NHANES needs urgent attention to ensure its future, which is facing emerging challenges associated with data collection, stagnant funding that has undercut innovation, and the increased call for granular data for subpopulations and groups at risk. The concerns do not rest merely on securing more funding but focus on the need for a constructive review of the survey to explore new approaches and identify appropriate change. This white paper, developed under the auspices of the ASN's Committee on Advocacy and Science Policy (CASP), is a call to the nutrition community to advocate for and support activities to prepare NHANES for future success in a changing nutrition world. Furthermore, because NHANES is much more than a nutrition survey and serves the needs of many in health fields and even commercial arenas, effective advocacy must be grounded in alliances among the survey's diverse stakeholders so that the full range of expertise and interests can engage. This article highlights the complicated nature of the survey along with key overarching challenges to underscore the importance of a measured, thoughtful, comprehensive, and collaborative approach to considering the future of NHANES. Starting-point questions are identified for the purposes of focusing dialog, discussion forums, and research. In particular, the CASP calls for a National Academies of Sciences, Engi-neering, and Medicine study on NHANES to articulate an actionable framework for NHANES going forward. With a well-informed and integrated set of goals and recommendations that could be provided by such a study, a secure future for NHANES is more readily achievable.

A recent report adds overwhelming weight to the imperative to reduce the amount of sodium in the U.S. food supply. Collaborations between the food industry and government agencies could help achieve this goal.

Accurate data for vitamin D in foods are essential to support epidemiological and clinical studies seeking to identify associations between total vitamin D “exposure” and health outcomes that require quantification of dietary intake and also to inform health professionals about wise food choices for clients. Challenges in developing reliable vitamin D food composition data encompass the existence of multiple forms of the vitamin including its hydroxylated metabolite 25(OH)D, variation among samples of the same food because of animal diet, ultraviolet exposure of provitamin D sterol-containing foods, and validation of analytical methods. Decisions made during development of vitamin D food composition databases also affect the representativeness of mean values used to calculate dietary intake in a specific context. These challenges are discussed, along with the potential impact on estimation of vitamin D intake from foods. Data for naturally occurring vitamin D3 and 25(OH)D3 in animal products are summarized.

Concerns about vitamin D deficiency and recommendations for vitamin D supplements have become very common in clinical practice in the past few years. Moreover, it is almost routine to order blood tests to screen for vitamin D status as part of a health care visit. A major clinical laboratory reported that since 2004, vitamin D laboratory tests increased by nearly eightfold, 1 Taylor C.L. Thomas P.R. Aloia J.F. Millard P.S. Rosen C.J. Questions about vitamin D for primary care practice: Input from an NIH conference. Am J Med. 2015; 128: 1167-1170 Abstract Full Text Full Text PDF PubMed Scopus (19) Google Scholar and the volume of Medicare reimbursement for vitamin D testing increased 83-fold between 2000 and 2010. 2 Shahangian S. Alspach T.D. Astles J.R. Yesupriya A. Dettwyler W.K. Trends in laboratory test volumes for Medicare Part B reimbursements, 2000–2010. Arch Pathol Lab Med. 2014; 138: 189-203 Crossref PubMed Scopus (32) Google Scholar However, evidence justifying such screening is limited. In 2014, the US Preventive Services Task Force stated that there was insufficient evidence to demonstrate benefit (or harm) from vitamin D screening among adults who were free of symptoms. 3 LeFevre M.L. Screening for vitamin D deficiency in adults: U.S. Preventive Services Task Force recommendation statement. Ann Intern Med. 2015; 162: 133-140 Crossref PubMed Scopus (145) Google Scholar At the same time, sales of vitamin D supplements have also increased dramatically, 4 Nutrition Business Journal NBJ’s Supplement Business Report 2014. Penton Media, Inc, New York, NY2014 Google Scholar and there is some evidence that the availability of high-dose vitamin D supplements is increasing. 5 Rooney M.R. Harnack L. Michos E.D. Ogilvie R.P. Sempos C.T. Lutsey P.L. Trends in use of high-dose vitamin D supplements exceeding 1000 or 4000 international units daily, 1999-2014. JAMA. 2017; 317: 2448-2450 Crossref PubMed Scopus (76) Google Scholar These trends are concerning because evidence of the benefit of such practices is lacking, and the potential adverse effects associated with too much vitamin D are seemingly disregarded.

