NUQUEST-NUtrition QUality Evaluation Strengthening Tools: development of tools for the evaluation of risk of bias in nutrition studies.

BACKGROUND: Dietary exposure assessments are a critical issue in evaluating human nutrition studies; however, nutrition-specific criteria are not consistently included in existing bias assessment tools. OBJECTIVES: Our objective was to develop a set of risk of bias (RoB) tools that integrated nutrition-specific criteria into validated generic assessment tools to address RoB issues, including those specific to dietary exposure assessment. METHODS: The Nutrition QUality Evaluation Strengthening Tools (NUQUEST) development and validation process included 8 steps. The first steps identified 1) a development strategy; 2) generic assessment tools with demonstrated validity; and 3) nutrition-specific appraisal issues. This was followed by 4) generation of nutrition-specific items and 5) development of guidance to aid users of NUQUEST. The final steps used established ratings of selected studies and feedback from independent raters to 6) assess reliability and validity; 7) assess formatting and usability; and 8) finalize NUQUEST. RESULTS: NUQUEST is based on the Scottish Intercollegiate Guidelines Network checklists for randomized controlled trials, cohort studies, and case-control studies. Using a purposive sample of 45 studies representing the 3 study designs, interrater reliability was high (Cohen's κ: 0.73; 95% CI: 0.52, 0.93) across all tools and at least moderate for individual tools (range: 0.57-1.00). The use of a worksheet improved usability and consistency of overall interrater agreement across all study designs (40% without worksheet, 80%-100% with worksheet). When compared to published ratings, NUQUEST ratings for evaluated studies demonstrated high concurrent validity (93% perfect or near-perfect agreement). Where there was disagreement, the nutrition-specific component was a contributing factor in discerning exposure methodological issues. CONCLUSIONS: NUQUEST integrates nutrition-specific criteria with generic criteria from assessment tools with demonstrated reliability and validity. NUQUEST represents a consistent and transparent approach for evaluating RoB issues related to dietary exposure assessment commonly encountered in human nutrition studies.

Surrogate disease markers as substitutes for chronic disease outcomes in studies of diet and chronic disease relations.

Surrogate biomarkers for clinical outcomes afford scientific and economic efficiencies when investigating nutritional interventions in chronic diseases. However, valid scientific results are dependent on the qualification of these disease markers that are intended to be substitutes for a clinical outcome and to accurately predict benefit or harm. In this article, we examine the challenges of evaluating surrogate markers and describe the framework proposed in a 2010 Institute of Medicine report. The components of this framework are presented in the context of nutritional interventions for chronic diseases. We present case studies of 2 well-accepted surrogate markers [blood pressure within sodium intake and cardiovascular disease (CVD) context and low density lipoprotein-cholesterol concentrations within a saturated fat and CVD context]. We also describe additional cases in which the evidence is insufficient to validate their surrogate status. Guidance is offered for future research that evaluates or uses surrogate markers.

Correspondence of folate dietary intake and biomarker data.

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.

Challenges and Lessons Learned in Generating and Interpreting NHANES Nutritional Biomarker Data.

For the past 45 y, the National Center for Health Statistics at the CDC has carried out nutrition surveillance of the US population by collecting anthropometric, dietary intake, and nutritional biomarker data, the latter being the focus of this publication. The earliest biomarker testing assessed iron and vitamin A status. With time, a broad spectrum of water- and fat-soluble vitamins was added and biomarkers for other types of nutrients (e.g., fatty acids) and bioactive dietary compounds (e.g., phytoestrogens) were included in NHANES. The cross-sectional survey is flexible in design, and biomarkers may be measured for a short period of time or rotated in and out of surveys depending on scientific needs. Maintaining high-quality laboratory measurements over extended periods of time such that trends in status can be reliably assessed is a major goal of the testing laboratories. Physicians, health scientists, and policy makers rely on the NHANES reference data to compare the nutritional status of population groups, to assess the impact of various interventions, and to explore associations between nutritional status and health promotion or disease prevention. Focusing on the continuous NHANES, which started in 1999, this review uses a "lessons learned" approach to present a series of challenges that are relevant to researchers measuring biomarkers in NHANES and beyond. Some of those challenges are the use of multiple related biomarkers instead of a single biomarker for a specific nutrient (e.g., folate, vitamin B-12, iron), adhering to special needs for specimen collection and handling to ensure optimum specimen quality (e.g., vitamin C, folate, homocysteine, iodine, polyunsaturated fatty acids), the retrospective use of long-term quality-control data to correct for assay shifts (e.g., vitamin D, vitamin B-12), and the proper planning for and interpretation of crossover studies to adjust for systematic method changes (e.g., folate, vitamin D, ferritin).

