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Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report (2022)

Chapter: 4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans

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Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
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4

Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans

INTRODUCTION

The second task of the Committee on Evaluating the Process of the Dietary Guidelines for Americans 2020–2025 was to respond to the question: Did the criteria for including scientific studies in the evidence base used to inform the 2020–2025 Dietary Guidelines for Americans (2020–2025 DGA) ensure that the evidence base was current with regard to publication date range, rigorous in study design, and generalizable or applicable with regard to the health status of study participants to public health nutrition guidance? (see Box 4-1). In the report Redesigning the Process for Establishing the Dietary Guidelines for Americans (2017 National Academies report), the Committee to Review the Process to Update the Dietary Guidelines for Americans (2017 committee) drew attention to the need for these guidelines to address the dietary needs of “the general public,” including the already high proportion of Americans with chronic diseases (NASEM, 2017).

To this end, the 2017 National Academies report proposed that a goal for the DGA should be to “Promote dietary intake that helps improve health and reduce the risk of chronic disease” (NASEM, 2017, p. 38). To achieve this goal, the 2017 National Academies report recognized the centrality of the systematic review process and the necessity to execute reviews within a framework. The 2017 National Academies report further noted that “The lack of an analytical framework guiding the overall structure of the evidence review makes it challenging to readily understand the overall approach and to identify gaps” (NASEM, 2017, p. 202). The

Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
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2017 committee regarded this deficiency to be “a major limitation of the current process, with respect to understanding the relationship between diet and chronic disease prevention” (NASEM, 2017, p. 202).

Key Considerations

In this chapter, the committee focused on the content of the systematic reviews that were conducted as part of the 2020–2025 DGA. The committee defined diet-related metabolic disease as those identified in the Scientific Report of the 2020 Dietary Guidelines Advisory Committee (2020 DGAC Scientific Report) (DGAC, 2020) as nutrition-related chronic health conditions. These include growth, size, and body composition, including overweight and obesity; metabolic syndrome; prediabetes and diabetes mellitus; reduced muscle strength and bone mass; gestational diabetes mellitus; chronic liver disease; cancer; cardiovascular disease; dental health; and food allergy. Using these definitions, 19 of the 34 systematic review questions in the 2020 DGAC Scientific Report were considered as specifically addressing diet-related metabolic diseases (see Appendix G). The committee acknowledges that other conditions in the systematic reviews used by the 2020 DGAC could be considered diet related (e.g., cognitive decline) (see Appendix G).

APPROACH TO TASK 2

The committee’s approach to task 2 is discussed in detail in Chapter 2. Briefly, task 2 assessed the systematic review processes used to identify the appropriate body of evidence. The committee also surveyed the systematic review practices from leading systematic review organizations

Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
×

(Agency for Healthcare Research and Quality [AHRQ, 2018], Cochrane Library [Higgins et al., 2021a], and Institute of Medicine report [IOM, 2011]) (see Chapter 3, Recommendation 4) to create a set of committee-identified practices to compare DGAC/Nutrition Evidence Systematic Review (NESR) practices (see Appendix E). To accomplish task 2, the committee developed and assessed the following questions:

  1. Committee-Identified Practices for Systematic Reviews:
    1. Did question development and systematic review protocols align with committee-identified practices for selecting the body of evidence?
    2. Was the search protocol peer reviewed prior to beginning the rest of the steps in the systematic review process in accord with committee-identified practices?
    3. When existing systematic reviews were used by DGAC, were systematic reviews updated to incorporate the full body of evidence in accord with committee-identified practices?
  2. Search Plan Inclusion and Exclusion Criteria:
    1. Was the overall NESR search strategy appropriate to achieve an evidence base that is generalizable and applicable for public health nutrition guidance?
    2. Was the search strategy appropriately tailored across those prepared for the 2020 DGAC?
    3. Was the date range of included research appropriate for all the systematic reviews prepared for the 2020 DGAC?
    4. Were the types of study designs specified appropriate for questions being answered across the systematic reviews prepared for the 2020 DGAC?

IDENTIFYING THE BODY OF EVIDENCE

Committee-Identified Practices for Systematic Reviews

Background

The practices for systematic reviews identified by the committee (Appendix E) include having a clear research question (population, intervention, control, outcome [PICO]), a descriptive analytical framework that illustrates the relationships between the research study data and outcomes of interest, and a clear description of the entire systematic review protocol. The systematic review protocol should include both a search plan to identify the evidence to be reviewed (including databases to be used, specific search terms, and inclusion/exclusion criteria based

Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
×

on the question and the analytical framework) and a description of data extraction methods with sufficient detail so the systematic review can be replicated. Data-extraction descriptions are important because inclusion and exclusion criteria may need to ensure availability of specific types of data (e.g., outcomes or confounding factors) that are critical for proper interpretation of the evidence. Data analysis plans would be included if quantitative analysis is planned (IOM, 2011).

