High-salt diets contribute to resistant hypertension, and modest reductions in salt can help reduce blood pressure in black, Asian, and white populations, according to two new studies published today in Hypertension.
In the first study, a randomized, cross-over study of a high-salt versus a low-salt diet in 12 patients with resistant hypertension, systolic blood pressure decreased 22.7 mm Hg and diastolic blood pressure decreased 9.1 mm Hg on the low-salt diet.
In the second study, a small reduction in salt intake from 9.7 to 6.5 grams per day resulted in a significant drop in blood pressure in 169 people with untreated mild hypertension, including blacks, Asians, and whites.
“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,” said Eduardo Pimenta, lead author of the first study, in an AHA press release. “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.”
[Update] Please note that Dick Hanneman, President of the Salt Institute, sent a comment to CardioBrief. Please scroll down to the comments section to read his response to these studies.
Salt Institute
Here are the AHA press releases:
High-salt diet contributes to resistance to blood pressure medications
DALLAS, July 20, 2009 — 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.
Resistant hypertension means that 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 millimeters of mercury (mm Hg) and 9.1 mm Hg 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 millimole (mmol) compared to 252 mmol per 24 hours during the high-salt diet.
- Plasma renin activity (PRA), a measure of the enzyme renin that plays a role in regulating blood pressure, 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 (measure of kidney function) 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 (about ½ teaspoon), 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 2 ½ teaspoons.
Patients were taking an average 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.
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Benefits of modest salt reduction in black, Asian and white populations
DALLAS, July 20, 2009 — A modest reduction in salt intake from an average of 9.7 to 6.5 grams per day, as measured by urinary sodium, decreases blood pressure significantly in blacks, Asians and whites, according to a new study reported in Hypertension: Journal of the American Heart Association.
Study co-authors, Graham A. MacGregor, M.D., Professor of Cardiovascular Medicine andFeng J. He, Ph.D., of St. George’s, University of London—tested the impact of salt reduction in 169 people, 113 men and 56 women aged 30 to 75 with untreated mild hypertension. The participants — 71 whites, 69 blacks and 29 Asians — had an average sitting systolic blood pressure of 147 millimeters of mercury (mm Hg) and an average diastolic reading of 91 mm Hg.
For six-week periods participants, while on reduced salt diet, were given either placebo or slow-release sodium tablets daily, then switched to the other type of tablet, preventing both researchers and subjects from knowing whether sodium intake was being reduced at a given time.
Among the findings:
- A reduction in daily salt intake of about one-third lowered blood pressure in all three ethnic groups. The average drop was 4.8 mm Hg systolic and 2.2 mm Hg diastolic.
- The salt reduction was accompanied by a drop in urinary excretion of albumin, a blood protein. The presence of albumin in urine can signal kidney damage and indicate cardiovascular risk.
- Urinary excretion of calcium, which may reflect osteoporosis risk, was also reduced.
- In blacks, the salt reduction was accompanied by a drop in carotid-femoral pulse wave velocity, a measure of large artery stiffness.
“A lower salt intake, in the long-term, could play an important role in the prevention of cardiovascular disease, renal disease and osteoporosis,” said Dr. He, a cardiovascular research fellow at St. George’s. “Our study provides further support for the current public health recommendations to reduce salt intake to less than 6 grams per day.”
Compared to whites and blacks there were a small number of Asians, yet the research still revealed a clear benefit of salt reduction in that group, said Dr. He. Most Asians studied were from the Indian subcontinent, while 59 percent of the blacks who participated were of African descent and 40 percent Caribbean descent.
“About 80 percent of salt intake in developed countries comes from sodium added by the food industry,” MacGregor said. “The best strategy to reduce salt intake in the population is to persuade the food industry to make a gradual and sustained reduction in the amount of salt added to food in a structured program across the whole of the food industry. This is now happening in the United Kingdom and the average salt intake has already fallen from 9.5 to 8.6 grams per day, and will continue to fall as more salt is taken out of all foods, with the saving of many lives. Even a small reduction in blood pressure in the whole population would have a large impact on reducing the number of people suffering and/or dying from strokes, heart attacks and heart failure.”
Other co-authors are: Maciej Marciniak, M.D.; Elisabeth Visagie, R.N.; Nirmala D. Markandu, R.N.; Vidya Anand, M.Sc.; and R. Neil Dalton, Ph.D. Individual author disclosures can be found on the manuscript. The Food Standards Agency funded the study.
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As president of the Salt Institute, I cannot respond for Dr. Oparil. I do not speak for her nor does she represent the Salt Institute. We obtain expert consultation to ensure our representations accurately reference the science. Our statements benefit from this process, and one of the benefits is to choose consultants with penetrating insights, not those who sacrifice their independent opinions. We do not advertise the names of our several consultants because we do not trade on their well-earned reputations. We value their expertise and diversity as we examine the evidence.
The evidence shows that the basic question asked in the Pimenta study is the wrong question. The right question is whether a recommendation to lower dietary sodium in the general population will improve public health, whatever the mechanisms involved. You might raise that question directly with Dr. Oparil before presuming the answer.
The evidence is that lowering dietary sodium triggers many changes in the body (and different people respond differently). Some will lower BP, especially if they are consuming only deficient intakes of K, Mg, and Ca. Most will also respond to lower Na intakes by increasing insulin resistance, plasma renin activity, aldosterone production and sympathetic nervous system activity. It is the NET EFFECT of all these changes that results when salt intake is reduced and it is the net effect that represents the health outcome.
Lower salt diets in the general population have been associated with increases in cardiovascular mortality, incidence of myocardial infarction and all-cause mortality. These, unfortunately, have all been observational studies; the government has refused to fund a trial of the question of whether reducing dietary salt achieves better outcomes.
An Italian team, however, HAS reported a clinical trial of the health outcomes of low-Na diets (in late 2008 and earlier this year). Their subjects were patients treated for congestive heart failure, a group that has routinely been treated with low-Na dietary therapy. These researchers found that, contrary to all expectations, those receiving low Na diets died more quickly and were re-hospitalized more frequently.
So as interesting as are studies that show that massive salt loadings raise BP and drastic Na reduction cuts BP, with respect, we already knew that fact. We need a trial in healthy subjects as to whether using salt reduction can reduce or prevent heart attacks and extend life.
That question is the key, but a second question would need to be answered even if the data were to return conclusions opposite those of the observational studies showing no health benefit. That second question is: can the general public sustain salt reduction by choosing low-Na foods (whether through education or “stealth” removal of NaCl from the food supply) or is there an inherent and neurally-modulated “central regulation of sodium appetite” as some researchers have proposed — where the brain unconsciously regulates desire to consume foods until the body’s need for this essential nutrient is satisfied.
So many questions, too few answers. But it will help if we answer the right questions.
Dick Hanneman
President
Salt Institute
[...] 2 studies show benefits of lowering salt (July 20, 2009) [...]