A high-salt diet may decrease the effectiveness of medications used to treat high blood pressure in patients with resistant hypertension, researchers report in a small study published in Hypertension: Journal of the American Heart Association.

In resistant hypertension, a patient's blood pressure remains above his/her treatment goals, despite using three different types of antihypertensive drugs at the same time. Twenty percent to 30 percent of high blood pressure patients may be resistant to multi-drug therapies.

"Our study, for the first time, demonstrates that a high-salt diet is an important contributor to resistant hypertension," said Eduardo Pimenta, M.D., lead author of the study and currently a clinical research fellow in the Endocrine Hypertension Research Centre in the University of Queensland School of Medicine in Australia. "Patients with resistant hypertension benefit substantially from intensive dietary salt restriction."

Researchers found:

• Patients resistant to high blood pressure medication had an average drop of 22.7 mmHg and 9.1 mmHg in office systolic and diastolic blood pressure respectively after eating a low-salt diet for seven days compared to a high-salt diet for seven days.
• A low-salt diet decreased office, daytime, nighttime and 24-hour systolic and diastolic blood pressure compared to a high-salt diet. The decrease in ambulatory blood pressure was persistent throughout the 24-hour period.
• Average urinary sodium excretion, a measure of salt intake, during the low-salt diet was 46 mmol compared to 252 mmol per 24 hours during the high-salt diet.
• Plasma renin activity (PRA) increased significantly after low-salt ingestion, while brain natriuretic peptide decreased significantly - indicating that plasma volume decreased when ingesting a low-salt diet.
• Body weight and creatinine clearance decreased significantly with a low-salt diet compared to a high-salt diet.

The study included 12 patients (eight women, six African Americans, average age 55) with resistant hypertension in a randomized, cross-over evaluation of a low-salt diet versus a high-salt diet. The sodium content of the low-salt diet was 50 mmol sodium per day, slightly below the 65 mmol a day recommended for people considered salt-sensitive, such as Africa Americans, middle aged and older individuals and people with hypertension, diabetes or chronic kidney disease. Calories were based on an amount to maintain weight. The sodium content of the high-salt diet was 250 mmol or about 2½ teaspoons.

Patients were taking an average of three or more high blood pressure medications, including a diuretic, and had an average office blood pressure of 145/83.9 mm Hg at the start of the study. They were randomized to a low- or high-salt diet for a week, then resumed their regular diet for two weeks, then crossed over to the opposite diet the final week. Patients with a history of heart attack or stroke in the previous six months, congestive heart failure or diabetes on insulin treatment were excluded.

All of the patients had been advised to reduce dietary salt intake and thought they had done so, but none received expert dietary consultation, said Pimenta, formerly a postdoctoral research fellow of the Vascular Biology and Hypertension Program at the University of Alabama at Birmingham (where this work was conducted).

Lifestyle modifications can reduce blood pressure and patients should discuss their options with their physicians. Those with resistant hypertension should be referred to a hypertension specialist for extensive and complex evaluation.

"Seventy-five percent of the daily intake of sodium in Westernized countries is from salt added during the commercial processing of foods and/or during food preparation by restaurants," Pimenta said. "Our findings lend additional support to efforts to decrease the salt content of prepared foods and support dietary guideline revisions that include salt restriction in the treatment of resistant hypertension. Further study is needed to determine the benefit and best approaches of long-term salt restriction."

Co-authors are: Krishna K. Gaddam, M.D.; Suzanne Oparil, M.D.; Inmaculada Aban, Ph.D.; Saima Husain, M.D.; Louis J. Dell'Italia, M.D.; and David A. Calhoun, M.D. Individual author disclosures can be found on the manuscript.

Suzanne Oparil is a consultant to the Salt Institute.

The National Heart, Lung, and Blood Institutes funded the study.