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Department of Nutrition, University of California, Davis, CA 95616;
* Block Dietary Data Systems, 2634 Le Conte Avenue, Berkeley, CA 94709; and
University of California, Berkeley School of Public Health, Berkeley, CA 94720
3To whom correspondence should be addressed. E-mail: ajclifford{at}ucdavis.edu.
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ABSTRACT |
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 TOP ABSTRACT SUBJECTS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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KEY WORDS: • RBC folate • analytical method • low-income women • folate-targeted food screener
Adequate intake of folate is important to minimize the risk of neural tube defects (NTDs)4 (1,2), cardiovascular disease (3), and some cancers (4). An adequate folate intake by women of child-bearing age is especially important because it can raise RBC folate levels, and has been shown to reduce the risk of NTDs by 
70% (5). The RBC folate level is rated a good biomarker for nutritional status of this important nutrient because of its correlation with liver folate, a significant tissue store (6). Cell accumulation of folate may be key to ameliorating NTD susceptibility with increased folate intake (7). Therefore, the strength of the relations between folate intake and biomarkers of folate status such as RBC folate is of considerable interest.
Women who fall below 185% of the Federal Poverty Level (FPL 
1.85) are an important target population group for improving folate status. They may not fully appreciate the importance of folate (8). The correlation between folate intake and RBC folate concentrations has not been adequately studied in this population; we were prompted to measure their folate status using their RBC folate levels and to examine its correlation with their folate intake. The intake was measured with a new abbreviated folate-targeted food/supplement frequency screener that was developed for this study.
Plasma and serum folate concentrations are commonly accepted to reflect recent dietary folate intake. Although erythrocyte (RBC) folate is considered to be a better indicator of body stores and hence nutritional status (9), there is considerable uncertainty about the reliability of the analytical methods for RBC folate (10,11). An interlaboratory comparison of popular analytical methods for folate revealed large differences among laboratories analyzing a common set of serum and whole-blood specimens (10). Results of the interlaboratory comparison prompted the recent development of promising new hyphenated methods that included LC-MS and LC-MS-MS for plasma and serum folate (12–15) and GC-MS for RBC folate (16–18). Like the microbiologic assay methods, the new hyphenated methods can be internally and externally standardized.
Here we present a feasibility study in which we compared a GC-MS assay to 3 other common assays for RBC folate. Also, we examined the correlation between RBC folate levels and folate intake as measured by an 18-item folate-targeted food/supplement Dietary Folate Equivalents (DFE) screener developed for this study. The time required to complete the DFE screener (6–12 min) makes it an attractive alternative to longer FFQs and for epidemiologic investigations focused on folate status. If further research demonstrates the validity of the DFE screener when it is self-administered, as opposed to administered by clinic staff via interview, this instrument may prove useful as a classification tool for clinical settings.
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SUBJECTS AND METHODS |
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TOPABSTRACTSUBJECTS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Subject enrollment. Enrollment occurred from March through September 2003. Pregnant women (n = 29), and nursing or nonpregnant low-income women (n = 39) who were at least 18 y old, and who were eligible or who had begun the Women, Infants, and Children (WIC) program during the preceding 2 wk, were enrolled in this study. All study instruments (consent, 1-page folate-targeted food screener) were translated into Spanish, back translated into English, and administered by bilingual research staff.
The pregnant women were enrolled primarily from 6 local community clinics in Sacramento, CA. The clinics were providers of the county Comprehensive Perinatal Services Program (CPSP) and Medi-Cal programs. CPSP and Medi-Cal are statewide government programs that provide comprehensive prenatal care to low-income women. To be eligible for these programs, a family’s income must be 185% of the Federal Poverty Level (FPL 1.85), i.e., gross monthly income of $1400-$5000 for a family unit of 1 to 8 persons, respectively. We targeted pregnant women who were scheduled for their first obstetrics appointment because they were most likely to have not started using supplements. We also targeted women who were newly recruited into the WIC program. WIC is a federal grant program that provides vouchers for supplemental foods, nutrition education, and prenatal counseling to low-income women. WIC food vouchers are special checks for purchasing health-promoting foods such as milk, juice, eggs, peanut butter, and so forth. The WIC authorized food list of July 2003 targeted foods rich in vitamins A and C, protein, and fiber, but not folate. Women enrolled in this federal program have been shown to improve their nutritional status and pregnancy outcome (19,20). Community healthcare practitioners refer pregnant women to the WIC program to initiate nutrition intervention.