Options for basing Dietary Reference Intakes (DRIs) on chronic disease endpoints: report from a joint US-/Canadian-sponsored working group.

Dietary Reference Intakes (DRIs) are used in Canada and the United States in planning and assessing diets of apparently healthy individuals and population groups. The approaches used to establish DRIs on the basis of classical nutrient deficiencies and/or toxicities have worked well. However, it has proved to be more challenging to base DRI values on chronic disease endpoints; deviations from the traditional framework were often required, and in some cases, DRI values were not established for intakes that affected chronic disease outcomes despite evidence that supported a relation. The increasing proportions of elderly citizens, the growing prevalence of chronic diseases, and the persistently high prevalence of overweight and obesity, which predispose to chronic disease, highlight the importance of understanding the impact of nutrition on chronic disease prevention and control. A multidisciplinary working group sponsored by the Canadian and US government DRI steering committees met from November 2014 to April 2016 to identify options for addressing key scientific challenges encountered in the use of chronic disease endpoints to establish reference values. The working group focused on 3 key questions: 1) What are the important evidentiary challenges for selecting and using chronic disease endpoints in future DRI reviews, 2) what intake-response models can future DRI committees consider when using chronic disease endpoints, and 3) what are the arguments for and against continuing to include chronic disease endpoints in future DRI reviews? This report outlines the range of options identified by the working group for answering these key questions, as well as the strengths and weaknesses of each option.

Applying inappropriate cutoffs leads to misinterpretation of folate status in the US population.

BACKGROUND: Folate cutoffs for risk of deficiency compared with possible deficiency were originally derived differently (experimental compared with epidemiologic data), and their interpretations are different. The matching of cutoffs derived from one assay with population-based data derived from another assay requires caution. OBJECTIVE: We assessed the extent of folate-status misinterpretation with the use of inappropriate cutoffs. DESIGN: In the cross-sectional NHANES, serum and red blood cell (RBC) folate were first measured with the use of a radioprotein-binding assay (RPBA) (1988-2006) and, afterwards, with the use of a microbiologic assay (2007-2010). We compared prevalence estimates for assay-matched cutoffs (e.g., with the use of an RPBA cutoff with RPBA data) and assay-mismatched cutoffs (e.g., with the use of microbiologic assay cutoff with RPBA data) for risk of deficiency on the basis of megaloblastic anemia as a hematologic indicator in persons ≥4 y of age (e.g., serum folate concentration <7 nmol/L and RBC folate concentration <305 nmol/L derived with the use of a microbiologic assay), possible deficiency on the basis of rising homocysteine as a metabolic indicator in persons ≥4 y of age (e.g., serum folate concentration <10 nmol/L and RBC folate concentration <340 nmol/L derived with the use of an RPBA), and insufficiency on the basis of elevated risk of neural tube defects in women 12-49 y old (e.g., RBC folate concentration <906 nmol/L derived with the use of a microbiologic assay). RESULTS: Pre-folic acid fortification (1988-1994), risks of deficiency for assay-matched compared with assay-mismatched cutoffs were 5.6% compared with 16% (serum folate), respectively, and 7.4% compared with 28% (RBC folate), respectively; risks declined postfortification (1999-2006) to <1% compared with <1% (serum folate), respectively, and to <1% compared with 2.5% (RBC folate), respectively. Prefortification (1988-1994), risks of possible deficiency for assay-matched compared with assay-mismatched cutoffs were 35% compared with 56% (serum folate), respectively, and 37% compared with 84% (RBC folate), respectively; risks declined postfortification (1999-2006) to 1.9% compared with 7.0% (serum folate), respectively, and to 4.8% compared with 53% (RBC folate), respectively. Postfortification (2007-2010), risks of insufficiency were 3% (assay matched) compared with 39% (assay mismatched), respectively. CONCLUSIONS: The application of assay-mismatched cutoffs leads to a misinterpretation of folate status. This confusion likely applies to clinical assays because no comparability data are available, to our knowledge.

