ABSTRACT
Background
Enriched cereal-grain products have been fortified in the United States for >20 y to improve folate status in women of reproductive age and reduce the risk of folic acid–responsive neural tube birth defects (NTDs).
Objectives
Our objectives were to assess postfortification changes in folate status in the overall US population and in women aged 12–49 y and to characterize recent folate status by demographic group and use of folic acid–containing supplements.
Methods
We examined cross-sectional serum and RBC folate data from the NHANES 1999–2016.
Results
Serum folate geometric means increased from 2007–2010 to 2011–2016 in persons aged ≥1 y (38.7 compared with 40.6 nmol/L) and in women (35.3 compared with 37.0 nmol/L), whereas RBC folate showed no significant change. Younger age groups, men, and Hispanic persons showed increased serum and RBC folate concentrations, whereas non-Hispanic black persons and supplement nonusers showed increased serum folate concentrations. The folate insufficiency prevalence (RBC folate <748 nmol/L; NTD risk) in women decreased from 2007–2010 (23.2%) to 2011–2016 (18.6%) overall and in some subgroups (e.g., women aged 20–39 y, Hispanic and non-Hispanic black women, and supplement nonusers). After covariate adjustment, RBC folate was significantly lower in all age groups (by ∼10–20%) compared with persons aged ≥60 y and in Hispanic (by 8.2%), non-Hispanic Asian (by 12.1%), and non-Hispanic black (by 20.5%) compared with non-Hispanic white women (2011–2016). The 90th percentile for serum (∼70 nmol/L) and RBC (∼1800 nmol/L) folate in supplement nonusers aged ≥60 y was similar to the geometric mean in users (2011–2014).
Conclusions
Blood folate concentrations in the US population overall and in women have not decreased recently, and folate insufficiency rates are ∼20%. Continued monitoring of all age groups is advisable given the high folate status particularly in older supplement users.
Keywords: serum folate, RBC folate, women of reproductive age, folate deficiency, folate insufficiency, supplement use, race-ethnicity
Introduction
Since 1998, folic acid fortification of enriched cereal-grain products has been mandatory in the United States (1) and is estimated to provide a median usual intake of 115 µg of folic acid per day in women of reproductive age, contributing to a total folic acid intake of ∼240 µg/d from enriched cereal-grain products and ready-to-eat cereals (2). Overall, a 28% reduction in birth prevalence of anencephaly and spina bifida, the 2 most common neural tube defects (NTDs), was observed from pre- to postfortification using data from various participating programs; a greater reduction was found for programs with rather than without prenatal ascertainment (35% compared with 21%) (3). The postfortification prevalence of NTDs remained fairly stable during a 13-y period up to 2011; however, Hispanic compared with non-Hispanic black or white women showed a higher NTD prevalence (3). In 2016, the FDA approved voluntary corn masa flour fortification to help increase folic acid intake in the Hispanic population (4).
Folate status, monitored through NHANES, improved greatly from pre- (1988–1994) to early postfortification (1999–2004) (5, 6). A longer-term analysis capturing more than a decade of postfortification showed a small but significant linear trend of decreasing blood folate concentrations during 1999–2010 (7). The prevalence of folate deficiency (risk of megaloblastic anemia) in the overall population aged ≥4 y was <1% for both serum and RBC folate during postfortification (1999–2010) (8). The prevalence of folate insufficiency (risk of NTDs) in women of reproductive age decreased from 59% prefortification (1988–1994) to 15% (1999–2006) and 23% (2007–2010) postfortification (8).
The goal of this study was to address some contemporary concerns regarding folate. Using the most recent NHANES data from 2011–2016, we assessed whether the decreasing trend in serum and RBC folate concentrations previously reported up to 2010 (7) continued in the overall US population and in women of reproductive age and also whether prevalence estimates of low folate concentrations changed during the postfortification period. We also provided serum and RBC folate reference data for major race-Hispanic origin groups, including new data for non-Hispanic Asians. Given that there are continued discussions regarding the safety of high folic acid intakes in older persons (9), we characterized folate status in persons aged ≥60 y by race-Hispanic origin and supplement use. The focus of this study is on newer data from 2007 to 2016 [generated with the microbiologic assay (MBA) and/or LC-MS/MS]. However, to provide context for nearly 2 decades of postfortification, we also included older radioassay data (1988–2006) after adjusting them to the MBA.
Methods
Participants and survey design
NHANES collects cross-sectional data on the health and nutritional status of the civilian noninstitutionalized US population. The National Center for Health Statistics conducts this survey, which uses a complex, stratified, multistage probability sample. Participants undergo a detailed in-home interview during which demographic characteristics, dietary supplement use, and health-related information are collected, followed by a visit to a Mobile Examination Center where a physical examination is performed and blood is collected. Prior to 1999, the survey was conducted periodically; 1988–1994 represented the last prefolic acid–fortification survey period, and folate status was assessed in persons aged ≥4 y. Since 1999, NHANES has been a continuous survey with data released in 2-y survey periods; 1999–2000 represented the first postfortification survey period. The NHANES sample design includes oversampling for particular population subgroups to increase the precision of estimates for these groups (Supplemental Methods). Folate status was assessed in persons aged ≥3 y during 1999–2002 and in persons aged ≥1 y during 2003–2016. All adult participants gave their informed consent. Parental consent was obtained from minors. The National Center for Health Statistics Research Ethics Review Board reviewed and approved the NHANES protocol. Interview and examination response rates for each survey period are publicly available on the NHANES website (10).
Laboratory methods and data adjustment
Different laboratory methods were utilized during various NHANES data collection periods to measure serum and whole-blood folate concentrations (Supplemental Table 1). Because the radioassay used during 1988–2006 produced much lower blood folate results than the MBA used during 2007–2010 (11, 12), assessment of long-term trends in folate status was only possible if radioassay data from 1988 to 2006 were converted to MBA-equivalent data (13, 14). The use of 5,10-methylenetetrahydrofolate reductase (MTHFR) genotype-specific regression equations would be preferable because the radioassay measured 31% lower than the MBA for whole-blood samples with T/T genotype but 48% lower for samples with C/C and C/T genotypes (14). However, MTHFR genotype information is currently not available for NHANES participants, so we used an “all genotype” equation (13, 15). During 2011–2016, LC-MS/MS was used to measure individual serum folate forms and to calculate serum total folate composed of biologically active folate forms. Excellent agreement between serum total folate measured by LC-MS/MS compared with MBA was demonstrated previously (16).
