Also, there are some hints, albeit from studies with serious flaws, that eating the lowest levels might actually harm certain people — those who are being aggressively treated for serious heart failure, the report added.
The prestigious group, which advises the government about health, urged more and better research to settle the best target range.
"We're not saying we shouldn't be lowering excessive salt intake," said Dr. Brian Strom of the University of Pennsylvania, who led the IOM committee. But below 2,300 mg a day, "there is simply a lack of data that shows it is beneficial."
The average American consumes more than 3,400 mg of sodium a day, equivalent to 1 ½ teaspoons. Current U.S. dietary guidelines say most people should limit that to 2,300 mg a day, while certain people — those older than 50, African-Americans, and people with high blood pressure, diabetes or chronic kidney disease — should aim for just 1,500 mg.
Tuesday's report sparked an immediate outcry from health organizations that have long battled to lower the nation's salt consumption.
The American Heart Association said it stood by its own recommendations, stricter than the government's, that everyone eat no more than 1,500 mg of sodium a day. Studies make clear that eating less sodium helps lower blood pressure, and lower blood pressure in turn leads to less heart disease, said heart association spokesman Dr. Elliott Antman, a cardiologist at Boston's Brigham and Women's Hospital.
"We're sticking where we have the most solid evidence," Antman said.
What to study is key to the debate: The IOM committee was asked to examine whether eating less salt directly affects longer-term outcomes such as heart attacks and death. That's harder to prove, especially since the panel stressed many of the studies it reviewed had quality problems. Among those problems, Antman said, was including some patients too sick for diet to matter.
Debating how little salt is too little is a moot point, added nutritionist Bonnie Liebman of the Center for Science in the Public Interest.
"The average American is still in the red zone, the danger zone," she said.
The salt industry, in contrast, has long opposed the push for sharp sodium reductions and welcomed the report.
"There is no scientific justification for population-wide sodium reduction to such low levels, and the recognition by the IOM experts that such low levels may cause harm may help steer overzealous organizations away from reckless recommendations," said Morton Satin of the Salt Institute.
Some salt is necessary for good health, although it's not clear exactly how much.
But today, it's very hard to cut back to 1,500 mg unless you always cook from scratch, or eat too little food in general because of illness, Liebman added. Three-quarters of people's daily sodium come from processed or restaurant food. The average sandwich in a restaurant has 1,000 mg or more, and some restaurant meals can provide a full day's supply of sodium in one entree, she said.
The Food and Drug Administration is working on a program to get food manufacturers to gradually lower sodium levels in their products, thus helping shoppers who don't scrutinize food labels to get closer to dietary guidelines. The FDA said Tuesday that it was reviewing the IOM report, calling it consistent with its own efforts "to work toward achievable and reasonable voluntary reductions in the sodium content of the U.S. food supply."
Some restaurants already are taking steps. The Boston Market chain, for instance, has begun reducing sodium in its most-sold items by about 20 percent — and removed salt shakers from the restaurant tables. "Where's the salt shaker?" read signs sitting next to the pepper that explain the chain's move to reduce sodium and advise that customers who really want an extra pinch will have to get up to find some at the counter.