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Folate Status in Women of Childbearing Age Residing in Southern California after Folic Acid Fortification

Food, Nutrition and Consumer Sciences Department (M.A.C., T.A., S.A.M., S.T.E.), California State Polytechnic University, Pomona, California
Animal and Veterinary Sciences Department (E.A.C.), California State Polytechnic University, Pomona, California

Address reprint requests to: Marie A. Caudill, PhD, RD, Food, Nutrition and Consumer Sciences Department, California State Polytechnic University, 3801 W. Temple Ave, Pomona, CA 91768. E-mail: macaudill{at}csupomona.edu

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Objective: The purpose of this study was to investigate folate status in healthy, nonpregnant women (18 to 45 years) following folic acid (FA) fortification of the food supply.

Design: This was a cross-sectional study design in which a fasting blood sample was obtained from socio-economically advantaged (n=85) and disadvantaged (n=50) women residing in Southern California who had not consumed supplemental FA within the past 12 months. Serum folate (SF), red cell folate (RCF) and plasma homocysteine (tHcy) concentrations were measured and methylene tetrahydrofolate reductase (MTHFR) genotype (C677T) was determined.

Results: SF and RCF concentrations (mean±SD) for socio-economically advantaged (54±18, 1387±329 nmol/L, respectively) and disadvantaged women (41±18, 1172±342 nmol/L, respectively) greatly exceeded the levels deemed acceptable for SF (13.6 nmol/L) and RCF (362 nmol/L). Moreover, 95% of socio-economically advantaged women and 78% of disadvantaged women achieved RCF concentrations 906 nmol/L, which are associated with very low risk of neural tube defects (NTD). Plasma tHcy concentrations for both socio-economically advantaged (5.2 ± 1.6 µmol/L) and disadvantaged women (6.1±1.6 µmol/L) were within the lower limit of normal range and indicative of adequate folate status. For the combined groups (n=135), the frequency of the C/C, C/T and T/T genotype was 56.0, 37.3 and 6.7%, respectively. MTHFR genotype was not associated with SF, RCF or tHcy.

Conclusions: These data suggest that women of childbearing age are achieving positive folate balance and RCF concentrations associated with reduced risk of NTD following FA fortification of the food supply.

Key words: folate, fortification, homocysteine, MTHFR, women, folic acid

	INTRODUCTION

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Randomized intervention trials have demonstrated that periconceptional consumption of folic acid reduces the incidence of neural tube defects (NTDs) by up to 75% [1,2], although the mechanism remains unknown. Maternal red cell folate concentrations 906 nmol/L (400 ng/mL) are associated with very low risk of NTDs [3], while elevated blood concentrations of homocysteine, indicative of poor folate status, are associated with increased risk [4]. A higher incidence of NTDs has been reported in women with a mutation (C677T) in the gene coding for methylene tetrahydrofolate reductase (MTHFR) [5], which functions to convert 5,10-methylene-tetrahydrofolate (THF) to 5-methyl-THF, the form of folate necessary for homocysteine metabolism and the major circulatory form. Under conditions of suboptimal folate intake, homozygosity for the MTHFR C677T mutation is associated with lower plasma folate and higher homocysteine concentrations [6,7].

In 1992, the US Public Health Service issued the recommendation that all women of childbearing age consume 400 µg of folic acid per day to reduce risk of having an NTD-affected pregnancy [8]. In an effort to help women of childbearing age increase folic acid consumption, the Food and Drug Administration (FDA) mandated that, beginning January 1998, all enriched cereal-grain products be fortified with folic acid [9]. The current level of fortification, 140 µg of folic acid per 100 grams of cereal-grain product [9], is expected to deliver an additional 80 to 100 µg of folic acid to the diets of the target population, women of childbearing age [10]. Because this level of intake is below the minimum amount reported to reduce incidence of NTDs (400 µg/day of folic acid), it has been criticized, and recommendations to fortify at twice the current level have been made [11].