Study variables
For the postfortification analysis (1999–2016) by 2-y survey periods, we assessed folate status in persons aged ≥3 y. We categorized the overall participant data into 4 age groups (3–11 y, 12–19 y, 20–59 y, and ≥60 y) and by sex (male and female). We used the 3 main race-Hispanic origin categories (Mexican American, non-Hispanic black, and non-Hispanic white) that can be compared over the time period covered in this analysis, but we included other Hispanic persons and persons of other non-Hispanic races in overall estimates. Folic acid supplement use was dichotomized (yes/no) based on self-reported consumption of folic acid–containing supplements in the past 30 d (only available for NHANES 1999–2014). For analysis of recent years (between 2007 and 2016), we assessed folate status in persons aged ≥1 y by age group (1–5 y, 6–11 y, 12–19 y, 20–39 y, 40–59 y, and ≥60 y), sex (male and female), race-Hispanic origin [Hispanic (Mexican American + other Hispanic), non-Hispanic Asian (2011–2016 only), non-Hispanic black, and non-Hispanic white], and supplement use. We also assessed folate status in women of reproductive age (aged 12–49 y), for brevity referred to as women from here on, by age group (12–19 y, 20–39 y, and 40–49 y), race-Hispanic origin, and supplement use.
Statistical analysis
We followed a data analysis plan approved by all authors prior to conducting the statistical analysis using SAS for Windows software version 9.4 (SAS Institute) and SAS callable SUDAAN software version 11 (RTI) to account for the complex survey design. Taylor series linearization was used to calculate variance estimates. We used Mobile Examination Center survey weights to account for unequal probabilities of selection, adjustment for nonresponse, and poststratification to estimate various descriptive statistics. To provide context for the pre- and postfortification patterns, we included prefortification data from NHANES III (1988–1994) and the entire postfortification period (1999–2016) for persons aged ≥3 y (Supplemental Figure 1; for postfortification sample sizes by survey period and variable categories, see Supplemental Table 2). However, our main analysis focused on changes in the more recent postfortification period (2007–2016), both for persons aged ≥1 y and for women. We used available-case analysis (“pairwise deletion”) to handle missing or incomplete data (e.g., folic acid supplement use) and excluded pregnant and/or lactating women.
Both serum and RBC folate concentrations were right-skewed; thus, we used a log transformation to make the data more symmetric and facilitate interpretation and statistical inference through the use of geometric mean, and we evaluated selected percentiles. We used commonly accepted cutoff values of <7 nmol/L (serum folate) and <305 nmol/L (RBC folate) (17) to assess the prevalence of folate deficiency (risk of megaloblastic anemia) using MBA-equivalent data. A cutoff value of <748 nmol/L for RBC folate in women was used to assess the prevalence of folate insufficiency (risk of NTDs) (8, 18). There are no accepted cutoff values for high folate status. To evaluate the upper end of serum and RBC folate concentrations, we assessed the 90th percentile overall, by age group, and by supplement use during the postfortification period. For more details on cutoff values, see Supplemental Methods.
To evaluate whether the decreasing trend in blood folate concentrations previously observed up to 2010 (7) continued, we compared the geometric mean concentrations of serum and RBC folate as well as the prevalence of folate insufficiency (RBC folate <748 nmol/L) for NHANES 2007–2010 and 2011–2016 to obtain estimates with greater statistical reliability. The 2007–2016 data were generated by MBA and/or LC-MS/MS and hence required no data adjustment.
The statistical analysis in this study is primarily descriptive, with less emphasis on hypothesis testing; for this reason, we used no adjustment for multiple comparisons. In places where a statistical hypothesis test was conducted, such as a test for change in log-transformed concentrations or prevalence (Wald F-test), we defined significance as a 2-sided P value of ≤0.05. We used multiple linear regression (log-transformed concentrations) for 2 different purposes: 1) to assess whether trends persisted either across the 1999–2016 survey cycles or between 2007–2010 and 2011–2016, and 2) to examine demographic differences in the most recent 2011–2016 combined period. All regression models included age, sex (if appropriate), race-Hispanic origin, and folic acid–containing supplement use as covariates, and we reported adjusted P values from the Wald F-test as well as percentage changes in adjusted geometric mean concentrations.
Results
Long-term patterns in serum and RBC folate concentrations by survey period in persons aged ≥3 y
Median and 90th percentile concentrations of serum folate (Figure 1A) and RBC folate (Figure 1B) sharply increased from pre- to postfortification, followed by comparatively minor fluctuations during the postfortification survey periods (1999–2016).
FIGURE 1.
Pre- and postfortification median and 90th percentile concentrations of serum (A) and RBC (B) folate in persons aged ≥3 y by survey period, NHANES 1988–2016. Folic acid fortification became mandatory in 1998. Lines between 1988–1994 and 1999–2000 are dashed to indicate that there were several years between these 2 surveys. Error bars represent 95% CIs. For NHANES 1988–1994, biological samples were collected from persons aged ≥4 y only. The 1988–1994 and 1999–2006 radioassay data were adjusted (13) to make them comparable to the 2007–2016 RBC microbiologic assay, 2007–2010 serum microbiologic assay, and 2011–2016 serum LC-MS/MS data. For sample sizes (n), see Supplemental Table 2 for 1999–2016; sample sizes for 1988–1994 were 22,939 (serum folate) and 22,438 (RBC folate).
The range of geometric mean serum folate concentrations across the postfortification cycles (1999–2016) was 38.0–45.7 nmol/L overall and 34.5–44.0, 30.5–38.6, and 40.5–47.9 nmol/L for Mexican-American, non-Hispanic black, and non-Hispanic white persons aged ≥3 y, respectively (Supplemental Table 3). We observed the highest serum folate concentrations in 1999–2000 for all 3 subgroups. Non-Hispanic black persons were the only race-Hispanic origin group that showed higher RBC folate concentrations in 2015–2016 compared with every previous cycle even after adjusting for age, sex, and supplement use (data not shown), although this difference was only statistically significant for 2001–2002, 2003–2004, 2009–2010, and 2011–2012.
When we stratified NHANES participants by use of folic acid–containing supplements, geometric mean serum folate concentrations in persons aged ≥3 y were 32.8–40.2 nmol/L (nonusers) and 51.3–61.2 nmol/L (users), whereas RBC folate concentrations were 943–1060 nmol/L (nonusers) and 1280–1490 nmol/L (users) (Supplemental Table 3). After adjusting for age, sex, race-Hispanic origin, and supplement use, serum folate concentrations in 1999–2000 were on average 8.6% (95% CI: 3.7%, 13.7%; Padjusted = 0.0005) higher compared with concentrations in 2001–2002 (model data not shown). After that, concentrations remained relatively stable with smaller fluctuations, showing an average concentration of 34.8 nmol/L (95% CI: 34.3, 35.2 nmol/L; nonusers) and 53.3 nmol/L (95% CI: 52.5, 54.2 nmol/L; users) for 2001–2014, respectively (model data not shown).
During the course of the 9 postfortification survey periods, the 90th percentile of serum folate concentrations ranged from 70.2 to 82.7 nmol/L for persons aged ≥3 y, whereas RBC folate concentrations ranged from 1710 to 1890 nmol/L (Supplemental Table 4). Persons aged ≥60 y had the highest serum and RBC folate concentrations compared to other age groups (Supplemental Table 4). Users of folic acid–containing supplements showed noticeably higher serum and RBC folate concentrations at the 90th percentile compared with nonusers (Supplemental Table 4).
