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Folate Status in Women of Childbearing Age, by Race/Ethnicity --- United States, 1999--2000

In September 1992, the U.S. Public Health Service (PHS) recommended that women of childbearing age (i.e., aged 15--44 years) who are capable of becoming pregnant should consume 400 µg of the B-vitamin folic acid to reduce the number of cases of spina bifida and anencephaly (neural tube defects [NTDs]) (1). Since then, an ongoing national effort has encouraged women to consume dietary supplements containing folic acid (2). In 1998, the Food and Drug Administration (FDA) required the fortification of enriched cereal grain products with folic acid (3), and manufacturers have voluntarily added more folic acid to many ready-to-eat breakfast cereals (CDC, unpublished data, 2002). To assess temporal changes in serum and red blood cell (RBC) folate concentrations among childbearing-aged women, CDC compared folate concentrations for childbearing-aged women who participated during 1988--1994 and 1999--2000 in the National Health and Nutrition Examination Survey (NHANES). This report outlines the results of this comparison and describes serum and RBC folate levels by race/ethnicity. The results indicate that over the period studied, the median serum folate concentration for women aged 15--44 years increased approximately threefold, and the median RBC folate concentration increased approximately twofold. These findings indicate that the national health objective for 2010 to increase the median RBC folate level among women of childbearing age to 220 ng/mL RBC (objective 16.16b) has been met for Mexican-American* and non-Hispanic white women but not for non-Hispanic black women. To reduce the number of pregnancies affected by NTDs further, all women of childbearing age capable of becoming pregnant should consume the PHS-recommended level of folic acid daily (1).

Both NHANES 1988--1994 and NHANES 1999--2000 used a stratified, multistage probability sample of the civilian, U.S. noninstitutionalized population. NHANES 1988--1994 included persons aged >2 months, and NHANES 1999--2000 included persons of all ages. A household interview and a physical examination were conducted for each survey participant. During the physical examination, blood was collected by venipuncture for all persons aged >1 year. Serum and RBC folate were measured in CDC's NHANES Central Laboratory for both NHANES 1988--1994 and NHANES 1999--2000 by using the Bio-Rad Quantaphase II simultaneous folate/vitamin B12 radioassay (Bio-Rad Laboratories, Hercules, California) (4). Long-term quality-control data for these assays, including overlapping control materials that were used in both surveys, indicated no analytical drift; results of all external proficiency testing challenges were graded as satisfactory.

From NHANES 1988--1994 to NHANES 1999--2000, median serum folate concentrations for women aged 15--44 years increased from 4.8 to 13.0 ng/mL. The 10th percentile increased from 2.3 to 6.4 ng/mL, and the 75th percentile increased from 7.8 to 18.1 ng/mL (Table 1). Serum folate concentrations increased for each of the three racial/ethnic populations for which estimates could be made (Table 2); in NHANES 1999--2000, the median serum folate concentration was highest for non-Hispanic whites (13.8 ng/mL) and lowest for non-Hispanic blacks (10.2 ng/mL). Similar results were obtained for RBC folate, a better measure of long-term folate status. Median RBC folate concentrations for women aged 15--44 years increased from 159.9 to 263.6 ng/mL RBC (Table 1). RBC folate concentrations increased for all racial/ethnic groups studied (Table 2). In NHANES 1999--2000, the highest RBC folate concentrations occurred among non-Hispanic white women (median: 278.1 ng/mL RBC) and were lowest among non-Hispanic black women (median: 213.8 ng/mL RBC).

Reported by: JD Erickson, DDS, J Mulinare, MD, Q Yang, PhD, National Center on Birth Defects and Developmental Disabilities; CL Johnson, MSPH, National Center for Health Statistics; C Pfeiffer, PhD, EW Gunter, National Center for Environmental Health; WH Giles, MD, BA Bowman, PhD, National Center for Chronic Disease and Health Promotion, CDC.

Editorial Note:

Results from NHANES 1999--2000, which was conducted after implementation of food fortification and educational efforts to increase folate consumption, indicate that these public health actions have been effective in increasing folate status among U.S. women of childbearing age. The findings are consistent with reports of improved folate status in selected subsets of the U.S. population (5,6) and the results from the 1999 sample of NHANES 1999--2000 (7). In 1999, the median serum folate concentration was 14.5 ng/mL, and the median RBC folate concentration was 293 ng/mL RBC, somewhat higher than the 1999--2000 results. Both serum and RBC folate concentrations for the 2000 sample of NHANES 1999--2000 were lower than the 1999 sample.