The importance of external peer review has been addressed in Chapter 3 (recommendations 3 and 4). Having a robust systematic review protocol includes having an informative peer review, ideally by individuals external to the research team, who have expertise designing and conducting literature search strategies. The Cochrane Library methodology requires external review of both systematic review protocols and the final reviews themselves (Lasserson et al., 2021). The practice identified by the committee is that used by the Cochrane Library, which requires external review of both the systematic review protocols and final reviews (Lasserson et al., 2021). Peer-reviewed Cochrane systematic review protocols include detailed descriptions of the search strategy; outcomes (primary and secondary) assessed; the data analysis process; assessment of risk of bias; and application of Grading of Recommendations Assessment, Development and Evaluation (GRADE) methodologies (Lasserson et al., 2021). Any changes from published protocols, including the rationale for any changes, are published as part of the final review (Lasserson et al., 2021). If the PROSPERO registry for systematic review protocols (see Box 4-2) is used as recommended in the 2017 National Academies report, the authors submit an amendment to the protocol along with rationale when changes are made. Additionally, during the peer-review process, a description of the response to reviewer input would be normally maintained as part of the documentation.

Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
×

The 2017 National Academies report suggested that the procedure for using existing DGAC systematic reviews and updating them should incorporate advances in systematic review methodology and that this would require a multi-pronged approach. The report further stated that the use of existing systematic reviews could be facilitated by employing an established method for ongoing evidence surveillance to identify and evaluate the research as it is being published (NASEM, 2017). The committee-identified practices for systematic reviews (see Appendix E) also echoed the need to synthesize the combined evidence. As noted by Cumpston and Chandler (2021):

Adding new studies and new data can substantively change the findings of the review. Even where the new studies observe results consistent with the existing data, increasing the number of studies can improve precision-of-effect estimates, demonstrate wider applicability of the effect, or enable additional comparisons or subgroup analyses to be performed. The introduction of new review methods, such as updated risk-of-bias assessment tools or improved statistical analysis methods, can also change both the results and the certainty of the review’s findings.

The 2017 National Academies report also indicated that, prior to including existing systematic reviews, the timeliness and methodological quality should be evaluated and a determination made about what types of updates may be needed.

Question 1a: Did question development and systematic review protocols align with committee-identified practices for selecting the body of evidence?

Review of evidence

The majority of the committee-identified practices that affect the body of evidence are documented in the NESR systematic reviews and the 2020 DGAC Scientific Report (DGAC, 2020). Briefly, the topics were identified by U.S. Department of Agriculture (USDA) and U.S. Department of Health and Human Services (HHS) staff.1 The list was posted for public input and the combined list was prioritized for inclusion using the following criteria: relevance, importance, potential impact to federal programs, and avoiding duplication (DGA, n.d.). Subsequently, the DGAC refined the questions, established priorities for which questions would be answered, created an analytical framework to describe each question, reviewed and approved the search protocol, and approved any subsequent modifications of the search protocol (DGAC, 2020).

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1 This text was modified after release of a prepublication version of the report to the sponsor to correct that topics were identified by USDA and HHS rather than the Interagency Committee on Human Nutrition Research (ICHNR).

Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
×

The refined questions and analytical framework developed by the DGAC informed the search criteria, the databases searched, and the inclusion and exclusion criteria. In addition, the NESR librarian used the DGAC’s protocol to develop the overall search strategy for the 2020 DGAC Scientific Report a priori; this was tailored by the DGAC to each systematic review question and analytical framework (DGAC, 2020). The complete systematic review reports included

  • a question developed with the PICO framework,
  • a detailed analytical framework,
  • the final search strategy specifying databases,
  • the final list of search terms used for each database,
  • the flow diagram showing the results of the search (number of full texts screened, number of included and excluded studies at each step of the screening and selection process), and
  • the list of articles excluded with the reasons for exclusion.

Documentation of the original protocol, dates, and the rationale for subsequent modification of the protocol were posted on the Dietary Guidelines website over the course of the DGA process but were later removed (see Appendix B). Protocol modifications were not consistently described in the posted systematic review reports, though they were discussed in varying levels of detail in the 2020 DGAC Scientific Report (DGAC, 2020).

Findings

The committee’s review of the evidence on the NESR and DGAC systematic review process identified an omission of a data analysis plan in the a priori systematic review protocol (DGAC, 2020). The committee further found unclear documentation on when the systematic review protocol was “approved” and on the tracking of subsequent modifications (e.g., changes to the search, inclusion/exclusion, etc.). Therefore, the committee was not able to determine the effect of protocol changes on the identification and synthesis of evidence.

Question 1b: Was the search protocol peer reviewed prior to beginning the rest of the steps in the systematic review process in accord with committee-identified practices?