Changes in postfortification blood folate concentrations from 2007–2010 to 2011–2016 in persons aged ≥1 y and in women
Serum folate geometric mean concentrations increased significantly from 2007–2010 to 2011–2016 in persons aged ≥1 y (38.7 compared with 40.6 nmol/L; Table 1) and in women (35.3 compared with 37.0 nmol/L; Table 2), whereas RBC folate concentrations showed no significant change. Some subgroups (e.g., younger age groups, males, and Hispanic persons) showed increases in both biomarkers, whereas non-Hispanic black persons and nonusers of folic acid–containing supplements showed increases in serum folate only. In no instance did either biomarker show significantly lower concentrations in the most recent period. After adjusting for age, sex, race-Hispanic origin, and supplement use, serum folate concentrations in 2011–2014 were on average 7.0% (95% CI: 4.0%, 10.0%; Padjusted < 0.0001) and 5.2% (95% CI: 1.6%, 9.0%; Padjusted = 0.0052) higher compared with concentrations in 2007–2010 in persons aged ≥1 y and in women, respectively (model data not shown). Adjusted RBC folate concentrations were not significantly different between the 2 time periods (Padjusted = 0.09 in persons aged ≥1 y and 0.33 in women; model data not shown).
TABLE 1.
Postfortification serum and RBC folate concentrations by demographic group in persons aged ≥1 y, NHANES 2007–2010 compared with 2011–20161
| Matrix | 2007–2010 | 2011–2016 | |||
|---|---|---|---|---|---|
| Population | n | nmol/L | n | nmol/L | P value |
| Serum | |||||
| All | 16,741 | 38.7 (37.7, 39.7) | 23,684 | 40.6 (39.8, 41.4) | 0.0039 |
| Age group, y | |||||
| 1–5 | 1606 | 52.0 (50.6, 53.5) | 2118 | 58.3 (56.6, 60.1) | <0.0001 |
| 6–11 | 1995 | 55.4 (54.0, 56.9) | 3022 | 57.5 (56.1, 58.9) | 0.0468 |
| 12–19 | 2254 | 37.9 (36.5, 39.2) | 3436 | 40.3 (39.1, 41.6) | 0.0088 |
| 20–39 | 3448 | 31.2 (30.3, 32.2) | 5017 | 34.2 (33.4, 34.9) | <0.0001 |
| 40–59 | 3636 | 36.2 (34.7, 37.8) | 5145 | 37.2 (36.2, 38.3) | 0.26 |
| ≥60 | 3802 | 48.1 (46.7, 49.6) | 4946 | 47.3 (45.9, 48.7) | 0.41 |
| Sex | |||||
| Male | 8451 | 36.3 (35.2, 37.4) | 11,840 | 38.8 (37.9, 39.7) | 0.0007 |
| Female | 8290 | 41.3 (40.3, 42.3) | 11,844 | 42.5 (41.6, 43.5) | 0.08 |
| Race-Hispanic origin2 | |||||
| Hispanic | 5498 | 36.0 (35.0, 37.1) | 6648 | 39.2 (38.3, 40.0) | <0.0001 |
| Non-Hispanic Asian | NA | 2599 | 40.2 (38.8, 41.7) | NA | |
| Non-Hispanic black | 3268 | 31.5 (30.7, 32.5) | 5475 | 33.8 (33.0, 34.7) | 0.0005 |
| Non-Hispanic white | 7100 | 40.9 (39.4, 42.5) | 7948 | 42.5 (41.6, 43.4) | 0.08 |
| Supplement use3 | |||||
| Yes | 4403 | 53.4 (52.0, 54.7) | 4079 | 54.6 (53.1, 56.2) | 0.22 |
| No | 12,229 | 33.3 (32.5, 34.0) | 11,543 | 36.4 (35.5, 37.2) | <0.0001 |
| RBC4 | |||||
| All | 16,829 | 1070 (1050, 1100) | 24,150 | 1110 (1090, 1130) | 0.0536 |
| Age group, y | |||||
| 1–5 | 1648 | 1040 (1020, 1060) | 2301 | 1120 (1090, 1140) | <0.0001 |
| 6–11 | 2018 | 1080 (1060, 1100) | 3134 | 1120 (1090, 1140) | 0.0283 |
| 12–19 | 2252 | 935 (904, 968) | 3491 | 982 (961, 1000) | 0.0183 |
| 20–39 | 3455 | 962 (940, 985) | 5047 | 999 (974, 1020) | 0.0294 |
| 40–59 | 3642 | 1090 (1060, 1140) | 5187 | 1120 (1100, 1140) | 0.29 |
| ≥60 | 3814 | 1330 (1300, 1370) | 4990 | 1310 (1280, 1340) | 0.32 |
| Sex | |||||
| Male | 8491 | 1050 (1020, 1080) | 12,091 | 1090 (1070, 1110) | 0.0435 |
| Female | 8338 | 1100 (1070, 1120) | 12,059 | 1120 (1100, 1150) | 0.10 |
| Race–Hispanic origin2 | |||||
| Hispanic | 5526 | 969 (939, 1000) | 6778 | 1010 (1000, 1030) | 0.0098 |
| Non-Hispanic Asian | NA | 2673 | 1010 (981, 1030) | NA | |
| Non-Hispanic black | 3289 | 884 (864, 905) | 5656 | 906 (887, 926) | 0.12 |
| Non-Hispanic white | 7138 | 1150 (1110, 1190) | 8006 | 1180 (1160, 1210) | 0.10 |
| Supplement use3 | |||||
| Yes | 4420 | 1330 (1290, 1370) | 4189 | 1340 (1300, 1380) | 0.66 |
| No | 12,299 | 970 (950, 991) | 11,850 | 999 (977, 1020) | 0.05 |
Values are weighted geometric means (95% CI). The 2007–2010 serum and RBC folate data were generated by microbiologic assay. The 2011–2016 serum folate data were generated by LC-MS/MS, and the RBC folate data were generated by microbiologic assay. NA, not applicable because no data are available for non-Hispanic Asians prior to 2011.
Persons of other non-Hispanic races are not shown but were included in overall estimates.
Use of folic acid–containing dietary supplements. Data are limited to 2011–2014 because information on dietary supplement use is not available for 2015–2016.
RBC folate estimates rounded to 3 significant figures.
TABLE 2.
