Special articleIn the face of contradictory evidence: Report of the Dietary Guidelines for Americans Committee
Introduction
What is required is less advice and more information.
—Gerald M. Reaven [1]
There is little disagreement that we have a nutritional crisis in the United States. One manifestation is confusion in the mind of the public as to what constitutes sound principles [2], [3]. Recent scientific advances have not led to consensus, but rather to substantial disagreement among experts and further uncertainty for the public. Nutritional health covers a wide range of concerns but foremost in the mind of the public are whether the standing recommendations for lowering fat intake and increasing carbohydrate intake were ever appropriate for the prevention of obesity, diabetes, and cardiovascular disease; whether the regulation of carbohydrates is more important; and what the role of protein, especially from animal sources, should be in the diet. These concerns were raised with the first national dietary recommendations 30 y ago and have yet to be adequately addressed even as the nutritional health of Americans continues to decline.
The 2010 Dietary Guidelines Advisory Committee (DGAC) Report [4], released on June 15, 2010, was expected to address these issues (sections of the report are indicated as part-section number, e.g., B2; pages in the report are denoted, e.g., B2-3.). The DGAC Report had the opportunity to review and evaluate the emerging science, to distinguish between established principles and ideas that are still areas of research or controversy, and to provide clear, consistent information for Americans. Instead, the 2010 DGAC Report continues to make one-size-fits-all recommendations that are based on evidence that is weak, fragmented, and even contradictory in nature.
Section snippets
Strong recommendations, weak evidence
Important aspects of the recommendations remain unproven. The DGAC Report provides several examples in the summary of “Needs for Future Research” in each section. In the carbohydrates section, a goal of that research would be to:
Develop and validate carbohydrate assessment methods. Explore and validate new and emerging biomarkers to elucidate alternative mechanisms and explanations for observed effects of carbohydrates on health [p. D5-43 [4]]. … Studies of carbohydrates and health outcomes on
Macronutrient proportion and health outcomes
A consistent theme in the 2010 DGAC Report is the statement that “very few American children or adults currently follow the US Dietary Guidelines” (p. D1-8) and that “the primary focus should be on reducing excessive calorie intake” (p. B2-3). However, according to the DGAC Report, caloric intake remains within recommended levels, and leisure-time physical activity has increased slightly (pp. D1-1, B2-3). Adult women on average consume at the lowest end of the recommended calorie range and yet
Macronutrients: Research questions are formulated in a way that prevents a thorough investigation of the literature
The 2005 Institute of Medicine Macronutrient Report states, “Compared to higher-fat diets, low-fat, high-carbohydrate diets may modify the metabolic profile in ways that are considered to be unfavorable with respect to chronic diseases such as coronary heart disease (CHD) and diabetes” [7]. The DGAC Report precludes the evaluation of the potential impact of high dietary carbohydrate on chronic disease by the way that research questions are formulated. The “Search Plan and Results” for the
Macronutrients and weight loss: Science is inaccurately summarized
Obesity and weight control are reasonably a major focus of the guidelines. However, the DGAC Report is hampered in its assessment of this issue by the common but overly simplified concept of weight loss as only a function of “calories in” versus “calories out.” In its answer to the question, “What is the relationship between macronutrient proportion and body weight in adults?” the DGAC Report concludes that:
There is strong and consistent evidence that when calorie intake is controlled,
Low-carbohydrate diets: Science is inaccurately represented
Low-carbohydrate diets are not recommended because they are “difficult to maintain over the long term.” Table 1 presents data from studies and meta-analyses included in the NEL showing that attrition rates are, if anything, lower for low-carbohydrate diets compared with low-fat diets [25], [26], [27], [28], [29], [30], [31]. An appropriate summary on adherence would state this. The DGAC Report suggests that the diet recommended in the current guidelines is difficult to follow (pp. D1-8, B3-4);
Low-carbohydrate diets: Conclusions do not reflect quantity and/or quality of relevant science
The DGAC Report’s conclusions also maintain that diets that stray from the recommended guidelines “may be less safe,” a claim made about low-carbohydrate diets for 40 y without supporting data. The DGAC Report relies on two studies that “found that diets lower in carbohydrate and higher in protein were associated with increased total and cardiovascular mortality” [32], [33] (p. D1-49). Both studies have considerable flaws. In Lagiou et al. [32], which the NEL gives a “neutral” quality rating,
Dietary fat and health outcomes
Prevention of chronic disease by manipulation of dietary fat and carbohydrate is a primary focus of the DGAC Report, although, as described above, evidence on the effects of macronutrient proportion on diseases is excluded. The section, “The Influence of Dietary Fats on Cardiovascular Disease (CVD) And Other Health Outcomes,” asks the question, “What is the effect of saturated fat intake on increased risk of cardiovascular disease or type 2 diabetes [T2D], including effects on intermediate
Effects of saturated fat: Answers based on an incomplete body of relevant science
The DGAC Report concludes that:
Strong evidence indicates that intake of dietary SFA [saturated fatty acids] is positively associated with intermediate markers and end point health outcomes for two distinct metabolic pathways: 1) increased serum total and LDL [low-density lipoprotein] cholesterol and increased risk of CVD and 2) increased markers of insulin resistance and increased risk of T2D. Conversely, decreased SFA intake improves measures of both CVD and T2D risk. The evidence shows that 5
Effects of saturated fat: Science is inaccurately represented or summarized
The conclusion of the DGAC Report suggests that the replacement of SFA with monounsaturated fatty acids or PUFA creates unequivocally positive cardiovascular risk factor outcomes; this is not the case. Studies cited by the DGAC Report demonstrate increases in atherogenic lipoprotein levels or triacylglycerols, decreases in high-density lipoprotein cholesterol, and varied metabolic responses to lowered dietary SFA in subpopulations [45], [46], [47], [48]. These controversies and uncertainties
Diabetes and fat: Science is inaccurately represented or summarized
With regard to diabetes, the DGAC Report concludes that:
The growing data to support a risk of T2D from SFA consumption supports the need for fat-modified diets in persons with pre-diabetes, including those with metabolic syndrome, and those with established diabetes [p. D3-15].