Annual variations in estimates from NHANES can be expected because of the size of the yearly sample and the nature of the sampling design, and these differences might be explained by sampling variability. Other possible explanations include reduced consumption of foods and supplements containing folic acid, a decrease in the folic acid content of fortified foods (8), or undetected variations in laboratory technique. Because of the high level of long-term assay quality-control, changes in laboratory technique probably do not account for the lower folate concentrations in 2000. Another line of evidence suggesting that variations in laboratory technique are not likely to explain the lower folate concentrations in 2000 is that vitamin B12 concentrations in women of childbearing age did not change from NHANES 1988--1994 to NHANES 1999--2000, even though they are measured simultaneously in the same assay as the folate concentrations.

Women of each of the three racial/ethnic populations compared had substantial increases in serum and RBC folate concentrations over time, indicating that women of all racial/ethnic groups examined have benefitted from the public health actions to improve folate status. However, disparities in blood folate concentrations among racial/ethnic groups remain.

Women of childbearing age in the United States who are capable of becoming pregnant should consume 400 µg (0.4 mg) of folic acid per day to reduce their risk for having a pregnancy affected with spina bifida or other NTDs (1). The use of vitamin supplements containing folic acid before and during early pregnancy reduces the risk for NTDs (1). Increases in the reported use of vitamin supplements have been small (9). Because approximately 50% of all pregnancies are unplanned, and because NTDs occur early in pregnancy before many women are aware that they are pregnant, food fortification is probably the most important approach to delivery of folic acid at conception and during early pregnancy.

The data presented in this report are subject to at least one limitation. Because the sample size in NHANES 1999--2000 is smaller than that of NHANES 1988--1994, more data are necessary to allow more detailed analyses of trends in blood folate concentrations in all population subgroups, particularly among young women of varying socioeconomic status.

One of the national health objectives for 2010 is to increase the proportion of pregnancies begun with an optimum folic acid level by increasing the median RBC folate level to 220 ng/mL RBC among women aged 15--44 years (objective 16.16b) (10). On the basis of NHANES 1999--2000, this objective has been met for non-Hispanic white and Mexican-American women but has not been met for non-Hispanic black women. For the number of pregnancies affected by NTDs to be reduced further, all women of childbearing age capable of becoming pregnant should consume the PHS-recommended level of folic acid daily (1).

References

  1. CDC. Recommendations for the use of folic acid to reduce the number of cases of spina bifida and other neural tube defects. MMWR 1992;41 (No. RR-14).
  2. CDC. Preventing neural tube defects: a prevention model and resource guide. Atlanta, Georgia: U.S. Department of Health and Human Services, CDC, 1998.
  3. Food and Drug Administration. Food standards: amendment of standards of identity for enriched grain products to require addition of folic acid. Federal Register 1996;61:8781--97.
  4. Wright JD, Bialstosky K, Gunter EW, et al. Blood folate and vitamin B12: United States, 1988--1994. U.S. Department of Health and Human Services, CDC, National Center for Health Statistics. Vital Health Stat 11(243), 1999.
  5. Jacques PF, Selhub J, Bostom AG, Wilson PW, Rosenberg IH. The effect of folic acid fortification on plasma folate and total homocysteine concentrations. N Engl J Med 1999;340:1449--54.
  6. Lawrence JM, Petitti DB, Watkins M, Umekubo MA. Trends in serum folate after food fortification. Lancet 1999;354:915--6.
  7. CDC. Folate status in women of childbearing age---United States, 1999. MMWR 2000;49:962--5.
  8. Rader JI, Weaver CM, Angyal G. Total folate in enriched cereal-grain products in the United States following fortification. Food Chem 2000;70:275--89.
  9. CDC. Knowledge and use of folic acid by women of childbearing age---United States, 1995 and 1998. MMWR 1999;48:325--7.
  10. U.S. Department of Health and Human Services. Healthy people 2010, 2nd ed. With understanding and improving health and objectives for improving health (2 vols). Washington, DC: U.S. Department of Health and Human Services, 2000.

* Numbers for other Hispanic women were too small for meaningful analysis.


Table 1

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Table 2

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