Background: Public health concerns with regard to both low and high folate status exist in the United States. Recent publications have questioned the utility of self-reported dietary intake data in research and monitoring.Objectives: The purpose of this analysis was to examine the relation between self-reported folate intakes and folate status biomarkers and to evaluate their usefulness for several types of applications.Design: We examined usual dietary intakes of folate by using the National Cancer Institute method to adjust two 24-h dietary recalls (including dietary supplements) for within-person variation and then compared these intakes with serum and red blood cell (RBC) folate among 4878 men and nonpregnant, nonlactating women aged ≥19 y in NHANES 2011-2012, a nationally representative, cross-sectional survey, with respect to consistency across prevalence estimates and rank order comparisons.Results: There was a very low prevalence (<1%) of folate deficiency when serum (<7 nmol/L) and RBC (<305 nmol/L) folate were considered, whereas a higher proportion of the population reported inadequate total dietary folate intakes (6%). Similar patterns of change occurred between intakes and biomarkers of folate status when distributions were examined (i.e., dose response), particularly when diet was expressed in μg. Intakes greater than the Tolerable Upper Intake Level greatly increased the odds of having high serum folate (OR: 17.6; 95% CI: 5.5, 56.0).Conclusions: When assessing folate status in the United States, where fortification and supplement use are common, similar patterns in the distributions of diet and biomarkers suggest that these 2 types of status indicators reflect the same underlying folate status; however, the higher prevalence estimates for inadequate intakes compared with biomarkers suggest, among other factors, a systematic underestimation bias in intake data. Caution is needed in the use of dietary folate data to estimate the prevalence of inadequacy among population groups. The use of dietary data for rank order comparisons or to estimate the potential for dietary excess is likely more reliable.

Iron is particularly important in pregnancy and infancy to meet the high demands for hematopoiesis, growth and development. Much attention has been given to conditions of iron deficiency (ID) and iron deficient anemia (IDA) because of the high global prevalence estimated in these vulnerable life stages. Emerging and preliminary evidence demonstrates, however, a U-shaped risk at both low and high iron status for birth and infant adverse health outcomes including growth, preterm birth, gestational diabetes, gastrointestinal health, and neurodegenerative diseases during aging. Such evidence raises questions about the effects of high iron intakes through supplementation or food fortification during pregnancy and infancy in iron-replete individuals. This review examines the emerging as well as the current understanding of iron needs and homeostasis during pregnancy and infancy, uncertainties in ascertaining iron status in these populations, and issues surrounding U-shaped risk curves in iron-replete pregnant women and infants. Implications for research and policy are discussed relative to screening and supplementation in these vulnerable populations, especially in developed countries in which the majority of these populations are likely iron-replete.

The NIH Office of Dietary Supplements convened a public workshop on iron screening and supplementation in iron-replete pregnant women and young children in 2016 in Bethesda, Maryland. The starting point for the workshop was the recent reports from the US Preventive Services Task Force concluding that there was insufficient evidence to evaluate the benefits and harms associated with iron screening and routine supplementation among asymptomatic pregnant women and young children (6-24 mo old) in the United States. The goal of the workshop was to explore and refine understanding about the existing knowledge gaps and research needs associated with these preventive services for these groups. Given the focus on the United States, planning for the workshop took into account the higher iron status in the United States compared with developing countries and, in turn, included a focus on iron-replete individuals consistent with the U-shaped risk curve for nutrient-health relations. Topic areas included adaptations in iron homeostasis associated with pregnancy and young childhood, the impact of inflammation, measurement of iron status, current estimates of iron status for pregnant women and young children in the United States and in Europe, and emerging evidence suggesting adverse effects associated with iron supplementation of iron-replete individuals. A crosscutting dialogue conducted at the close of the workshop formed the basis for a workshop summary that specified evidence gaps and research needs in a range of areas centered on the relation of these adaptations of iron homeostasis with the response to and risk from iron supplementation as well as the need for indicators informative of the full continuum of iron status and based on health outcomes, not just erythropoiesis.

