved that it works in a large research study that was performed between 1994 and 1996. The results of DASH are taken so seriously because of the way the study was conducted: a "controlled clinical trial" – medical language for a foolproof way to test a treatment – that featured both a large number of participants and close monitoring, two factors that ensure accurate results.
Out of 8,800 people who volunteered for the DASH study, 459 were chosen to participate. Some of the participants had high blood pressure and some did not. The average age of the participants was 46 and the average blood pressure reading was 132/85, 49% were women, nearly 60% were African American.
The 11–week study was divided into two parts. There was a “baseline” three–week period when all 459 participants ate a "typical American diet." This was done to get all the participants to the same nutritional status.
Following this, the participants were randomly assigned to one of three different types of diets for eight weeks:
To assure they weren't doing anything else that might lower their blood pressure, participants were asked not to make any major changes in the physical activity levels during the study. The participants were weighed frequently to make sure their weight stayed constant; if someone gained or lost weight, they were given a bit less or more food to eat to get their weight back to what it was. Salt/sodium intake was the same in all three diets – slightly lower than the U.S. average, but still higher than what most guidelines recommended.
To prevent bias, researchers were not allowed to see or know what the results of the study were until it was completed (in scientific terms, this is called "blinding"). This meant that they were unaware of the results until the study was completely over in 1996. However, the researchers knew when they saw the results it had been worth the wait!
The effects of three different diets were tested – 1) the so–called "typical American diet," 2) a diet rather like the typical American diet but with large added amounts of fruits and vegetables, and 3) the DASH diet.
Those who ate the typical American diet did not see a change in their blood pressure. Those on the fruit and vegetable diet experienced a significant lowering of their systolic blood pressure – the upper number, which is a measurement of blood pressure in the arteries when the heart contracts to pump out blood – but little change in their diastolic pressure.
But the men and women who ate the DASH diet for eight weeks experienced a significant drop in both their systolic and diastolic blood pressure readings. Changes occurred within a week of starting the diet, stabilized within two weeks, and stayed lowered for the remainder of the eight weeks. On average, blood pressure fell 5.5 mmHg (systolic) and 3.0 mmHg (diastolic) among all participants (including both those with normal blood pressure and those with hypertension). In participants with high blood pressure, blood pressure dropped an average of 11.4 mmHg (systolic) and 5.5 mmHg (diastolic). These improvements in blood pressure are about the same as what can be achieved with a single antihypertensive medication.
There were positive health outcomes of the DASH diet beyond lowering high blood pressure. Most importantly, perhaps, the DASH diet lowered the study participants' cholesterol levels. When blood cholesterol is high, cholesterol and other fatty substances collect on the walls of your blood vessels and in time restrict or block the flow of blood to your heart. High cholesterol, which is generally caused by a diet high in saturated fats, is a major risk factor for heart disease. The DASH diet is low in total fat and saturated fat, and in people who ate the diet during the study, cholesterol dropped 14 points. The "bad" cholesterol (LDL) fell 11 points. The level of good cholesterol (HDL) also fell by 3.7 points (this type of drop in HDL is seen when people reduce their overall fat intake). Combining all the effects (changes in blood pressure, LDL, and HDL), there was an important improvement in overall cardiac risk with DASH.
Another notable feature of the DASH diet was that it lacked harmful or unpleasant side–effects. There was no downside to the DASH diet, except for a very small number of the study participants who suffered from mild gastrointestinal upset (gas and loose stools) and that resolved itself as soon as they got used to the increased amounts of healthy fruits and vegetables in their diet.
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