High blood pressure is the leading cause of preventable deaths around the world. But did the Institute of Medicine (IOM) really say that lowering salt consumption is not the answer?
“Lowering daily sodium intake below 2,300 milligrams may do more harm than good,” reported CBS News in May 2013. “No benefit in sharply restricting salt, panel finds,” said The New York Times. “Is eating too little salt risky?” asked National Public Radio. “New report raises questions.”
Questions, indeed. But not so much about salt as about what the IOM was asked, what it concluded, and whether it relied on discredited data.
When the Centers for Disease Control and Prevention asked the Institute of Medicine to quickly review the impact of very low-salt diets on health in 2012, the CDC expected to clear up some confusion. Instead, the report caused more. Here’s what the IOM did—and didn’t—say.
The report wasn’t about whether we eat too much salt.
The question wasn’t whether typical U.S. sodium intakes —at least 3,400 mg a day, not including what we get from the salt shaker—are healthy. They’re not.
Instead, the CDC wanted to know what happens at 1,500 mg to 2,300 mg a day.
While that matters for setting daily sodium targets, it applies to few people, since only about 5 percent of adults get less than 2,300 mg of sodium a day (not counting what comes from the salt shaker).
The report found no harm for most people.
There’s “insufficient and inconsistent” evidence that very low-salt diets cause harm in the “general population,” the IOM concluded.
Why insufficient? Studies that have observed a higher risk of disease or death in people who eat very low-salt diets have weaknesses.
One example: “People who report eating very little sodium are more likely to be ill,” explains Stephen Havas of the Northwestern University Feinberg School of Medicine. Odds are, it’s illness that raises their risk of disease and death and makes them eat so little salt (and food).
Since all the evidence of harm in the general population was based on studies that had that or some other weakness, it added up to “insufficient,” said the IOM.
The report found evidence of harm in people with heart failure, but that’s irrelevant, and the evidence is suspect.
The most persuasive evidence of harm came from a group of Italian researchers who randomly assigned patients with heart failure to normal or very-low-sodium diets. Those restricting sodium were more likely to be readmitted to the hospital or to die.
Alarming? Not quite. First, the researchers restricted not just salt, but also how much water the patients could consume. And they put them on high doses of diuretics. That regimen isn’t used in this country.
“This hyper-aggressive treatment may have caused severe depletion of blood volume,” explains Frank Sacks of the Harvard School of Public Health.
More troubling, in June 2013 the journal Heart retracted a meta-analysis on sodium and heart failure by one of the Italian researchers, Pietro Di Pasquale of the University of Palermo. Each of the six studies in the meta-analysis was done by his research group.
In March 2013 Heart’s editor warned that two of the studies had duplicate data. The editor explained that when its ethics committee asked to see the raw data, the researchers said it was “lost as a result of computer failure.” The IOM didn’t cite the meta-analysis, but it did cite the two studies with duplicate data.
“It’s hard to believe that data can be lost from two trials,” says Lawrence Appel of Johns Hopkins University in Baltimore.
“The editors from the journals that published these papers should ask for the data and conduct an independent analysis of all trials from this research group.”
What’s the bottom line? We should all aim for 1,500 mg of sodium a day, says the American Heart Association. But since at least 95 percent of adults exceed 2,300 mg a day, most of us should be eating less salt.
Sources: www.iom.edu/Reports/2013/Sodium-Intake-in- Populations-Assessment-of-Evidence.aspx; MMWR 60: 1413, 2011; Clin. Sci. 114: 221, 2008; Heart 99: 820, 2013.
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