The vitamin D status of the US population from 1988 to 2010 using standardized serum concentrations of 25-hydroxyvitamin D shows recent modest increases.

BACKGROUND: Temporal trends in the US population's vitamin D status have been uncertain because of nonstandardized serum 25-hydroxyvitamin D [25(OH)D] measurements. OBJECTIVE: To accurately assess vitamin D status trends among those aged ≥12 y, we used data from the cross-sectional NHANESs. DESIGN: A liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for measuring 25(OH)D (sum of 25-hydroxyvitamin D2 and 25-hydroxyvitamin D3), calibrated to standard reference materials, was used to predict LC-MS/MS-equivalent concentrations from radioimmunoassay data (1988-2006 surveys; n = 38,700) and to measure LC-MS/MS concentrations (2007-2010 surveys; n = 12,446). Weighted arithmetic means and the prevalence of 25(OH)D above or below cutoff concentrations were calculated to evaluate long-term trends. RESULTS: Overall, mean predicted 25(OH)D showed no time trend from 1988 to 2006, but during 2007-2010 the mean measured 25(OH)D was 5-6 nmol/L higher. Those groups who showed the largest 25(OH)D increases (7-11 nmol/L) were older, female, non-Hispanic white, and vitamin D supplement users. During 1988-2010, the proportions of persons with 25(OH)D <40 nmol/L were 14-18% (overall), 46-60% (non-Hispanic blacks), 21-28% (Mexican Americans), and 6-10% (non-Hispanic whites). CONCLUSIONS: An accurate method for measuring 25(OH)D showed stable mean concentrations in the US population (1988-2006) and recent modest increases (2007-2010). Although it is unclear to what extent supplement usage compared with different laboratory methods explain the increases in 25(OH)D, the use of higher vitamin D supplement dosages coincided with the increase. Marked race-ethnic differences in 25(OH)D concentrations were apparent. These data provide the first standardized information about temporal trends in the vitamin D status of the US population.

Safe use of high intakes of folic acid: research challenges and paths forward.

Adequate folic acid intake is an effective dietary-based prevention tool for reducing the risk of neural tube defects. Achieving adequate intake for the prevention of neural tube defects frequently requires the consumption of foods fortified with folic acid and/or the use of folic acid-containing dietary supplements. To date, research on the potential for adverse effects of high intakes of folic acid has been limited. Without such research, it is difficult to define a value for high intake. In May 2015, an expert panel was tasked with examining the available scientific literature and making research recommendations within 4 general categories of potential folate-related adverse health effects: cancer, cognition in conjunction with vitamin B12 deficiency, hypersensitivity-related outcomes, and thyroid and diabetes-related disorders. This article summarizes the expert panel's conclusions, outlines the challenges faced when reviewing the literature, and examines some of the panel's recommendations for research.

A Method-bridging Study for Serum 25-hydroxyvitamin D to Standardize Historical Radioimmunoassay Data to Liquid Chromatography-Tandem Mass Spectrometry.

