A DASH OF SALT AND TONE: RECENT TRIALS PROVIDING NEW INSIGHTS INTO THE DIET-BLOOD PRESSURE-HEALTH LINK

Myron H. Weinberger, M.D.
Hypertension Research Center
Indiana University School of Medicine

Several recent trials have provided new information regarding the relationship between dietary constituents and the sensitivity of blood pressure to salt to blood pressure levels over a short term of intervention, and to mortality over the long-term. Since lifestyle modifications and dietary changes are currently recommended as the first step in the prevention or treatment of high blood pressure and since such approaches are efficacious in lowering blood pressure, relatively economical and risk-free for most individuals, they have great appeal. This article will address the substance of some of these recent findings.

Salt sensitivity of blood pressure can be demonstrated in the majority of hypertensive patients and in as many as 25% of the normotensive population (1). Moreover, in normotensive subjects who are salt-sensitive there is a greater blood pressure increase with time (age) than among those who are not salt-sensitive (2). Thus the age-related increase in blood pressure, even among normotensives, is related to salt intake. Salt-sensitivity of blood pressure is more frequently observed in older subjects (2) and among those of African-American descent (1). A recent follow-up study of individuals who were initially classified with respect to salt responsiveness of blood pressure as long as 25 years ago or more, has revealed that normotensive salt-sensitive individuals have the same decreased survival as those who are hypertensive, whether salt-sensitive or salt-resistant and all 3 groups have reduced survival compared to those who are salt-resistant and normotensive (3). It is speculated that the reduced survival of these salt-sensitive normotensive subjects may be related to an increased risk for the subsequent development of hypertension. It should be emphasized that salt sensitivity is a continuous variable and thus the separation of individuals into salt sensitive and salt resistant groups is based on arbitrary criteria. The differences between the groups, therefore, are of degree rather than being qualitative ones. But what of those who are already hypertensive or who have blood pressure in the "high-normal" (130-139/85-89) range?

The TONE (Trial of Nonpharmacologic Interventions in the Elderly) Trial included 681 hypertensive subjects aged 60-80 who participated in a 2 x 2 factorial trial of weight loss + reduced salt intake, reduced salt intake alone, weight loss alone, or neither (4). When those assigned to reduced salt intake alone were compared to those maintaining their usual lifestyle (neither) an average reduction in dietary sodium intake of 40 mmol/d was observed based on 24-hour urine collections. This modest change in salt intake was associated with a significant (p<0.001) decrease in both systolic and diastolic blood pressure for the 30-month duration of the study. The lowering of blood pressure permitted reduction or elimination of antihypertensive medications (p<0.001) as well as demonstrating a trend to reduced cardiovascular events over the short study period in the group assigned to dietary salt reduction compared to the Usual Lifestyle group. More importantly, no adverse effects were observed related to the modest degree of dietary salt reduction in these older subjects. These observations speak directly to the benefit and safety of reducing dietary salt intake in elderly hypertensives and the possibility that this approach may be useful or preferable among those in whom medical therapy is not feasible or desirable.

The DASH (Dietary Approaches to Stop Hypertension) trial (5) was conducted in individuals with "high-normal" and Stage 1 hypertension randomly assigned to receive a "normal" diet, a diet enriched in fresh fruits and vegetables and a diet enriched in fresh fruits and vegetables and low-fat dietary products. Estimates of sodium intake indicated a modest reduction to an average of about 130 mmol/d. The group receiving the diet enriched with fresh fruits and vegetables and low-fat dairy products had the lowest blood pressure levels during the study followed by the group assigned to fresh fruits and vegetables which was lower than the blood pressure observed with the control diet. These findings supported the concept that dietary enhancement of potassium, magnesium and calcium intake contributed to blood pressure reduction. However, because of the modest reduction of dietary salt intake among all three groups, the possibility of an interaction of the study diets with a reduced salt intake could not be excluded. This set the stage for yet another study to examine this issue.

The DASH-II Trial assigned individuals with blood pressure ranging from 120/80 to159/95 to a control diet or the DASH combination diet randomized to one of three dietary sodium levels, high (150 mmol/d), intermediate (100 mmol/d) or low (50 mmol/d) in a randomized, crossover design (6). Adherence to the goal sodium intake was quite good as evidenced by urinary sodium excretion of 143 mmol/d on the high, 107 mmol/d on the intermediate and 66 mmol/d on the low sodium diets, respectively. There was a stepwise reduction in blood pressure with each level of reduced salt intake on both the control and DASH combination diets. The difference in blood pressure between those on the highest and lowest levels of sodium intake receiving the DASH diet averaged 12 mm Hg, a response equal to that of potent antihypertensive drugs. The DASH diet had a greater effect to reduce blood pressure on the two higher levels of salt intake in comparison to the control diet. Individuals of African-American background had the greatest decrease in blood pressure with dietary salt reduction when compared to the other subgroups examined. As in the earlier DASH trial, no adverse effects of dietary salt reduction were observed.

What then are the implications of these studies? They demonstrate that dietary interventions consisting of reduced salt intake and increased use of fresh fruits, vegetables and low-fat dairy products can effectively reduce blood pressure in substantial proportions of the population with no adverse effects noted at the levels evaluated. Further, individuals typically considered to be at the highest risk for hypertension and cardiovascular events, the elderly and African-Americans, had the greatest reduction in blood pressure with these dietary alterations. These findings have great implications for the prevention of hypertension as well as for non-pharmacologic treatment for those with established hypertension and the possibility of reducing the need for expensive and problematic antihypertensive drug therapy. Moreover, the adoption of diets modestly reduced in salt content holds out the potential of preventing the future development of hypertension in susceptible individuals and populations with virtually no risk.

BIBLIOGRAPHY
1) Weinberger MH, Miller JZ, Luft FC et al. Hypertension 1986:8(Pt 2):127-134.
2) Weinberger MH, Fineberg NS.Hypertension 1991;18:67-71.
3) Weinberger, MH, Fineberg NS, Fineberg SE, Weinberger M. Hypertension 2001;37(Pt2):429-432.
4) Appel LJ, Espeland MA, Easter L et al. Arch Intern Med 2001;161:685-693.
5) Appel LJ, Moore TJ, Obarzanek E, et al. New Engl J Med 1997;336:1117-1124.
6) Sacks FM, Svetkey LP, Vollmer WM, et al. New Engl J Med 2001;344:3-

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