High blood pressure affects about one in four in the United States and United Kingdom and is defined as blood pressure consistently above 140/90 mmHg. The top number, 140, is the systolic pressure exerted by the blood against the arteries while the heart is contracting. The bottom number, 90, is the diastolic pressure in the arteries while the heart is relaxing or between beats. The concern is the higher the blood pressure, the greater the risk for developing heart and kidney disease and stroke. High blood pressure is known as the silent killer as it has no symptoms or warning signs.
The DASH study by the National Lung, Blood and Heart Institute (NHLBI), published in the New England Journal of Medicine in 1977, was the first study to look at the effect a whole diet rich in potassium, magnesium and calcium foods, not supplements, had on blood pressure.
The study involved 459 adults with and without high blood pressure. Systolic blood pressures had to be less than 160 mm Hg and diastolic pressures 80 to 95 mm Hg. Approximately half the participants were women and 60% were African Americans. Three eating plans were compared. The first was similar to a typical American diet—high in fat (37% of calories) and low in fruit and vegetables. The second was the American Diet, but with more fruits and vegetables. The third was a plan rich in fruits, vegetables, and low fat dairy foods and low fat (less than 30% of calories). It also provided 4,700 mg potassium, 500 mg magnesium and 1,240 mg calcium per 2,000 calories. This has become known as the DASH diet. All three plans contained equal amounts of sodium, about 3,000 mg of sodium daily, equivalent to 7 g of salt. This was approximately 20% below the average intake for adults in the United States and close to the current salt recommendations of 5–6 g. Calorie intake was adjusted to maintain each person”s weight. These two factors were included to eliminate salt reduction and weight loss as potential reasons for any changes in blood pressure. All meals were prepared for the participants in a central kitchen to increase compliance on the diets.
Results showed that the increased fruit and vegetable and DASH plans lowered blood pressure, but the DASH plan was the most effective. It reduced blood pressure by 6 mmHg for systolic and 3 mmHg for diastolic, those without high blood pressure. The results were better for those with high blood pres-sure–the drop in systolic and diastolic was almost double at 11 mmHg and 6 mmHg respectively. These results showed that the DASH diet appeared to lower blood pressure as well as a 3 g salt restricted diet, but more importantly, had a similar reduction as seen with the use of a single blood pressure medication. The effect was seen within two weeks of starting the DASH plan, which is also comparable to treatment by medication, and continued throughout the trial. This trial provided the first experimental evidence that potassium, calcium, and magnesium are important dietary factors in determinants of blood pressure than sodium alone.
The original DASH plan did not restrict sodium. As a result, a second DASH-Sodium trial from 1997-1999 (published 2001) looked at the effect the DASH diet with different sodium levels (3,300, 2,300 or 1,500mg) had on blood pressure. This is known as the DASH-sodium diet. The highest amount recommended by the 2005 U.S. dietary guidelines is 2,300 mg. The amount recommended by the Institute of Medicine, as a minimum to replace the amount lost through urine and to achieve a diet that provides sufficient amounts of essential nutrients, is 1,500 mg. The results showed that the combined effect of a lower sodium intake with the DASH diet was greater than just the DASH diet or a low salt diet. Like earlier studies, the greatest effect was with the lower sodium intake of 1,500mg (4 g or 2–3 tsp of salt), particularly for those without hypertension. For this group, the systolic dropped about 7.1 mmHg and the diastolic about 3.7 mmHg. However, the reduction in blood pressure for hypertensives was 11.5 mmHg for systolic and 5.7 mmHg for diastolic, quite similar to the reductions seen with the DASH diet.
July 4th, 2008 at 3:08 am
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