The nonpregnant women were enrolled from a local Woman’s Center. This Center is a nonprofit organization in Sacramento, CA that serves low-income women and their children by providing them a daily meal, and referrals to local state agencies for food, housing, and shelter.
Clinic visits. Women enrolled at the 1st clinic visit (baseline values) were questioned about their general medical history, dietary habits, length of time on WIC, and demographics. Each woman completed a 1-page folate-targeted food/supplement intake screener, and blood was drawn as described below. Pregnant women began taking a standard over-the-counter prenatal vitamin supplement (800 µg or 1814 nmol folic acid/tablet) as prescribed by their healthcare provider. All nonpregnant women received one bottle of 100 prenatal vitamin tablets (with the same amount of folic acid/tablet, Longs Drug Store Brand) to match the pregnant group. In addition to folic acid, each tablet also supplied vitamin B-12, 4 µg (3 nmol); vitamin B-6, 2.6 mg (11 µmol); and iron, 27 mg (490 µmol). All women were instructed to take 1 prenatal tablet/d until their 2nd clinic visit. A $25 gift certificate to a local supermarket (for food only) was given to each woman at the end of the visit.
The 2nd clinic visit occurred 30–60 d after supplementation first began. Only 3 of 38 nonpregnant women were lactating at this visit. All women again completed the 1-page folate-targeted food/supplement intake screener, and blood was drawn again from each woman as described above. All women were queried about compliance with supplement use, i.e., how often they took/missed taking the supplement. The overall compliance was determined from the 1-page folate-targeted food/supplement intake screener, interviews, and queries,. Pregnant women were asked about their start date for receiving WIC benefits. At the end of the visit, another $25 gift certificate to a local supermarket (for food only) was given to each woman.
Folate-Targeted Semiquantitative Block DFE screener.
The semiquantitative Block Dietary Folate Equivalents (DFE) screener was designed to measure usual and customary intake of DFEs and was optimized for use in low-income populations.1 The questions on the screener were identified by examining the food records reported by individuals 
17 y old in the National Health and Nutrition Examination Survey (NHANES) 1999–2000. To develop the screener, all 4368 food items and beverages in that dataset were divided into 152 food groups (questions). Each question represented a similar type of food item (for example, pinto, red, black, or refried beans, separately, or as part of another food such as a "burrito," were coded together as beans). The folate contributed as DFE by each food in the NHANES dataset was calculated by multiplying, for each NHANES respondent, the mass of the food consumed by its folic acid concentration, and then summing across all of the respondents. To calculate DFEs from the NHANES data, the estimated synthetic folic acid (SFA) content of each food was multiplied by 1.7, the bioavailability correction factor provided by the Institute of Medicine (21). Food groups (questions) were then ranked according to their contribution to total DFE intake; 19 questions, representing foods contributing the top 60% of U.S. DFE intake, were included in the Block DFE screener. The total DFE consumed by each woman enrolled in our study was calculated as the cumulative sum of DFEs contributed by each food group on the screening questionnaire. Folate values for the questions on the DFE screener were identified by analysis of the USDA Nutrient Database for Standard Reference, Release 15 (August, 2002). Synthetic folic acid values used for the computation of DFEs from vitamin supplements were identified by reference to label claims on commonly available dietary supplements.
Blood drawing and analysis.
