1

Introduction

Obesity has come to the forefront of the American public health agenda. The increased attention has led to a growing interest in quantifying obesity prevalence and determining how the prevalence has changed over time. Consequently, a wide range of reports have been published that present estimates of prevalence and trends in obesity for various population groups throughout the United States. The data sources from which such estimates are derived range from local initiatives to national surveillance programs, differing from each other in terms of resources, intent, funding, and approach. Differences extend into the analyses, which are driven by the specific question(s) being asked, the quantity and quality of the data, and the analytic capabilities of the investigators.

Challenges exist in the collection and analysis of data across all population groups and in subgroups of the population. Many are rooted in epidemiological and statistical principles, and are independent of the age of the participants. Evaluations inclusive of children, however, have some distinct considerations. Body composition, for instance, changes as a child grows and maturates, which affects the way obesity status must be classified. It can be difficult to obtain accurate measurements on young children, an age group in which small differences can change obesity status classification. For many studies, data collection often must include gathering information, in part or entirely, from the child’s parent or guardian. Although far from exhaustive, these considerations must be factored into the study design, the data collection procedures, and the analytic approach. How investigators overcome methodological challenges ultimately informs the interpretation of estimates.

Accurate and meaningful estimates of obesity prevalence and trends are fundamental to understanding and describing the scope of the issue. Policy makers, program planners, and other stakeholders at the national, state, and local levels are among those who search for estimates relevant to their population(s) of interest to inform their decision making. The differences in the collection, analysis, and interpretation of data have given rise to a body of evidence that is inconsistent and has created barriers to applying published reports (see examples in Box 1-1). As such, there is a need to provide guidance to those who seek to better understand and use estimates of obesity prevalence and trends.

STUDY CHARGE, APPROACH, AND SCOPE

Study Charge

To better understand, assess, and apply the current literature and to consider strategies for future research, the Robert Wood Johnson Foundation (RWJF) asked the National Academies of Sciences, Engineering, and

Medicine to convene an expert committee to examine the approaches to data collection, analysis, and interpretation that have been used in recent reports on obesity prevalence and trends at the local, state, and national levels, particularly among children, adolescents, and young adults (see Box 1-2).

Study Approach

The committee was comprised of 12 members with expertise in public health nutrition, epidemiology, pediatrics, public policy, health disparities, obesity prevention and treatment, statistics and biostatistics, health assessment, and data collection and analysis methodologies (see Appendix E for biographies of the committee members).

The committee performed a comprehensive review and assessment of sources directly relevant to its task. To be inclusive, the committee considered a wide range of materials from the peer-reviewed literature,

along with publicly available national, state, and local research and surveillance sources. The review of the evidence allowed the committee to broadly examine the landscape of the collection of data and the reporting of results related to obesity prevalence and trends. In addition to reviewing the literature, the committee held a public workshop that included the perspectives of investigators who collect and analyze obesity prevalence and trends data, along with stakeholders who rely on reports of such analyses to inform decision making (see Appendix B for the workshop agenda). The committee also considered public comments received through an online submission system. From these activities, the committee developed a framework for assessing and interpreting reports on obesity prevalence and trends and recommendations for evaluating published reports, and filling data gaps improving future data collection efforts.

Scope

This report delineates the current practices, challenges, and considerations related to data collection and analysis that ultimately affect the interpretation of estimates of obesity prevalence and trends across population groups, with a focus on children, adolescents, and young adults. Study design and data collection options are described individually and as they exist in a range of common data sources. Insight is offered into the methodological approaches and analytical procedures that lead to subgroup estimates and comparisons. Concepts related to subgroup differences are discussed broadly and in the context of health disparities, a type of difference that exists due to social disadvantage. Because interpretation of estimates requires the assessment of elements both narrow and broad in scope, the committee explores analytic approaches specific to the assessment of obesity and statistical considerations generally applicable to any analyses of prevalence and trends. To synthesize the landscape of the literature and provide guidance on how to assess reports for decision making, the committee offers a conceptual framework. Finally, by identifying the inconsistencies that exist and exploring why they exist, the evidence presented throughout this report not only offers insight into the present state of data collection efforts but also highlights opportunities for improvement.