BACKGROUND: Serum concentrations of 25-hydroxyvitamin D (25OHD) were measured for the National Health and Nutrition Examination Survey (NHANES) over the 1988-2006 period using a radioimmunoassay (RIA). In 2010, the Centers for Disease Control and Prevention (CDC) reissued RIA-harmonized 25OHD for NHANES 2004 and 2006, and advised users to adjust the original RIA data from 1988-1994 by using an equation. Beginning with NHANES 2007-2008, a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method measured 25OHD. METHODS: A method comparison (bridging) study was designed to convert original RIA 25OHD to LC­MS/MS-equivalents. This report compares the predictive ability of a competitor regression model (Model 2) to the equations that CDC publicly released in 2015 (Model 1). The models differ by time period variable and use of transformations. RESULTS: The two models provided similar adjusted R(2) (Model 1: 88.9%, Model 2: 90.4%) and root mean square error of prediction (plus or minus 9 to 10 nanomoles per liter [nmol/L]). Applying these models to NHANES 1988­2006 RIA 25OHD, the Pearson correlation of LC­MS/MS-equivalent concentrations was 0.99; the median difference between models was 0 nmol/L (interquartile range: ­2.8 to 1 nmol/L). In contrast to declining RIA-harmonized 25OHD, both models showed little change in LC­MS/MS-predicted 25OHD over the 1988­2006 period. For 2001­2006, both models predicted similar prevalences of 25OHD less than 30 nmol/L, which were lower than the prevalence estimates based on RIA-harmonized data. Mean weighted LC­MS/MS-equivalent concentrations based on either model were about 3 nmol/L lower for the 1988­1994 survey and about 3 nmol/L higher for the 2001­2006 surveys, effectively smoothing out temporal trends observed using the harmonized RIA data. CONCLUSIONS: Given minimal differences between models, final selection was based on public availability of the regression data. The bridging equations provide a way to use the previous RIA results to obtain LC­MS/MS-equivalent concentrations and evaluate temporal trends in vitamin D status.

National Estimates of Serum Total 25-Hydroxyvitamin D and Metabolite Concentrations Measured by Liquid Chromatography-Tandem Mass Spectrometry in the US Population during 2007-2010.

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.

Folate status and concentrations of serum folate forms in the US population: National Health and Nutrition Examination Survey 2011-2.

Serum and erythrocyte (RBC) total folate are indicators of folate status. No nationally representative population data exist for folate forms. We measured the serum folate forms (5-methyltetrahydrofolate (5-methylTHF), unmetabolised folic acid (UMFA), non-methyl folate (sum of tetrahydrofolate (THF), 5-formyltetrahydrofolate (5-formylTHF), 5,10-methenyltetrahydrofolate (5,10-methenylTHF)) and MeFox (5-methylTHF oxidation product)) by HPLC-MS/MS and RBC total folate by microbiologic assay in US population ≥ 1 year (n approximately 7500) participating in the National Health and Nutrition Examination Survey 2011-2. Data analysis for serum total folate was conducted including and excluding MeFox. Concentrations (geometric mean; detection rate) of 5-methylTHF (37·5 nmol/l; 100 %), UMFA (1·21 nmol/l; 99·9 %), MeFox (1·53 nmol/l; 98·8 %), and THF (1·01 nmol/l; 85·2 %) were mostly detectable. 5-FormylTHF (3·6 %) and 5,10-methenylTHF (4·4 %) were rarely detected. The biggest contributor to serum total folate was 5-methylTHF (86·7 %); UMFA (4·0 %), non-methyl folate (4·7 %) and MeFox (4·5 %) contributed smaller amounts. Age was positively related to MeFox, but showed a U-shaped pattern for other folates. We generally noted sex and race/ethnic biomarker differences and weak (Spearman's r< 0·4) but significant (P< 0·05) correlations with physiological and lifestyle variables. Fasting, kidney function, smoking and alcohol intake showed negative associations. BMI and body surface area showed positive associations with MeFox but negative associations with other folates. All biomarkers showed significantly higher concentrations with recent folic acid-containing dietary supplement use. These first-time population data for serum folate forms generally show similar associations with demographic, physiological and lifestyle variables as serum total folate. Patterns observed for MeFox may suggest altered folate metabolism dependent on biological characteristics.