This statement shows the same disregard for physiologic mechanisms as before: all effects of saturated fat are measured in the presence of recommended (high) levels of carbohydrate intake. Because digestible dietary
Dietary carbohydrate and health outcomes
The data in Figure 1 show that the increase in calories during the previous 30 y is almost entirely due to carbohydrate. The effectiveness of carbohydrate restriction for weight loss and improved markers of chronic disease when compared in head-to-head trials with low-fat diets continues to be newsworthy. With the established biochemistry of the glucose–insulin axis, dietary carbohydrate is a topic of increasing relevance to a public battling obesity and diabetes.
The DGAC Report describes it
Dietary fiber and whole grains: Conclusions do not reflect the quantity and/or quality of science
The section on carbohydrate begins its evaluation of dietary carbohydrates with the question, “What are the health benefits of fiber?,” a question that presumes that health benefits have already been established. In fact, evidence supporting the health benefits of fiber with regard to obesity, diabetes, and bowel health is limited, as acknowledged in the American Dietetics Association (ADA) position paper on which much of the fiber information in the DGAC Report is based [63].
The ADA position
Glycemic load/index: Answers based on an incomplete body of relevant science
Based on the same physiologic principle as total carbohydrate decreases, the glycemic index (GI) and the glycemic load (GL) were designed to measure the impact of food on blood glucose levels. In practice, the GI/GL cannot always be reproduced consistently from person to person or even in the same person at different times. Glycemic impact can vary with a food’s ripeness, physical form, preparation, and foods with which it is consumed; research discussing the limitations of the GI/GL is not
Dietary protein and health outcomes
For the first time, the DGAC Report makes the case for protein as the most important macronutrient in the diet (pp. D4-1, 3). The DGAC Report establishes the role of protein and protein quality in human health, discussing the essentiality of nine amino acids, which must be obtained from the diet, and the conditional essentiality of another six, pointing out that a dietary source of these amino acids may also be necessary (p. D4-1). The DGAC Report concludes that:
Protein quality varies greatly
Animal versus plant protein: Recommendations do not reflect limitations and uncertainties of the science
The DGAC Report explores a possible relation between the intake of animal protein products and CHD, CVD, T2D, and prostate, colorectal, and breast cancers. The DGAC Report concludes that 1) moderate evidence found no clear association between intake of animal protein products and blood pressure, 2) limited inconsistent evidence suggests that mainly processed meat may have a link to T2D, 3) there is insufficient evidence to link animal protein to body weight, 4) there are inconsistent positive
Salt: Recommendations do not reflect limitations and uncertainties of the science
There is probably no more telling example of the limitations of the DGAC Report than the recommendations on salt. The DGAC Report states that a “strong body of evidence has documented that in adults, as sodium intake decreases, so does blood pressure” (p. D6-2). Strong evidence is what is needed to make dietary recommendations but that evidence is lacking. A Cochrane review, for example, concluded that “intensive interventions, unsuited to primary care or population prevention programs, provide
Summary: What can be done?
Is there nothing of value in the DGAC Report? On the contrary, there are valuable suggestions made regarding improving nutritional literacy and cooking skills; restructuring the food environment, including farmers, agricultural producers, and food manufacturers; and improving the availability of affordable fresh produce. However, none of these recommendations makes sense in the context of nutritional guidance that is not based on sound scientific principles and demonstrable results. Reforming
Acknowledgments
The authors are grateful to Dr. Alan Titchenal and Dr. Joannie Dobbs of the University of Hawaii for valuable discussion and suggestions.
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