By GINA KOLATA
Published: May 14, 2013 312 Comments
In a report that undercuts years of public health warnings, a prestigious group convened by the government says there is no good reason based on health outcomes for many Americans to drive their sodium consumption down to the very low levels recommended in national dietary guidelines.
Those levels, 1,500 milligrams of sodium a day, or a little more than half a teaspoon of salt, were supposed to prevent heart attacks and strokes in people at risk, including anyone older than 50, blacks and people with high blood pressure, diabetes or chronic kidney disease — groups that make up more than half of the American population.
Some influential organizations, including the American Heart Association, have said that everyone, not just those at risk, should aim for that very low sodium level. The heart association reaffirmed that position in an interview with its spokesman on Monday, even in light of the new report.
But the new expert committee, commissioned by the Institute of Medicine at the behest of the Centers for Disease Control and Prevention, said there was no rationale for anyone to aim for sodium levels below 2,300 milligrams a day. The group examined new evidence that had emerged since the last such report was issued, in 2005.
“As you go below the 2,300 mark, there is an absence of data in terms of benefit and there begin to be suggestions in subgroup populations about potential harms,” said Dr. Brian L. Strom, chairman of the committee and a professor of public health at the University of Pennsylvania. He explained that the possible harms included increased rates of heart attacks and an increased risk of death.
The committee was not asked to specify an optimal amount of sodium and did not make any recommendations about how much people should consume. Dr. Strom said people should not eat too much salt, but he also said that the data on the health effects of sodium were too inconsistent for the committee to say what the upper limit of sodium consumption should be.
Until about 2006, almost all studies on salt and health outcomes relied on the well-known fact that blood pressure can drop slightly when people eat less salt. From that, and from other studies linking blood pressure to risks of heart attacks and strokes, researchers created models showing how many lives could be saved if people ate less salt.
The United States dietary guidelines, based on the 2005 Institute of Medicine report, recommend that the general population aim for sodium levels of 1,500 to 2,300 milligrams a day because those levels will not raise blood pressure. The average sodium consumption in the United States, and around the world, is about 3,400 milligrams a day, according to the Institute of Medicine — an amount that has not changed in decades.
But more recently, researchers began looking at the actual consequences of various levels of salt consumption, as found in rates of heart attacks, strokes and death, not just blood pressure readings. Some of what they found was troubling.
One 2008 study the committee examined, for example, randomly assigned 232 Italian patients with aggressively treated moderate to severe congestive heart failure to consume either 2,760 or 1,840 milligrams of sodium a day, but otherwise to consume the same diet. Those consuming the lower level of sodium had more than three times the number of hospital readmissions — 30 as compared with 9 in the higher-salt group — and more than twice as many deaths — 15 as compared with 6 in the higher-salt group.
Another study, published in 2011, followed 28,800 subjects with high blood pressure ages 55 and older for 4.7 years and analyzed their sodium consumption by urinalysis. The researchers reported that the risks of heart attacks, strokes, congestive heart failure and death from heart disease increased significantly for those consuming more than 7,000 milligrams of sodium a day and for those consuming fewer than 3,000 milligrams of sodium a day.
There are physiological consequences of consuming little sodium, said Dr. Michael H. Alderman, a dietary sodium expert at Albert Einstein College of Medicine who was not a member of the committee. As sodium levels plunge, triglyceride levels increase, insulin resistance increases, and the activity of the sympathetic nervous system increases. Each of these factors can increase the risk of heart disease.
“Those are all bad things,” Dr. Alderman said. “A health effect can’t be predicted by looking at one physiological consequence. There has to be a net effect.”
Medical and public health experts responded to the new assessment of the evidence with elation or concern, depending on where they stand in the salt debates.