Unmetabolized folic acid is detected in nearly all serum samples from US children, adolescents, and adults.

BACKGROUND: Serum total folate consists mainly of 5-methyltetrahydrofolate (5-methylTHF). Unmetabolized folic acid (UMFA) may occur in persons consuming folic acid-fortified foods or supplements. OBJECTIVES: We describe serum 5-methylTHF and UMFA concentrations in the US population ≥1 y of age by demographic variables and fasting time, stratified by folic acid-containing dietary supplement use. We also evaluate factors associated with UMFA concentrations >1 nmol/L. METHODS: Serum samples from the cross-sectional NHANES 2007-2008 were measured for 5-methylTHF (n = 2734) and UMFA (n = 2707) by HPLC-tandem mass spectrometry. RESULTS: In supplement users compared with nonusers, we found significantly higher geometric mean concentrations of 5-methylTHF (48.4 and 30.7 nmol/L, respectively) and UMFA (1.54 and 0.794 nmol/L, respectively). UMFA concentrations were detectable (>0.3 nmol/L) in >95% of supplement users and nonusers, regardless of demographic or fasting characteristics; concentrations differed significantly by age and fasting time, but not by sex and race-ethnicity, both in supplement users and nonusers. The prevalence of UMFA concentrations >1 nmol/L was 33.2% overall and 21.0% in fasting (≥8 h) adults (≥20 y of age). Using multiple logistic regression analysis, UMFA concentrations >1 nmol/L were associated with being older, non-Hispanic black, nonfasting (<8 h), having smaller body surface area, higher total folic acid intake (diet and supplements), and higher red blood cell folate concentrations. In fasting adults, a decrease in the mean daily alcohol consumption was also associated with increased odds of UMFA concentrations >1 nmol/L. CONCLUSIONS: UMFA detection was nearly ubiquitous, and concentrations >1 nmol/L were largely but not entirely explained by fasting status and by total folic acid intake from diet and supplements. These new UMFA data in US persons ≥1 y of age provide much-needed information on this vitamer in a fortified population with relatively high use of dietary supplements.

Including food 25-hydroxyvitamin D in intake estimates may reduce the discrepancy between dietary and serum measures of vitamin D status.

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.

Modeling a methylmalonic acid-derived change point for serum vitamin B-12 for adults in NHANES.

BACKGROUND: No consensus exists about which cutoff point should be applied for serum vitamin B-12 (SB-12) concentrations to define vitamin B-12 status in population-based research. OBJECTIVE: The study's aim was to identify whether a change point exists at which the relation between plasma methylmalonic acid (MMA) and SB-12 changes slope to differentiate between inadequate and adequate vitamin B-12 status by using various statistical models. DESIGN: We used data on adults (≥19 y; n = 12,683) from NHANES 1999-2004-a nationally representative, cross-sectional survey. We evaluated 6 piece-wise polynomial and exponential decay models that used different control levels for known covariates. RESULTS: The MMA-defined change point for SB-12 varied depending on the statistical model used. A linear-splines model was determined to best fit the data, as determined by the approximate permutation test; 3 slopes relating SB-12 and MMA and resulting in 2 change points and 3 subgroups were shown. The first group (SB-12 <126 pmol/L) was small and had the highest MMA concentration (median: 281 nmol/L; 95% CI: 245, 366 nmol/L; n = 157, 1.2%); many in this group could be considered at high risk of severe deficiency because combined abnormalities of MMA and homocysteine were very frequent and the concentrations themselves were significantly higher. The highest SB-12 group (SB-12 >287 pmol/L; n = 8569, 67.6%) likely had adequate vitamin B-12 status (median MMA: 120 nmol/L; 95% CI: 119, 125 nmol/L). The vitamin B-12 status of the sizable intermediate group (n = 3957, 33%) was difficult to interpret. CONCLUSIONS: The 3 distinct slopes for the relation between SB-12 and MMA challenges the conventional use of one cutoff point for classifying vitamin B-12 status. In epidemiologic research, the use of one cutoff point would fail to separate the small, severely deficient group from the intermediate group that has neither normal nor clearly deficient vitamin B-12 concentrations (ie, unknown vitamin B-12 status). This intermediate group requires further characterization.