Postfortification serum and RBC folate concentrations by demographic group in women aged 12–49 y, NHANES 2007–2010 compared with 2011–20161
| Matrix | 2007–2010 | 2011–2016 | |||
|---|---|---|---|---|---|
| Population | n | nmol/L | n | nmol/L | P value |
| Serum | |||||
| All | 3792 | 35.3 (34.2, 36.5) | 5530 | 37.0 (36.2, 38.0) | 0.0204 |
| Age group, y | |||||
| 12–19 | 1049 | 38.1 (36.5, 39.7) | 1681 | 41.2 (39.8, 42.7) | 0.0040 |
| 20–39 | 1728 | 34.0 (32.6, 35.4) | 2442 | 36.0 (35.1, 37.0) | 0.0174 |
| 40–49 | 1015 | 35.8 (33.8, 38.0) | 1407 | 36.0 (34.7, 37.4) | 0.85 |
| Race-Hispanic origin2 | |||||
| Hispanic | 1297 | 34.0 (32.9, 35.1) | 1602 | 37.1 (35.9, 38.3) | 0.0002 |
| Non-Hispanic Asian | NA | 728 | 40.4 (38.7, 42.3) | NA | |
| Non-Hispanic black | 759 | 28.6 (27.4, 29.9) | 1274 | 30.6 (29.4, 31.9) | 0.0231 |
| Non-Hispanic white | 1524 | 37.4 (35.5, 39.4) | 1692 | 38.2 (37.2, 39.3) | 0.46 |
| Supplement use3 | |||||
| Yes | 867 | 46.6 (44.3, 49.1) | 897 | 46.1 (44.3, 47.9) | 0.71 |
| No | 2904 | 31.6 (30.8, 32.4) | 2781 | 34.3 (33.5, 35.3) | <0.0001 |
| RBC4 | |||||
| All | 3800 | 995 (972, 1020) | 5583 | 1020 (998, 1040) | 0.13 |
| Age group, y | |||||
| 12–19 | 1047 | 937 (907, 968) | 1710 | 978 (952, 1010) | 0.0428 |
| 20–39 | 1735 | 983 (958, 1010) | 2460 | 1010 (986, 1040) | 0.11 |
| 40–49 | 1018 | 1060 (1010, 1110) | 1413 | 1070 (1030, 1100) | 0.80 |
| Race–Hispanic origin2 | |||||
| Hispanic | 1301 | 956 (925, 987) | 1613 | 1000 (979, 1020) | 0.0176 |
| Non-Hispanic Asian | NA | 740 | 981 (953, 1010) | NA | |
| Non-Hispanic black | 758 | 843 (814, 873) | 1298 | 872 (845, 900) | 0.15 |
| Non-Hispanic white | 1529 | 1050 (1020, 1090) | 1694 | 1070 (1040, 1100) | 0.52 |
| Supplement use3 | |||||
| Yes | 869 | 1210 (1170, 1250) | 908 | 1170 (1130, 1210) | 0.20 |
| No | 2909 | 920 (900, 941) | 2820 | 947 (923, 971) | 0.09 |
Values are weighted geometric means (95% CI). The 2007–2010 serum and RBC folate data were generated by microbiologic assay. The 2011–2016 serum folate data were generated by LC-MS/MS, and the RBC folate data were generated by microbiologic assay. NA, not applicable because no data are available for non-Hispanic Asians prior to 2011.
Persons of other non-Hispanic races are not shown but were included in overall estimates.
Use of folic acid–containing dietary supplements. Data are limited to 2011–2014 because information on dietary supplement use is not yet available for 2015–2016.
RBC folate estimates rounded to 3 significant figures.
Postfortification prevalence of low folate concentrations
The prevalence of folate deficiency (serum folate <7 nmol/L or RBC folate <305 nmol/L) representing risk of megaloblastic anemia was <1% in persons aged ≥3 y in each survey cycle during the period 1999–2016 (data not shown). The prevalence of folate insufficiency in women (RBC folate <748 nmol/L; NTD risk) ranged from 13.5% to 26.6% during the period 1999–2016, with similar prevalence estimates for Mexican-American (12.0–25.5%) and non-Hispanic white (9.46–21.3%) women but higher estimates for non-Hispanic black women (26.5–43.4%) (data not shown). Where data were available for Hispanic and non-Hispanic Asian women, the prevalence of folate insufficiency ranged from 14.6% to 25.2% (2007–2016) and from 18.7% to 23.4% (2011–2016), respectively (data not shown). The prevalence of folate insufficiency in women decreased from 2007–2010 (23.2%) to 2011–2016 (18.6%) overall and in some subgroups (e.g., 20- to 39-y-old women, Hispanic and non-Hispanic black women, and nonusers of folic acid–containing supplements), whereas it did not change significantly in other subgroups (Table 3).
TABLE 3.
Postfortification prevalence of folate insufficiency by demographic group in women 12–49 y, NHANES 2007–2010 compared with 2011–20161
| Matrix | 2007–2010 | 2011–2016 | |||
|---|---|---|---|---|---|
| Population | n | % | n | % | P value |
| All | 3800 | 23.2 (21.0, 25.4) | 5583 | 18.6 (16.8, 20.6) | 0.0028 |
| Age group, y | |||||
| 12–19 | 1047 | 23.9 (20.0, 28.3) | 1710 | 19.2 (16.4, 22.2) | 0.07 |
| 20–39 | 1735 | 24.2 (21.6, 27.0) | 2460 | 19.0 (17.0, 21.1) | 0.0029 |
| 40–49 | 1018 | 21.0 (16.8, 25.8) | 1413 | 17.7 (14.8, 21.2) | 0.24 |
| Race–Hispanic origin2 | |||||
| Hispanic | 1301 | 22.7 (19.6, 26.2) | 1613 | 17.0 (15.1, 18.8) | 0.0017 |
| Non-Hispanic Asian | NA | 740 | 21.3 (18.5, 24.3) | NA | |
| Non-Hispanic black | 758 | 39.0 (36.0, 42.1) | 1298 | 33.8 (30.7, 37.1) | 0.0223 |
| Non-Hispanic white | 1529 | 18.8 (16.2, 21.8) | 1694 | 15.6 (13.2, 18.2) | 0.09 |
| Supplement use3 | |||||
| Yes | 869 | 9.55 (7.42, 12.2) | 908 | 9.69 (7.40, 12.6) | 0.94 |
| No | 2909 | 28.8 (26.1, 31.6) | 2820 | 23.8 (21.2, 26.5) | 0.0089 |
Values are weighted percentage RBC folate concentrations <748 nmol/L (95% CI). The RBC folate data were generated by microbiologic assay. NA, not applicable because no data are available for non-Hispanic Asians prior to 2011.
Persons of other non-Hispanic races are not shown but were included in overall estimates.
Use of folic acid–containing dietary supplements. Data are limited to 2011–2014 because information on dietary supplement use is not yet available for 2015–2016.
Postfortification reference data for serum and RBC folate for 2011–2016 in persons aged ≥1 y, in women, and in older persons
After combining the serum folate data from the 3 most recent survey periods, we observed a U-shaped age pattern, higher concentrations in females compared with males, and lowest concentrations in non-Hispanic black persons compared with other race-Hispanic origin groups at the geometric mean and throughout the entire distribution (Table 1, Supplemental Table 5). For RBC folate, we also observed the same age and race-Hispanic origin patterns, but the sex differences were less pronounced. We quantified the magnitude of demographic differences relative to a reference category after covariate adjustment (Table 4). Females had 1.8% higher RBC folate concentrations and 6.9% higher serum folate concentrations compared with males after adjusting for age, race-Hispanic origin, and supplement use. Non-Hispanic Asian and Hispanic persons had similar serum folate concentrations compared with non-Hispanic white persons (Padjusted = 0.80 and 0.36, respectively), whereas non-Hispanic black persons had 17.3% lower concentrations after adjusting for age, sex, and supplement use. In contrast, adjusted RBC folate concentrations were significantly lower in all 3 race-Hispanic origin groups compared with concentrations in non-Hispanic white persons by 8.2% (Hispanics), 12.1% (non-Hispanic Asians), and 20.5% (non-Hispanic blacks).