To date, only two studies have examined the impact of folic acid fortification of the food supply on folate status. Jacques and co-workers [12] reported significant increases in plasma folate concentrations and decreases in plasma homocysteine values in the Framingham offspring cohort after folic acid fortification. Substantial increases in serum folate concentrations after fortification in blood specimens submitted to Kaiser Permanente’s Southern California Endocrinology Laboratory were also observed by Lawrence and colleagues [13]. However, because only a small proportion of the populations studied consisted of women younger than 40 years, the impact of folic acid fortification on folate status in the target population, women of reproductive age, could not be assessed.

The primary purpose of this study was to provide preliminary data on folate status in women of childbearing age (18 to 45 years of age, n=135) post folic acid fortification of the food supply. Folate status was assessed by measurement of serum and red cell folate and plasma tHcy concentrations, a functional index of folate status. In addition, MTHFR genotype (C677T) was determined and associations with blood folate concentrations, plasma homocysteine and race/ehtnic group examined.

	MATERIALS AND METHODS

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Subjects
A total of 135 healthy women aged 18 to 45 years participated in this study. Eighty-five of the women were socio-economically advantaged women (>200% federal poverty level 1999) and were recruited among students attending Cal Poly Pomona University in Southern California. The remainder (n<50) were socio-economically disadvantaged women (200% federal poverty level 1999) and were recruited from Pomona and surrounding areas in Los Angeles county. Women were excluded from participation if they were smokers, pregnant, lactating, consuming supplements containing folic acid within the past twelve months, taking medication known to interfere with folate metabolism or had a history of chronic disease.

Protocol
This was an observational study design using a non-random, convenience sample. Socio-economically advantaged women volunteers were recruited from Cal Poly Pomona University from 1/99 to 6/99 via flyers. Socio-economically disadvantaged women were recruited from 7/99 to 12/99 by means of newspaper advertisements and were paid (gift certificates or monetary compensation) for their participation. After obtaining informed consent, background information about the subjects was acquired via questionnaire. Socio-economically advantaged women completed the Block Food Frequency Questionnaire which was analyzed by Block Dietary Data Systems [14]. Because of their low literacy rate, socio-economically disadvantaged women completed a 24-hour dietary recall. Nutrient intake obtained via the 24-hour dietary recall was computed by THE FOOD PROCESSOR® (Version 7.4; Nutrient Analysis System, ESHA Research, Salem, OR). A one-time fasting (ten-hour) blood sample was collected for determination of serum folate, red cell folate, and plasma tHcy concentrations as well as MTHFR (C677T) genotype. The study protocol was approved by the Cal Poly Pomona University Institutional Review Board.

Sample Analyses
Blood (30 mL) was collected into serum separator tubes for serum folate analysis and into EDTA containing tubes for red cell folate, tHcy and MTHFR genotype analyses. The EDTA tube for tHcy analysis was immediately put on ice and centrifuged within one hour of the blood draw. Whole blood for determination of red cell folate concentrations was diluted 1:20 with freshly prepared ascorbic acid (0.1%) and mixed continuously at room temperature for 30 minutes. White blood cells containing genomic DNA (500 µL) were collected following centrifugation and removal of plasma for future analyses of MTHFR genotype. Dimethyl sulfoxide (50 µL) was added to each sample to preserve the DNA. All samples were stored at -30°C until analyzed. Serum and red cell folate concentrations were determined in triplicate by microbial (Lactobacillus casei) assay in 96 well plates [15]. The intra- and inter-assay CV of the positive control were 8% and 9%, respectively. Total plasma homocysteine concentrations were measured in duplicate by a modified high-performance liquid chromatography with fluorometric detection [16,17] and had an intra- and inter-assay CV of 3% and 5%, respectively. The three genotypes for MTHFR (C/C, C/T and T/T) were determined by agarose (3%) gel electrophoresis with ethidium bromide following genomic DNA extraction from white blood cells, PCR-amplification and treatment with HinfI [18].

Statistical Analyses
All data summarization and analyses were performed using SPSS® 9.0.0 for WINDOWS. Summaries of scale variables are presented as means±SD, unless otherwise noted, and nominal variables as number (n) and/or percentage (%). Statistical significance was set at p 0.05. For comparisons of folate status between levels of categorical variables (race, genotype and socio-economic standing), independent t tests (levels=2) or one-way ANOVA (levels >2) were used. Where a significant effect was detected by ANOVA, the Duncan’s New Multiple Range test was used for mean separation. The Levene’s test was used to test the equality of variance assumption. A Pearson correlation coefficient (r) was used to measure association between serum folate, red cell folate, tHcy and folate intake. The relationship between MTHFR genotype and race was tested using the Pearson chi-square for contingency tables for the combined genotype data.