Is there a reverse J-shaped association between 25-hydroxyvitamin D and all-cause mortality? Results from the U.S. nationally representative NHANES.

CONTEXT: A reverse J-shaped association between serum 25-hydroxyvitamin D (25[OH]D) concentration and all-cause mortality was suggested in a 9-year follow-up (1991-2000) analysis of the Third National Health and Nutrition Examination Survey (NHANES III, 1988-1994). OBJECTIVE: Our objective was to repeat the analyses with 6 years additional follow-up to evaluate whether the association persists through 15 years of follow-up. PARTICIPANTS: The study included 15 099 participants aged ≥ 20 years with 3784 deaths. MAIN OUTCOME MEASURE: Relative risk (RR) of death from all causes was adjusted for age, sex, race/ethnicity, and season using 2 Poisson regression approaches: traditional categorical and cubic splines. Results were given for 9 25(OH)D levels: <20, 20 to 29, 30 to 39, 40 to 49, 50 to 59, 60 to 74, 75 to 99 (reference), 100 to 119, and ≥ 120 nmol/L. RESULTS: The reverse J-shaped association became stronger with longer follow-up and was not affected by excluding deaths within the first 3 years of follow-up. Similar results were found from both statistical approaches for levels <20 through 119 nmol/L. Adjusted RR (95% confidence interval [CI]) estimates for all levels <60 nmol/L were significantly >1 compared with the reference group. The nadir of risk was 81 nmol/L (95% CI, 73-90 nmol/L). For 25(OH)D ≥ 120 nmol/L, results (RR, 95% CI) were slightly different using traditional categorical (1.5, 1.02-2.3) and cubic splines approaches (1.2, 0.9-1.4). The association appeared in men, women, adults ages 20 to 64 years, and non-Hispanic whites but was weaker in older adults. The study was too small to evaluate the association in non-Hispanic black and Mexican-American adults. CONCLUSIONS: A reverse J-shaped association between serum 25(OH)D and all-cause mortality appears to be real. It is uncertain whether the association is causal.

Appropriateness of the probability approach with a nutrient status biomarker to assess population inadequacy: a study using vitamin D.

BACKGROUND: There are questions about the appropriate method for the accurate estimation of the population prevalence of nutrient inadequacy on the basis of a biomarker of nutrient status (BNS). OBJECTIVE: We determined the applicability of a statistical probability method to a BNS, specifically serum 25-hydroxyvitamin D [25(OH)D]. The ability to meet required statistical assumptions was the central focus. DESIGN: Data on serum 25(OH)D concentrations in adults aged 19-70 y from the 2005-2006 NHANES were used (n = 3871). An Institute of Medicine report provided reference values. We analyzed key assumptions of symmetry, differences in variance, and the independence of distributions. We also corrected observed distributions for within-person variability (WPV). Estimates of vitamin D inadequacy were determined. RESULTS: We showed that the BNS [serum 25(OH)D] met the criteria to use the method for the estimation of the prevalence of inadequacy. The difference between observations corrected compared with uncorrected for WPV was small for serum 25(OH)D but, nonetheless, showed enhanced accuracy because of correction. The method estimated a 19% prevalence of inadequacy in this sample, whereas misclassification inherent in the use of the more traditional 97.5th percentile high-end cutoff inflated the prevalence of inadequacy (36%). CONCLUSIONS: When the prevalence of nutrient inadequacy for a population is estimated by using serum 25(OH)D as an example of a BNS, a statistical probability method is appropriate and more accurate in comparison with a high-end cutoff. Contrary to a common misunderstanding, the method does not overlook segments of the population. The accuracy of population estimates of inadequacy is enhanced by the correction of observed measures for WPV.