TABLE 4.
Estimated percentage difference in serum and RBC folate concentrations relative to a reference category in persons aged ≥1 y and women aged 12–49 y, NHANES 2011–20161
| Population subgroup | Serum folate, % | P adjusted | RBC folate, % | P adjusted |
|---|---|---|---|---|
| Persons aged ≥1 y | ||||
| Age group, y | ||||
| 1–5 | 29.4 (23.9, 35.1) | <0.0001 | −10.5 (−13.8, −7.0) | <0.0001 |
| 6–11 | 28.4 (22.9, 34.2) | <0.0001 | −10.1 (−13.4, −6.7) | <0.0001 |
| 12–19 | −6.3 (−9.5, −2.9) | 0.0004 | −18.6 (−21.0, −16.1) | <0.0001 |
| 20–39 | −23.1 (−25.8, −20.4) | <0.0001 | −19.7 (−22.4, −17.0) | <0.0001 |
| 40–59 | −19.2 (−22.1, −16.3) | <0.0001 | −12.4 (−15.3, −9.4) | <0.0001 |
| ≥60 | Reference | Reference | ||
| Sex | ||||
| Female | 6.9 (5.0, 8.9) | <0.0001 | 1.8 (0.6, 2.9) | 0.0028 |
| Male | Reference | Reference | ||
| Race–Hispanic origin | ||||
| Hispanic | −1.5 (−4.7, 1.8) | 0.36 | −8.2 (−10.7, −5.7) | <0.0001 |
| Non-Hispanic Asian | −0.4 (−3.7, 3.0) | 0.80 | −12.1 (−14.6, −9.5) | <0.0001 |
| Non-Hispanic black | −17.3 (−19.9, −14.6) | <0.0001 | −20.5 (−22.7, −18.3) | <0.0001 |
| Non-Hispanic white | Reference | Reference | ||
| Supplement use2 | ||||
| Yes | 46.3 (42.6, 50.3) | <0.0001 | 28.6 (25.9, 31.5) | <0.0001 |
| No | Reference | Reference | ||
| Women aged 12–49 y | ||||
| Age group, y | ||||
| 12–19 | 23.6 (17.6, 30.0) | <0.0001 | −4.0 (−7.8, 0.0) | 0.0501 |
| 20–39 | 4.2 (−0.3, 8.8) | 0.07 | −2.4 (−6.3, 1.8) | 0.26 |
| 40–49 | Reference | Reference | ||
| Race–Hispanic origin | ||||
| Hispanic | 2.7 (−1.6, 7.3) | 0.22 | −3.3 (−7.4, 0.9) | 0.12 |
| Non-Hispanic Asian | 9.2 (3.7, 15.0) | 0.0011 | −7.2 (−11.5, −2.7) | 0.0026 |
| Non-Hispanic black | −17.7 (−22.3, −12.9) | <0.0001 | −18.4 (−22.3, −14.3) | <0.0001 |
| Non-Hispanic white | Reference | Reference | ||
| Supplement use2 | ||||
| Yes | 35.9 (30.8, 41.2) | <0.0001 | 21.6 (18.2, 25.0) | <0.0001 |
| No | Reference | Reference | ||
Values are percentage difference (95% CI) in weighted geometric mean concentrations derived from a multiple linear regression model (log-transformed concentrations) that adjusted for age, sex (if appropriate), race–Hispanic origin, and folic acid–containing supplement use. Wald F- test was used to determine the adjusted P value (Padjusted). The sample size and R2 for the 4 models were n = 15,622, R2 = 24.3% (serum folate in persons aged ≥1 y); n = 16,039, R2 = 20.0% (RBC folate in persons aged ≥1 y); n = 3678, R2 = 12.5% (serum folate in women aged 12–49 y); n = 3728, R2 = 9.6% (serum folate in women aged 12–49 y). The 2011–2016 serum folate data were generated by LC-MS/MS, and the RBC folate data were generated by microbiologic assay.
Use of folic acid–containing dietary supplements. Data are limited to 2011–2014 because information on dietary supplement use is not yet available for 2015–2016.
In the combined 3 most recent survey periods, the central 95% reference intervals for serum and RBC folate were 14.0–107 and 505–2510 nmol/L in persons aged ≥1 y, respectively, and 14.2–87.7 and 466–2270 nmol/L in women, respectively (Supplemental Table 5). We observed large differences in the reference intervals by age group (e.g., serum folate for persons aged 1–5 y compared with persons aged 20–39 y was 24.6–129 nmol/L compared with 12.7–81.5 nmol/L) and by supplement use (e.g., serum folate for users compared with nonusers was 19.5–136 nmol/L compared with 13.6–89.4 nmol/L).
During 2011–2014, geometric mean serum folate concentrations in persons aged ≥60 y were nearly twice as high in supplement users compared with nonusers, whereas RBC folate concentrations were ∼50% higher (Table 5). In fact, the 90th percentile in nonusers was just slightly higher than the geometric mean in supplement users, both for serum and for RBC folate. The 90th percentile serum and RBC folate concentrations in supplement users were ∼100 nmol/L and between 2000 and ∼3000 nmol/L, respectively, depending on the race-Hispanic origin group.
TABLE 5.
