By AMERICAN HEART ASSOCIATION NEWS
An exaggerated response to too much sodium in the diet may be as dangerous as high blood pressure when it comes to cardiovascular diseases. But researchers are still years away from being able to detect and treat so-called salt sensitivity in everyday practice, according to a statement released Thursday by the American Heart Association.
An estimated one in four Americans are salt-sensitive despite having normal blood pressure, according to a 1996 study. About half of people with high blood pressure are sensitive to salt. That jumps to nearly three out of four for African-Americans with high blood pressure, another 1996 study found.
Salt sensitivity is determined when a person’s blood pressure goes higher when consuming the same amount of salt as a salt resistant person.
Cheryl Laffer, M.D., Ph.D., who led the committee that wrote the new statement, advises her patients with high blood pressure to avoid eating too much salt because of their increased risk for salt sensitivity.
But the condition has broader health implications, she said.
“The important thing is not just that [salt sensitivity] affects blood pressure, but it’s a marker of cardiovascular risk,” said Laffer, director of hypertension service at Vanderbilt University School of Medicine in Nashville. “People who are salt-sensitive, even if they don’t have high blood pressure, are at increased cardiovascular risk.”
The condition is considered as strong a risk factor as high blood pressure, yet there are many unknowns, said Fernando Elijovich, M.D., coauthor of the statement and a professor of medicine at Vanderbilt.
The lack of standardized cutoffs for salt sensitivity and methods for testing salt sensitivity prompted the new statement, Elijovich said. “It’s a mess the way it’s being investigated in humans.”
For example, researchers have been using different thresholds to define salt sensitivity. In the statement, it is defined as a sustained drop in systolic blood pressure — the top number of a reading — of at least 10 mmHg, following testing that involves raising sodium, severely restricting sodium, then measuring the resulting blood pressure changes, and continuous measurements of sodium in the urine.
Changes in blood pressure are measured for up to 24 hours after lowering sodium.
The process is “incredibly laborious” and confined to research settings, Elijovich said. It requires three days of hospitalization. Some researchers used outpatient tests requiring several weeks of careful dietary monitoring.
Finding a simple lab test to detect salt sensitivity is “our dream,” Laffer said.
“Perhaps in less than a decade we’ll have a reasonable test that could be used in a clinic,” Elijovich said. But for now, he said, “it has no diagnosis of certainty with a test, and there is no specific treatment.”
The statement details how the body handles excess sodium, with the kidneys, heart and nervous system all potentially impacted by salt sensitivity’s influence on blood pressure.
“The prevailing theory until very recently was that [salt sensitivity] was all the kidneys’ fault,” Laffer said.
Researchers thought blood pressure rose to force the kidneys to excrete excess sodium, but “it’s not that simple,” Laffer said. For example, the body might store the extra sodium in muscle and skin cells.
Although studies have uncovered numerous genetic links in rats pointing to salt sensitivity, researchers have yet to discover a genetic component in humans.
“We have hints, but it hasn’t been studied because it’s hard to do,” Laffer said.
Unlike with rats, “you can’t inbreed humans” to produce someone who is 100 percent salt-sensitive, Elijovich said. He suspects several genes interact to influence salt sensitivity in people.
Further complicating research are the many factors that can alter the effects of salt on a person’s blood pressure: age, sex, race and ethnicity, and existing conditions such as high blood pressure, diabetes and chronic kidney disease.
Due to the absence of screening tests or specific medications, knowing a patient with high blood pressure was salt sensitive wouldn’t change the treatment approach for Elliott Antman, M.D., associate dean for clinical/translational research at Harvard Medical School and senior physician in the Cardiovascular Division of Brigham and Women’s Hospital in Boston. Antman said he has to use the same high blood pressure medications and advise the same limits on sodium.
“Our efforts in precision medicine may help refine the genotype and phenotype of salt sensitivity in the future, we all need to ingest much less salt than we do now. The AHA recommends less than 1500 mg a day of sodium from all sources,” said Antman.