Dietary Guidelines May Have a Downside

By Judith Groch
NEW YORK, Jan. 22 -- With their weak standards of evidence, national dietary guidelines might actually do more harm than good, said researchers here.
Using guidelines against dietary fat promulgated in the late 1970s as a case in point, Paul R. Marantz, M.D., of Albert Einstein College of Medicine, and colleagues, noted that people may have been led to believe that limiting dietary fat gave them a green light to eat their fill of carbohydrates.
This association, they wrote online in the American Journal of Preventive Medicine, may have contributed to the current epidemic of obesity and overweight in the U.S.
National dietary guidelines have been promulgated based on scientific reasoning and indirect evidence, the researchers said.
In general, they said, weak evidentiary support has been accepted as adequate justification for the guidelines.
"This low standard of evidence is based on several misconceptions, most importantly the belief that such guidelines could not cause harm."
But, the researchers noted, in 2000, the Dietary Guideline Advisory Committee reversed the earlier 1995 recommendation to lower fat intake, saying that it might have been ill-advised and might actually have some potential for harm.
They quoted the 2000 committee statement that "an increasing prevalence of obesity in the United States has corresponded roughly with an absolute increase in carbohydrate consumption."
This increase may also have been influenced by marketing trends for low-fat foods, which although not entirely driven by the U.S. Dietary Guidelines, may have used an apparent governmental "seal of approval" for products such as fat-free cookies.
From 1971 through 2001, the researchers noted, absolute fat intake decreased by only 5% in men while fat as a percent of total calorie intake declined by 11%. In women, they said, "the difference was even starker: absolute fat intake actually increased by 11% while relative fat intake declined by 9%."
The temporal association between the recommendations and the increase in total calorie consumption does not prove causation, the researchers were careful to point out, but it raises the possibility of a net harmful effect of seemingly innocuous dietary advice, they said.
The researchers also pointed to advice on salt intake as another example of unintended consequences of a seemingly sensible recommendation, noting, however, that a blood pressure benefit may be trumped by harmful effects on plasma renin, insulin resistance, sympathetic nerve activity, and aldosterone levels.
A recent finding of no difference in total mortality between randomized sodium-intake groups has left the value of sodium reduction an open question, they said.
The trans fat issue, they said, provides another example. Although there is good evidence linking dietary trans fats to cardiovascular disease, though not with obesity, the net effect of the campaign against trans fats is that these fats will be replaced by "something else (unclear what)," they wrote.
Furthermore, as the vilification of dietary trans fats continues and marketing of other high-calorie foods with "0 grams trans fats" takes off, it should not be forgotten that trans fat consumption increased dramatically as margarine was promoted as the healthy alternative to butter.
To avoid possible unanticipated and adverse effects, specific and transparent classification of the quality of the evidence should attend guideline development and promulgation, the researchers said.
"When adequate evidence [for a guideline recommendation] is not available," they wrote, "the best option may be to issue no guideline."
In an accompanying comment, Steven Woolf, M.D., of Virginia Commonwealth University in Richmond, and Marion Nestle, Ph.D., of New York University, differed with the investigators, maintaining that the dietary guidelines are not the culprit in the obesity epidemic.
"The larger concerns," they said, "are poverty and an environment that promotes overeating and inactivity."
"Although we agree with many of the themes described by the authors of this study," Drs. Woolf and Nestle wrote, "we strongly disagree with their depiction of how the guidelines were developed, their characterization of the evidence on which the guidelines were based, and their indictment of public health guidance in general."
Nutrition research has had its share of "flip-flops" and is challenging perhaps more so than in other areas of public health or medicine, they wrote. For example, the time needed to measure health outcomes is so long that studies must often rely on surrogate measures, and every study is complicated by the enigma of whether foods or some combination or interactions of their nutrients are responsible for the outcomes.
It is true that messages about fat must be accompanied by messages about caloric balance and deal with other complex issues. But the solution is "not to abandon the enterprise but to reshape the message," they wrote.
A guideline cannot single-handedly change a nation's eating habits. Food preferences, portion sizes, and physical activity levels are products of advertising, the environment, and a milieu of other obesogenic influences, "not the consequence of a poorly distributed federal publication," they wrote.
"To scapegoat guidelines is to oversimplify the complex and to obfuscate the necessary -- albeit difficult -- task of confronting these larger determinants of obesity," Drs. Woolf and Nestle concluded.
No financial disclosures were reported by the authors of the paper or by the authors of the comment.
Primary source: American Journal of Preventive MedicineSource reference:Marantz P, et al "A call for higher standards of evidence for dietary guidelines" Am J Prev Med 2008: DOI: 1016/j.amepre.2007.11.017. Additional source: American Journal of Preventive MedicineSource reference: Woolf S, Nestle M, "Do dietary guidelines explain the obesity epidemic? Am J Prev Med 2008; DOI: 1016/j.amepre.2007.12.002.