Although the committee’s charge is circumscribed, the task encompasses tremendous complexity. To evaluate the evidence, the committee defined the following elements of its task:

• Types of Recent Reports: Because the literature on obesity is expansive and estimates reside in a variety of publication types, the committee chose to draw its evidence regarding the current practices of “reports” primarily from peer-reviewed research and academic

• journals.¹ In considering population-based estimates of obesity prevalence and trends, the committee limited the reports to those that were observational in design, rather than interventional. The committee, however, has made a statement on the role of intervention studies in the assessment of obesity prevalence (see Box 3-1). Although this evidence base misses some relevant publications, such as those produced at the state and local levels, the committee considered the impact of their omission to be minimal for several reasons. First, the reports the committee evaluated encompass a range of locations, geographic regions, and jurisdictions. Second, some reports describing obesity at the local and state levels are summaries of analyses conducted on data collected through national surveillance systems, such as the Youth Risk Behavior Surveillance System (YRBSS) or Pediatric Nutrition Surveillance System (PedNSS). These summaries often present the findings, but may not include the details of the approaches used for data collection or analysis (see Box 1-1). Reports containing the primary analyses of such data sources were included in the committee’s evidence base. Because the intent of this report was not to be exhaustive, the committee considered the published reports it evaluated illustrative of the various methodological and analytic approaches that exist in peer-reviewed literature and beyond, but recognizes throughout that other options and approaches exist. A detailed description of the selection of published reports can be found in Appendix C, while summary tables of key characteristics of the published reports can be found in Appendix D.

• Focus on Children, Adolescents, and Young Adults: Consistent with its charge, the committee primarily evaluated data collection and analysis methodologies in published reports that included population groups ages 18 years and younger. Brief discussions about how specific methodologies differ for the assessment of children and adults have been incorporated throughout the report, but are not a main focus. In considering assessment of the weight status of children, the age group birth to 2 years posed a challenge. As will be further explained in Chapter 2 (see Box 2-1), current recommendations and practices in the published literature do not support the use of the term “obesity” or the assessment of body mass index (BMI) for the purposes of weight status classification for this age group. Because this committee was tasked with evaluating

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¹ As described in Appendix C, most, but not all, of the reports were published in peer-reviewed journals. For brevity, the reports that served as the evidence base will be referred to as “published” throughout this report.

The intended audience of this report includes a wide range of policy makers, program planners, and other stakeholders who seek to better understand and appropriately use estimates of obesity prevalence and trends. This includes organizations, groups, and individuals with diverse resources, skill-sets, and expertise. Because the audience is broad, it is expected that levels of comfort with reading and interpreting published reports will range the gamut. This report serves as common ground, bringing together evidence and key concepts that are intended to be widely applicable.

Many important topics are inherently linked to the content of this report, but are beyond the scope of the committee’s task. This report serves as an appraisal of the landscape of the literature, and is intended to facilitate the interpretation and application of estimates presented in published reports. Although recommendations for advancing the field are offered, the committee has not been charged with developing a prescriptive set of best practices for data collection, analysis, and reporting or identifying what resources would be required to implement changes. Furthermore, this report is not intended to provide an overview of the etiology of obesity, comprehensively evaluate its association with chronic disease, or provide guidance on effective initiatives, interventions, policies, or other solutions. Concepts related to health disparities are included, but the committee has not been charged with examining the breadth of obesity disparities or explaining how or why such disparities exist. Readers who are interested in these topics should refer to the section in this chapter on Notable Past Work for additional resources.

DEFINITIONS OF KEY TERMS

This section provides definitions of key terms that will be used throughout this report. A full glossary can be found in Appendix A.

Obesity

Obesity describes a state of excess adiposity. Although various approaches exist for assessing adiposity and describing obesity (see Chapter 2), published reports on prevalence and trends most frequently use BMI. For adults, a BMI

≥30 kg/m² is considered obese. For children, adolescents, and young adults, a BMI must be compared to a BMI-for-age distribution seen in a reference population and a cut point must be used for classification (see Chapter 2 and Chapter 5). The prevailing cut point and reference population used in the United States is the 95th percentile on the 2000 Centers for Disease Control and Prevention (CDC) sex-specific BMI-for-age growth charts. As will be discussed in Chapters 2 and 5, the 95th percentile describes the distribution of the population from which the growth charts were derived, not the population being evaluated. The prevalence of obesity within a sample different than the reference population may be less than or greater than 5 percent. Unless otherwise noted, the term “obesity” with respect to adults will refer to a BMI ≥30 kg/m² and with respect to children, adolescents, and young adults will refer to ≥95th percentile on the CDC BMI-for-age growth charts.