Understanding the iron status in pregnant women in Europe provides a foundation for considering the role of iron screening and supplementation. However, available reports and studies have used different approaches that challenge the devising of overall summaries. Moreover, data on pregnant women are limited, and thus, data on women of reproductive age provide useful background information including baseline iron stores in pregnant women. This review considered data that are available from >15 European countries including national surveys and relevant clinical studies. In European women of reproductive age, median or geometric mean serum ferritin (SF) concentrations were estimated at 26–38 μg/L. Approximately 40–55% of this population had small or depleted iron stores (i.e., SF concentration ≤30 μg/L), and 45–60% of this population had apparently replete iron stores. The prevalence of iron deficiency (ID) and iron deficiency anemia (IDA) was 10–32% and 2–5%, respectively, depending on the cutoffs used. Approximately 20–35% of European women of reproductive age had sufficient iron stores (SF concentration >70 μg/L) to complete a pregnancy without supplementary iron. During pregnancy, European women in controlled supplementation trials who were not receiving iron supplements displayed increasing prevalences of ID and IDA during pregnancy, which peaked in the middle to late third trimester. Available evidence has suggested that, in gestational weeks 32–39, the median or geometric mean SF concentrations were 6–21 μg/L, and prevalences of ID and IDA were 28–85% and 21–35%, respectively. Women who were taking iron supplements had higher iron status and lower prevalences of ID and IDA, which were dependent on the dose of iron and compliance. The data suggest that, in Europe, the iron status of reproductive-aged women varies by region and worsens in pregnancy without iron supplementation.

This report addresses the evidence and the uncertainties, knowledge gaps, and research needs identified by participants at the NIH workshop related to iron screening and routine iron supplementation of largely iron-replete pregnant women and young children (6-24 mo) in developed countries. The workshop presentations and panel discussions focused on current understanding and knowledge gaps related to iron homeostasis, measurement of and evidence for iron status, and emerging concerns about supplementing iron-replete members of these vulnerable populations. Four integrating themes emerged across workshop presentations and discussion and centered on 1) physiologic or developmental adaptations of iron homeostasis to pregnancy and early infancy, respectively, and their implications, 2) improvement of the assessment of iron status across the full continuum from iron deficiency anemia to iron deficiency to iron replete to iron excess, 3) the linkage of iron status with health outcomes beyond hematologic outcomes, and 4) the balance of benefit and harm of iron supplementation of iron-replete pregnant women and young children. Research that addresses these themes in the context of the full continuum of iron status is needed to inform approaches to the balancing of benefits and harms of screening and routine supplementation.

The science surrounding vitamin D presents both challenges and opportunities. Although many uncertainties are associated with the understandings concerning vitamin D, including its physiological function, the effects of excessive intake, and its role in health, it is at the same time a major interest in the research and health communities. The approach to evaluating and interpreting the available evidence about vitamin D should be founded on the quality of the data and on the conclusions that take into account the totality of the evidence. In addition, these activities can be used to identify critical data gaps and to help structure future research. The Office of Dietary Supplements (ODS) at the National Institutes of Health has as part of its mission the goal of supporting research and dialogues for topics with uncertain data, including vitamin D. This review considers vitamin D in the context of systematically addressing the uncertainty and in identifying research needs through the filter of the work of ODS. The focus includes the role of systematic reviews, activities that encompass considerations of the totality of the evidence, and collaborative activities to clarify unknowns or to fix methodological problems, as well as a case study using the relationship between cancer and vitamin D.