Review of evidence

For the process to develop the 2020–2025 DGA, NESR search strategies were peer reviewed by librarians from the National Institutes of Health and portions of protocols were posted on the Dietary Guidelines website for public comment (an improvement from previous DGA cycles) (DGAC, 2020). The portion of the protocol posted for public comment did not include search terms used for each database, but the

Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
×

protocols stated that the complete search strategies were available upon request and USDA and HHS indicated that only one request for search terms was received and processed (see Appendix B). In addition, USDA and HHS also included an additional peer review of the DGAC systematic reviews by other federal scientists (DGAC, 2020). When revisions were required based on peer-review comments, NESR staff reviewed and resolved some typographical or editorial issues and asked the DGAC to respond to comments that were more substantive in nature (DGAC, 2020). The list of peer reviewers is included in the 2020 DGAC Scientific Report (DGAC, 2020). The committee was unable to determine if the peer reviewers had expertise in both the content area of the review and systematic review methodology. Additionally, the tools used by peer reviewers (e.g., PRISMA) as well as the peer-review comments and responses to them are not publicly available (DGAC, 2020).

Findings

In its assessment of evidence, the committee found that NESR made a number of improvements over previous editions of the DGA. These included posting protocols and modifications to protocols for public comment, which improved transparency. However, it is unclear that what they did at the stage of a priori review of the systematic review protocol is equivalent to a peer review completed by reviewers with expertise in the content area of the review and systematic review methodology, which is the committee-identified practice. The committee was unable to determine if any of the comments from the new peer review after the systematic reviews were completed addressed the search plans. The committee was unable to compare the earlier protocol versions to the final version, and therefore any changes resulting from peer-review input that may have affected the body of evidence in the final systematic review cannot be fully assessed.

Question 1c: When existing systematic reviews were used by DGAC, were systematic reviews updated to incorporate the full body of evidence in accord with committee-identified practices?

Review of evidence

In its assessment of evidence for the design and conduct of systematic reviews during the process to develop the 2020–2025 DGA, the committee focused on reviews with implications for diet-related metabolic conditions. Data were extracted from the individual systematic review reports and separate analysis of those with implications for diet-related metabolic conditions was conducted (see Appendixes C and G). The committee found 11 systematic reviews used existing systematic reviews (see Appendix H). The 2020 DGAC Scientific Report indicated that 16 existing systematic reviews were used (DGAC, 2020). The 2017 National Academies report identified the need for an ongoing

Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
×

surveillance system to monitor and inform the need to update systematic reviews. USDA indicated that this is under consideration but has not yet begun (Stoody, 2019).

The committee identified three types of searches that were completed when using existing systematic reviews: no change (no new search), updated search of existing systematic reviews (same search strategy but extending the date from the last search to a current date with modifications of search terms appropriate to the database), or adding a new element to an existing systematic review search (e.g., adding a search term to a new search from the original to the current date) (see Appendix C, Figure C-1). The description of the updated systematic review searches indicated that some were carried out from the original search date (e.g., from 2000 to 2019/2020) while others began from the date of the last systematic review (e.g., from 2014 to 2019/2020). The systematic review report template did not have a standard location or terminology to be used to clearly describe if or how existing systematic reviews were used. In addition, documentation varied across the systematic reviews as well as between the systematic reviews and the 2020 DGAC Scientific Report (DGAC, 2020).

The description of the methodology for updating existing systematic reviews described the decision considerations for updating reviews, but it did not fully describe the decision criteria that were used to determine the types of updates that were referenced in the 2020 DGAC Scientific Report (DGAC, 2020). The considerations listed were date range, the DGAC’s knowledge of advancement in a particular field of research, conclusion statements assigned a grade of Limited or grade Not Assignable, and results from an evidence scan (DGAC 2020). The report described two options that were applied: (1) an assessment of new evidence as it related to existing conclusions or (2) a separate synthesis of new evidence (DGAC, 2020). The documented methodology also indicated that the synthesis took different forms (DGAC, 2020). However, it did not fully describe whether or how the studies from existing and new searches were combined and resynthesized. The risk-of-bias assessment for research studies included in an existing systematic review was not reassessed using current risk-of-bias tools.