Blood was drawn from each woman into 4 separate 4-mL tubes that contained spray-dried K2-EDTA and one SST tube for vitamin B-12. The tubes were inverted gently to prevent clotting. All EDTA tubes for whole-blood folate were promptly wrapped in aluminum foil (to exclude light) and immediately stored on dry ice. One EDTA tube and the SST tube were stored at 5°C during transit (2 h) to the University of California Davis, Sacramento Medical Center (UCDMC) Pathology Department Laboratory where complete blood count, packed cell volume (PCV), and serum vitamin B-12 (SimulTRAC Radioassay Kit, Vitamin B-12[57Co]/Folate[125I] from ICN Diagnostics) measurements were made. Also, one frozen tube of whole blood was analyzed for folate using chemiluminescence technology (Bayer Diagnostics ADVIA Centaur, Bayer Corporation Diagnostics Division) at the UCDMC Pathology Department Laboratory. The remaining 3 EDTA tubes were stored on dry ice during transit (2 h) to the laboratories where they were stored at –70°C for RBC folate analysis using GC-MS (16–18), Lactobacillus casei (22), and radioassay (RA; SPNB Dual Count 125I, Diagnostic Products) methods. The L. casei and RA assays were conducted at the UCD-Clinical Nutrition Research Unit (UCD-CNRU). The Chemiluminescence, L. casei, and RA assays were performed on whole-blood lysates (prepared with ascorbic acid). Folate concentrations of whole blood were divided by the PCV (fraction) and expressed as nmol folate/L erythrocytes. A pool of freshly drawn blood was promptly divided into small aliquots and stored –70°C; aliquots from the pool were analyzed with each batch of study samples to serve as a control monitor for day-to-day variation. Finally, all RBC folate assays were performed within 3 wk from the date the blood was drawn.
Calculations and data analysis. The data were checked for normality, and descriptive statistics were calculated for each response variable. The data were analyzed by repeated-measures ANOVA. The 3-way interaction (assay x supplementation x pregnancy status) was not significant; thus, only main effects (Table 1) and the 2-way interactions, assay x supplement and assay x pregnancy status (Figs. 1, 2) are presented. Regression analysis of RBC folate levels by the 4 assay methods and by folate intake were also calculated. All statistical analyses were conducted with StatView 5.0.1 software program (Abacus Concepts).
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RESULTS |
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TOPABSTRACTSUBJECTS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Other characteristics of the study participants are summarized in Table 1. The pregnant women were younger than the nonpregnant women by approximately a decade. The pregnant women were in wk 18 ± 2 of gestation and they were currently not receiving WIC benefits. The pregnant group had fewer children (P = 0.0140) than the nonpregnant group, but the difference would likely disappear when the pregnant group was as old as the nonpregnant group.
As expected, the pregnant women had a lower PCV (P < 0.0001), RBC count (P < 0.0001), blood hemoglobin (P < 0.0001), and plasma vitamin B-12 levels (P = 0.0749) compared with nonpregnant women. Also, the pregnant women had a higher white blood cell (WBC) count (P = 0.0073) than the nonpregnant women. Folic acid supplementation did not significantly affect PCV, WBC count, RBC count, hemoglobin, or plasma vitamin B-12. The study population was normal.
RBC folate levels were dependent on assay method and supplement (Table 1, Fig. 1). The overall mean concentrations (main effects of assay method ranked low to high) were as follows: 1278 ± 39 nmol/L by L. casei < 1450 ± 32 nmol/L by chemiluminescence < 1646 ± 34 nmol/L by GC-MS < 1877 ± 46 nmol/L by RA (P < 0.001). In addition, the magnitude of the increases in RBC folate (nmol/L) in response to supplementation (ranked low to high) were as follows: 138 ± 63 nmol/L by chemiluminescence < 267 ± 64 nmol/L by GC-MS = 285 ± 75 nmol/L by L. casei < 351 ± 87 nmol/L by RA. RBC folate levels were dependent on assay method and pregnancy status (Table 1, Fig. 2) with pregnant women having a higher RBC folate concentration only when analyzed by chemiluminescence and RA.
