c.2013 New York Times News Service
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 — a group that makes up more than half of the U.S. population.
Some influential organizations, including the American Heart Association, have said everyone, not just those at risk, should aim for that very low sodium level. The heart association reaffirmed that position in an interview 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 lower than 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. 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.
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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 U.S. 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 3,700 milligrams a day, 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 re-admissions — 30 as compared with nine in the higher-salt group — and a more than twice as many deaths — 15 as compared with six in the higher-salt group.
Another study, published in 2011, followed 28,800 subjects with high blood pressure aged 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 less than 3,000 milligrams of sodium a day.
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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,” 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.
“What they have done is earth-shattering,” Alderman said. “They have changed the paradigm of this issue. Until now it was all about blood pressure. Now they say it is more complicated.”
The report, he predicted, “will have a big impact.”
But Bonnie Liebman, director of nutrition at the Center for Science in the Public Interest, an advocacy group that has taken a strong position against excessive salt consumption, worried that the public would get the wrong message.
“It would be a shame if this report convinced people that salt doesn’t matter,” Liebman said.
The American Heart Association agrees with her. Dr. Elliott Antman, a spokesman for the American Heart Association and a professor of medicine at Brigham and Women’s Hospital, said the association remained concerned about the large amount of sodium in processed foods, which makes it almost impossible for most Americans to cut back. People should aim for 1,500 milligrams of sodium a day, he said.
“The American Heart Association is not changing its position,” Antman said.
The association rejects the Institute of Medicine’s conclusions because the studies on which they were based had methodological flaws, he said. The heart association’s advice to consume 1,500 milligrams of sodium a day, he added, is based on epidemiological data and studies that assessed the effects of sodium consumption on blood pressure.
The Institute of Medicine committee said it was well aware of flaws in many of the studies of sodium, especially ones that the previous Institute of Medicine committee relied on for its 2005 recommendations. Much of that earlier research, committee members said, looked for correlations between what people ate and their health. But people with different diets can differ in many ways that are hard to account for — for example, the amount of exercise they get. And relying on people’s recall of how much salt they consumed can be unreliable.
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Even the ways previous studies defined high and low sodium consumption varied widely.
“In one study it was high if it hit 2,700 milligrams a day. In another study it was high if it hit 10,000 milligrams a day,” said Cheryl A.M. Anderson, a committee member who is an associate professor of family and preventive medicine at the University of California, San Diego.
The committee said it found more recent studies, published since 2005 that were more careful and rigorous. Much of the new research found adverse effects on the lower end of the sodium scale and none showed a benefit from consuming very little salt.
Although the advice to restrict sodium to 1,500 milligrams a day has been enshrined in dietary guidelines, it never came from research on health outcomes, Strom said. Instead, it is the lowest sodium consumption can go if a person eats enough food to get sufficient calories and nutrients to live on. As for the 2,300-milligram level, that was the highest sodium levels could go before blood pressure began inching up.
In its 2005 report, the Institute of Medicine’s committee said that sodium consumption between 1,500 and 2,300 milligrams a day would not raise blood pressure.
That range, Strom said, “was taken by other groups and set in stone.” Those other groups included the Department of Agriculture and the Department of Health and Human Services, which formulated dietary guidelines in 2005.
But those dietary guidelines will soon be revised, with new recommendations to be issued in 2015.