	RESULTS

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Socio-Economically Advantaged Women
Eighty-five socio-economically advantaged women participated in this study. Most of the subjects were Asian (n=44) or Caucasian (n=25), although twelve were Hispanic and four were African-American. The women averaged 26 years in age (range=18 to 44 years) and had a BMI (kg/m²) of 22.7 (range=16.0 to 39.5). The average family size was three, and household incomes ranged from less than $24,000 (n=20) to greater than $72,000 (n=14).

Mean serum and red cell folate concentrations as well as plasma tHcy are shown in Table 1. Total dietary folate intake was 386±172 µg/day and correlated (r=0.22, p 0.05) with serum folate. Red cell and serum folate were strongly correlated (r=0.375, p<0.01), but neither were correlated with plasma tHcy. Asian women had lower (p0.05) red cell folate concentrations compared to Hispanic women (Table 2). Hispanic and African-American (4.0±1.1 and 4.5±1.5 µmol/L, respectively) women had lower (p0.05) tHcy compared to Caucasian women (5.9±1.3 µmol/L; Table 2). Fifty of the women were homozygous wildtype (C/C) for the MTHFR C677T genotype, 29 were heterozygous (C/T), and five were homozygous mutant (T/T). Data on genotype for one of the participants was missing. The frequency of the C/C, C/T and T/T genotype by race/ethnic group is illustrated in Table 3. No differences (p>0.05) were found in serum folate, red cell folate or tHcy between the three different genotypes (Table 4).

Serum and red cell folate concentrations as well as plasma tHcy are shown in Table 1. Total dietary folate intake was 297±137 µg/day and did not correlate with indices of folate status. Red cell and serum folate were weakly correlated (r=0.251, p=0.079) and neither correlated with tHcy. No differences (p>0.05) in the variables measured (serum folate, red cell folate and tHcy) were detected between races/ethnic groups (Table 2). Twenty-five of the women were homozygous wildtype (C/C) for the MTHFR C677T genotype, 21 were heterozygous (C/T), and four were homozygous mutant (T/T). The frequency of the C/C, C/T and T/T genotype by race/ethnic group is detailed in Table 3. All of the four participants with the T/T MTHFR genotype were Hispanic. No differences (p>0.05) were found in serum folate, red cell folate or tHcy concentrations between the three different genotypes (Table 4).

Combined Groups
Combined mean serum folate, red cell folate and tHcy are listed in Table 1. Socio-economically advantaged women had higher (p0.05) serum and red cell folate concentrations and lower (p0.05) tHcy levels than socio-economically disadvantaged women (Table 1). Caucasian women had higher (p0.05) serum folate concentrations than either Hispanic or African-American women and had the highest (p0.05) red cell folate values (Table 2). No differences (p>0.05) were found between races/ethnic groups in tHcy (Table 2). Seventy-five of the women were homozygous wildtype (C/C) for the MTHFR C677T genotype, 50 were heterozygous (C/T), and nine were homozygous mutant (T/T) (Table 3). There was a significant (p0.05) interaction between race and genotype, with the Hispanic women showing the highest incidence of the T/T genotype and the African-American women the lowest (Table 3). The interaction persisted when the African-American women were excluded. No differences (p>0.05) were found in serum folate, red cell folate or tHcy between the three different genotypes (Table 4).