The Block DFE screener had 21 questions (19 diet, 2 supplement) and measured usual and customary intake of DFEs. It included the 19 questions that account for 60% of total folate intake as DFEs in the U.S. diet, and 2 questions concerning vitamin supplements. The women enrolled in our study completed the screener in 6–12 min.
The intake of natural food (NF) folate was the same for the pregnant and nonpregnant women (Table 1). Because the pregnant group had a higher intake of SFA from fortified foods (breakfast cereals) than the nonpregnant group (P = 0.0269), there was a trend (P = 0.0644) for them to have a higher intake of total food folate (NF + SFA). Differences in the food (NF + SFA) intake did not differ between the baseline and supplementation periods (P = 0.5742).
The overall compliance with taking the folic acid supplement was as follows: 64% of women reported taking the supplement every day; 21% reported taking it 4–6 d/wk; 7.5% reported taking it 1–3 d/wk; and 7.5% reported that they had stopped taking it 30–60 d before giving birth. This level of compliance resulted in the supplement providing 596 ± 56 and 680 ± 39 µg/d (1351 ± 127 and 1542 ± 88 nmol/d) of SFA to pregnant and nonpregnant women (Table 1).
The interassay correlation coefficients (r) for RBC folate ranged from 0.4679 (GC-MS vs. chemiluminescence) to 0.8261 (L. casei vs. RA) (Fig. 3). The r-value for RA vs. GC-MS was 0.7063, the 4th highest of 6.
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DISCUSSION |
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TOPABSTRACTSUBJECTS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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1.85) before and after each received a daily supplement of 1814 nmol folic acid for 30–60 d. Also, we measured folate intake with a 1-page folate-targeted food/supplement frequency screener designed specifically for our study (Block DFE screener), and determined whether folate intake was correlated with RBC folate. Maternal RBC folate > 906 nmol/L, measured with Quantaphase IR Folate Radioassay (5), is considered optimal for prevention of folate-responsive neural tube defects (28). When we used chemiluminescence, GC-MS, and RA methods, only 5, 1, and 3 women, respectively, had RBC folate < 906 nmol/L. However, when we used L. casei, 24 women had RBC folate < 906 nmol/L. A goal of Healthy People 2010 is to increase RBC folate of nonpregnant women to 500 nmol/L (29) and only the L. casei assay identified women (n = 3) with RBC folate < 500 nmol/L in our study. Another research group (30) used the L. casei assay to determine RBC folate status in a similar population of women residing in California postfortification and not taking any supplements. Their respective levels for socioeconomically disadvantaged and advantaged women were 1172 ± 342 nmol/L and 1387 ± 329 nmol/L, levels similar to ours. Our mean RBC folate levels and those of Caudill et al. (29) both far exceeded 362 nmol/L (9), a concentration that is widely considered an acceptable cutoff value.
Only 1 subject in our present study had an RBC folate level < 362 nmol/L; the lowest individual RBC folate values were 333 nmol/L by L. casei, 653 nmol/L by chemiluminescence, 806 nmol/L by RA, and 894 nmol/L by GC-MS. Of the women in our study, 65% had RBC folate > 906 nmol/L, which compares well with the value of 78% reported by another research group (30). Using the L. casei method, the mean RBC folate concentration in the Framingham Offspring Cohort study after fortification was 1020 nmol/L (31), which agreed with L. casei-measured baseline values in Table 1. Our 30- to 60-d supplementation period was chosen to equal or exceed the recommended 4-wk period of folic acid use before conception to improve folate status (21,32,33) and/or the 4-wk period used in another study (34).
