Previously, the Dietary Approaches to Stop Hypertension (DASH) diet has demonstrated substantial blood pressure-lowering effects, but the reason for the beneficial effects was unclear. Results from the new study indicate that the DASH diet promotes salt excretion, similar to diuretic drugs, which increase urine production. Diuretic medications are widely used to treat high blood pressure. The effects appear most pronounced in people who are sensitive to the blood pressure effects of salt.
“Among lifestyle interventions, the DASH diet is the first to have a known mechanism of action, to my knowledge,” says Genjiro Kimura, M.D., professor and chairman of internal medicine and pathophysiology at Nagoya City University Graduate School of Medical Sciences in Japan. “Therefore, the indication for this diet is now clear. If people followed the DASH diet from childhood, I think hypertension would not occur in adulthood, although we do not have data available to prove that.”
The blood pressure-lowering effects of the DASH diet were first reported six years ago. A subsequent study known as DASH-Sodium evaluated the blood pressure effects of the diet at different levels of salt intake and showed that the blood pressure benefits of the DASH diet were greatest at the highest level of salt intake. The current study used data from DASH-Sodium to clarify and confirm the relationship between blood pressure and salt excretion, or natriuresis.
The new analysis involved 375 adults whose blood pressure ranged between normal and mildly elevated without blood pressure medication. The patients were randomly assigned to consume a DASH diet or a control diet for three consecutive 30-day periods.
During each 30-day evaluation, patients in both dietary groups had a different level of salt intake. The patients’ blood pressure was measured on five of the last nine days of each 30-day feeding period. In addition, a 24-hour urine sample was obtained during the last week of each study period to determine salt excretion.
Blood pressure and salt excretion were plotted graphically for each patient, blood pressure along a horizontal line or axis and salt excretion along a vertical line. A curve that shifted to the left indicated that the relationship was affected mainly by widening of the vessel opening. However, if the curve did not shift but the slope of the curve became steeper, the relationship was influenced by enhanced salt excretion.
Data analysis showed patients assigned to the DASH diet had significantly steeper slopes on the plots without shifting along the horizontal axis. People on the DASH diet excreted salt easier and in greater amounts as well as reduced their blood pressure. Results also show that the impact of the DASH diet decreased as the level of salt intake decreased. These findings suggest that the DASH diet lowers blood pressure more effectively in people with high sodium sensitivity, through its diurection action.
Collectively, the findings led Kimura and his colleagues to conclude that the DASH diet lowers blood pressure by an action that involves salt excretion. A similar effect has been shown for diuretic drugs.
“It is well known that diuretics exert strong blood pressure-lowering effects in subjects who have sodium-sensitive hypertension and whose blood pressure is very sensitive to changes in the amount of sodium intake,” Kimura says. “African-Americans and elderly people are two groups that are sodium sensitive. Overall, about half of all hypertensive patients are sodium sensitive.”
No one knows whether the blood pressure effects of the diet derive from specific foods or the combination. The diet is rich in potassium and calcium, two nutrients that cause natriuresis, but the effects on salt excretion seem much greater than either of those, suggesting a combination of factors are involved in the blood pressure effects, Kimura says.
At least one study has shown that patients with mildly elevated hypertension might be able to reduce blood pressure with the diet alone. For patients who require blood pressure-lowering medication, the DASH diet would probably enhance the effects of the medication, much as diuretics do when added to other blood pressure medication, Kimura says.
Results of the recent Antihypertensive and Lipid Lowering to Prevent Heart Attack Trial (ALLHAT) indicated that diuretics should be considered the first choice of blood pressure-lowering medication for most patients who require medication. Noting that diuretics can cause side effects, Kimura suggested the DASH diet offers a safer alternative to diuretic drugs.
“I think the DASH diet acts as a natural diuretic without the adverse effects,” he says. “The DASH diet provides healthy food for people of all ages. I think the diet should be recommended for everyone, regardless of age, not only for reducing blood pressure but also to prevent heart disease and cancer.”
The first author of the report is Sachie Akita, M.D. and other co-authors are Frank M. Sacks, M.D.; Paul R. Conlin, M.D.; and Laura P. Svetkey, M.D.
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