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
We investigated folate status following folic acid fortification of the food supply in socio-economically advantaged (n=85) and disadvantaged women (n=50) residing in Southern California who had not consumed supplemental folic acid for at least twelve months. The mean serum folate concentration when all subjects were included was 50 nmol/L and greatly exceeded the lower limit of the acceptable range (ie, 13.6 nmol/L, [19]). No subject was found to have a serum folate value indicative of compromised folate status. The mean red cell folate concentration (data combined) was 1307 nmol/L and was four times higher than the level deemed acceptable, 362 nmol/L [19]. Eighty-eight percent of all participants had red cell folate concentrations >906 nmol/L, which is associated with very low NTD risk [3]. After separating subjects by race/ethnicity, mean red cell folate concentration was >906 nmol/L in each ethnic group, with Caucasian women having the highest (p0.05) concentrations. Mean plasma tHcy concentrations (5.5 µmol/L) fell within the lower end of normal range [20] and reflected the positive folate balance of these participants. Of the 134 women who were genotyped, nine were homozygous for the MTHFR C677T mutation (ie, T/T). No association was found between the T/T genotype and tHcy; this is consistent with the finding that individuals who are homozygotes for the MTHFR mutation (T/T) have elevated homocysteine concentrations only when plasma folate concentration are low-normal, i.e., <15.4 nmol/L [6]. The incidence of the T/T genotype was substantially more frequent (p0.05) among the Hispanic women than among non-Hispanic women; this supports the findings of Shaw and associates [21]. However, it should be noted that the Caucasian and Asian women had a lower than expected T/T genotype frequency [18,22], which may have resulted from the relatively small sample size of the present study.

To date, two studies have investigated the effect of folic acid fortification of the food supply on folate status. Using post-fortification blood samples obtained from September 1997 to March 1998, Jacques and associates [12] reported an increase in mean plasma folate from 10 to 23 nmol/L in a population of middle aged to older adults. Similarly, using clinical specimens obtained from 1994 through 1998, Lawrence and co-workers [13] reported an increase in median serum folate values from 29 to 41 nmol/L in Kaiser clients residing in Southern California. The post-folic acid fortification status of the present study is more comparable to the Kaiser study, possibly due to geographic location and/or the fact that folic acid fortification had been in place for longer. The length of exposure to folic acid may result in higher blood folate levels, as suggested by data from the Kaiser study, in which median serum folates increased from 32 nmol/L in 1997 to 41 nmol/L in 1998.

Previous findings indicate that the incidence of NTDs is greater in socio-economically disadvantaged women [23,24], possibly related to less supplemental multivitamin consumption during the periconceptional period [25]. Our data suggest that socio-economically disadvantaged women are benefitting from folic acid fortification. Mean serum and red cell folate concentrations were higher (p0.05) than pre-fortification data reported in NHANES III (14 and 399 nmol/L, respectively) [26] and/or the Kaiser study [13]. Although these differences may be partially explained by differences in sampling and analytical methods, it is clear that folic acid fortification is reaching at least some segments of the lower income, minority women of child-bearing age. Moreover, 78% of the socioeconomically disadvantaged women had red cell folates >906 nmol/L, and none had elevated homocysteine values. Compared to socio-economically advantaged women, disadvantaged women had lower (p0.05) blood folate concentrations. This observation may be related to the racial/ethnic differences between the two groups. The socio-economically advantaged women were mostly Asian or Caucasian in contrast to the disadvantaged participants who were primarily Hispanic. Ford and colleagues [27] found higher serum and red cell folate concentrations in Caucasian women compared to Mexican-American women.

The current level of folic acid fortification (140 µg of folic acid per 100 g cereal-grain product) is estimated to deliver an additional 100 µg/day of folic acid to the diets of women of childbearing age [10]. Since 400 µg/day of folic acid is the quantity recommended for reduction of NTDs [28], it would appear necessary for the majority of women to consume supplemental folic acid in order to achieve this goal. The high concentrations of folate found in the red cell and serum in the present study indicate that folic acid fortification is delivering greater than the estimated 100 µg/day of folic acid. For example, Daly and associates [29] randomly assigned women to consume either 0, 100, 200 or 400 µg/day of folic acid in addition to their usual dietary folate intake. At the end of the six-month treatment period, median red cell folate concentrations measured with the microbiological assay were 705, 850, 1076 and 1294 nmol/L, respectively, and differed significantly from each other (p=0.001). The median RCF concentration for all women in the present study was 1246 nmol/L (95% CI for median: 1242–1360 nmol/L); this implies that fortification is delivering an additional 200 to 400 µg/day of folic acid. Although total dietary folate intake was assessed in the present study and was 100–200 µg/day higher than nationwide pre-fortification intakes [30], independent determination of folic acid intake could not be performed because food composition tables do not distinguish between food folate and synthetic folic acid [31].

