July 20, 2009 -- Lowering daily salt intake may reduce the need to prescribe additional medications to control high blood pressure, according to a new study.
Patients with resistant hypertension are those who take three or more medicines to try and control their blood pressure, but their readings are still high. "These patients especially benefit from a low-salt diet," says study lead author Eduardo Pimenta, MD, a clinical research fellow in the hypertension department of the University of Queensland in Brisbane, Australia.
"Doctors tend to add more and more antihypertensive medications," he says, but ''these patients could have their blood pressure controlled with a low-salt diet and fewer medications." Based on his study, he says, doctors should consider additional lifestyle intervention, reinforcing to patients the importance of a low-salt diet before adding more drugs.
The study is published in Hypertension: Journal of the American Heart Association. In the same issue, another study found that modest salt reduction reduced blood pressure in blacks, whites, and Asians who had mildly elevated pressures, and that the low-salt diet also produced other health benefits.
Salt and Resistant Blood Pressure Study: Details
While many studies have found a link between dietary sodium and blood pressure, exactly how dietary sodium affects the resistant form of high blood pressure isn't well-known, according to Pimenta.
In his study, he assigned 12 men and women, average age 55, all with high blood pressure even while taking an average of 3.4 medicines, to eat a high-salt diet for one week and a low-salt diet for one week, separating the two diet experiments by a two-week "washout" period.
The average body mass index (BMI) was nearly 33, considered obese. At the study start, the average blood pressure while taking the medications was about 146/84. (Ideal blood pressures are below 120/80. If pressures are repeatedly over 140/90, it is considered hypertension.)
When the participants were on the high-salt diet, they took in about 7,000 milligrams of sodium per day, according to Pimenta; while on the low-salt diet they took in about 2,000 to 3,000 milligrams of sodium. Under U.S. dietary guidelines, less than 2,300 milligrams of sodium a day, or about one teaspoon of salt, is recommended for the general population; 1,500 milligrams is recommended for those with high blood pressure. The average American gets 3,436 milligrams of sodium a day, according to the American Heart Association.
Salt and Resistant Blood Pressure Study: Results
Compared to the high-salt diet, after being on the low-salt diet for a week, the participants had an average drop of 22.7 points for systolic blood pressure (the top number) and 9.1 for diastolic blood pressure (the bottom number).
The drop, Pimenta writes, is larger than what has been found in other blood pressure studies, suggesting that those with resistant hypertension may be especially sensitive to high salt intake.
"Doctors should reinforce the importance of a low-salt diet," Pimenta tells WebMD. "I think they should refer these patients to a nutritionist."
Salt and Blood Pressure: Across Populations
In another study in the same issue, U.K. researchers found that a modest reduction in salt intake reduces blood pressures in Asians, blacks, and whites."The vast majority of previous studies have only been in white subjects," study co-author Graham A. MacGregor, MD, professor of cardiovascular medicine at St. George's, University of London, tells WebMD.
This study tested the impact of salt reduction in 169 men and women, ages 30 to 75, who had mild high blood pressure but weren't on blood pressure medications. They reduced salt from an average of 9.7 grams a day to 6.5. That translates to dropping sodium intake from about 3,800 milligrams a day to about 2,400 milligrams, according to MacGregor. (Salt is different than sodium. Salt is about 40% sodium; the rest is chloride.)
At the study start, participants had an average blood pressure of 147/91. After being on the low-salt diet, their blood pressure dropped to an average of about 141/88.
''There were other benefits of salt reduction other than blood pressure," MacGregor tells WebMD. They found less calcium in the urine when the low-salt diet was followed. Over the long haul, reducing calcium loss through the urine would be expected to reduce osteoporosis risk. They also found less albumin in the urine. High levels of albumin in the urine can signal kidney damage and indicate a higher risk of cardiovascular disease.
"Some people have a bigger fall [in blood pressure] than others," MacGregor says. But salt reduction, he adds, will benefit everyone. "Even if you have very low blood pressure, you are less likely to get osteoporosis."
High blood pressure affects more than 1 billion people worldwide. Even modest reductions in blood pressure readings would be expected to have a large impact on rates of blood-pressure-related diseases such as heart attack and stroke when spread over such a large population.
While the study of those with resistant hypertension included only a dozen patients, the reduction in blood pressure was "striking," says Lawrence J. Appel, MD, MPH, a professor of medicine and epidemiology at the Johns Hopkins School of Medicine and School of Public Health in Baltimore. He wrote an editorial for the journal.
The blood pressure drop seen in the Pimenta study, according to Appel, is equivalent to what would be expected if two more blood-pressure-lowering medications were added.
The study of those with mild high blood pressure, Appel tells WebMD, points out not only that different ethnic groups can benefit from lowering salt, but that salt reduction has effects beyond blood pressure, such as potential protection from kidney and heart disease.
Reducing salt, he says, will not be easy for many Americans. He suggests first buying lower-salt breads and cereals and limiting consumption of processed foods such as luncheon meats, which have high amounts of salt.
"However, if we are to succeed at lowering sodium consumption as a society, ultimately significant changes will need to be made in our food supply," he writes.
A co-author on the Pimenta study has served as a consultant for the Salt Institute; Appel has received research grants from King-Monarch Pharmaceuticals, which makes a blood pressure-lowering medicine.