Postfortification serum and RBC folate concentrations by sex and race-Hispanic origin and stratified by supplement use in persons aged ≥60 y, NHANES 2011–20141
| Supplement use2 | ||||||
|---|---|---|---|---|---|---|
| Matrix | Nonuser | User | ||||
| Population | n | Geometric mean, nmol/L | 90th percentile, nmol/L | n | Geometric mean, nmol/L | 90th percentile, nmol/L |
| Serum | ||||||
| All | 2077 | 37.4 (35.9, 38.9) | 74.3 (70.5, 79.1) | 1106 | 70.7 (67.5, 74.0) | 117 (110, 124) |
| Sex | ||||||
| Male | 1066 | 35.6 (33.3, 37.9) | 69.4 (62.3, 75.2) | 493 | 64.7 (59.8, 70.1) | 109 (100, 122) |
| Female | 1011 | 39.1 (37.5, 40.8) | 78.1 (73.3, 85.1) | 613 | 75.4 (72.6, 78.3) | 124 (116, 131) |
| Race–Hispanic origin3 | ||||||
| Hispanic | 464 | 38.8 (36.3, 41.4) | 68.8 (64.7, 72.9) | 148 | 62.3 (58.7, 66.2) | 97.7 (89.5, 114) |
| Non-Hispanic Asian | 205 | 41.4 (37.8, 45.4) | 75.9 (67.3, 96.6) | 93 | 71.0 (67.3, 74.9) | 1074 (98.8, 121) |
| Non-Hispanic black | 512 | 32.4 (31.2, 33.6) | 64.3 (59.0, 67.3) | 215 | 57.2 (53.0, 61.7) | 103 (99.9, 115) |
| Non-Hispanic white | 862 | 37.7 (35.8, 39.8) | 76.9 (71.3, 85.1) | 636 | 72.2 (68.3, 76.4) | 120 (111, 126) |
| RBC5 | ||||||
| All | 2110 | 1100 (1070, 1130) | 1870 (1790, 1970) | 1112 | 1680 (1590, 1760) | 2750 (2570, 2920) |
| Sex | ||||||
| Male | 1083 | 1070 (1020, 1130) | 1850 (1670, 1970) | 494 | 1610 (1500, 1720) | 2610 (2440, 2910) |
| Female | 1027 | 1120 (1090, 1160) | 1930 (1790, 2070) | 618 | 1730 (1650, 1820) | 2820 (2640, 2960) |
| Race–Hispanic origin 2 | ||||||
| Hispanic | 472 | 1020 (968, 1080) | 1620 (1490, 1780) | 151 | 1440 (1350, 1530) | 2140 (2010, 2820) |
| Non-Hispanic Asian | 210 | 1030 (959, 1110) | 1670 (1530, 2030) | 92 | 1590 (1510, 1680) | 22404 (2080, 2710) |
| Non-Hispanic black | 524 | 924 (887, 963) | 1560 (1490, 1700) | 220 | 1360 (1280, 1450) | 2240 (2090, 2500) |
| Non-Hispanic white | 869 | 1140 (1100, 1190) | 1940 (1850, 2070) | 634 | 1720 (1620, 1830) | 2810 (2620, 2980) |
Values are weighted geometric means (95% CI) and 90th percentiles (95% CI). The 2011–2014 serum folate data were generated by LC-MS/MS, and the RBC folate data were generated by microbiologic assay.
Use of folic acid–containing dietary supplements. Data are limited to 2011–2014 because information on dietary supplement use is not yet available for 2015–2016.
Persons of other non-Hispanic races are not shown but were included in overall estimates.
Estimates are subject to greater variability due to small cell size.
RBC folate estimates rounded to 3 significant figures.
Discussion
This study presents new information on postfortification concentrations of serum and RBC folate over nearly 2 decades (1999–2016) in a representative sample of the US population. The previously shown decline in blood folate concentrations from 1999–2010 (6) did not continue into more recent years. Serum folate concentrations in the overall population and in women showed minor statistically significant increases from 2007–2010 to 2011–2016, whereas RBC folate concentrations trended in the same direction but were borderline nonsignificant (for both crude and covariate-adjusted estimates). Similarly, the prevalence of folate insufficiency decreased from 2007–2010 to 2011–2016 in all women and in subgroups of women who showed an increase in blood folate concentrations. Yet despite the decreases in insufficiency prevalence, disparities among race-Hispanic origin groups remain, with non-Hispanic black women having the highest prevalence (34% in 2011–2016). Still, this group generally had the lowest prevalence of NTDs during 1995–2011 compared with other race-Hispanic origin groups (3), which may be explained by the multifactorial etiology of NTDs, with factors such as vitamin B-12 status and genetic background playing additional roles (19). Compared with non-Hispanic whites, non-Hispanic blacks have better vitamin B-12 status (20, 21) and a lower population frequency of the MTHFR C677T polymorphism (22).
The prevalence of folate insufficiency in US women has been, on average, 18.6% (2011–2016). Unfortunately, there is a paucity of global data on folate insufficiency due to issues with noncomparability of assays and cutoff values used in national surveys (23). Nonetheless, we compared our NHANES estimate to data from 5 countries that used the MBA and corresponding cutoff values (<748 or <906 nmol/L for calibration with 5-methyltetrahydrofolate or folic acid, respectively). Two Central American countries with voluntary wheat flour fortification (1.8 mg folic acid/kg), Guatemala [47% (24)] and Belize [49% (25)], had lower insufficiency prevalence compared with Ireland [64% (26)] and New Zealand [73% (27)], where voluntary folic acid fortification is permitted. Of note, the 2009–2010 Guatemala National Micronutrient Survey reported a wide range of folate insufficiency—19% in the Metropolitan region (similar to the US data) and 81% in the Northern region—suggesting problems with limited access to fortified products (24). The United Kingdom has no folic acid fortification, and the UK National Diet and Nutrition Survey Rolling Programme Years 7 and 8 reported an insufficiency prevalence of 91% (28). This indicates that a mandatory and well-controlled folic acid fortification program can be effective in reducing the prevalence of folate insufficiency and keeping it at a fairly constant level of ∼20%.
Minor fluctuations in blood folate concentrations across survey periods are expected because the policies that regulate folic acid fortification are not the only factors associated with blood folate concentrations. In addition to folic acid intake from fortification and supplements, blood folate concentrations depend on folate intake from natural foods and genetic variation in the population. Shifts in eating habits [e.g., low-carbohydrate diets (29)] and demographic shifts with resulting changes in frequencies of the MTHFR variant [e.g., indigenous Mexican populations (30)] are some of the factors that may contribute to the variation and obscure underlying shifts. Consistent with our findings of small changes in blood folate concentrations since 2007, USDA statistics on wheat flour supply and disappearance showed similar market disappearance rates between 2004 and 2016 (range: 134.6–138.3 pounds/capita) (31). Similarly, USDA's “What We Eat in America” NHANES data tables on nutrient intakes from foods showed relatively stable mean folic acid intakes for persons aged ≥2 y by survey period during 2005–2012 (199, 193, 196, and 202 µg/d) followed by a slight decrease during 2013–2016 (186 and 180 µg/d) (32).
Nonusers of folic acid–containing supplements showed slight increases in serum (statistically significant) and RBC (borderline nonsignificant) folate concentrations from 2007–2010 to 2011–2014, whereas supplement users showed no change. Given the debate about high folic acid intakes in supplement users, this is a welcome finding and it confirms that these 2 groups should be reviewed separately when evaluating nutrition policies because they likely behave differently. In the most recent 2011–2014 combined period, we observed 46.3% higher serum folate but only 28.6% higher RBC folate concentrations in supplement users compared with nonusers after adjusting for covariates, exemplifying differences between these 2 biomarkers. In older persons, we observed even larger differences between supplement users and nonusers for both folate biomarkers, but the sex and race-Hispanic origin patterns were similar in supplement users and nonusers. The serum folate concentrations we observed at the 90th percentile in older persons (∼100 nmol/L) were comparable to the median (103 nmol/L) reported in older German adults after a 3-wk intervention with 5 mg folic acid/d in a country with no folic acid fortification but much lower than the 90th percentile (281 nmol/L) (33). Thus, in the presence of continuous folic acid intake from fortified foods and supplements, serum folate concentrations are much lower than those observed after short-term high-dose folic acid supplementation.
