Do dietary guidelines explain the obesity epidemic

Two decades ago, the movements known as evidence-based medicine and evidence-based public health arose out of concerns that the road to bad interventions is paved with good intentions, which sometimes result in more harm than good. Since then, both medicine and public health have critiqued scientific evidence more carefully and weighed benefits against harms before making recommendations to practitioners and the public. This analytic rigor is reflected in the work of groups such as the U.S. Preventive Services Task Force (USPTSF) and the Task Force on Community Preventive Services. Attention to the same concerns has long been evident in the development of dietary guidelines. For decades, researchers, clinicians, and policymakers have considered the potential harms, along with the benefits, of advocating changes in the diet. The need for a systematic review of these issues was explicit as long ago as the late 1980s, when the Surgeon General’s Report on Nutrition and Health and the National Research Council’s Diet and Health reports together devoted more than 1400 pages to meticulous analyses of the scientific evidence supporting each recommendation and to cautious, nuanced critiques of the quality and limitations of the data. Today’s readers enjoy access to an extensive literature regarding the potential harms of dietary modifications, of dietary guidelines, and of practice guidelines in general. Science is not the only consideration in developing dietary guidelines, however. Government agencies and the constituents they serve invariably bring an agenda to statements of nutrition advice. Dietary guidelines have implications at every level of government, from federal agencies such as the U.S. Department of Agriculture (USDA) to the local school board. In the private sector, the food industry, ranchers, restaurateurs, and beverage producers—along with their lobbyists—have famously exerted pressure to eliminate or soften language in the guidelines that might harm commercial interests. Accommodating the realities of public acceptance has also been a factor, as when the best advice based on science was considered too unrealistic for widespread adoption. The critical appraisal of nutrition research is inherently challenging, perhaps moreso than in other areas of public health or medicine. The time needed to measure meaningful health outcomes is so long that studies often must rely on questionable surrogate measures. Almost every study is complicated by the enigma of whether foods, some combination of their nutrients, or interactions between foods and other covariates are responsible for observed outcomes. Randomized trials are challenging, not least because of the inherent difficulty of controlling and measuring subjects’ diets and the prohibitive costs and logistic challenges of measuring long-term outcomes. Interventions and outcomes in diet research may be nebulously defined. Even the best dietary trials lack the crisp adherence and evidentiary strength of gold standard efficacy trials of drugs and medical devices. Nutrition research has had its share of “flip-flops” in which dietary practices and nutrients that were supposedly healthful were ultimately shown to offer little benefit or to cause harm (e.g., beta-carotene). Given such difficulties, whether the evidence is good enough to recommend population-based dietary changes comes down to a matter of subjective judgment. Despite these challenges and the enduring pressure of special interests, what is most striking is that for nearly half a century almost every authoritative government or professional committee that has reviewed research on diet and chronic disease ultimately has arrived at the same basic dietary advice: eat less; move more; eat more fruits, vegetables, and whole grains; and avoid junk food. One need not know the data to infer from the consistency itself and from the number and quality of expert panels over these years that the scientific evidence must be compelling. Against this backdrop appears an article in this issue of the American Journal of Preventive Medicine, in which Marantz et al. discuss the potential harm of dietary guidelines in general and of advice about dietary fat in particular. Although the authors give little attribution to the extensive prior writing on this topic, they do invoke many of the themes we have just mentioned and with which we agree. We strongly disagree, however, 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. Department of Family Medicine, Virginia Commonwealth University (Woolf), Richmond, Virginia; Department of Nutrition, Food Studies, and Public Health, New York University (Nestle), New York, New York Address correspondence and reprint requests to: Steven H. Woolf, MD, MPH, Department of Family Medicine, Virginia Commonwealth University, West Hospital, 1200 East Broad Street, P.O. Box 980251, Richmond VA 23298-0251. E-mail: swoolf@vcu.edu.