They're updated every five years, and new guidelines are expected this fall, after a report from the Dietary Guidelines Advisory Committee.
The committee's leader has emphasized the importance of including only the best medical evidence and said all committee members are vetted.
The 2015 report "used weak scientific standards (which) seems to have made the report vulnerable to internal bias as well as outside agendas," food journalist Nina Teicholz wrote in the British Medical Journal.
Nutrition experts said the debate likely won't change the guidelines or the way consumers eat -- but it may spark frustration as they try to get a clear answer: What does a healthy diet look like?
The research behind a healthy diet
The 2015 dietary guidelines report relied on existing reviews of the evidence base, rather than conducting original reviews, but the committee left out important studies, according to Teicholz, who wrote the book, "The Big Fat Surprise: Why Butter, Meat and Cheese Belong in a Healthy Diet
." The reviews have examined, for example, clinical trials and observational studies on whether there is a link between saturated fat consumption and heart disease and have come up with conflicting results. The British Medical Journal issued a correction to Teicholz's article
stating that evidence to date does in fact suggest a link between saturated fat and heart disease (though not necessarily between saturated fats and death from heart disease).
The result is that the 2015 dietary guidelines report overstated the evidence for the health risks of saturated fats and understated the importance of a low-carbohydrate diet, Teicholz said. Although the recent report broke with earlier publications by exonerating cholesterol
and avoiding a limit on total fats in the diet
, it did state that saturated fats should not exceed 10% of total calories.
"I think (Teicholz) has really put her finger on something important here," said Jeff Volek, a professor of human sciences at Ohio State University, who has criticized the dietary guidelines
in the past. "The committee certainly missed the boat on a lot of existing data" that suggest saturated fats are not associated with heart disease and other chronic health problems, Volek said. He argued that the advisory committee didn't give enough credit to some large analyses, as well as some of his own studies on saturated fat and heart disease.
Advice to restrict saturated fat has backfired, Volek said, because people replaced those calories with carbohydrates and sugar, and rates of obesity and diabetes in the country continue to rise. Instead, diets low in carbohydrates and without limits on saturated fats should be an option, especially for people with diabetes and prediabetes, he said. He noted that the 2015 report does include some changes he sees as positive, such as not recommending a limit on total fat.
Barbara Millen, chairwoman of the most recent advisory committee, told CNN that all of the best studies on saturated fat and carbohydrates were considered for the report. To be included, studies had to meet a long list of criteria, such as looking at health effects of dietary components over a long enough period of time.
The committee relied on existing analyses of research only if they included the most relevant studies, said Millen, who is president of Millennium Prevention, a company that develops technologies to manage diseases such as obesity, diabetes and heart disease. If not, the advisory committee commissioned its own original analyses.
Criticism of the British Medical Journal piece was published in The Verge
, which said some of the studies Teicholz noted were not relevant or were outside of the purview of the report. The focus of the committee's report, the Verge writer noted, is on maintaining health and not on preventing disease.
Exploring conflicts on the committee
The British Medical Journal article raises the possibility of conflicts of interest among members of the Dietary Guidelines Advisory Committee. For example, Millen's Millennium Prevention sells mobile apps for monitoring health, and the report recommends self-monitoring technologies. Other members of the committee have received research funding from companies that make vegetable oil products and groups that advocate eating nuts.
There could be pressure to maintain status quo guidelines, possibly because scientists have staked their careers on certain dietary paradigms or because of pressure from food industry groups, Volek said.
The committee might not be in the best position to judge the most relevant studies, said Edward Archer, an obesity theorist at the Obesity and Nutrition Research Center at the University of Alabama-Birmingham. The U.S. Department of Health and Human Services and the Department of Agriculture "choose people who are going to agree with previous guidelines," Archer said.
But Millen said consultants and committee members were extensively vetted and "determined to have no conflicts that were related to this report, end of story."
"What is important is that you are very transparent about (potential conflicts of interest), and the vetting the federal government does is extraordinary," Millen said.
The points raised in the journal article probably won't change much about how consumers eat, said Jeanne P. Goldberg, professor of nutrition at Tufts University and director of the graduate program in nutrition communication and behavior change. These reports used to set the guidelines are too complicated for the average person to use as dietary guides, and professionals and policymakers have probably already formed their opinions, Goldberg said. She is not on the Dietary Guidelines Advisory Committee, but two of her Tufts colleagues are.
She said the report is a good update, and science will always involve adding more data and gathering expert opinions.
What the controversy could do is cause "consumers to want to throw up their hands (because) it plays out in the media as 'the science can't get it right,'" Goldberg said.