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.
Remote tribes, politics and science
Since their inception more than 30 years ago, the salt guidelines have drawn criticism.
Some of the earliest notions that Americans were eating too much salt arose from international comparisons.
It turned out that in some cultures, especially isolated ones, people consumed less salt and had lower blood pressure.
In one influential 1973 paper, University of Michigan anthropologist Lillian Gleiberman collected statistics for 27 different populations. It showed the lowest blood pressures were among African Bushmen, the Chimbu of New Guinea, the Caraja of Brazil and Eskimos. Each consumed exceptionally low levels of salt.
Maybe, Gleiberman suggested, human bodies had not adapted to the higher salt available in modern societies.
“My major hypothesis was that people ate much less salt in prehistoric times,” Gleiberman, now retired, said by phone recently. “And that our bodies may not be prepared for the larger amounts of salt now available to us.”
But she said her paper was intended to inspire more research, not to serve as the basis of dietary guidelines. Those remote peoples, she said, are too different from modern populations to make sound comparisons.
“They have a simpler life,” Gleiberman said. “They don’t have the obesity, the diabetes and the other problems we have. We can’t look at a no-salt culture and say, 'If we just do that, we’d be okay.'
“I have friends who won’t eat anything with salt,” she said. “I tell them they’re foolish.”
Nevertheless, when a Senate committee led by Sen. George McGovern (D-S.D.) in 1977 set out to issue national dietary goals, the international comparisons played a key role.
There was not much else to go on. Scientists told the committee there was general agreement that very high salt consumption could be harmful. But were Americans eating too much? That was a matter of dispute.
“There is no doubt that excess salt, gross excesses, can produce high blood pressure in specific populations,” Robert I. Levy, director of the National Heart, Lung and Blood Institute testified to the committee, according to a transcript. “The problem is demonstrating the efficacy of salt lowering in the American free-living population.”
In formulating their salt recommendations, the committee looked to the work of George R. Meneely and Harold D. Battarbee, researchers at Louisiana State University who made two arguments for restricting salt.
First, they said that our ancestors, “a primal herbivorous people,” probably consumed no more than 600 milligrams of salt per day - far less than today - and that our bodies have yet to adjust to the amount of salt available in modern society.
Second, they noted that, in some people, lowering salt consumption lowered blood pressure. Since about 20 percent of American adults at the time had high blood pressure, reducing salt consumption would “result in the amelioration of much suffering.”
But even Meneely and Battarbee noted the complexity of the issue, noting that it was difficult to to say just how poisonous salt is, or, as they put it, to “document its toxicity.”
Despite the uncertainty, the committee advised Americans in “Dietary Goals” to reduce their salt consumption to a very, very low level - 1,200 milligrams of sodium per day. That is even lower than today’s most restrictive recommendations.
Wherever that figure came from, it didn’t last long. By November, the committee issued another set of guidelines. They raised the daily amount to 2,000 milligrams. But that didn’t last long, either. Three years later, the federal bureaucracy - not the Senate committee - issued its dietary advice. It was the first version of the “Dietary Guidelines.” It advised people to lower their salt consumption, but it did not specify an upper limit.
Intersalt, more doubts and the revival of a salt limit
So the question lingered. The Dietary Guidelines said Americans were eating too much salt. But how much was “too much”?
In 1984, a major worldwide study known as Intersalt was launched, with scientists testing more than 10,000 people from 52 different populations. The study was funded by the U.S. government, a British charitable trust and other world groups.
Yet Intersalt, too, failed to settle the argument. When the results were published in 1988, many of the findings undercut the salt orthodoxy. In the comparisons of populations, there was little proof that societies that consumed more salt suffered from higher blood pressure. For example, while South Koreans consumed vast amounts of salt and had low blood pressures, the opposite was true for a Belgian population.
One other item, however, did favor salt restrictions: In places where more salt was consumed, blood pressures rose more with age.
Both sides declared victory. Then, despite the muddle, Intersalt became the basis for strict U.S. salt guidelines, with the 1995 Dietary Guidelines recommending holding to just about today’s limit of 2,300 milligrams per day.
(It takes some tracing back to see that Intersalt is the basis of those guidelines, but the academic references wind back as follows: The 1995 Dietary Guidelines cited an FDA report, which cited a report called “Diet and Health” from the National Academy of Sciences, which in turn cited the Intersalt findings. At each step, scientists had declared their uncertainty.)
The strict salt limit had become a fixture of U.S dietary advice.
Many experts expect that the 2015 Dietary Guidelines will stick to the existing 2,300 milligram limit.
Already, the 15-member advisory panel in February recommended keeping the limit, though it withdrew support for the even stricter 1,500 milligram limit for African Americans and people over 50. It further called for measures to remove salt from American foods.
Cheryl Anderson, a nutrition expert at the University of California at San Diego who led the advisory panel’s sodium working group, said the government should continue to offer salt guidelines despite some of the recent findings that have called them into question.
One of the major critical studies was the PURE investigation, which incuded more than 100,000 people and was published in the New England Journal of Medicine. Anderson said it was “respectable” and “an important contribution.”
But she said such studies may be misleading because researchers take only a limited number of urine samples. And, she said, that type of research, known as observational studies, may suffer from a problem known as “reverse causality.” That is, while the data suggested that low-salt diets may have caused cardiovascular problems, maybe it was just the reverse - that preexisting cardiovascular problems had led people to eat low-salt diets.
The authors of the PURE study took steps to minimize such bias, but at the end, Anderson said, the weight of the evidence favored the old salt warning.
“We can’t take any one study in isolation,” Anderson said. “We placed the new ones in the context of the body of literature on sodium - and we put the strongest recommendations forward.”