With vitamin D at the center of considerable research and clinical interest, it is concerning that the measurement of its metabolized form, 25-hydroxyvitamin D [(25(OH)D], presents such a conundrum. First, available assays for 25(OH)D do not consistently produce the same measured outcomes, and some have reported quantitative amounts ranging from 17.1 to 35.6 ng/mL for the same sample analyzed depending on the assay used (1). The serum concentration of 25(OH)D is the best measure of vitamin D status, and the variation introduced by the assay differences not only impedes pooling 25(OH)D results and cross-comparing research outcomes, it can also cause differences in clinical determinations and resulting interventions. Fortunately, in 2010, a collaborative effort was established to promote the standardization of laboratorymeasures of serum 25(OH)D (2, 3). Although still a work in progress, these efforts enhance the likelihood that questions about vitamin D benefits and/or adverse effects will be clarified (3). Second, tables of food composition in the United States do not include the 25(OH)D contained in foods.Many animals consumed as human food metabolize some of the vitamin D in their diets to 25(OH)D, which is the major transport form of vitamin D and can be found in many tissues after slaughter (4). Therefore, animalderived foods contain 25(OH)D along with the nonmetabolized form.Moreover, 25(OH)D absorbed from the diet has been found to be more potent in increasing serum concentrations of 25(OH)D than is the equivalent amount of nonmetabolized dietary vitamin D (4). When the 25(OH)D content of animal-derived foods is not measured and included in tables of food composition, the ability to obtain a reasonable estimate of total vitamin D intake, and in turn its impact on vitamin D status, is not only uncertain but very likely to lead to underestimates of true intake. There have been reports concerning the discrepancy between recommended intakes for vitamin D and the reported vitamin D intakes based on national surveys that are coupled to available tables of food composition (5). This has caused public health concern and led to discussions about the need to increase vitamin D fortification of the food supply. However, tables of food composition in the United States currently report quantitative amounts in foods for only the nonmetabolized forms of vitamin D. This may explain, in part, the discrepancy between the reported population-based intakes of vitamin D and the measures of serum 25(OH)D from national surveys, the latter beingmuch higher thanwould be expected given the estimates of intake (6). We, in turn, agree with the conclusion that some, but not all, of the difference between intake and serum measures can be attributed to the contribution of sunlight (7), and the failure to take into account the contributions of 25(OH)D from food is likely a significant factor in this regard. Recently, the USDA and the Office of Dietary Supplements, NIH, worked collaboratively to demonstrate that there could be meaningful underestimates of vitaminD intake in the United States if 25(OH)D in foods is ignored. With the use of the only available US data, which were preliminary and limited, we concluded that current estimates of vitamin D intake among US men and women aged $20 y would be increased by 1.7–2.9 mg/d [or 15– 30% of the Estimated Average Requirement (6)] if the 25(OH)D in the diet were included in the intake estimates (8). Those who rely on vitamin D intake estimates should be mindful of this data gap. Just as importantly, our work (8) has made it clear that, similar to the challenges facing those measuring 25(OH)D in serum, the determination of 25(OH)D amounts in foods requires exploration and standardization of laboratory methodologies. The USDA has conducted initial studies to examine consistency among analytical laboratories in assaying 25(OH)D in various food materials based on the laboratories’ existing methods (9). Up to now, the lack of well-characterized control and reference materials, as well as the lack of validated methodologies, has presented analytical challenges. However, the results of our exploration suggest the likelihood of agreement among at least some of the existing analytical laboratories and have yielded several potential standard reference materials for measuring 25(OH)D in foods. Such work needs to continue so that vitamin D intake can be more accurately assessed relative to measures of status and better used to simulate or model the impact of modifying the nutrient content of the food supply. Such modeling is an essential task before health policies and related dietary interventions can be determined as appropriate. Targeted efforts and collaborative activities are needed now to allow for systematic analyses of 25(OH)D in foods and to elevate the determination of 25(OH)D in foods to a higher rung within the nutrition research agenda.

BACKGROUND The 2007-2010 NHANES provides the first US nationally representative serum 25-hydroxyvitamin D [25(OH)D] concentrations measured by standardized liquid chromatography-tandem mass spectrometry. OBJECTIVE We describe patterns for total 25(OH)D and individual metabolites in persons aged ≥1 y stratified by race-ethnicity and grouped by demographic, intake, physiologic, and lifestyle variables. METHODS We measured 25-hydroxycholecalciferol [25(OH)D3], 25-hydroxyergocalciferol [25(OH)D2], and C3-epimer of 25(OH)D3 [C3-epi-25(OH)D3] in serum samples (n = 15,652) from the 2007-2010 cross-sectional NHANES [total 25(OH)D = 25(OH)D3 + 25(OH)D2]. RESULTS Concentrations (median, detection rate) of 25(OH)D3 (63.6 nmol/L, 100%) and C3-epi-25(OH)D3 (3.40 nmol/L, 86%) were generally detectable; 25(OH)D2 was detectable in 19% of the population. Total 25(OH)D, 25(OH)D3, and C3-epi-25(OH)D3 displayed similar demographic patterns and were strongly correlated (Spearman's r > 0.70). Concentrations of 25(OH)D2 (90th percentile) were much higher in persons aged ≥60 y (17.3 nmol/L) than in younger age groups (≤4.88 nmol/L). We noted significant race-ethnicity differences in mean total 25(OH)D [non-Hispanic blacks (NHBs), Hispanics, and non-Hispanic whites (NHWs): 46.6, 57.2, and 75.2 nmol/L, respectively] and in the prevalence of total 25(OH)D <30 nmol/L overall (24% of NHBs, 6.4% of Hispanics, and 2.3% of NHWs) as well as stratified by season (winter months: 30% of NHBs, 7.5% of Hispanics, and 3.8% of NHWs; summer months: 17% of NHBs, 4.4% of Hispanics, and 1.6% of NHWs). Persons with higher vitamin D intakes (diet, supplements, or both) and those examined during May-October had significantly higher total 25(OH)D. Significant race-ethnicity interactions in a multiple linear regression model confirmed the necessity of providing race-ethnicity-specific estimates of total 25(OH)D. CONCLUSIONS Race-ethnicity differences in the prevalence of low total 25(OH)D remained strong even after adjustment for season to account for the NHANES design imbalance between season, latitude, and race-ethnicity. The strong correlation between C3-epi-25(OH)D3 and 25(OH)D3 may be because the epimer is a metabolite of 25(OH)D3. The presence of 25(OH)D2 mainly in older persons is likely a result of high-dose prescription vitamin D2.

