Review and special article
A Call for Higher Standards of Evidence for Dietary Guidelines

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Abstract

Dietary guidelines, especially those designed to prevent the diseases of dietary excess, are a relatively new phenomenon in the United States. National dietary guidelines have been promulgated based on scientific reasoning and indirect evidence. In general, weak evidentiary support has been accepted as adequate justification for these guidelines. This low standard of evidence is based on several misconceptions, most importantly the belief that such guidelines could not cause harm. Using guidelines against dietary fat as a case in point, an analysis is provided that suggests that harm indeed may have been caused by the widespread dissemination of and adherence to these guidelines, through their contribution to the current epidemic of obesity and overweight in the U.S. An explanation is provided of what may have gone wrong in the development of dietary guidelines, and an alternative and more rigorous standard is proposed for evidentiary support, including the recommendation that when adequate evidence is not available, the best option may be to issue no guideline.

Section snippets

A Brief History of Dietary Guidelines for Fat Intake

National dietary guidelines were introduced in 1894. Initially designed to prevent dietary deficiency, the goals were modified in the late 1970s to include recommendations designed to reduce the burden of cardiovascular disease (CVD).2 This made sense because, first, heart attacks, strokes, and renal disease had become the leading causes of morbidity and mortality in the United States, and second, the importance of CVD risk factors, including hyperlipidemia, had been firmly established. Since

Health Trends

As these dietary fat guidelines were promulgated from 1980 to 2000, cardiovascular mortality rates continued their established decline in the U.S.9 A Cochrane Collaboration meta-analysis, published in 2001, showed that low-fat diets had a marginally (and nonsignificantly) favorable effect on CVD mortality in randomized trials,10 consistent with the notion that if a population-wide reduction in dietary fat had occurred, it might have contributed to this decline in mortality. Of course, this

What Went Wrong?

The dietary fat story underscores several weaknesses in public health decision making. The following attempts to explain where current thinking might go wrong.

What Can Be Done?

Dietary guidelines need explicit standards of evidence. Rating scales, such as those used by the U.S. Preventive Services Task Force (USPSTF) and other bodies, have been reviewed recently.27 Rather than bury the complexity and uncertainty that may underlie dietary recommendations in order to make the message clear and unambiguous,7 guidelines should always include a clear and thoughtful assessment and disclosure of the evidence.

For dietary guidelines, the strengths and limitations of evidence

The Current War on Trans Fat

Unfortunately, neither humility nor caution characterizes much of the current public health approach to diet. The campaign against trans fat is a case in point. Although there is good evidence linking dietary trans fat and CVD,29 the situation is analogous to that for dietary fat guidelines: (1) trans fat intake has been associated with CVD (which is already in decline), but not with obesity (which is an epidemic); (2) trans fats will be replaced by something (unclear what), and the net effect

Conclusion

Ironically, it now seems that the U.S. dietary guidelines recommending fat restriction might have worsened rather than helped the obesity epidemic and, by so doing, possibly laid the groundwork for a future increase in CVD. Unfortunately, when public health officials recognized this possibility, they did not cease issuing guidelines; they issued new ones. The new recommendation was: “choose a diet low in saturated fat and cholesterol, and a diet moderate in fat.”35 In addition, the food

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