One in four adult Americans has hypertension (high blood pressure), a major risk for heart disease and stroke. Dietary modifications, particularly sodium (salt) restriction, continue to play a key role in the management of hypertension. Over the past decade, however, a number of researchers have challenged the wisdom of advising the general public—or even those with hypertension—to limit dietary sodium. Their argument centers on two issues: 1) not everyone has an elevated blood pressure response to sodium, and 2) the results of human studies examining the relationship between sodium and hypertension are not consistent.
The Dietary Approaches to Stop Hypertension (DASH) Study is an ongoing effort to determine the effect of dietary factors, particularly calcium, potassium, and magnesium, on blood pressure. The first phase of the study showed that a diet low in total and saturated fat and rich in vegetables, fruits, and low-fat dairy products substantially decreased blood pressure compared with the typical U.S. diet. The second phase of the project, the DASH-sodium trial, added sodium to the mix. A recent study published in the
Annals of Internal Medicine
reports that the DASH diet and reduced dietary sodium lowered blood pressure among all population groups tested—whether or not they had high blood pressure.
The DASH-Sodium Trial was a multi-center, 14-week study during which participants received all their food from the study. It involved 412 people, aged 22 and older, with systolic (top number) blood pressures of 120-160 and diastolic (bottom number) blood pressures of 80-95. Fifty-two percent of the participants were women and 48 percent were men; 54 percent were African American, 42 percent were white, and 10 percent were of other races. Forty-one percent had hypertension and 59 percent did not. People were excluded from the study if they had a history of heart disease, poorly controlled hyperlipidemia, kidney insufficiency, insulin-dependent diabetes, special dietary requirements, or an alcohol intake of more than 14 drinks per week. People who took blood pressure medication or medications that might affect their blood pressure were also excluded.
For the first two weeks of the study, all participants ate a control diet that contained 3500 mg of sodium per day. For the remainder of the study (3 months), participants ate either the DASH diet—rich in fruits, vegetables, low-fat dairy foods, grains, poultry, fish and nuts—or a control diet, considered to be a typical American diet. In addition to the DASH diet or the control diet, participants were also randomly assigned to varied sodium intakes of approximately 3500 mg/day, 2300 mg/day, or 1500 mg/day for each month-long feeding period. Caloric intake was adjusted during the study period to keep participants’ body weights stable.
After detailed analysis, researchers found that both the DASH diet and salt restriction lowered blood pressure compared with the typical U.S. diet and normal salt intake. However, the DASH diet combined with the lowest sodium intake consistently produced the greatest reductions in blood pressure than either measure alone. All groups—men and women, African Americans and non-African Americans, people with normal and high blood pressure, and people older and younger than 45—experienced decreases in blood pressure on the lower-sodium diets. The effects of sodium restriction were the greatest in people with existing hypertension, those older than 45, and women (systolic pressure only).
Although these results sound clear-cut, there are several limitations to this study. First, the researchers did not look at whether the effect of the sodium restriction lasted more than the month-long study periods. Second, the researchers were able to control the diets of the participants more strictly than "free-living" people could on their own. The lowest level of salt provided in this diet—approximately 1500 mg—is probably not feasible for anyone who eats processed food such as frozen dinners, canned soups, cheeses, or cold cuts. Because evidence from other studies suggests that sodium restriction will continue to lower blood pressure only if patients adhere to the sodium restriction over time, there is some concern that diets low enough in sodium to make a difference may prove difficult for people to follow over the long term.
The authors of the study state that the DASH diet plus decreased sodium intake should be broadly recommended for prevention and treatment of high blood pressure and its complications. However, not everyone shares this belief. There is a small, but vocal, body of researchers who feel that low-sodium diets should not be routinely recommended for the prevention of hypertension. It is difficult to interpret the studies on which they base their objections, however, because the studies used very low sodium intakes and were not adjusted for other factors that might indicate higher cardiovascular risk.
It is important to remember that the maximum benefit came from the DASH diet combined with a salt-restricted diet. There is no controversy over the benefit of the DASH diet, which is rich in fruits, vegetables, low-fat dairy products, and whole grains. And there is little controversy over a recommendation that you avoid salting your food and avoid processed foods when you can.
But diet is just one component of blood pressure treatment. Some evidence shows that other therapies—including drug therapy, not smoking, and an active lifestyle—are equally or more important than low-salt and very low-salt diets in treating high blood pressure. If you have high blood pressure, or a family history of cardiovascular disease, speak with your health care provider about all the measures available to keep your blood pressure down and reduce your risk of cardiovascular disease.
Vollmer WM, et al. Effects of diet and sodium intake on blood pressure: subgroup analysis of the DASH-Sodium Trial.
Annals of Internal Medicine
. 2001. 135(12):1019-1028.
Mulrow, C. Sound clinical advice for hypertensive patients.
Annals of Internal Medicine
. 2001. 135(12):1084-1086.
Last reviewed Dec 18, 2001
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