Dietary Guidelines May Have a Downside

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.


Source: Judith Groch, Senior Writer, MedPage Today
Published: January 22, 2008
http://www.medpagetoday.com/PrimaryCare/DietNutrition/dh/8064

Primary source: American Journal of Preventive Medicine
Source 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 Medicine
Source reference:
Woolf S, Nestle M, "Do dietary guidelines explain the obesity epidemic?
Am J Prev Med 2008; DOI: 1016/j.amepre.2007.12.002.

Additional source: American Journal of Preventive Medicine
Source reference:
Marantz P, et al "The authors respond" Am J Prev Med 2008; DOI:
1016/j.amepre.2007.12.003.