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Review
.2022 Aug 22:42:423-452.
doi: 10.1146/annurev-nutr-043020-091647.

Folic Acid and the Prevention of Birth Defects: 30 Years of Opportunity and Controversies

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Review

Folic Acid and the Prevention of Birth Defects: 30 Years of Opportunity and Controversies

Krista S Crider et al. Annu Rev Nutr..

Abstract

For three decades, the US Public Health Service has recommended that all persons capable of becoming pregnant consume 400 μg/day of folic acid (FA) to prevent neural tube defects (NTDs). The neural tube forms by 28 days after conception. Fortification can be an effective NTD prevention strategy in populations with limited access to folic acid foods and/or supplements. This review describes the status of mandatory FA fortification among countries that fortify (n = 71) and the research describing the impact of those programs on NTD rates (up to 78% reduction), blood folate concentrations [red blood cell folate concentrations increased ∼1.47-fold (95% CI, 1.27, 1.70) following fortification], and other health outcomes. Across settings, high-quality studies such as those with randomized exposures (e.g., randomized controlled trials, Mendelian randomization studies) are needed to elucidate interactions of FA with vitamin B12 as well as expanded biomarker testing.

Keywords: folic acid; fortification; meta-analysis; neural tube defects; vB12.

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Figures

Figure 1
Figure 1
Hierarchy of evidence for folic acid and neural tube defects (NTDs). It is crucial to keep the basics of the hierarchy of evidence in mind when reviewing any new study. Policy changes generally require randomized controlled trials (RCTs) and preferably meta-analyses of multiples, given the lack of causal inference without randomization. The level of evidence is a critical issue in nutrition because both supplementation and nutritional patterns are heavily confounded by innumerable health conditions and demographic characteristics. Folic acid is one of the best-studied molecules in the published literature, with more than 93,000 peer-reviewed publications; however, no fortification studies of NTDs (only initial supplementation trials) have been conducted because of the ethical and logistical issues surrounding withdrawal of an established, successful intervention. Additional trials are generally limited to intermediate biomarkers. In contrast to other nutritional interventions, there are large RCTs that show the prevention of NTDs by periconceptional folic acid supplementation. New studies are weighted not only against these critical metrics but also against the totality of the evidence in the field.
Figure 2
Figure 2
Folic acid and food folates: metabolic pathway, one-carbon pathway, and biomarker measures. (❶). In addition to naturally occurring food folates from diet (available primarily from leafy greens and liver), it is recommended that all women of reproductive age consume 400 μg/day of folic acid, a synthetic isomer of folate. In the USA, there are three primary sources of folic acid: cereal grain products labeled as enriched (ECGP) containing 140 μg of folic acid per 100 g of flour, ready-to-eat cereals (RTE) containing up to 400 μg of folic acid per serving, and supplements (SUPP) (45). These three sources generate four mutually exclusive folic acid consumption groups with varying median usual intakes of folic acid in adults over 16 years of age: ECGP, ECGP+RTE, ECGP+SUPP, and ECGP+RTE+SUPP (169). (❷) Food folates and folic acid are absorbed primarily from the small intestine via proton-coupled folate receptors (PCFRs), which have a high affinity to folic acid, and reduced folate carriers (RFCs), which have a lower affinity to folic acid (167). Once absorbed, folates reach systemic circulation and are further processed by peripheral tissue. (❸) Absorbed folates are partially removed by the liver. In the liver, folic acid may undergo biotransformation to 5-methyltetrahydrofolate (5-MTHF) via dihydrofolate reductase (DHFR), where it is highly expressed; however, the final biotransformation to 5-MTHF may be limited byMTHFR genotype (❹), leading to ~16%-lower circulating folates inTT genotypes (31, 164). 5-MTHF may be partially released into bile, allowing for further reabsorption in the small intestine. Folylpolyglutamate synthetase (FPGS) in the liver enables long-term storage of folates within the liver. γ-Glutamyl hydrolase (GGH) in the liver enables hydrolysis of stored folate polyglutamates back into bioavailable monoglutamate folate forms, which then reenter the systemic circulation via the hepatic vein (183). (❺) Circulating folates are transported from serum into newly created red blood cells (RBCs) in the bone marrow via membrane-associated folate-binding proteins with a half-life of approximately 120 days (5). Serum folates that are not bound to proteins are filtered by the kidney. (❻) Folate receptor α (FRα), which has high affinities to both folic acid and 5-MTHF, is highly expressed along tubule epithelial cells, allowing for efficient reabsorption of folates (79). Excess folates not reabsorbed and reduced folate forms with lower affinities to FRα are then eliminated into the urine by the kidney (79). In pregnant women, FRα is also highly expressed in the placenta, enabling nutritional transfer of folates to the fetus throughout pregnancy (158). (❼) RBC and serum folate measurements reflect the biological processing of dietary food folates and folic acids. The median serum folate concentration following fortification is ~42 nmol/L, and the median RBC folate concentration is ~1,200 nmol/L. Both comprise primarily 5-MTHF (41, 131). Asterisks indicate active transport via membrane-associated folate-binding proteins. Abbreviations: DHF, dihydrofolate; DHFR, dihydrofolate reductase; DNMT, DNA methyltransferase; dTMP, deoxythymidine monophosphate; dUMP, deoxyuridine monophosphate; MS, methionine synthase; SAH,S-adenosylhomocysteine; SAM,S-adenosylmethionine; SHMT, serine hydroxymethyltransferase; THF, tetrahydrofolate; TS, thymidylate synthase; UMFA, unmetabolized folic acid.
Figure 3
Figure 3
Impact of folic acid fortification on neural tube defect (NTD) prevalence, red blood cell (RBC) folate concentrations, and serum/plasma folate concentrations. (a) NTD rates per 10,000 live births before and after the implementation of mandatory folic acid fortification from population and hospital surveillance programs. Results for studies that monitored all NTDs in either population or hospital surveillance programs are compared during pre- and postfortification periods. For these studies, the postfortification period lasted at least 2 years following the implementation of mandatory folic acid fortification. The standard error was calculated using a Poisson distribution for rare events. Results for studies comparing pre- and postfortification (b) RBC folate concentrations and (c) serum folate concentrations are summarized as a ratio of means (ROM) corresponding to the fold change in post-versus prefortification folate concentration levels. Asterisks indicate concentrations measured via radioimmunological assays, as opposed to microbiological assays. Despite the high level of heterogeneity across studies, the overall effect using a random effects model demonstrated a significant fold change in folate concentration following the introduction of folic acid fortification (P < 0.001 in both cases). RBC folate concentrations increased approximately 1.70-fold (95% CI, 1.24, 2.35), and serum folate concentrations increased approximately 2.12-fold (95% CI, 1.54, 2.91). A subanalysis excluding countries that had fortification levels>400 μg per 100 g of product was also conducted; RBC folate concentrations increased approximately 1.47-fold (95% CI, 1.27, 1.70) and serum folate concentrations increased approximately 1.86-fold (95% CI, 1.48, 2.34) following folic acid fortification. For the search strategy and references, see Supplemental Appendix 3.
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