Study: Reducing salt really does lower blood pressure

By Elizabeth Cohen
CNN Medical Correspondent

(CNN) -- Cutting back on salt not only lowers your blood pressure, but it lowers it by much more than previously thought, according to a new study in the New England Journal of Medicine.

The study "lays to rest the long-standing controversy over whether sodium reduction lowers blood pressure," said Dr. Claude Lenfant, director of the National Heart, Lung, and Blood Institute, which funded the study in part.

AUDIO Elizabeth Cohen discusses the correlation between salt and blood pressure on CNN How much salt is too much? WAV sound Even people with normal blood pressure should watch their salt intake WAV sound     PORTION GUIDE The American Heart Association recommends adults get no more than 2,400 milligrams of sodium a day and offers these guidelines: 1/4 teaspoon salt = 500 mg sodium 1/2 teaspoon salt = 1000 mg sodium 3/4 teaspoon salt = 1500 mg sodium 1 teaspoon salt = 2000 mg sodium 1 teaspoon baking soda = 1000 mg sodium Source: American Heart Association

The reductions seen in the study were so great, researchers said, that some people with hypertension could avoid taking drugs if they just lowered the amount of salt in their diet -- and they'd get an even bigger reduction in blood pressure if they followed a low-fat diet, too.

"This is really groundbreaking. We were very surprised by this data," said Dr. Frank Sacks, lead author of the study and associate professor of medicine at Harvard Medical School.

The researchers at Harvard and the NHLBI, which is part of the U.S. National Institutes of Health, wanted to see what would happen to blood pressure levels when people followed different diets with different levels of salt.

The 412 study participants were randomly assigned to follow either a typical American diet or the so-called DASH (Dietary Approaches to Stop Hypertension) diet, which is rich in fruits, vegetables and low-fat dairy products and low in red meat and sweets.

The study subjects followed their assigned diet for 30 days at each of three sodium levels: at 3,300 milligrams per day, the U.S. average; at 2,400 milligrams per day, the maximum recommended by the American Heart Association and the National High Blood Pressure Education Program; and at 1,500 milligrams per day.

About 41 percent of the participants had high blood pressure (defined as blood pressure over 140/90 mm Hg). The others had blood pressure on the high end of normal.

When the study subjects with normal blood pressure reduced their sodium intake but still ate a typical high-fat U.S. diet, systolic blood pressure (the top number) went down 6.7 mm. When they ate the low-fat diet but didn't change their sodium intake, systolic blood pressure went down 3 mm. When they did both, blood pressure went down 7.1 mm.

When people with high blood pressure tried the dietary approaches, the results were even more dramatic. Sodium reduction alone lowered blood pressure by 8.3 mm. Going on the low-fat diet alone lowered blood pressure by 5 mm, and together they lowered blood pressure by 11.5 mm.

In comparison, high blood pressure drugs lower mildly elevated blood pressure by about 9 mm, according to the National Heart, Lung, and Blood Institute.

Making these changes "is going to prevent people from getting into trouble later in life, having heart attacks and strokes," said Sacks. "It's mind-boggling the public health benefit this could have if people can motivate themselves to make these changes."

But that's a big "if." Participants in the study were provided with their food. Other people would have to make permanent changes on their own -- and many studies have shown that Americans are not adept at making those kinds of changes on a permanent basis.

Moreover, the people in the study got the greatest benefits when they reduced their salt intake dramatically -- to 1,500 milligrams per day. Some foods already have almost 1,500 milligrams of sodium in just one serving -- for example, one McDonald's Crispy Chicken sandwich alone has 1,180 milligrams of sodium and a serving of Campbell's Chicken Noodle Soup has 890 milligrams. The American Heart Association recommends adults consume no more than 2,400 milligrams of sodium each day.

ALSO How much sodium is in your food?  • The National Institutes of Health has information on heart-healthy foods  • Cyberdiet.com has nutrient information for hundreds of foods     ALSO Learn more about sodium and other dietary minerals with our Mineral Guide     ALSO Find out more about hypertension

Making such a reduction in sodium "would be very difficult to achieve without cooperation from the food industry, since much of the salt in the U.S. diet comes from prepared foods, rather than from salt added in cooking or at the table," wrote Northwestern University Medical School's Dr. Philip Greenland in an editorial in the journal.

The Salt Institute -- and researchers who are paid consultants to the group -- didn't argue with the study results, but disagreed with the authors' interpretation of the data.

Dr. David McCarron, professor of medicine at the Oregon Health Sciences University, said the study results show that going on the low-fat diet does a much better job of lowering blood pressure than cutting back on salt.

According to the study, cutting back on salt lowered blood pressure four points when people with high blood pressure were already on the low-fat diet, and by two points for people with normal blood pressure.

Since a low-fat diet has been shown to help prevent many diseases -- from high cholesterol to cancer to heart disease -- McCarron said it makes more sense to just encourage people to change their overall diet.

"When you tell people to cut something out, they say 'I don't want to hear that,'" McCarron said. "People are more willing to listen to positive messages."

McCarron is on the Salt Industry's scientific advisory board and has been a paid consultant to the group.

Study author Sacks said both approaches -- the low-fat diet and salt reduction -- are important, and even just a four-point reduction in blood pressure is significant.

"It's shameful that anyone in the field would say [a four point decrease] is unimportant. That's more than half the effect of a drug," he said.

The president of the Salt Institute, Richard Hanneman, said he thinks it's useless telling Americans to cut back on salt. "We don't think it's possible for the government to legislate or regulate a reduction in sodium content. It would be like legislating morality," he said.

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Dietary Approaches to Stop Hypertension
Cardiovascular Information from the National Heart, Lung and Blood Institute
USDA Center for Nutrition Policy and Promotion
USDA Nutrient Data Laboratory
The Salt Institute
International Food Information Council

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