Harvard Diet Review: DASH
Finding yourself confused by the seemingly endless promotion of weight-loss strategies and diet plans? In this series, we take a look at some popular diets—and review the research behind them.
What Is It?
The DASH (Dietary Approaches to Stop Hypertension) diet is sometimes prescribed by doctors to help treat high blood pressure. Blood pressure is the amount of pressure that blood places against the walls of arteries. It will normally vary throughout the day but if it remains too high, this is called high blood pressure or hypertension. Untreated high blood pressure can lead to heart disease, stroke, congestive heart failure, kidney disease, and blindness.
DASH was first introduced at a meeting of the American Heart Association in 1996 and later published in the New England Journal of Medicine in 1997. The DASH trial randomly assigned 456 people to different diets to test the effects of dietary patterns on lowering blood pressure. The authors surmised that eating a diet with many different foods with blood pressure-lowering nutrients would show a greater effect on blood pressure than eating single nutrients, such as found in supplements or in a limited diet. Three diets were tested: 1) a control diet, or a standard American diet, 2) a fruits and vegetables diet, similar to the control diet but providing more fruits and vegetables and less snacks and sweets, and 3) a combination diet rich in fruits, vegetables, nuts, and low-fat dairy foods with reduced amounts of saturated fat, total fat, and cholesterol. The last two diets were richer in nutrients associated with lower blood pressure, such as potassium, magnesium, calcium, fiber, and protein. All three diets provided about 3000 mg sodium, which is more than the recommended amount from the Dietary Guidelines for Americans but less than the average sodium intake for Americans.
Despite no weight changes, the combination diet reduced blood pressure more than the other two diets. Those with hypertension showed greater decreases in blood pressure than those without hypertension. The reduction of blood pressure in the DASH combination diet was comparable to that of people on medication for stage 1 hypertension.
The results of this landmark study contributed much of the scientific basis for the Dietary Guidelines for Americans 2010 and later editions.
How It Works
DASH is based on the following foods: fruits, vegetables, low fat milk, whole grains, fish, poultry, beans, and nuts. It recommends reducing sodium, foods and beverages with added sugars, and red meat. The diet is heart-friendly as it limits saturated and trans fat, while increasing the intake of potassium, magnesium, calcium, protein, and fiber, nutrients believed to help control blood pressure.
The diet suggests a specific number of servings of the recommended foods listed above. The sample plans provided by the National Heart Lung and Blood Institute (NHLBI) are based on 1600, 2000, or 2600 calories daily. For 2000 calories a day, this translates to about 6-8 servings of grains or grain products (whole grains recommended), 4-5 servings vegetables, 4-5 fruits, 2-3 low fat dairy foods, 2 or fewer 3-ounce servings of meat, poultry, or fish, 2-3 servings of fats and oils, and 4-5 servings of nuts, seeds, or dry beans per week. It advises limiting sweets and added sugars to 5 servings or less per week. The plan defines the serving sizes of each these food groups.
To follow the plan, one must decide their calorie level and then divide the suggested servings of each food group throughout the day. This requires meal planning ahead of time. The NHLBI guide provides many tips on how to incorporate DASH foods and to lower sodium intake; a one-day sample menu following a 2300 mg sodium restriction and a 1500 mg sodium restriction; and one week’s worth of recipes. The NHLBI also publishes an online database of “heart healthy” recipes.
The Research So Far
Numerous studies show wide-ranging health benefits of the DASH diet. A consistent body of research has found that DASH lowers blood pressure in people with high blood pressure but also normal blood pressure even without lowering sodium intake. It can produce greater reductions in blood pressure if sodium is restricted to less than 2300 mg a day, and even more so with a 1500 mg sodium restriction. When compared with a standard American diet (e.g., high intake of red and processed meats, beverages sweetened with sugar, sweets, refined grains) DASH has also been found to lower serum uric acid levels in people with hyperuricemia, which places them at risk for a painful inflammatory condition called gout. Because people with gout often also have high blood pressure and other cardiovascular diseases, DASH is optimal in improving all of these conditions.
Adherence to the DASH-style pattern may also help prevent the development of diabetes, as analyzed in a recent meta-analysis, and kidney disease as found in the Atherosclerosis Risk in Communities (ARIC) cohort that followed more than 3700 people who developed kidney disease. Dietary components of DASH that were protective in the ARIC cohort included a high intake of nuts, legumes, and low-fat dairy products. A high intake of red meat and processed meats increased kidney disease risk.
- DASH requires each person to plan their own daily menus based on the allowed servings. People who are not used to meal planning or cooking may need more specific guidance.
- The types of foods listed are not comprehensive. For example, avocados are not included so it is not clear if they would be categorized as a fruit or a fat serving. Certain foods are placed into questionable categories: pretzels are placed in the grain group even though they have fairly low nutrient content and no fiber; frozen yogurt is placed in the dairy group even though most brands contain little calcium and vitamin D and are high in added sugar. The general term “cereals” are placed in the grain group but different types of cereals can be highly variable in nutrient and sugar content.
- Those with lactose intolerance or food allergies (e.g., nuts) may need to modify the diet to include lactose-free alternatives to dairy and seeds instead of nuts.
- Some people may experience gas and bloating when starting the diet due to the high fiber content of plant foods like fruits, vegetables, and whole grains. This can be minimized by adding one or two new high fiber foods a week instead of all at once.
Research supports the use of the DASH diet as a healthy eating pattern that may help to lower blood pressure, and prevent or reduce the risk of cardiovascular diseases, diabetes, kidney disease, and gout.