Abstract/Summary

Population-wide dietary guidance for the prevention of chronic diseases was first introduced in the United States in 1961, by the American Heart Association, followed by the U.S. government in 1980. Today, more than 100 countries world-wide have such guidelines, most following the lead of the U.S. in terms of the basic recommendations to emphasize a diet based on fruits, vegetables, whole grains, legumes, nuts and seeds, with limited amounts of lean meat and low-fat dairy. For added fats, vegetable oils have been preferred over animal-based sources, and for many years, most nutrition recommendations favored a diet low in total fat overall. These guidelines were initially issued with the aim of preventing cardiovascular disease and cancer; Later, they were also promoted as the best possible approach to prevent obesity and type 2 diabetes. The dilemma with this now-global nutritional advice is that it cannot be said to be “evidence based.” When first launched by the U.S., these recommendations were not based on strong scientific evidence that such a diet could prevent any type of chronic disease, i.e., there was a lack of randomized controlled clinical trials (RCTs) to demonstrate the purported benefits of this nutritional approach for either adults or children. Moreover, there was significant evidence to the contrary. Specifically, in 1980, when the U.S. guidelines were first issued, there existed a large body of clinical-trial data demonstrating that the recommendation to replace saturated fats with polyunsaturated vegetable oils would not, in fact, have the desired effect on heart disease prevention. In addition, the available clinical trial and observational data in 1980 found that lower consumption of dietary cholesterol was not linked to reduced cardiovascular mortality. These data on both saturated fats and cholesterol were either not considered or dismissed as unconvincing by experts when the U.S. guidelines were originally formulated. The low-fat diet, another longtime key pillar of population-wide dietary guidelines was not properly tested in large clinical trials until the early 1990s, more than ten years after this advice had already been enshrined as official policy. The clinical trial results suggested that the diet had virtually no effect on the prevention of obesity, diabetes, heart disease or the several types of cancer measured. However, these results have also been repeatedly dismissed by nutrition experts. Subsequent dietary advice to follow a Mediterranean diet was adopted almost entirely on the basis of a single large clinical trial in Spain, yet this study was subsequently retracted and reissued due to problems with randomization. A common feature of all these dietary recommendations is the lack of sufficient rigorous, clinical trial evidence to support their implementation as population-wide guidance and in some cases, the existence of significant bodies of contrary data that were –and continue to be—ignored. This thesis traces the history of each of these recommendations and offers some explanations as to why authorities and experts have been unresponsive to the scientific evidence. In contrast to dietary recommendations that have remained intractable despite weak or contrary evidence, the dietary guidance on industrially produced trans fatty acids did change in response to evidence of potential health harms that emerged in the early 2000s. The fact that U.S. authorities reacted with relative alacrity to impose limits and ultimately ban the use of these fatty acids in the food supply can likely be explained by the absence of an existing recommendation on trans fatty acids, which therefore avoided any challenge to an existing status quo. However, the guidance on trans fatty acids remains problematic in Europe and at the World Health Organization, where guidelines have erroneously included trans fatty acids from ruminant, animal-derived foods in their overall intake limits. In conclusion, U.S. population-wide dietary guidance as well as similar such advice dispensed by nations world-wide lacks a rigorous scientific foundation and therefore cannot be called “evidence based,” according to international standards in the field. This observation must be seen in the context of ever-rising rates of diet-related diseases, which national nutritional guidelines have self-evidently failed to prevent. A policy based on weak evidence, with failed outcomes, necessarily requires reevaluation of its foundational scientific evidence. Dietary guidelines are the most important levers that nations can employ to direct populations towards solving the root cause of obesity, diabetes, and other chronic diseases. The need for these policies to provide evidence-based dietary guidance that can successfully prevent and reverse these diseases is paramount.