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
These preliminary data suggest that folic acid fortification of the food supply is resulting in significant improvements in folate status in women of reproductive age and, based on folate status, greater than 100 µg/day of folic acid is being delivered to the target population. Caution, however, is warranted in that these results may not be generalized to all women of childbearing age, as this was a non-random, convenience sample from one geographic location. In addition, this study lacked a pre-folic acid fortified control group and comparisons of serum and red blood cell folate results obtained from different laboratories and methods may not be directly comparable [32]. Future data generated by NHANES will provide more definitive data on the impact of folic acid fortification on folate status nationwide. Importantly, studies are still needed to address the impact of folic acid fortification on incidence of NTDs.

	ACKNOWLEDGMENTS

 
Supported in part by the National Institute of Health (S06GM53933) and the California Agricultural Research Initiative. We thank Walmart® and Target® for their donations of gift certificates for low-income participants. We also gratefully acknowledge the Student Health Center at Cal Poly Pomona University for its assistance in the blood draws.

Received November 1, 2000. Accepted January 22, 2001.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

1. MRC Vitamin Study Research Group: Prevention of NTDs: results of the Medical Research Council Vitamin Study. Lancet 338: 131–137, 1991.[Medline]
2. Czeizel AE, Dudas I: Prevention of the first occurrence of neural tube defects by periconceptional vitamin supplementation. N Engl J Med 327: 1832–1835, 1992.[Abstract]
3. Daly LE, Kirke PN, Molloy A, Weir DG, Scott JM: Folate levels and neural tube defects, implications for prevention. JAMA 274: 1696–1702, 1995.
4. Vollset SE, Refsum H, Irgens LM, Emblem BM, Tverdal A, Gjessing HK, Monsen ALB, Ueland PM: Plasma total homocysteine, pregnancy complications, and adverse pregnancy outcomes: the Hordaland Homocysteine Study. Am J Clin Nutr 71: 962–968, 2000.[Abstract/Free Full Text]
5. van der Put NM, Eskes TK, Blom HJ: Is the common 677CT mutation in the methylenetetrahydrofolate reductase gene a risk factor for neural tube defects? A meta-analysis. QJM 90: 111–115, 1997.[Abstract/Free Full Text]
6. Jacques PF, Bostom AG, Williams RR, Ellison RC, Eckfeldt JH, Rosenberg IH, Selhub J, Rosen R: Relation between folate status, a common mutation in methylenetetrahydrofolate reductase, and plasma homocysteine concentrations. Circulation 93: 7–9, 1996.[Abstract/Free Full Text]
7. Zittoun J, Tonetti C, Bories D, Pignon J, Tulliez M: Plasma homocysteine levels related to interactions between folate status and methylenetetrahydrofolate reductase: a study in 52 healthy subjects. Metabolism 47: 1413–1418, 1998.[Medline]
8. Anonymous: Recommendations for the use of folic acid to reduce the number of cases of spina bifida and other neural tube defects. MMWR Morb Mortal Wkly Rep 41: 1–7, 1992.
9. US Department of Health and Human Services, Food and Drug Administration: Food standards: Amendment of standards of identity for enriched grain products to require addition of folic acid. Fed Regist 61: 8781–8797, 1996.
10. Crane NT, Wilson DB, Cook A, Lewis CJ, Yetley EA, Rader JI: Evaluating food fortification options: general principles revisited with folic acid. Am J Public Health 85: 660–666, 1995.[Abstract/Free Full Text]
11. Oakley GP, Adams MJ, Dickinson CM: More folic acid for everyone, now. J Nutr 126: 751S–755S, 1996.
12. Jacques PF, Selhub J, Bostom AG, Wilson PWF, Rosenberg IH: The effect of folic acid fortification on plasma folate and total homocysteine concentrations. N Engl J Med 340: 1449–1454, 1999.[Abstract/Free Full Text]