Published Reports

Unless otherwise noted, the term “published report” describes a publication, peer-reviewed or otherwise, with original analysis that produces an estimate of obesity prevalence or trend. Some publications are summaries of primary analyses and may not contain details needed to adequately assess the findings. In these instances, the reader is directed to the primary source of the statistic, if available.

Estimate of Obesity Prevalence or Trend

The committee uses the phrase “estimates of obesity prevalence or trend” or simply “estimate” to describe a statistic about the proportion or number of individuals affected with obesity at one point in time (prevalence) or over time (trend).

Social Disadvantage and Health Disparities

Social disadvantage is a complex, multidimensional construct. A host of factors, including but not limited to race, ethnicity, socioeconomic status, age, gender, geography, religion, and sexual orientation create social disadvantage. Many of the underlying factors have both biological and social components (e.g., gender, race). Others are socially constructed (e.g., state boundaries). Social disadvantage is of particular concern in relation to obesity because disadvantage can create health disparities. The committee uses the phrase “health disparities” to describe a health difference caused by the systematic marginalization, discrimination, or vulnerabilities that could be avoided through different, more equitable circumstances (Braveman et al., 2011).

End Users

The committee uses the term “end user” to describe individuals, groups, or organizations who use reports on obesity prevalence and trends to inform a decision.

NOTABLE PAST WORK

This report is the first consensus study conducted by the National Academies of Sciences, Engineering, and Medicine to examine the effects of data collection methodologies and analytic approaches on interpreting estimates of obesity prevalence and trends. In addition to this report, a brief summary of highlights from the committee’s public workshop has been published (IOM, 2015b).

The Academies have previously convened and continue to engage in activities exploring various aspects of obesity. Two earlier reports that informed the committee’s work were Evaluating Obesity Prevention Efforts: A Plan for Measuring Progress (IOM, 2013) and Bridging the Evidence Gap in Obesity Prevention: A Framework to Inform Decision Making (IOM, 2010). The Academies also have recently evaluated policy-related strategies to preventing obesity (Early Childhood Obesity Prevention Policies [IOM, 2011a]; Legal Strategies in Childhood Obesity Prevention: Workshop Summary [IOM, 2011b]; Local Government Actions to Prevent Childhood Obesity [IOM et al., 2009]), assessed prevention efforts (Cross-Sector Responses to Obesity: Models for Change: Workshop Summary [IOM, 2015a]; The Current State of Obesity Solutions in the United States: Workshop Summary [IOM, 2014]; Accelerating Progress in Obesity Prevention: Solving the Weight of the Nation [IOM, 2012]), and explored etiology of obesity related to fetal and early life exposures (Examining a Developmental Approach to Childhood Obesity: The Fetal and Early Childhood Years: Workshop Summary [IOM, 2015c]).

In addition to this previous work by the Academies, the committee was informed by stakeholder perspectives, peer-review reports on obesity prevalence and trends, and data collection protocol manuals. This report reviews this evidence in the following chapters.

ORGANIZATION OF THE REPORT

This introductory chapter has provided background information related to the issues that led to the development of this study and the approach the committee took to address the Statement of Task. Chapter 2 provides additional contextual information related to the definition of obesity, obesity across the lifespan, and differences in obesity status by demographic char-

acteristics. Chapter 3 discusses methodological considerations related to the design and execution of data collection. Chapter 4 describes and compares a range of common data sources and identifies existing gaps. Chapter 5 discusses the various analytic approaches that have been used in recent reports and describes interpretive considerations associated with statistical analyses. Chapter 6 presents a conceptual framework for interpreting and determining the utility of reports on obesity prevalence and trends. Chapter 7 presents the committee’s findings, conclusions, and recommendations. Finally, Chapter 8 discusses innovations on the horizon for the field.

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