The new data for non-Hispanic Asian persons showed lower geometric mean serum folate concentrations compared with those of non-Hispanic white persons, but this difference disappeared after adjusting for covariates. Furthermore, at higher percentiles (e.g., 75th and 97.5th), non-Hispanic Asian persons had more similar serum folate concentrations compared with non-Hispanic white persons. Others have shown that supplement use in NHANES 2011–2014 was not statistically different between non-Hispanic Asians and non-Hispanic whites for older adults (aged ≥60 y: 67.7% and 72.5%, respectively) and for women (aged 20–44 y: 52.0% and 53.1%, respectively) (34, 35).
Our analysis is subject to commonly known limitations pertaining to cross-sectional study data. One caveat is that we adjusted the early postfortification radioassay data in order to assess long-term folate status. Using regression equations to adjust survey data has limitations, such as an underestimation of the SEs impacting statistical inferences and potentially biased reference intervals or prevalence estimates at a defined cutoff around the tails of the distribution (36). Such data adjustment should not be used to associate individual participant data with health outcomes because this would likely result in misclassification and biased or incorrect findings, particularly for RBC folate without adjustment for MTHFR genotype (37). Furthermore, other laboratories should not adjust their radioassay data using the reported regression equations because these equations describe the relation between the Bio-Rad radioassay and the CDC MBA (13).
This study has several strengths. We used originally measured blood folate data (2007–2016) to assess more recent changes in serum and RBC folate concentrations, and we based our interpretation of change on combined data from multiple cycles to improve the reliability of inferences. The availability of postfortification folate status data during 9 survey periods and the new information for non-Hispanic Asian persons during 3 survey periods provide unique and important data to US public health officials but also to countries that are considering folic acid fortification. Furthermore, the current data on folate status of Hispanic persons will serve as an important “baseline” for an impact assessment of corn masa flour fortification once new NHANES data become available.
In summary, the long-term monitoring of folate status in NHANES has provided critical data to policymakers and the research community. Although our results indicate that blood folate concentrations have not decreased in recent years and folate insufficiency rates appear to be stable at ∼20%, monitoring of folate status should be continued in all age groups, particularly given the high folate status in older supplement users.
Supplementary Material
ACKNOWLEDGEMENTS
We thank Neelima Paladugula, Ralph D Whitehead, Bridgette Haynes, Daniel Rabinowitz, LeighAnne Hurst, and Mary Xu (National Center for Environmental Health, CDC) for performing the serum and RBC folate analysis.
The authors’ responsibilities were as follows—CMP: developed the study concept; MRS: performed the statistical data analysis; CMP: was responsible for drafting the manuscript; CMP, MRS, MZ, ZF, RJS, KSC, SY, JJG, WYJ, C-YW, NP, and JW: participated in the study design and interpretation of the data; and all authors: were responsible for critical revision of the manuscript and read and approved the final manuscript. None of the authors declared any personal or financial conflicts of interest.
Notes
This work was performed under employment of the US federal government and the authors did not receive any outside funding.
The findings and conclusions in this report are those of the authors and do not necessarily represent the views of the Centers for Disease Control and Prevention, the National Institutes of Health, the Food and Drug Administration, or the Department of Health and Human Services.
Supplemental Methods, Supplemental Tables 1–5, and Supplemental Figure 1 are available from the “Supplementary data” link in the online posting of the article and from the same link in the online table of contents at https://academic.oup.com/ajcn/.
Abbreviations used: MBA, microbiologic assay; MTHFR, 5,10-methylenetetrahydrofolate reductase; NTD, neural tube defect.
References
- 1. FDA. Food standards: amendment of standards of identity for enriched grain products to require addition of folic acid. Fed Regist. 1996;61(44):8781–97. [Google Scholar]
- 2. Crider KS, Qi YP, Devine O, Tinker SC, Berry RJ. Modeling the impact of folic acid fortification and supplementation on red blood cell folate concentrations and predicted neural tube defect risk in the United States: have we reached optimal prevention?. Am J Clin Nutr. 2018;107:1027–34. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 3. Williams J, Mai CT, Mulinare J, Isenburg J, Flood TJ, Ethen M, Frohnert B, Kirby RS. Updated estimates of neural tube defects prevented by mandatory folic acid fortification—United States, 1995–2011. MMWR Morb Mortal Wkly Rep. 2015;64(1):1–5. [PMC free article] [PubMed] [Google Scholar]
- 4. FDA. Food additives permitted for direct addition to food for human consumption: folic acid. Fed Regist. 2016;81(73):22176–83. [PubMed] [Google Scholar]
- 5. Ganji V, Kafai MR. Trends in serum folate, RBC folate, and circulating total homocysteine concentrations in the United States: analysis of data from National Health and Nutrition Examination Surveys, 1988–1994, 1999–2000, and 2001–2002. J Nutr. 2006;136:153–8. [DOI] [PubMed] [Google Scholar]
- 6. Pfeiffer CM, Johnson CL, Jain RB, Yetley EA, Picciano MF, Rader JI, Fisher KD, Mulinare J, Osterloh JD. Trends in blood folate and vitamin B12 concentrations in the United States, 1988–2004. Am J Clin Nutr. 2007;86:718–27. [DOI] [PubMed] [Google Scholar]
- 7. Pfeiffer CM, Hughes JP, Lacher DA, Bailey RL, Berry RJ, Zhang M, Yetley EA, Rader JI, Sempos CT, Johnson CL. 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. J Nutr. 2012;142:886–93. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 8. Pfeiffer CM, Sternberg MR, Hamner HC, Crider KS, Lacher DA, Rogers LM, Bailey RL, Yetley EA. Applying inappropriate cutpoints leads to misinterpretation of folate status in the US population. Am J Clin Nutr. 2016;104:1607–15. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 9. Field MS, Stover PJ. Safety of folic acid. Ann NY Acad Sci. 2018;1414:59–71. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 10. CDC, National Center for Health Statistics. NHANES response rates and population totals. [Internet]. [cited 10 February, 2019].Available from: https://wwwn.cdc.gov/nchs/nhanes/ResponseRates.aspx. [Google Scholar]
- 11. Fazili Z, Pfeiffer CM, Zhang M. Comparison of serum folate species analyzed by LC-MS/MS with total folate measured by microbiologic assay and BioRad radioassay. Clin Chem. 2007;53:781–4. [DOI] [PubMed] [Google Scholar]
- 12. Fazili Z, Pfeiffer CM, Zhang M, Jain RB, Koontz D. Influence of 5,10-methylene-tetrahydrofolate reductase polymorphism on whole blood folate concentrations measured by LC-MS/MS, microbiologic assay and BioRad radioassay. Clin Chem. 2008;54:197–201. [DOI] [PubMed] [Google Scholar]
