Jennifer Domenick/The Washington Post

For two decades, the government has been warning that most Americans are eating dangerous amounts of salt. Contrary evidence has emerged in recent years, however, that suggests the government's advice may be unfounded.

Yet speakers at a recent Capitol Hill briefing behaved as if the matter remains settled.

“There is no, or minimal, debate about whether to lower sodium intake,” Lawrence Appel a medical professor at Johns Hopkins University, one of the leading advocates for salt restrictions.

“Excess sodium is killing more Americans than anything else in food,” according to Michael Jacobson, executive director of the Center for Science in the Public Interest, a public interest lobbying group, which organized the gathering.

And Rep. Rosa DeLauro (D-Conn.), speaking generally of the U.S. dietary guidelines, suggested at the meeting that no more research is necessary to know what should be done.

“We don’t need these studies!” she said, throwing a fist in the air.  “We have the data.”

Once every five years, the U.S. updates the so-called “Dietary Guidelines,” and when it does, the question of what Americans should eat enters the political realm, where an air of forceful certainty gains more traction than ordinary scientific doubt.

Friday is the deadline for public comments about the 2015 version of the dietary guidelines, the federal government's diet advice book, and the prosecutions of the usual suspects - saturated fats, red meat, caffeine - are being revived with special intensity.

Few nutrients provoke debates as heated, however, as salt. Despite the assertions on Capitol Hill last week that there is a scientific consensus, a major study published last year in the esteemed New England Journal of Medicine, as well as a 2013 report from the Institute of Medicine, have renewed questions about exactly what the government should be telling people.

Generally speaking, both sides in the salt debate agree that eating too much can be harmful. The critical disagreement arises over how much is "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.)

Americans typically go way over that limit, ingesting about 3,500 milligrams per day. If the U.S. salt warnings are correct, in other words, Americans are indeed endangering themselves on a massive scale.

If the skeptics on the other side of the argument are correct, on the other hand, the amount of salt eaten by most Americans is nothing to worry about. In this view, the typical American consumption of about 3,500 milligrams per day is fine -in fact, a typical healthy person can consume as much as 6,000 milligrams daily without significantly raising health risks.  This side argues, moreover, that  consuming too little - somewhere below 3,000 milligrams - also raises health risks.

If they are right, the current U.S. dietary guideline to eat less than 2,300 milligrams may be harming people.

For anyone seeking practical advice, the debate can be frustrating.

It goes back at least as far as the  ‘70s, however, and arises in part because of the difficulty in studying nutrition, and salt in particular. This is partly because so many foods contain salt and most people have little idea how much they consume on a given day. Moreover, individual reactions to salt vary widely.

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While dozens of studies have been conducted on the subject, a look at two of the major research efforts cited recently in the debate can give an idea of the contrasting positions.

One of the most important studies underlying the current U.S. advice on salt, which calls for Americans to cut way back, is known as the “DASH - Sodium” study. Published in the New England Journal of Medicine in 2001, it is often cited as a key reference for those who support the idea that Americans consume far too much salt, and that this over-consumption is provoking strokes and heart attacks.

In the DASH-Sodium trial, researchers fed specially prepared diets to 412 people for about three months. The diets had either high, medium or low levels of salt. Their blood pressures were recorded.

What the research found is that average blood pressures rose with higher levels of salt, though the changes were not huge. For example, the results indicated that a  person on a typical diet who cut his or her salt intake about 25 percent would see their blood pressure drop from about 120/80 to 118/79.

Appel notes that high blood pressure,  which raises the risk for heart attack and stroke, is a critical health problem in the U.S. According to Centers for Disease Control figures, nearly one in three American adults has high blood pressure.

The trouble with that key trial, however, is that the patients who were enrolled did not closely match the demographics of the U.S. population. Instead, most of the people enrolled in the trial were from populations that are considered to be “salt-sensitive” - that is, their blood pressures react more when salt levels change. More than half of the participants in the trial were African-American, and about 40 percent already had high blood pressure - and these populations generally react more sharply to salt, scientists said.

“The deck was stacked to have an exaggerated response,” said Andrew Mente, a professor at McMaster University in Ontario, an expert in the field.

Research by Mente and his colleagues argues in the other direction - that is, it suggests that the very low levels of salt currently recommended by the government could harm people.

Their research, known as the PURE study and published last year in the New England Journal of Medicine, followed more than 100,000 people around the world for more than three years. Because it was longer in duration than DASH-Sodium, it could record not just blood pressure levels, but whether people actually suffered strokes and heart attacks.

The results published last year agreed with the DASH-Sodium findings in one way. They suggested that blood pressures decline somewhat when a person lowers his or her salt intake. But it also suggested that people who consumed the very low levels of sodium recommended by the government had more heart attacks and strokes.

And what does Appel think of this research?

“It’s bunk,” he said flatly during the press briefing.

Appel criticized the study because researchers measured salt intake with only one urine sample, and salt intakes can vary wildly from day-to-day. The PURE researchers note, however, that the large number of people enrolled would have muted any random errors that would have been introduced this way.

Whatever the case, each side in the salt debate has its advocates, some of whom are forceful, even passionate. But in matters of nutrition, as the past shows,  a sense of certainty is no guarantee of accuracy.

Way back in 1979, as the government was developing the very first Dietary Guidelines, Jacobson, who now leads the charge against salt, also played a leading role in the debate. At the time, he offered this rhyming slogan to a reporter covering the debate surrounding the 1980 Dietary Guidelines.

“Eat less sugar. Eat less fat. Bread and potatoes are where its at.”

Because of new research, however, that advice has not weathered well. Some critics have come to blame the  obesity epidemic at least partly on Americans who avoided more fat only to eat too many carbohydrates - like bread and potatoes. And current guidelines take a more nuanced approach to fat, advising people to replace saturated fats with polyunsaturated and monounsaturated fats.

Given the past missteps, how do we know the current research is valid?

“There’s so much more evidence on these issues than in the late 1970s," Jacobson said. "There are more randomized trials. There is far more research...That was how many years ago? I certainly didn’t know as much as I thought I knew back then.”

When will we really be sure of the diet answers?

"On Judgment Day," he said.