But unknown to many shoppers urged to buy foods that are “low sodium” and “low salt,” this longstanding warning has come under assault by scientists who say that typical American salt consumption is without risk.
Moreover, according to studies published in recent years by pillars of the medical community, the low levels of salt recommended by the government might actually be dangerous.
“There is no longer any valid basis for the current salt guidelines,” said Andrew Mente, a professor at McMaster University in Ontario and one of the researchers involved in a major study published last year by the New England Journal of Medicine. “So why are we still scaring people about salt?”
But the debate over dietary salt is among the most contentious in the field of nutrition, and other scientists, including the leadership of the American Heart Association, continue to support the decades-old warning.
The result is that as the federal government prepares its influential Dietary Guidelines for 2015, bureaucrats confront a quandary: They must either retract one of their oldest dietary commandments - or overlook these prominent new doubts.
The U.S. Dietary Guidelines cover an array of nutritional issues including cholesterol, fat and sugars. They have broad effects on American menus, shaping school lunches, guiding advertisers, and serve as a touchstone for reams of diet advice.
Dennis Bier, a professor at Baylor College of Medicine said that as the editor of the American Journal of Clinical Nutrition, he has been trying to stay neutral in what he considers the “hot buttonest” of topics.
“When you are making recommendations for 300 million people, you have to be concerned about any data that suggests harm,” Bier said.
A spokesperson for the Department of Health and Human Services said that the federal guidelines will consider comments from the public and the advice of its science panel. Known as the Dietary Guidelines Advisory Committee, that panel in February generally reaffirmed the current salt warning.
No matter what the government comes up with on salt, however, Americans may be left confused.
The scientific question: How much is too much?
There is one area of consensus: Both sides agree that eating too much salt, especially for people with high blood pressure, can be dangerous.
The critical disagreement concerns how to define “too much.”
Under the current dietary guidelines, too much is more than 2,300 milligrams of sodium per day - the amount of sodium in a teaspoon of salt. (For people over 50, and for African-Americans, the current recommended intake is even lower - 1,500 milligrams per day.)
If the U.S. salt warnings are correct, Americans are indeed endangering themselves on a massive scale. Americans typically go way over the limit, ingesting about 3,500 milligrams per day.
If the skeptics are correct, on the other hand, most Americans are fine. In their view, a typical healthy person can consume as much as 6,000 milligrams per day without significantly raising health risks. But consuming too little - somewhere below 3,000 milligrams - also raises health risks, they say.
To understand how divided scientists are on salt, consider that even authorities with the American Heart Association, one of the organizations promoting the current salt limits, don’t agree.
“The totality of the evidence strongly suggests that Americans should be lowering their sodium intake,” said Elliott Antman, the president of the American Heart Association. “Everyone agrees that current sodium intake is too high.”
This is the long-established view. It is based on the observation that, in some people, reducing salt consumption can lower blood pressure. Because high blood pressure is common and raises the risk of cardiovascular troubles, strict salt limits will benefit society, according to this view.
None of this is persuasive to people like Suzanne Oparil, a former president of the American Heart Association.
For one thing, the blood-pressure reductions that come from abstaining from salt are relatively small on average, because individuals vary widely in their reactions. (An average person who reduces his or her salt intake from median levels to the U.S. recommended levels may see a drop in blood pressure from 120/80 to 118/79, according to American Heart Association figures.)
“The current [salt] guidelines are based on almost nothing,” said Oparil, a distinguished professor of medicine at the University of Alabama at Birmingham. “Some people really want to hang onto this belief system on salt. But they are ignoring the evidence.”
How could something as simple as salt stymie scientists for so long? The answer is that, despite the dietary claims that are made for all kinds of foods, actually substantiating how eating influences human health is notoriously difficult.
While the diets and lifestyles of test animals are easily controlled, humans and their whims introduce an array of murky variables, making people less-than-ideal subjects for what scientists call randomized controlled trials, their preferred form of research. This is especially true when these experiments go on for years, as diet research often does.
In the absence of such experiments, scientists are forced to consider lesser types of evidence. And in recent years, the debate appears to have tilted in the skeptics' favor.
In 2013, the Institute of Medicine published a major review of the evidence connecting salt consumption and health outcomes. There was insufficient proof, the panel concluded, that heeding the U.S. recommended limit on sodium consumption improved health outcomes.
Then, this past August, the New England Journal of Medicine published the results of a massive research effort known as the PURE study. It indicated that people who conform to the U.S. recommended limits actually have more heart trouble.
To explain their findings, these researchers pointed to studies suggesting that low sodium may stimulate the production of renin, a hormone that may have harmful effects on blood vessels.
While food studies are often financed by the industry, the PURE study in the New England Journal of Medicine and the Institute of Medicine study were funded by governmental and other sources.