- 13. Pfeiffer CM, Hughes JP, Durazo-Arvizu RA, Lacher DA, Sempos CT, Zhang M, Yetley EA, Johnson CL. 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. J Nutr. 2012;142:894–900. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 14. Yetley EA, Pfeiffer CM, Phinney KW, Fazili Z, Lacher DA, Bailey RL, Blackmore S, Bock JL, Brody LC, Carmel R et al.. Biomarkers of folate status in the National Health and Nutrition Examination Survey (NHANES): a roundtable summary. Am J Clin Nutr. 2011;94(Suppl):303S–12S. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 15. CDC. National Health and Nutrition Examination Survey: 2007–2008 Data Documentation, Codebook, and Frequencies: Folate—RBC & Serum. [Internet]. [cited 10 February, 2019].Available from: https://wwwn.cdc.gov/nchs/nhanes/2007-2008/FOLATE_E.htm. [Google Scholar]
- 16. Zhang M, Sternberg MR, Pfeiffer CM. Harmonizing the calibrator and microorganism used in the folate microbiologic assay leads to improvements in serum and whole blood folate comparability as shown in a CDC Round Robin study. J Nutr. 2018;148:807–17. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 17. Institute of Medicine, Food and Nutrition Board. Dietary Reference Intakes: Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline. Washington (DC): National Academies Press; 1998. [PubMed] [Google Scholar]
- 18. WHO. Guideline: Optimal Serum and Red Blood Cell Folate Concentrations in Women of Reproductive Age for Prevention of Neural Tube Defects. Geneva (Switzerland): WHO; 2015. [cited 10 February, 2019].Available from: http://apps.who.int/iris/bitstream/10665/161988/1/9789241549042_eng.pdf?ua=1. [PubMed] [Google Scholar]
- 19. Molloy AM. Should vitamin B12 status be considered in assessing risk of neural tube defects?. Ann NY Acad Sci. 2018;1414:109–25. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 20. Ganji V, Kafai MR.. Population preference values for serum methylmalonic acid concentrations and its relationship with age, sex, race-ethnicity, supplement use, kidney function and serum vitamin B-12 in the post-folic acid fortification period. Nutrients. 2018;10(1):74. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 21. Mineva EM, Sternberg MR, Zhang M, Aoki Y, Storandt R, Bailey RL, Pfeiffer CM. Age-specific reference ranges are needed to interpret serum methylmalonic acid concentrations in the US population. Am J Clin Nutr. 2019; 110:(1):158–68. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 22. Botto LD, Yang Q.. 5,10-Methylenetetrahydrofolate reductase gene variants and congenital anomalities: a HuGE review. Am J Epidemiol. 2000;151:862–77. [DOI] [PubMed] [Google Scholar]
- 23. Rogers LR, Cordero AM, Pfeiffer CM, Hausman DB, Tsang BL, De-Regil LM, Rosenthal J, Razzaghi H, Wong EC, Weakland AP et al.. Global folate status in women of reproductive age: a systematic review with emphasis on methodological issues. Ann NY Acad Sci. 2018;1431:35–57. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 24. Rosenthal J, Reeve M-E, Ramirez N, Crider KS, Sniezek J, Vellozzi C, Devine O, Lopez-Pazos E. Red blood cell folate insufficiency among nonpregnant women of childbearing age in Guatemala 2009 to 2010: prevalence and predicted neural tube defects risk. Birth Defects Res (Part A). 2016;106:587–95. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 25. Rosenthal J, Largaespada N, Bailey LB, Cannon M, Alverson CJ, Ortiz D, Kauwell GPA, Sniezek J, Figueroa R, Daly R et al.. Folate deficiency is prevalent in women of childbearing age in Belize and is negatively affected by coexisting vitamin B-12 deficiency: Belize National Micronutrient Survey 2011. J Nutr. 2017;147:1183–93. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 26. Hopkins SM, Gibney MJ, Nugent AP, McNulty H, Molloy AM, Scott JM, Flynn A, Strain JJ, Ward M, Walton J et al.. Impact of voluntary fortification and supplement use on dietary intakes and biomarker status of folate and vitamin B-12 in Irish adults. Am J Clin Nutr. 2015;101:1163–72. [DOI] [PubMed] [Google Scholar]
- 27. University of Otago, Ministry of Health (New Zealand). A Focus on Nutrition: Key Findings of the 2008/09 New Zealand Adult Nutrition Survey. Wellington (New Zealand): Ministry of Health;2011. [Google Scholar]
- 28. Public Health England. National Diet and Nutrition Survey: Results from Years 7 and 8 (combined) of the Rolling Programme (2014/2015–2015/2016). [Internet]. 2018; [cited 19 May, 2019]. Available from: https://www.gov.uk/government/statistics/ndns-results-from-years-7-and-8-combined. [Google Scholar]
- 29. Hursh H, Martin J. Low-carb and beyond: the health benefits of inulin. Cereal Foods World. 2005;50(2):57–61. [Google Scholar]
- 30. Contreras-Cubas C, Sánchez-Hernández BE, García-Ortiz H, Martínez-Hernández A, Barajas-Olmos F, Cid M, Mendoza-Caamal EC, Centeno-Cruz F, Ortiz-Cruz G, Jiménez-López JC et al.. Heterogenous distribution of MTHFR gene variants among Mestizos and diverse Amerindian groups from Mexico. PLoS One. 2016;11(9):e0163248. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 31. US Department of Agriculture. Wheat Data—All Years: Wheat Yearbook Table 32—Wheat Flour: Supply and Disappearance, United States, 1989–2017. [Internet]. [cited 12 February, 2019].Available from: https://www.ers.usda.gov/data-products/wheat-data. [Google Scholar]
- 32. US Department of Agriculture, Agricultural Research Service. Nutrient Intakes from Food and Beverages: Mean Amounts Consumed Per Individual, by Gender and Age, What We Eat in America, NHANES. [Internet]. [cited 14 March, 2019].Available from: https://www.ars.usda.gov/northeast-area/beltsville-md-bhnrc/beltsville-human-nutrition-research-center/food-surveys-research-group/docs/wweia-data-tables. [Google Scholar]
- 33. Obeid R, Kirsch SH, Kasoha M, Eckert R, Herrmann W. Concentrations of unmetabolized folic acid and primary folate forms in plasma after folic acid treatment in older adults. Metabolism. 2011;60:673–80. [DOI] [PubMed] [Google Scholar]
- 34. Gahche JJ, Bailey RL, Potischman N, Dwyer JT. Dietary supplement use was very high among older adults in the United States in 2011–2014. J Nutr. 2017;147:1968–76. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 35. Gupta PM, Gahche JJ, Herrick KA, Ershow AG, Potischman N, Perrine CG. Use of iodine-containing dietary supplements remains low among women of reproductive age in the United States: NHANES 2011–2014. Nutrients. 2018;10:422. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 36. Sternberg M. Multiple imputation to evaluate the impact of an assay change in national surveys. Stat Med. 2017;36:2697–719. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 37. Tsang BL, Devine OJ, Cordero AM, Marchetta CM, Mulinare J, Mersereau P, Guo J, Qi YP, Berry RJ, Rosenthal J et al.. Assessing the association between the methylenetetrahydrofolate reductase (MTHFR) 677 C>T polymorphism and blood folate concentrations: a systematic review and meta-analysis of trials and observational studies. Am J Clin Nutr. 2015;101:1286–94. [DOI] [PubMed] [Google Scholar]
Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.