The claim that large proportions of North American and other populations are deficient in vitamin D is based on misinterpretation and misapplication of the Institute of Medicine reference values for nutrients — misunderstandings that can adversely affect patient care.

Assessment of total vitamin D intake from foods and dietary supplements (DSs) may be incomplete if 25-hydroxyvitamin D [25(OH)D] intake is not included. However, 25(OH)D data for such intake assessments are lacking, no food or DS reference materials (RMs) are available, and comparison of laboratory performance has been needed. The primary goal of this study was to evaluate whether vitamin D3 and 25(OH)D3 concentrations in food and DS materials could be measured with acceptable reproducibility. Five experienced laboratories from the United States and other countries participated, all using liquid chromatography tandem-mass spectrometry but no common analytical protocol; however, various methods were used for determining vitamin D3 in the DS. Five animal-based materials (including three commercially available RMs) and one DS were analyzed. Reproducibility results for the materials were acceptable. Thus, it is possible to obtain consistent results among experienced laboratories for vitamin D3 and 25(OH)D3 in foods and a DS.

The participant will be able to explain the relevance of obtaining 25(OH) D values for animal-based foods for total vitamin D assessment and name some foods with significant 25(OHD) content.

Fortification is the process of adding nutrients or non-nutrient bioactive components to edible products (e.g., food, food constituents, or supplements). Fortification can be used to correct or prevent widespread nutrient intake shortfalls and associated deficiencies, to balance the total nutrient profile of a diet, to restore nutrients lost in processing, or to appeal to consumers looking to supplement their diet. Food fortification could be considered as a public health strategy to enhance nutrient intakes of a population. Over the past century, fortification has been effective at reducing the risk of nutrient deficiency diseases such as beriberi, goiter, pellagra, and rickets. However, the world today is very different from when fortification emerged in the 1920s. Although early fortification programs were designed to eliminate deficiency diseases, current fortification programs are based on low dietary intakes rather than a diagnosable condition. Moving forward, we must be diligent in our approach to achieving effective and responsible fortification practices and policies, including responsible marketing of fortified products. Fortification must be applied prudently, its effects monitored diligently, and the public informed effectively about its benefits through consumer education efforts. Clear lines of authority for establishing fortification guidelines should be developed and should take into account changing population demographics, changes in the food supply, and advances in technology. This article is a summary of a symposium presented at the ASN Scientific Sessions and Annual Meeting at Experimental Biology 2014 on current issues involving fortification focusing primarily on the United States and Canada and recommendations for the development of responsible fortification practices to ensure their safety and effectiveness.

There is considerable consumer and physician interest in vitamin D as a possible therapeutic agent for a range of clinical conditions and, despite mixed evidence, the interest does not appear to lessen. Some clinicians believe that consumption of vitamin D is inadequate and, in turn, advocate vitamin D supplementation to increase serum levels of the nutrient. However, evidence concerning the role of vitamin D in health and disease is conflicting, and primary care physicians have little time to sort through the data and may find it difficult to advise their patients. To better understand the challenges that primary care physicians face regarding vitamin D, and to help inform those who provide guidance for clinical decision-making, the Office of Dietary Supplements at the National Institutes of Health, with co-sponsorship from other federal health agencies, held a conference titled Vitamin D: Moving Toward Evidence-based Decision Making in Primary Care in December 2014. More than 20 invited presenters and panelists considered laboratory methods for measuring vitamin D status, discussed how clinical studies of vitamin D should be evaluated and used in developing recommendations, noted the role of values and preferences in clinical decision-making, debated the current science related to at-risk groups, and described emerging data about health risks of excessive intakes of vitamin D. Eight questions about vitamin D stem from the Conference presentations as well as other expert sources.