Since 1998, women capable of becoming pregnant have been advised to consume 907 nmol folic acid/d from fortified foods and supplements, in addition to dietary folate (21). The fortified foods and supplements together provided the pregnant and nonpregnant groups with 939 ± 55 and 950 ± 64 µg/d (2129 ± 125 and 2154 ± 145 nmol/d) of SFA, respectively (Table 1). These quantities of SFA almost matched those recommended since 1998 (21).
Participants in the present study consumed 600 µg/d (1360 nmol/d) supplement (SFA) and experienced (mean) increases in RBC folate of 138–351 nmol/L, depending on the assay used. Had we doubled the duration of the supplementation period to match the 120-d lifespan of RBCs, and/or increased the amount of the SFA supplement, we might have achieved a higher rise in RBC folate in response to supplementation. The rise in RBC folate remained linear with no evidence of saturation as total folate intake increased irrespective of the assay method (Fig. 3); therefore, the plateau level of RBC folate remains unknown. Further investigation is warranted to establish a reference range for RBC folate levels to prevent anemia and another to minimize the risk of more serious defects (35).
The protein binding folate assays (chemiluminescence and RA) are facile compared with the L. casei and GC-MS assays, which require several steps in sample preparation (16,17,36). Among the protein-binding assays, the magnitude of the increases in RBC folate (nmol/L) upon supplementation had a lower P-value with chemiluminescence (P < 0.0171) than with the RA assay (P < 0.0001), whereas the magnitude of the rise with the L. casei and GC-MS assays did not differ (Table 1, Fig. 1). When the protein-binding assays were employed, pregnant women had higher RBC folate levels than nonpregnant women (Table 1, Fig. 2), suggesting that these assays may be hormone sensitive. Finally, RBC folate, as determined by L. casei was correlated with the concentration measured by GC-MS with an r = 0.7185, the 4th highest of 6 (Fig. 3, Panel B). Therefore, it seems that the L. casei and GC-MS methods may offer hope for accurate RBC folate determinations that can serve as guides to optimal folate status.
The RBC folate as determined by RA was correlated with the concentration measured by GC-MS with an r = 0.7063 (Fig. 3, Panel C); this did not differ from the r = 0.7010 value reported by another group (26) who compared a number of commercially available protein-binding assay kits.
The Block DFE screener was effective in assessing dietary intake of folate because the DFEs measured by the screener were positively (and modestly) correlated with the results of all 4 folate assay methods (r = 0.26–0.46) and the r-values were significant (P < 0.0001) for all measures (Fig. 4). Yen et al. (37) found a similarly modest correlation (r = 0.354, P = 0.06) when they compared folate intake measured by 7 d of "focused recalls" with plasma folate levels. They also found no significant correlations between the Fred Hutchinson Cancer Research Center FFQ and plasma folate (37). Comparisons between correlations reported by Yen et al. (37) and those presented here for the Block DFE screener should be interpreted with caution, because the 2 dietary instruments have different purposes. The instrument of Yen et al. (37) was designed to measure short-term folate intake and to compare it with plasma folate, whereas the Block DFE screener was designed to measure usual, long-term folate intake and to compare it with RBC folate, a good biomarker of long-term folate nutritional status.
The Block DFE screener was designed to capture foods contributing the top 60% of folate intake (as DFE) in the United States, and was not intended to capture a point estimate that would reflect 100% of folate intake. Longer FFQ, however, are often designed to capture 80–90% of nutrient intake. The Block DFE screener produces lower estimates than a full-length FFQ, or than the truth, by design. The purpose was to create an instrument that would rank subjects well, while being brief. Block et al. (38,39) showed that shorter questionnaires such as the DFE screener rank subjects similarly to full-length FFQ.
We conclude that RBC folate and its response to folic acid supplementation was assay dependent; thus, method-specific RBC folate ranges may be necessary in evaluating published data at this time and setting cutoff values. Further comparisons among assay methods are warranted to identify the method that most accurately determines the actual RBC folate value. The folate screener was effective in assessing dietary folate intake. The adequacy of the folate nutritional status (of our subjects) reflects the fortified diets.