Findings

The committee found that the process of establishing systematic review questions, the analytical framework, search strategy, and the examination of how closely the existing systematic review mirrored its current question were in accord with committee-identified practices (see Appendix E). However, the committee was unable to discern which systematic reviews included updates and which prior systematic reviews were updated for the cycle to develop the 2020–2025 DGA. This resulted from inconsistent and/or incomplete reporting of how existing systematic

Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
×

reviews were used in the final reports.2 In addition, the committee found that the current methodology for deciding if an existing systematic review needed a full update appeared to be based only on those conclusion statements that had been rated with lower grades of evidence (DGAC, 2020). Moreover, no example was provided of when or if a conclusion statement that had been previously rated as “strong evidence” could warrant an update based on a new understanding or the emergence of new research methodologies (DGAC, 2020; Obbagy, 2019; see Appendix E). The system for ongoing surveillance of published research recommended in the 2017 National Academies report, but not yet implemented, would provide invaluable input to decisions about the need to update existing systematic reviews. These deviations from committee-identified practices decrease the transparency and rigor of the decision-making process. Higgins et al. noted, a change in risk-of-bias ratings could be affected in either direction depending on differences in risk-of-bias tools (Higgins et al., 2021b). Previous NESR systematic reviews used the Nutrition Evidence Library (NEL) Bias Assessment Tool that applied specific items/questions from other accepted risk-of-bias tools specific to the type of research design being evaluated. The 2020 DGAC process used separate tools (e.g., Cochrane Risk of Bias 2.0, the ROBINS-I for non-randomized controlled trials [RCTs], and a modified ROBINS-I designed for observational studies) (DGAC, 2020). It is unclear whether the risk-of-bias rating of these tools has been compared to previous ratings from the NEL Bias Assessment Tool to determine whether these tools would affect risk-of-bias rating. Risk of bias is one of five elements in the NESR grading rubric used to determine the grades of evidence.3 If the conclusion grade was on the border between two grades (e.g., limited or moderate), a change in the risk of bias in a large enough number of studies could lead to a change to either a higher or lower grade.

Conclusions for Committee-Identified Practices

The committee concluded that the process to identify questions and create analytical frameworks was implemented, but the external peer review of systematic review protocols and use of existing systematic reviews was only partially implemented. The addition of posting portions of the search protocol for public comment enhanced transparency. The internal peer review of the search strategy and terms by a second NESR librarian enhanced rigor of the process. The addition of agency-level

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2 Final systematic reviews are available at https://nesr.usda.gov/2020-dietary-guidelines-advisory-committee-systematic-reviews (accessed December 1, 2021).

3 See https://nesr.usda.gov/sites/default/files/2019-03/Nutrition%20Evidence%20Systematic%20Review%20Grading%20Rubric.pdf (assessed December 1, 2021).

Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
×

peer review after the systematic review was completed also enhanced perceived integrity. However, the committee was unable to determine whether all these processes together would be equivalent to having an entire systematic review protocol externally reviewed by peer reviewers with expertise in the content area and systematic review methodology. The lack of consistent and thorough documentation of the protocol modifications that led to changes in the studies selected for review decreases transparency and increases the potential for bias. Similarly, the lack of decision guides for determining the need for and type of updates contributed to inconsistent documentation of how existing systematic reviews were updated. The absence of the ongoing surveillance system for published research recommended in the 2017 National Academies report makes it difficult to determine the need for and type of updates to be done. Taken together, these remaining obstacles compromise the integrity, transparency, and deliberative process, and leave open the possibility of introduction of bias into the NESR systematic review process.

Search Plan Inclusion and Exclusion Criteria

Background

The [DGAC] established inclusion and exclusion criteria to provide an objective, consistent, and transparent framework for determining which articles to include in each systematic review. These criteria were developed before any studies were reviewed to guide selection of the most relevant and appropriate body of evidence for each systematic review question. Additionally, these criteria were framed to increase the utility of the systematic review to inform U.S. federal policy and programs. (DGAC, 2020, Part C, p. 20)

The Cochrane Library methodology, one of the authoritative bodies the committee reviewed when collating its identified practices, specifies that predefining a study design and rationale should be a mandatory component of a systematic review protocol (McKenzie et al., 2021). It also specifies that protocols should also include a rationale that justifies the study design and considers both the appropriateness of the protocol for answering the question and the potential for bias (McKenzie et al., 2021).

It is an accepted principle in systematic reviews that there is a hierarchy of evidence that reflects the rigor of the research design to answer specific types of questions. Much of the scientific literature in nutrition that is evaluated to support the DGA process comes from observational studies (e.g., association of some aspect of dietary intake with a specific health outcome). These studies are valuable for many purposes but are not sufficient for causal inference. For systematic review questions that depend on establishing a cause-and-effect relationship, the highest-level,

Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
×

most rigorous research design is an RCT. The importance of specifying the highest level of scientific evidence for use in creating the DGA was acknowledged in the 2017 National Academies report (see Box 4-3).

Question 2a: Was the overall NESR search strategy appropriate to achieve an evidence base that is generalizable and applicable for public health nutrition guidance?