13. Lawrence JM, Petitti DB, Watkins M, Umekubo MA: Trends in serum folate after food fortification. Lancet 354: 915–916, 1999.[Medline]
14. Block G, Hartman AM, Dresser CM, Carroll MD, Gannon J, Gardner L: A data-based approach to diet questionnaire design and testing. Am J Epidemiol 124: 453–469, 1986.[Abstract/Free Full Text]
15. Tamura T: Microbiological assay of folates. In Picciano MF, Stokstad ELR, Gregory JF (eds): "Folic Acid Metabolism in Health and Disease." New York: John Wiley & Sons, pp 121–137, 1990.
16. Vester B, Rasmussen K: High performance liquid chromatography method for rapid and accurate determination of homocysteine in plasma and serum. Eur J Clin Chem Clin Biochem 29: 549–554, 1991.[Medline]
17. Pfeiffer CM, Huff DL, Gunter EW: Rapid and accurate HPLC assay for plasma total homocysteine and cysteine in a clinical setting. Clin Chem 45: 290–292, 1999.[Free Full Text]
18. Frosst P, Blom H, Milos R, Goyette P, Sheppard CA, Matthews RG, Boers GJH, den Heijer M, Kluijtmans LA, van den Heuvel LP, Rozen R: A candidate genetic risk factor for vascular disease: a common mutation in methylenetetrahydrofolate reductase. Nat Genet 10: 111–113, 1995.[Medline]
19. Sauberlich HE, Dowdy RP, Skala JH: Folacin. "Laboratory Tests for the Assessment of Nutritional Status." Cleveland: CRC Press Inc, pp. 49–57, 1974.
20. Ueland PM, Refsum H, Stabler SP, Malinow MR, Andersson A, Allen RH: Total homocysteine in plasma or serum: methods and clinical applications. Clin Chem 39: 1764–1779, 1993.[Abstract]
21. Shaw GM, Rozen R, Finnell RH, Wasserman CR, Lammer EJ: Maternal vitamin use, genetic variation of infant methylenetetrahydrofolate reductase, and risk for spina bifida. Am J Epidemiol 148: 30–37, 1998.[Abstract/Free Full Text]
22. Botto LD, Yang Q: 5,10-methylenetetrahydrfolate reductase gene variants and congenital anomalies. a HuGE review. Am J Epidemiol 151: 862–877, 2000.[Abstract/Free Full Text]
23. Nevin NC, Johnston WP, Merrett JD: Influence of social class on the risk of recurrence of anencephalus and spina bifida. Develop Med Child Neurol 23: 155–159, 1981.[Medline]
24. Elwood JM, Little J, Elwood JH: Epidemiology and control of neural tube defects. New York: Oxford University, 1992.
25. Friel JK, Frecker M, Fraser CF: Nutritional patterns of mothers of children with neural tube defects in Newfoundland. Am J Med Genet 55: 195–199, 1995.[Medline]
26. Wright JD, Bialostosky K, Gunter EW, Carroll MD, Najjar MF, Bowman BA, Johnson CL: Blood folate and vitamin B12: United States, 1988–1994. Vital Health Stat 11.243: 1ndash;78, 1998.
27. Ford ES, Bowman BA: 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, 1999.[Abstract/Free Full Text]
28. Institute of Medicine. Subcommittee on Folate, Other BVitamins, and Choline. Dietary Reference Intakes: Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline. Washington, DC: National Academy Press, 1998.
29. Daly S, Mills JL, Molloy AM, Conley M, Lee YJ, Kirke PN, Weir DG, Scott JM: Minimum effective dose of folic acid for food fortification to prevent neural-tube defects. Lancet 350: 1666–1669, 1997.[Medline]
30. Alaimo K, McDowell MA, Briefell RR, Bischof AM, Gaughman GR, Loria CM, Johnson GL: Dietary intake of vitamins, minerals and fiber of persons ages 2 months and over in the United States: Third National Health and Nutrition Examination Survey, Phase 1, 1988-1991. Advance Data from Vital and Health Statistics, No. 258. Hyattsville, MD: National Center for Health Statistics, 1994.
31. Suitor CW, Bailey LB: Dietary folate equivalents: interpretation and application. J Am Diet Assoc 100: 88–94, 2000.[Medline]
32. Gunter EW, Bowman BA, Caudill SP, Twite DB, Adams MJ, Sampson IJ: Results of an international round robin for serum and whole-blood folate. Clin Chem 42: 1689–1694, 1996.[Abstract/Free Full Text]

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