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FOOTNOTES |
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2 Supported by Grant/Cooperative Agreement (RO1 8928) from the Centers for Disease Control and Prevention (CDC) and National Institutes of Health DK 45939. The contents are the sole responsibility of the authors and do not necessarily represent the official views of the CDC. The microbiologic assays were conducted in the Clinical Nutrition Research Unit, at the University of California, Davis, which is supported by Grant DK 35747 from NIH.
4 Abbreviations used: DFE, dietary folate equivalents; FPL, federal poverty level; NF, natural food; NHANES, National Health and Nutrition Examination Survey; NTD, neural tube defect; PCV, packed cell volume; RA, radioassay; SFA, synthetic folic acid; WBC, white blood cell; WIC, Women, Infants, and Children.
Manuscript received 4 August 2004. Initial review completed 8 September 2004. Revision accepted 1 October 2004.
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LITERATURE CITED |
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TOPABSTRACTSUBJECTS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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1. Anonymous (1991) Prevention of neural tube defects: results of the Medical Research Council Vitamin Study. MRC Vitamin Study Research Group. Lancet 338:131-137.[Medline]
2. Czeizel, A. E., Dudas, I. & Metneki, J. (1994) Pregnancy outcomes in a randomised controlled trial of periconceptional multivitamin supplementation. Final report. Arch. Gynecol. Obstet. 255:131-139.
3. Boushey, C. J., Beresford, S. A., Omenn, G. S. & Motulsky, A. G. (1995) A quantitative assessment of plasma homocysteine as a risk factor for vascular disease. Probable benefits of increasing folic acid intakes. [see comments]J. Am. Med. Assoc. 274:1049-1057.
4. Jennings, E. (1995) Folic acid as a cancer-preventing agent. Med. Hypotheses 45:297-303.[Medline]
5. Brown, J. E., Jacobs, D. R., Jr, Hartman, T. J., Barosso, G. M., Stang, J. S., Gross, M. D. & Zeuske, M. A. (1997) Predictors of red cell folate level in women attempting pregnancy. J. Am. Med. Assoc. 277:548-552.
6. Wu, A., Chanarin, I., Slavin, G. & Levi, A. J. (1975) Folate deficiency in the alcoholic-its relationship to clinical and haematological abnormalities, liver disease and folate stores. Br. J. Haematol. 29:469-478.[Medline]
7. Molloy, A. M. (2000) Is impaired folate absorption a factor in neural tube defects?. Am. J. Clin. Nutr. 72:3-4.
8. Kloeblen, A. S. (1999) Folate knowledge, intake from fortified grain products, and periconceptional supplementation patterns of a sample of low-income pregnant women according to the Health Belief Model. J. Am. Diet Assoc. 99:33-38.[Medline]
9. Sauberlich, H. E. (1995) Folate status of U.S. population groups. Bailey, L. B. eds. Folate in Health and Disease 1995:171-194 Marcel Dekker New York, NY. .
10. Gunter, E. W., Bowman, B. A., Caudill, S. P., Twite, D. B., Adams, M. J. & Sampson, E. J. (1996) Results of an international round robin for serum and whole-blood folate. Clin. Chem. 42:1689-1694.
11. Wright, A. J., Finglas, P. M. & Southon, S. (1998) Erythrocyte folate analysis: a cause for concern?. Clin. Chem. 44:1886-1891.
