U.S. Agriculture Secretary Tom Vilsack and Health and Human Services Secretary Sylvia Burwell testify during a hearing before the House Agriculture Committee on Capitol Hill on Wednesday. The committee held the hearing to review the development of the 2015 Dietary Guidelines for Americans. (Alex Wong/Getty Images)

The quality of the evidence supporting the Dietary Guidelines for Americans, the influential nutritional advice from the federal government, came under steady attack at a Congressional hearing Wednesday, with representatives complaining that the credibility of the national advice has been eroded by shifts in science.

Salt? Saturated fat? Eggs? Meat? Opinions about each of these were aired as members of Congress directed their skepticism at the two cabinet secretaries who oversee the development of the nutritional guidelines, Health and Human Services Secretary Sylvia Burwell and Agriculture Secretary Tom Vilsack.

"My concern is we have these guidelines that have pushed people away from eggs and butter and milk and so forth and then they come back and say, 'Well, we're wrong.' You know?" said  Rep. Collin C. Peterson (D-Minn.), ranking member of the House Agriculture Committee, which held the hearing on the Dietary Guidelines. "Why are we going off on these tangents if we have a [scientific] process that is so heavily vetted?"

"I want you to understand, from my constituents, most of them don't believe this stuff anymore. You have lost your credibility with a lot of people. They are just flat out ignoring this stuff, and so that's why I say I wonder why we are doing this."

"Uncertainty in the process leads to concern about whether the [Dietary Guidelines] recommendations will maintain the scientific integrity necessary," committee chair K. Michael Conaway (R-Tex.) told the secretaries.

Burwell and Vilsack defended the nutritional guidelines, saying they have been based on the best available science. Evidence changes, they noted, and isn't always clear-cut.

But the Dietary Guidelines, due to be updated later this year, have come under increasingly heavy scrutiny in recent months because of doubts about their scientific underpinnings. The government's long-standing guidance about nutritional basics such as fat, salt and cholesterol have been undermined by recent research. Members of the public filed 29,000 comments on the report filed in February by the advisory panel advising the government on the development of the guidelines.

A group of academics and other scientists, calling itself the Nutrition Coalition, has also formed to push for stronger science in the guidelines. The coalition is funded by the Action Now Initiative, an advocacy group supported by Houston-based philanthropists Laura and John D. Arnold. No financial support is provided by industry groups of any kind, the group said. Three of the group members are former members of the Dietary Guidelines Advisory Committee, and another is former chair of the American Heart Association’s nutrition committee.

Glenn Thompson (R-Pa.) raised the possibility that the guidelines, which the government has published since 1980, might be rated a failure, given the nation's high rates of obesity, diabetes and other chronic health problems.

The guidelines have come out for decades, but "are Americans healthier or less healthy since the guidelines have been published?" he asked Burwell and Vilsack. "In some ways, haven't these guidelines somewhat failed? . . . They don't seem like they are accomplishing their objective."

Burwell seemed to acknowledge the nation's continuing health problems and answered with a question about what American health would have been like without the guidelines.

"We are on the wrong trajectory, but would the trajectory have been worse?" she said.

Vilsack repeatedly emphasized the difficulty of making recommendations when the science sometimes provides only hunches, not proof about the best diet. He noted, too, that the legislation creating the Dietary Guidelines calls for guidelines based on the "preponderance" of the evidence, not guidelines based on evidence that might be considered "beyond a reasonable doubt."

"This is really about well-informed opinion," Vilsack told the commiteee. "I wish there were scientific facts. But the reality is stuff changes, right? Stuff changes. The key here is taking a look at the preponderance, the greater weight of the evidence, and trying to make a judgement. "