MONDAY, July 20, 2009 (HealthDay News) -- Not only does a high-salt diet contribute to hypertension, but it can also reduce the effectiveness of blood pressure medications, a new study finds.
"What is striking about these results is the degree of the effect," said Dr. David A. Calhoun, a professor of medicine at the University of Alabama at Birmingham, and a member of the team reporting the finding in the July 20 online issue of Hypertension.
The study evaluated 12 people with resistant hypertension, high blood pressure that can't be controlled by a three-drug regimen. Because the study was so small, the results can't be easily applied to everyone with high blood pressure, but "anyone with high blood pressure certainly benefits from a low-salt diet," Calhoun said.
High blood pressure is a major risk factor for heart attack, stroke and other cardiovascular problems, and heavy salt intake has long been known to contribute to the condition, Calhoun said.
Half of the study participants were black, a group that is especially prone to high blood pressure. They were taking an average of three blood pressure medications. In the trial, half were assigned to a low-salt diet containing 50 millimoles of salt a day (about a half teaspoon), slightly below the 65 millimole intake recommended for salt-sensitive people. The other half had a high-salt diet, containing 250 millimoles (2.5 teaspoons) daily. They all spent time on each diet, and continued to take their prescribed medications.
People on the high-salt diet had an average 24-hour systolic blood pressure (the top reading, measured when the heart is contracting) of 150.3, compared to 130.0 for those on the low-salt diet, the researchers report. When it came to readings taken in a doctor's office, patients on the low-salt diet had an average drop of 22.7 points in systolic blood pressure and 9.1 points in diastolic blood pressure.
"The effect of the low-salt diet was greater than has previously been described," Calhoun said.
Another study reported in the same issue of the journal described a significant reduction in high blood pressure from a modest reduction in salt intake in a group that included whites, blacks and Asians.
That study, done at St. George's University of London in England, had 169 participants, all of whom had moderately high blood pressure. After reducing their salt intake from 9.7 grams a day to 6.5 grams a day, the average reduction in a six-week period was 4.8 points in systolic pressure and 2.2 points in diastolic pressure.
Both studies emphasize the importance of controlling salt intake to keep blood pressure at safe levels, said Dr. Martha Daviglus, a professor of preventive medicine and medicine at the Northwestern University Feinberg School of Medicine in Chicago and a spokeswoman for the American Heart Association.
Between 20 percent and 30 percent of Americans have resistant hypertension, and the emphasis for them has been on drug treatment, Daviglus said. "When a patient comes to a physician's office with hypertension, we start with one drug, then add another," she said. "We often forget about lifestyle interventions because they are so difficult."
The two studies show that attention must be paid to both drug treatment and diet, Daviglus said.
"They give us some hope that by doing a combination of both, we will be able to reach our goal," she said.
For Americans, most salt comes in processed foods, Daviglus and Calhoun said. "It is extremely difficult to avoid high salt intake when you eat these processed foods," Calhoun said.
People have to be aware of the salt content of all the food products they buy, Daviglus said. "I always say to them, 'you have to look at the labels,'" she said. "All these foods are loaded with salt, and we don't realize it -- even ice cream."
The emphasis should be on eating fresh foods, Daviglus said. The heart association provides a list of foods, "how to cook them, what spices to use instead of salt," she said.
Calhoun recommends adherence to the Dietary Approaches to Stop Hypertension (DASH) diet, which is low in salt and fat and high in fiber. Such a diet reduces cardiovascular risk in several ways, he said.
The DASH diet is described by the U.S. National Heart, Lung, and Blood Institute.