Review of evidence

The NESR overall search strategy included the standard inclusion and exclusion criteria (see Table 4-1; see Appendix C). All systematic reviews included studies that enrolled participants who were healthy and/or at risk for chronic disease, as well as studies that enrolled “some” participants diagnosed with a disease or classified with severe undernutrition or as having underweight, stunting, or obesity (DGAC, 2020) (see Appendix C, Table C-3). In other systematic reviews, the inclusion criteria were often more specific than “some” and included a percentage, such as 25 percent of individuals included. The overall search strategy further specifically excluded studies in which 100 percent of the study population had the endpoint of interest. For example, the

Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
×

systematic review on Dietary Patterns and Bone Health included studies that enrolled some participants diagnosed with low bone mineral density, low bone mineral content, osteoporosis, osteopenia, rickets, and fracture, but excluded studies that exclusively enrolled participants who had been diagnosed with osteoporosis, osteopenia, rickets, and fracture (i.e., studies that aimed to treat participants who had already been diagnosed with the outcome of interest) (DGAC and NESR, 2020a). The health status overall inclusion and exclusion criteria excluded most studies on secondary prevention interventions.

The exclusion of countries ranked as medium or low on the Human Development Index (HDI)4 resulted in excluding research from other countries with populations that are not primarily European in ethnic origin. For example, India, many Central American countries, and most African countries have a medium or low HDI. This issue was not always identified as a limitation of the source literature. For example, in the review titled Dietary Patterns and Growth, Size, Body Composition, and/or Risk of Overweight or Obesity: A Systematic Review for children, 10 of the 12 studies included were from countries outside of the United States (Australia, Portugal, the Netherlands, the United Kingdom) (DGAC and NESR, 2020b). However, this systematic review report indicated “Generalizability: The study participants, interventions and/or exposures, comparators, and outcomes examined in the body of evidence are applicable to the U.S. population” (DGAC and NESR, 2020b, p. 26).

Findings

The committee found that by excluding secondary-prevention interventions, the body of evidence may be limited as individuals with some specific health problems would be excluded from the systematic reviews. The committee also found that NESR’s overall systematic review criterion to include only countries with a high or very high HDI (DGAC, 2020) may emphasize white European ethnicities and those with higher incomes and underrepresent other ethnicities and those with lower incomes in the data considered in the systematic reviews. As a result, this may mean that the data considered in some of the systematic reviews may not be fully generalizable to the U.S. population. In many but not all cases,

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4 The Human Development Index (HDI) is used by the United Nations Development Programme to measure a country’s development. The HDI assesses three key dimensions of human development: life expectancy, access to education, and standard of living. The HDI produced by the United Nations is released annually. The United Nations release for the 2020 HDI includes ratings for 189 countries placed into four categories, very high (0.80–1.00), high (0.70–0.79), medium (0.55–0.70), and low (below 0.55). Other organizations may produce similar calculations from other data sources that can be placed into similar categories. See http://hdr.undp.org/en/content/latest-human-development-index-ranking (accessed December 9, 2021).

Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
×

TABLE 4-1 Inclusion and Exclusion Criteria Typically Used in the Dietary Guidelines Advisory Committee’s Nutrition Evidence Systematic Review Systematic Reviews

Inclusion and Exclusion Criteria
Study Design Included studies typically are RCTs (e.g., individual, cluster, and crossover trials), non-RCTs, PCSs, retrospective cohort studies, and nested case-control studies.
Excluded studies generally are uncontrolled trials, cross-sectional studies, and case-control studies.
Language Included studies published in English
Excluded studies published in languages other than English
Publication Status Included peer-reviewed studies
Excluded grey or unpublished literature
Health Status of Study Participants Included studies done with participants who are healthy and/or who are at risk for a chronic disease, including those with obesity
Included studies that enrolled some subjects with a disease
Excluded studies that exclusively enrolled participants with a disease or health outcome of interest, or who were hospitalized with an illness or injury (i.e., studies designed to medically treat individuals who already have the disease outcome of interest)
Country Included studies conducted in countries ranked as high or very high on the Human Development Index
Excluded studies conducted in countries ranked as medium or low

NOTE: PCS = prospective cohort study; RCT = randomized controlled trial.

SOURCE: DGAC, 2020.

statements about this limitation were included in the systematic review report as appropriate.5

Question 2b: Was the search strategy appropriately tailored across those prepared for the 2020 DGAC?6

Review of evidence

The committee’s assessment of the DGAC methodology indicated that the NESR analysts supported the DGAC’s work to establish the standard inclusion and exclusion criteria to the specific ques-

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5 Final systematic reviews are available at https://nesr.usda.gov/2020-dietary-guidelines-advisory-committee-systematic-reviews (accessed December 1, 2021).

6 Text in this section was modified after release of a prepublication version of the report to the sponsor to correct an error that was in the published systematic review on dietary patterns and cardiovascular disease posted on the NESR website. Based on that information, the report previously indicated that this systematic review excluded studies of populations if 100 percent of the participants already had been classified as having obesity. The information has been removed from this section. Related changes were made in Table C-3 in Appendix C.

Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
×

tion being answered (DGAC, 2020). The committee extracted the inclusion and exclusion criteria for the systematic reviews to evaluate if tailoring had occurred (see Appendix C). An example of how tailoring the overall search strategy was based on the question and analytical framework intermediate and endpoint outcomes is provided in Table 4-2.

Another form of tailoring used by NESR was the exclusion of populations in which 100 percent had a disease (DGAC, 2020). In the specific instance of obesity, this exclusion was generally lifted as long as the outcome of interest was not specifically related to weight (see Appendix C). A majority of the systematic reviews allowed the inclusion of study populations that included 100 percent of individuals with obesity (see Appendix C). However, in the case of cardiovascular disease, studies were not excluded if 100 percent of participants had a cardiovascular risk factor, such as hypertension or hyperlipidemia, and the goal of the study was to prevent cardiovascular disease rather than to treat the condition (see Appendix C). The committee did not find evidence of tailoring related to the study setting; all reviews included only studies conducted in settings with high or very high HDI (see Appendix C).

TABLE 4-2 Two Examples of Nutrition Evidence Systematic Review Overall Search Strategy: Health Status

Inclusion and Exclusion Criteria Questions
What is the relationship between dietary patterns consumed and risk of cardiovascular disease? (Children and Adults) What is the relationship between types of dietary fat consumed and risk of cardiovascular disease? (Children and Adults)
Intermediate Outcomes
  • Total Cholesterol
  • LDL Cholesterol
  • HDL Cholesterol
  • TC:HDL and LDL:HDL ratios
  • Triglycerides
  • Blood pressure
  • Total Cholesterol
  • LDL Cholesterol
  • HDL Cholesterol
  • TC:HDL and LDL:HDL ratios
  • Triglycerides
  • Blood pressure
Endpoint Outcomes
  • Cardiovascular disease (myocardial infarction, coronary heart disease, coronary artery disease, congestive heart failure, peripheral artery disease)
  • Stroke
  • Venous thrombosis
  • Cardiovascular disease-related mortality
  • Cardiovascular disease (myocardial infarction, coronary heart disease, coronary artery disease, congestive heart failure, peripheral artery disease)
  • Stroke
  • Venous thrombosis
  • Cardiovascular disease-related mortality
Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
×
Inclusion Criteria Tailored Inclusion Criteriaa
  • Healthy people and those with a range of diet-related chronic diseases, including obesity and type 2 diabetes
  • Included studies that comprise participants who are representative of the general public
  • Included studies done in participants who are healthy and/or who are at risk for a chronic disease
  • Included studies that enroll some subjects with a disease, including those with obesity, or with the health outcome of interest (intermediate or health outcomes)
No tailoring
  • Studies that enroll infants born full term (≥37 weeks and 0/7 days gestational age)b
  • Studies that exclusively enroll participants with high blood pressure or high cholesterol and are evaluating cardiovascular disease endpoint outcomesc
  • Studies that enroll some infants born preterm (gestational age <37 weeks and 0/7 days), infants with low birth weight (<2,500 grams), and/or infants born small for gestational ageb
Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
×
Exclusion Criteria Tailored Exclusion Criteriaa
  • Studies that exclusively enroll participants diagnosed with a disease, or hospitalized with an illness or injury
  • Exclude studies focused solely on people who already have a diet-related chronic disease and are being treated for that disease
  • Excluded studies that exclusively enrolled participants with a disease, who were hospitalized or who were diagnosed the health outcome of interest (i.e., studies designed to medically treat individuals who already have the disease outcome of interest)
  • Interventions designed to induce weight loss or treat overweight and obesity through energy restriction/hypocaloric diets for the purposes of treating additional or other medical conditionsb
  • For this criterion, studies that exclusively enroll participants with obesity will not be excludedc
  • Studies that exclusively enroll participants with high blood lipids (e.g., TC, LDL cholesterol, HDL cholesterol, or triglycerides) and are evaluating any blood lipid intermediate outcome (i.e., studies that aim to treat participants who already have high cholesterol or blood lipids)b

NOTE: HDL = high-density lipoprotein; LDL = low-density lipoprotein; TC = total cholesterol.

a Tailoring can include elaborating on how the NESR Overall Search Strategy: Health Status applies to a specific systematic review question OR it can make an exception to the NESR Overall Search Strategy: Health Status criteria.

b This tailoring represents an application of the NESR Overall Search Strategy: Health Status in this systematic review.

c This tailoring represent an exception to the NESR Overall Search Strategy: Health Status for this systematic review.

SOURCES: DGAC and NESR, 2020c,d.

Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
×
Findings

The committee found that the NESR search strategies were tailored to match some aspects of the questions and analytical framework (see Appendix C). However, the committee was unable to determine how much flexibility was allowed. The committee further found that how the general search strategy for addressing chronic diseases was tailored varied by search question (see Appendix C). The search strategy was often modified based on the aim of the study or population of interest for the specific search, although the committee was not able to find specific justification for these modifications. The committee noted that the modifications of the search strategy often supported the goal of including rather than excluding individuals with chronic health conditions or risk factors for such conditions.