Background: The purpose of food fortification is to affect those at the lower end of the distribution curve for nutrient status while avoiding unintended consequences for those at the high end of the distribution. Vitamin D presents challenges in this regard.Objectives: We used scenarios to model changes in concentrations of serum 25-hydroxyvitamin D [(25(OH)D] based on increases made because of assumptions about fortification. We then examined the outcomes for balance between improving serum (25(OH)D status for those at risk of inadequacy while avoiding high concentrations for those not at risk.Methods: Data from NHANES 2001-2006 served as baseline serum (25(OH)D concentrations and were used to model shifts in serum (25(OH)D distribution after application of 3 fortification scenarios, including conceptual scenarios and an experiential predictive scenario we developed with the use of statistical modeling of changes in NHANES serum folate concentrations between prefortification and postfortification time periods.Results: All scenarios suggested the possibility of increasing serum (25(OH)D above 125 nmol/L among the proportion of the population at the high end of baseline serum (25(OH)D distribution. The scenario based on serum folate change struck a middle ground between the 2 conceptual scenarios. It predicted a prevalence of 11%, <40 nmol/L serum 25(OH)D compared with 17% currently (study baseline), and 8% prevalence of serum (25(OH)D >125 nmol/L compared with <1% currently (study baseline). It also confirmed that fortification affects those at the low end of the status distribution curve differently from those at the high end.Conclusions: Nutrient inadequacy of the type demonstrated by vitamin D-in which the risk is not universal-requires a thorough exploration of the unintended consequences of the overall shift in the distribution of serum (25(OH)D if efforts are made to use fortification to increase the status of persons at risk of deficiency. Fortification is at best a blunt instrument that must be implemented with caution. Moreover, fortification must be preceded by more research to elucidate the dose-response relation between intake and changes in serum 25(OH)D.

The discrepancy between the commonly used vitamin D status measures-intake and serum 25-hydroxyvitamin D [25(OH)D] concentrations--has been perplexing. Sun exposure increases serum 25(OH)D concentrations and is often used as an explanation for the higher population-based serum concentrations in the face of apparently low vitamin D intake. However, sun exposure may not be the total explanation. 25(OH)D, a metabolite of vitamin D, is known to be present in animal-based foods. It has been measured and reported only sporadically and is not currently factored into U.S. estimates of vitamin D intake. Previously unavailable preliminary USDA data specifying the 25(OH)D content of a subset of foods allowed exploration of the potential change in the reported overall vitamin D content of foods when the presence of 25(OH)D was included. The issue of 25(OH)D potency was addressed, and available commodity intake estimates were used to outline trends in projected vitamin D intake when 25(OH)D in foods was taken into account. Given the data available, there were notable increases in the total vitamin D content of a number of animal-based foods when potency-adjusted 25(OH)D was included, and in turn there was a potentially meaningful increase (1.7-2.9 μg or 15-30% of average requirement) in vitamin D intake estimates. The apparent increase could reduce discrepancies between intake estimates and serum 25(OH)D concentrations. The relevance to dietary interventions is discussed, and the need for continued exploration regarding 25(OH)D measurement is highlighted.

Decisions related to a spectrum of nutrition-related public health and clinical concerns must consider many factors and are best informed by evaluating the totality and quality of the evidence. Systematic review (SR) is a structured process to evaluate, compare, and synthesize relevant evidence for the SR-specific question(s). Applications of SR are exemplified here through the discussion of four case studies: research agendas, nutrient reference intakes, dietary guidance, and practice guidelines. Concerns that SR cannot be effectively applied to nutrition evidence because of the lack of an unexposed comparator and the complex homeostasis in nutrition are discussed. Central to understanding the applicability of SR is its flexibility in defining key inclusion criteria and rigorous elements as appropriate for the SR-specific question(s). Through the reduction of bias and random error by explicit, reproducible, comprehensive, and rigorous examination of all of the evidence, SR informs the scientific judgment needed for sound evidence-based public health nutrition.