Changes in measurement procedure from a radioassay to a microbiologic assay necessitate adjustment of serum and RBC folate concentrations in the U.S. population from the NHANES 1988-2010.

The NHANES measured serum and RBC folate concentrations by using a radioassay during prefortification (1988-1994) and postfortification (1999-2006) periods followed by the use of a microbiologic assay (MBA) from 2007-2010. The MBA produces higher concentrations than does the radioassay and is considered to be more accurate. To allow for accurate long-term trending (1988-2010), we evaluated different regression models (linear, piecewise linear, and fractional polynomial) to assay-adjust the radioassay results to be comparable to the MBA results. The data used to derive the regression models originated from 2 crossover studies in which the 2 assays were applied to a set of 325 serum and 171 whole-blood samples. Fractional polynomial regression of logarithmically transformed data provided the best fit for serum folate. Linear regression of logarithmically transformed whole-blood data provided an equally good fit compared with the other models and was the simplest to apply for RBC folate. Prefortification serum and RBC folate geometric mean concentrations increased after adjustment from 13.0 to 16.7 nmol/L and from 403 to 747 nmol/L, respectively. Postfortification serum folate concentrations increased from ~30 to ~43 nmol/L, and RBC folate concentrations increased from ~600 to ~1100 nmol/L after adjustment, with some variation across survey cycles. The presented regression equations allow the estimation of more accurate prevalence estimates and long-term trends in blood folate concentrations in the U.S. population by using results that are equivalent to the MBA. This information will be useful to public health officials in the United States who are dealing with folic acid fortification issues.

Estimation of trends in serum and RBC folate in the U.S. population from pre- to postfortification using assay-adjusted data from the NHANES 1988-2010.

The NHANES has monitored folate status of the U.S. population from prefortification (1988-1994) to postfortification (1999-2010) by measuring serum and RBC folate concentrations. The Bio-Rad radioassay (BR) was used from 1988 to 2006, and the microbiologic assay (MBA) was used from 2007 to 2010. The MBA produces higher concentrations than the BR and is considered to be more accurate. Thus, to bridge assay differences and to examine folate trends over time, we adjusted the BR results to be comparable to the MBA results. Postfortification, assay-adjusted serum and RBC folate concentrations were 2.5 times and 1.5 times prefortification concentrations, respectively, and showed a significant linear trend (P < 0.001) to slightly lower concentrations during 1999-2010. The postfortification prevalence of low serum (<10 nmol/L) or RBC (<340 nmol/L) folate concentrations was ≤ 1%, regardless of demographic subgroup, compared with 24% for serum folate and 3.5% for RBC folate prefortification, with substantial variation among demographic subgroups. The central 95% reference intervals for serum and RBC folate varied by demographic subgroup during both pre- and postfortification periods. Age and dietary supplement use had the greatest effects on prevalence estimates of low folate concentrations during the prefortification period. In summary, the MBA-equivalent blood folate concentrations in the U.S. population showed first a sharp increase from pre- to postfortification, then showed a slight decrease (17% for serum and 12% for RBC folate) during the 12-y postfortification period. The MBA-equivalent pre- and postfortification reference concentrations will inform countries that plan folic acid fortification or that need to evaluate its impact.

Monitoring folate status in population-based surveys.

Folate status assessments depend primarily on the measurement of biomarkers such as serum and red blood cell folate. Lessons learned from a large national monitoring system such as the National Health and Nutrition Examination Survey and a public health intervention such as the implementation of folic acid fortification in the United States have provided useful insights into the challenges of assessing folate status and possible solutions for addressing these challenges. © 2011 International Union of Biochemistry and Molecular Biology, Inc. 2011.