Question 2c: Was the date range of included research appropriate for all the systematic reviews prepared for the 2020 DGAC?

Review of evidence

As summarized in Figures C-1 and C-2 (see Appendix C), the searches in support of the systematic reviews created during the 2020–2025 DGA were conducted for new and/or updated existing systematic reviews from June 2019 through February 2020.

Findings

The body of evidence was current up to the date of the systematic review search, in some cases including research published as recently as 6 months prior to the release of the 2020 DGAC Scientific Report in July 2020.

Question 2d: Were the types of study designs specified appropriate for questions being answered across the systematic reviews prepared for the 2020 DGAC?

Review of evidence

When the posted systematic review reports were assessed, the committee determined that NESR systematic review searches included the following study designs: RCTs, non-RCTs (including quasi-experimental and controlled before-and-after studies), prospective cohort studies, retrospective cohort studies, and nested case-control studies (see Appendix C, Table C-4). Some searches included other study designs that are typically considered to be rigorous (e.g., Mendelian randomization for the search on alcohol consumption and all-cause mortality) (see Appendix C). Others included less rigorous designs, such as the systematic review titled Folic Acid from Fortified Foods and/or Supplements During Pregnancy and Lactation and Health Outcomes, which included uncontrolled

Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
×

before-and-after studies and cross-sectional studies (DGAC and NESR, 2020e; see Appendix C).7

Findings

The committee’s review found that the NESR systematic reviews were based on the most rigorous study designs available. There were few exceptions and, in these cases, the additional types of research designs included were appropriate to the questions being addressed.

Conclusions for Search Plan Inclusion and Exclusion Criteria

With regard to inclusion of evidence that is rigorous, up to date, and applicable to public health nutrition, the committee concluded that the overall search strategy was well described and justified and was generally implemented as proposed in the 2017 National Academies report. To the question of whether the NESR systematic reviews were appropriately tailored, the committee concluded that, in many cases, tailoring was used to include populations with some chronic health conditions, such as obesity. However, these criteria also excluded individuals with chronic health conditions who participated in studies of secondary prevention. The inclusion criterion based on a country’s HDI was applied uniformly. However, use of this criterion may have created limitations in the evidence base by limiting the racial, ethnic, cultural, and socioeconomic diversity of included populations. The descriptions of the limitations in systematic reviews were not transparent about this because they did not always include information about the diversity of the included study populations.

To the questions of the appropriateness of systematic review date ranges and study design, the committee concluded that the appropriate date range was implemented and that the evidence base included in the systematic reviews was current. The committee also concluded that the appropriate study designs for the specific questions were implemented.

Thus, the committee concluded that the exclusion of prevention-intervention research and research from countries not classified with a high HDI may reduce the generalizability of the outcomes of the systematic reviews to the U.S. population.

___________________

7 This text was modified after release of a prepublication version of the report to the sponsor to clarify that there was more than one systematic review that contained less rigorous designs.

Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
×

OVERARCHING CONCLUSION

Overall, the committee concluded that the overall search strategy was well described and justified and was generally implemented as proposed in the 2017 National Academies report. The methodology for new systematic reviews was more carefully developed than the methodology for using existing systematic reviews. The absence of the recommended ongoing surveillance system for published research hampers the ability to determine the need for and the type of updates that will be required. Finally, the exclusion of prevention-intervention research and research from countries not classified with a high HDI may reduce the generalizability of the outcomes of the systematic reviews to the U.S. population.

REFERENCES

AHRQ (Agency for Healthcare Research and Quality). 2018. Nutritional research series. https://www.ahrq.gov/research/findings/evidence-based-reports/tr17-series.html (accessed December 8, 2021).

Ayorinde, A. A., I. Williams, R. Mannion, F. Song, M. Skrybant, R. J. Lilford, and Y. Chen. 2020. Assessment of publication bias and outcome reporting bias in systematic reviews of health services and delivery research: A metaepidemiological study. PLoS ONE 15(1):e0227580. https://doi.org/10.1371/journal.pone.0227580.

Chandler, J., M. Cumpston, H. J. Thomas, J. P. T. Higgins, J. J. Deeks, and M. Clarke. 2021. Chapter 1: Introduction. In Cochrane handbook for systematic reviews of interventions version 6.2, edited by T. J. Higgins, J. Chandler, M. Cumpston, T. Li, M. J. Page, and V. A. Welch. https://www.training.cochrane.org/handbook (accessed December 8, 2021).