12. Pawlosky, R. J., Flanagan, V. P. & Pfeiffer, C. M. (2001) Determination of 5-methyltetrahydrofolic acid in human serum by stable-isotope dilution high-performance liquid chromatography-mass spectrometry. Anal. Biochem. 298:299-305.[Medline]
13. Nelson, B. C., Pfeiffer, C. M., Margolis, S. A. & Nelson, C. P. (2003) Affinity extraction combined with stable isotope dilution LC/MS for the determination of 5-methyltetrahydrofolate in human plasma. Anal. Biochem. 313:117-127.[Medline]
14. Pfeiffer, C. M., Fazili, Z., McCoy, L., Zhang, M. & Gunter, E. W. (2004) Determination of folate vitamers in human serum by stable-isotope-dilution tandem mass spectrometry and comparison with radioassay and microbiologic assay. Clin. Chem. 50:423-432.
15. Nelson, B. C., Pfeiffer, C. M., Margolis, S. A. & Nelson, C. P. (2004) Solid-phase extraction-electrospray ionization mass spectrometry for the quantification of folate in human plasma or serum. Anal. Biochem. 325:41-51.[Medline]
16. Dueker, S. R., Lin, Y., Jones, A. D., Mercer, R., Fabbro, E., Miller, J. W., Green, R. & Clifford, A. J. (2000) Determination of blood folate using acid extraction and internally standardized gas chromatography-mass spectrometry detection. Anal. Biochem. 283:266-275.[Medline]
17. Lin, Y., Dueker, S. R., Jones, A. D. & Clifford, A. J. (2002) A parallel processing solid phase extraction protocol for the determination of whole blood folate. Anal. Biochem. 301:14-20.[Medline]
18. Lin, Y., Dueker, S. R. & Clifford, A. J. (2003) Human whole blood folate analysis using a selected ion monitoring gas chromatography with mass selective detection protocol. Anal. Biochem. 312:255-257.[Medline]
19. Swensen, A. R., Harnack, L. J. & Ross, J. A. (2001) Nutritional assessment of pregnant women enrolled in the Special Supplemental Program for Women, Infants, and Children (WIC). J. Am. Diet Assoc. 101:903-908.[Medline]
20. Owen, A. I. & Owen, G. M. (1997) Twenty years of WIC: a review of some of the effects of the program. J. Am. Diet Assoc. 97:777-782.[Medline]
21. Food and Nutrition Board, Institute of Medicine (1998) A Report of the Standing Committee on the Scientific Evaluation of Dietary References Intakes and Its Panel on Folate, Other B Vitamins and Choline and Subcommittee on Upper Reference Levels of Nutrients. 1998 Dietary Reference Intakes: Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Panthothenic Acid, Biotin, and Choline 1998 National Academy Press Washington, DC.
22. Anonymous (1992) The USDA Nutrient Database for Standard Reference. Online 1992 [http://www.nal.usda.gov/fnic/foodcomp/search/].
23. O’Broin, S. & Kelleher, B. (1992) Microbiological assay on microtitre plates of folate in serum and red cells. J. Clin. Pathol. 45:344-347.
24. Ford, E. S. & Bowman, B. A. (1999) Serum and red blood cell folate concentrations, race, and education: findings from the third National Health and Nutrition Examination Survey. Am. J. Clin. Nutr. 69:476-481.
25. Shane, B. (1980) Folate assay: a comparison of radioassay and microbiological methods. Clin. Chim. Acta 100:13-19.[Medline]
26. Brown, R. D., Jun, R., Hughes, W., Watman, R., Arnold, B. & Kronenberg, H. (1990) Red cell folate assays: some answers to current problems with radioassay variability. Pathology 22:82-87.[Medline]
27. Owen, W. E. & Roberts, W. L. (2003) Comparison of five automated serum and whole blood folate assays. Am. J. Clin. Pathol. 120:121-126.[Medline]
28. Daly, L. E., Kirke, P. N., Molloy, A., Weir, D. G. & Scott, J. M. (1995) Folate levels and neural tube defects, implications for prevention. J. Am. Med. Assoc. 274:1698-1702.
29. National Center for Health Statistics () Healthy People 2010. Summary of Objectives, Maternal, Infant and Child Health http:/www.health.state.mo.us/GLRequest/MCH/hp2010.html [accessed 07/30/04].
