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.
“What they have done is earth-shattering,” Dr. 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.”