Cumpston, M., and J. Chandler. 2021. Chapter IV: Updating a review. In Cochrane handbook for systematic reviews of interventions version 6.2, edited by T. J. Higgins, J. Chandler, M. Cumpston, T. Li, M. J. Page, and V. A. Welch. https://www.training.cochrane.org/handbook (accessed December 8, 2021).

DGA(Dietary Guiedlines for Americans). n.d. Topics and questions for review organized by topic. https://www.dietaryguidelines.gov/resources/about-process/process-identify-topics-and-questions/topics-and-questions-review-organized (accessed December 10, 2021).

DGAC (Dietary Guidelines Advisory Committee). 2015. Scientific report of the 2015 Dietary Guidelines Advisory Committee: Advisory report to the Secretary of Agriculture and the Secretary of Health and Human Services. Washington, DC: Agricultural Research Service, U.S. Department of Agriculture.

DGAC. 2020. Scientific report of the 2020 Dietary Guidelines Advisory Committee: Advisory report to the Secretary of Agriculture and the Secretary of Health and Human Services. Washington, DC: Agricultural Research Service, U.S. Department of Agriculture.

DGAC and NESR (Nutrition Evidence Systematic Review Team). 2020a. Dietary patterns and bone health: A systematic review. Alexandria, VA: Center for Nutrition Policy and Promotion.

DGAC and NESR. 2020b. Dietary patterns and growth, size, body composition, and/or risk of overweight or obesity: A systematic review. Alexandria, VA: Center for Nutrition Policy and Promotion.

DGAC and NESR. 2020c. Dietary patterns and risk of cardiovascular disease: A systematic review. Alexandria, VA: Center for Nutrition Policy and Promotion.

Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
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DGAC and NESR. 2020d. Types of dietary fat and cardiovascular disease: A systematic review. Alexandria, VA: Center for Nutrition Policy and Promotion.

DGAC and NESR. 2020e. Folic acid from fortified foods and/or supplements during pregnancy and lactation and health outcomes: A systematic review. Alexandria, VA: Center for Nutrition Policy and Promotion.

Higgins, J. P. T., J. Thomas, J. Chandler, M. Cumpston, T. Li, M. J. Page, and V. A. Welch. 2021a. Cochrane handbook for systematic reviews of interventions version 6.2. https://www.training.cochrane.org/handbook (accessed December 8, 2021).

Higgins, J., J. Savović, M. Page, R. Elbers, and J. Sterne. 2021b. Chapter 8: Assessing risk of bias in a randomized trial. In Cochrane handbook for systematic reviews of interventions version 6.2. www.training.cochrane.org/handbook (accessed December 8, 2021).

IOM (Institute of Medicine). 2011. Finding what works in health care: Standards for systematic reviews. Washington, DC: The National Academies Press.

Lasserson, T., J. Thomas, and J. P. T. Higgins. 2021. Chapter 1: Starting a review. In Cochrane handbook for systematic reviews of interventions version 6.2. https://www.training.cochrane.org/handbook (accessed December 8, 2021).

McKenzie, J. E, S. E. Brennan, R. E. Ryan, H. J. Thomson, R. V. Johnston, and J. Thomas. 2021. Chapter 3: Defining the criteria for including studies and how they will be grouped for the synthesis. In Cochrane handbook for systematic reviews of interventions version 6.2 (updated February 2021). https://www.training.cochrane.org/handbook (accessed December 8, 2021)

NASEM (National Academies of Sciences, Engineering, and Medicine). 2017. Redesigning the process for establishing the Dietary Guidelines for Americans. Washington, DC: The National Academies Press. https://doi.org/10.17226/24883.

Obbagy, J. 2019. Nutrition evidence systematic review. Presentation to the Dietary Guidelines Advisory Committee: Meeting 1, Washington, DC.

Stoody, E. 2019. Responding to the National Academies of Sciences, Engineering, and Medicine study on the process to update the Dietary Guidelines for Americans. Presentation to the 2020 Dietary Guidelines Advisory Committee. Washington, DC. March 28. https://www.dietaryguidelines.gov/sites/default/files/2019-05/Day%201%20Response%20to%20NASEM%20Study.pdf (accessed December 8, 2021).

Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
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Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
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Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
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Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
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Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
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Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
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Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
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Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
×
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Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
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Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
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Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
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Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
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Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
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Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
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Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
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Suggested Citation:"4 Analysis of the Scientific Studies Used to Develop the Dietary Guidelines for Americans." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluating the Process to Develop the Dietary Guidelines for Americans, 2020-2025: A Midcourse Report. Washington, DC: The National Academies Press. doi: 10.17226/26406.
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This midcourse report provides an initial assessment of how the process used to develop the Dietary Guidelines for Americans, 2020-2025 (DGA) compares to the recommendations in the 2017 National Academies report on redesigning the process for establishing the DGA. It also assesses the criteria and processes for including the scientific studies used to develop the guidelines. The scope of this study was to address the process and not the content of the guidelines.

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