30. Caudill, M. A., Le, T., Moonie, S. A., Esfahani, S. T. & Cogger, E. A. (2001) Folate status in women of childbearing age residing in Southern California after folic acid fortification. J. Am. Coll. Nutr. 20:129-134.
31. Choumenkovitch, S. F., Jacques, P. F., Nadeau, M. R., Wilson, P. W., Rosenberg, I. H. & Selhub, J. (2001) Folic acid fortification increases red blood cell folate concentrations in the Framingham study. J. Nutr. 131:3277-3280.
32. Czeizel, A. E. & Dudas, I. (1992) Prevention of the first occurrence of neural-tube defects by periconceptional vitamin supplementation. N. Engl. J. Med. 327:1832-1835.[Abstract]
33. Anonymous (1992) Recommendations for the use of folic acid to reduce the number of cases of spina bifida and other neural tube defects. Morb. Mortal. Wkly. Rep. 41:1-7.
34. Brouwer, I. A., van Dusseldorp, M., Thomas, C. M., Duran, M., Hautvast, J. G., Eskes, T. K. & Steegers-Theunissen, R. P. (1999) Low-dose folic acid supplementation decreases plasma homocysteine concentrations: a randomized trial. Am. J. Clin. Nutr. 69:99-104.
35. Green, N. S. (2002) Folic acid supplementation and prevention of birth defects. J. Nutr. 132:2356S-2360S.
36. Tamura, T. (1990) Microbiological assay of folates. Picciano, M. F. Stokstad, E.L.R. Gregory, J. F., III eds. Folic Acid Metabolism in Health and Disease. Contemporary Issues in Clinical Nutrition 13:121-137 Wiley-Liss New York, NY. .
37. Yen, J., Zoumas-Morse, C., Pakiz, B. & Rock, C. L. (2003) Folate intake assessment: validation of a new approach. J. Am. Diet. Assoc. 103:991-1000 discussion 1000.[Medline]
38. Block, E., Hartman, A. M. & Naughton, D. (1990) A reduced dietary questionnaire: development and validation. Epidemiology 1:58-64.[Medline]
39. Cummings, S. R., Block, G., McHenry, K. & Baron, R. B. (1987) Evaluation of two food frequency methods of measuring dietary calcium intake. Am. J. Epidemiol. 126:796-802.
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 S.S. Young, B. Eskenazi, F.M. Marchetti, G. Block, and A.J. Wyrobek The association of folate, zinc and antioxidant intake with sperm aneuploidy in healthy non-smoking men Hum. Reprod., May 1, 2008; 23(5): 1014 - 1022. [Abstract] [Full Text] [PDF]
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 Z. Fazili, C. M. Pfeiffer, M. Zhang, and R. Jain Erythrocyte Folate Extraction and Quantitative Determination by Liquid Chromatography-Tandem Mass Spectrometry: Comparison of Results with Microbiologic Assay Clin. Chem., December 1, 2005; 51(12): 2318 - 2325. [Abstract] [Full Text] [PDF]
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 L. L. Iannotti, K. O. O'Brien, S.-C. Chang, J. Mancini, M. Schulman-Nathanson, S. Liu, Z. L. Harris, and F. R. Witter Iron Deficiency Anemia and Depleted Body Iron Reserves Are Prevalent among Pregnant African-American Adolescents J. Nutr., November 1, 2005; 135(11): 2572 - 2577. [Abstract] [Full Text] [PDF]
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T. L. Yang, J. Hung, M. A. Caudill, T. F. Urrutia, A. Alamilla, C. A. Perry, R. Li, H. Hata, and E. A. Cogger A Long-Term Controlled Folate Feeding Study in Young Women Supports the Validity of the 1.7 Multiplier in the Dietary Folate Equivalency Equation J. Nutr., May 1, 2005; 135(5): 1139 - 1145. [Abstract] [Full Text] [PDF]
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