Healthnotes Newswire (January 4, 2001)—Both a low-salt diet and a diet emphasizing fruits, vegetables, grains, and low-fat dairy products have been shown to lower blood pressure in people with hypertension (elevated blood pressure) and in those with normal blood pressure, according to a study published in today’s New England Journal of Medicine.¹ The two diets appear to work best when used together.

A total of 412 participants were randomly assigned to eat either a standard American diet (SAD) or a more healthful diet (called the DASH diet, described below). Within the assigned diet, participants ate foods with high, intermediate, or low levels of salt for 30 days each in random order. Reducing salt intake significantly lowered blood pressure in both the SAD group and the DASH group. Similarly, the DASH diet alone, as compared with the SAD, significantly lowered blood pressure, regardless of salt intake. Both diets were more successful in reducing blood pressure in people with hypertension than in people with normal blood pressure. And most important, the combined effects on blood pressure of low-salt intake and the DASH diet were substantial, and greater than the effects of either approach alone.

As noted in an accompanying editorial,² the current study found that the combined effects of the DASH diet and salt restriction on blood pressure worked as well as standard prescription blood pressure-lowering drugs.^{3 4}

A DASH of Common Sense

In 1997, the Dietary Approaches to Stop Hypertension (DASH) trial⁵ achieved an important advance in the dietary treatment of hypertension. The diet used in the DASH trial emphasized fruits, vegetables, and low-fat dairy products. It also included whole grains, poultry, fish, and nuts. The DASH diet contained only small amounts of lean red meat and few sweets or sugar-containing beverages. It also contained lower amounts of total fat, saturated fat (e.g., fat from dairy and animal products), and cholesterol than were found in the SAD.

The original DASH trial lowered blood pressure both in people with hypertension and in those without the disease. The results were so impressive that they were incorporated into the report of the Joint Committee on Prevention, Detection, Evaluation and Treatment of High Blood Pressure⁶ and were adopted by the American Heart Association.⁷ The new study confirms that the benefits of the DASH diet apply throughout the range of salt intakes, with best results at the lowest salt intake.

Salt in Your Food: How Low Can You Go?

How low-in-salt is a low-salt diet? The average intake of dietary salt among U.S. adults is 8.75 grams per day. The recommended upper limit of salt intake, based on the recommendations of the Joint Committee on Prevention, Detection, Evaluation and Treatment of High Blood Pressure, is 5.75 grams per day. The low-salt diet used in the new trial provided only 3 grams of salt per day. Achieving a low-salt intake by these standards is not easy. Simply avoiding the saltshaker is not enough. Much of the salt in the U.S. diet comes from prepared foods, rather than from salt added in cooking or at the table.⁸ Hidden sources of dietary salt are widespread and include many processed foods and foods served in restaurants and fast-food outlets.

So just how much salt is in some of the foods we eat? A McDonald’s hamburger contains 1.3 g of salt; a chicken pie frozen dinner contains 3.8 g of salt; 1 cup of commercial vegetable beef soup contains 5.3 g of salt; 2 pieces of skinless chicken contain .06 g of salt; 1 cup dried, unsalted beans contains .03 g of salt; and ½ cup of fresh, shredded cabbage contains .02 g of salt.

A person with hypertension may find sufficient motivation to make the somewhat difficult choices that can effectively reduce dietary salt intake (for example, avoiding most pre-packaged and fast foods, and much restaurant food). But the person with normal blood pressure might very well balk at such challenges. This is even more likely since, until the publication of this new study, it had not been definitively established that salt restriction prevents hypertension by lowering blood pressure in people with normal blood pressure. This question has been the source of controversy for more than 50 years. Previous salt-restriction research has produced somewhat conflicting results.^{9 10 11} The inconsistent outcomes of salt-restriction trials could be due to many factors, such as non-compliance with prescribed dietary recommendations, poor study design, small study populations, or other limitations in research design or analysis.

The new research appears to put this issue to rest. The authors clearly demonstrated that reducing salt intake lowered blood pressure, not only in people with hypertension, but also in people without hypertension who were eating the SAD. Because the new research is based on a single, large, well-controlled trial with a diverse population, it provides a more reliable estimate of the effects of salt restriction than do data taken from smaller, incompletely controlled clinical trials.

The new research confirms that blood pressure can be lowered, in consumers of either a SAD or a DASH diet, by reducing salt intake. The lower the salt level, the greater was the reduction in blood pressure—even at levels below those currently recommended. These findings suggest that current recommendations for salt intake (5.75 grams per day) should be more stringent.

The Pressure is Killing Us

Half of the adult U.S. population, and 80% of those over 50 years old, have a blood pressure at the upper limit of normal (120/80 millimeters of mercury on a blood pressure gauge).¹² Over 50 million people in the United States have hypertension, which puts them at increased risk of other diseases.⁶ Even small reductions in blood pressure can have sweeping effects in large populations, significantly reducing the incidence of stroke, heart attacks, and atherosclerosis (hardening of the arteries).¹³

The new study suggests that optimal blood pressure lowering is comparable to modern drug therapy and can be obtained by combining DASH dietary recommendations with a low-salt diet. The new data provide no justification for backing away from a firm recommendation to reduce salt to less than 6 grams per day. In fact, they suggest that such a recommendation may not go far enough. To achieve greater salt restriction in the general population, however, will require cooperation from food manufacturers and fast-food chains. In the meantime, an overall dietary pattern characterized by higher intake of vegetables, fruit, legumes, whole grains, fish, and poultry (instead of beef and pork), and a low intake of pre-packaged, salt-rich foods may be the best protection against hypertension and other diseases of the heart and blood vessels.