Special Diets Decrease Blood Pressure

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The summary below is from the full report titled "Effects of Diet and Sodium Intake on Blood Pressure: Subgroup Analysis of the DASH-Sodium Trial." It is in the 18 December 2001 issue of Annals of Internal Medicine (volume 135; pages 1019-1028). The authors are WM Vollmer, FM Sacks, J Ard, LJ Appel, GA Bray, DG Simons-Morton, PR Conlin, LP Svetkey, TP Erlinger, TJ Moore, and N Karanja, for the DASH- Sodium Trial Collaborative Research Group.

What is the problem and what is known about it so far?

The "DASH" diet, which was studied in the Dietary Approaches to Stop Hypertension (DASH) Trial, decreases blood pressure compared with a typical U.S. diet. The DASH diet emphasizes fruits, vegetables, low-fat dairy foods, whole grains, poultry, and fish. It allows smaller amounts of red meat and sweets than does the typical U.S. diet. The DASH-Sodium trial found that another way to decrease blood pressure is to decrease salt intake below the average daily amount consumed by Americans (8 to 10 grams, or about 2 to 3 teaspoons of salt, which is about 4 to 6 grams of sodium). Experts currently recommend consuming no more than 6 grams of salt per day (1 teaspoon or less).

Why did the researchers do this particular study?

To compare the ability of salt reduction and the DASH diet to decrease blood pressure in certain groups of people—men and women, older (over 45 years of age) and younger (45 years or age or younger) adults, African Americans and non–African Americans, and people with and without high blood pressure.

Who was studied?

Of the 412 U.S. adults included in the study, 57% were women, 57% were African American, and 41% had high blood pressure.

How was the study done?

Participants were randomly assigned to eat a typical U.S. diet or the DASH diet. The researchers provided all of the participants' food, including snacks and cooked meals, for 90 days (three consecutive 30-day periods). During the 90 days, both diet groups were randomly assigned to sodium intake of 3.5 grams, 2.3 grams, or 1.2 grams for each 30-day feeding period.

What did the researchers find?

People who ate the DASH diet had lower systolic blood pressure (by about 5 to 6 mm Hg) and diastolic blood pressure (by about 3 mm Hg) than did those who ate the typical U.S. diet. Blood pressure was lower by 5 to 8 mm Hg during periods of lower sodium intake than during periods of higher sodium intake in people eating either the DASH diet or the typical diet. People who followed the DASH diet and had the lowest salt intake experienced the largest decrease in blood pressure. All groups (men and women, African Americans and non–African Americans, older and younger persons, and those with and without high blood pressure) experienced a decrease the blood pressure while eating diets that were low in salt. Salt restriction had the largest effect on blood pressure in African Americans, persons older than 45 years of age, and those with high blood pressure.

What were the limitations of the study?

The researchers did not look at whether the effect of lower-salt diets lasted longer than the 30-day test periods. The researchers controlled the diets more strictly than most people could on their own. The lowest-salt diet (1.2 grams sodium) is probably not feasible for many Americans because they eat a lot of processed foods, such as frozen dinners, restaurant meals, and canned food, which are the main sources of salt in the diet.

What are the implications of the study?

The DASH diet combined with salt restriction decreases blood pressure more than either measure alone. Salt restriction, with or without the DASH diet, decreases blood pressure in many different groups of people. People with high blood pressure, African Americans, and people older than 45 years of age have the greatest reductions in blood pressure with salt restriction.