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Journal of Perinatal Medicine

Official Journal of the World Association of Perinatal Medicine

Editor-in-Chief: Dudenhausen, MD, FRCOG, Joachim W.

Editorial Board Member: / Bancalari, Eduardo / Greenough, Anne / Genc, Mehmet R. / Chervenak, Frank A. / Chappelle, Joseph / Bergmann, Renate L. / Bernardes, J.F. / Bevilacqua, G. / Blickstein, Isaac / Cabero Roura, Luis / Carbonell-Estrany, Xavier / Carrera, Jose M. / D`Addario, Vincenzo / D'Alton, MD, Mary E. / Dimitrou, G. / Grunebaum, Amos / Hentschel, Roland / Köpcke, W. / Kawabata, Ichiro / Keirse, Marc J.N.C. / Kurjak M.D., Asim / Lee, Ben H. / Levene, Malcolm / Lockwood, Charles J. / Marsal, Karel / Makatsariya, Alexander / Nishida, Hiroshi / Papp, Zoltán / Pejaver, Ranjan Kumar / Pooh, Ritsuko K. / Romero, Roberto / Saugstad, Ola D. / Schenker, Joseph G. / Sen, Cihat / Seri, Istvan / Vetter, Klaus / Winn, Hung N. / Young, Bruce K. / Zimmermann, Roland

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Volume 41, Issue 5 (Sep 2013)


Is 5-methyltetrahydrofolate an alternative to folic acid for the prevention of neural tube defects?

Rima Obeid
  • Corresponding author
  • Department of Clinical Chemistry and Laboratory Medicine, Medical School, Saarland University, Building 57, D-66421 Homburg, Germany
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/ Wolfgang Holzgreve / Klaus Pietrzik
  • Department of Nutrition and Food Science, Rheinische Friedrich-Wilhelms University, Bonn, Germany
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Published Online: 2013-03-13 | DOI: https://doi.org/10.1515/jpm-2012-0256


Women have higher requirements for folate during pregnancy. An optimal folate status must be achieved before conception and in the first trimester when the neural tube closes. Low maternal folate status is causally related to neural tube defects (NTDs). Many NTDs can be prevented by increasing maternal folate intake in the preconceptional period. Dietary folate is protective, but recommending increasing folate intake is ineffective on a population level particularly during periods of high demands. This is because the recommendations are often not followed or because the bioavailability of food folate is variable. Supplemental folate [folic acid (FA) or 5-methyltetrahydrofolate (5-methylTHF)] can effectively increase folate concentrations to the level that is considered to be protective. FA is a synthetic compound that has no biological functions unless it is reduced to dihydrofolate and tetrahydrofolate. Unmetabolized FA appears in the circulation at doses of >200 μg. Individuals show wide variations in their ability to reduce FA. Carriers of certain polymorphisms in genes related to folate metabolism or absorption can better benefit from 5-methylTHF instead of FA. 5-MethylTHF [also known as (6S)-5-methylTHF] is the predominant natural form that is readily available for transport and metabolism. In contrast to FA, 5-methylTHF has no tolerable upper intake level and does not mask vitamin B12 deficiency. Supplementation of the natural form, 5-methylTHF, is a better alternative to supplementation of FA, especially in countries not applying a fortification program. Supplemental 5-methylTHF can effectively improve folate biomarkers in young women in early pregnancy in order to prevent NTDs.

Keywords: 5-MethylTHF; neural tube defects; pregnancy


Neural tube defects (NTDs) are serious congenital birth defects affecting the brain or the spinal cord. NTDs arise as a consequence of the failure of or delay in the fusion of the neural tube early in embryogenesis (days 22–28 after conception). Failure to close the neural tube can cause NTDs at levels of the body axis that undergo primary neurulation (the brain and the cervical, thoracic, lumbar, and upper sacral spine). A significant number of first occurrences [28, 45, 143] or recurrent NTDs [80] can be prevented by periconceptional supplementation of folic acid (FA) in mothers. The percentage reduction of the NTD risk by supplementing with FA depends on both the genetic background and other factors such as dietary folate intake or deficiency of other related micronutrients in a certain population. Since NTDs are multifactorial, not all cases can be prevented by supplementing with FA.

Folate functions and requirements

Folate (vitamin B9 or vitamin B11) is a water-soluble B-vitamin that functions as an acceptor or donor of one-carbon groups. 5-Methyltetrahydrofolate [5-methylTHF, (6S)-5-methyltetrahydrofolate, or l-5-methyltetrahydrofolate] is the most available folate form in plants [110], human plasma [82], and human whole blood [67]. 5-MethylTHF constitutes 95–98% of folate in serum or red blood cells (RBCs) [67].

Folates play crucial roles during cell growth and division. They are involved in the de novo synthesis of thymidylate and purine nucleotides and in delivering methyl groups (Figure 1). 5-MethylTHF donates a methyl group to homocysteine (Hcy) that is converted into methionine and further to S-adenosylmethionine (SAM), the main methyl donor in the cell. The methylation of Hcy is mediated by the vitamin B12-dependent enzyme, methionine synthase. 5-MethylTHF is formed from the reduction of methylene-THF in a nicotinamide adenine dinucleotide phosphate-dependent reaction catalyzed by methylenetetrahydrofolate reductase (MTHFR). Folate deficiency causes a wide range of diseases like anemia, depression, pregnancy complications, and poor pregnancy outcomes. Folate deficiency disrupts DNA synthesis and methylation, and causes hyperhomocysteinemia. Since hyperhomocysteinemia is related to pregnancy complications and poor outcome [52, 142], its concentrations should be as low as possible during pregnancy.

Folate and folic acid metabolism. MTHFR=methylenetetrahydrofolate reductase, DHFR=dihydrofolate reductase, Hcy=homocysteine, DHF=dihydrofolate, Meth=methionine, SAM=S-adenosylmethionine, SHMT=serine hydroxymethyl transferase.
Figure 1

Folate and folic acid metabolism.

MTHFR=methylenetetrahydrofolate reductase, DHFR=dihydrofolate reductase, Hcy=homocysteine, DHF=dihydrofolate, Meth=methionine, SAM=S-adenosylmethionine, SHMT=serine hydroxymethyl transferase.

The dietary reference intake for folate for adults is 400 μg of dietary folate equivalents (DFEs) [86, 111]. The main sources of folate in the diet are green leafy plants. Because of the active transport to the child (via the placenta or the milk), pregnant and lactating women have higher requirements for folate.

Folates differ in the one-carbon substituent, the number of polyglutamate residues, and the oxidation state (the natural forms are reduced derivatives). In serum or plasma, folates are present as monoglutamates. Folic acid is the synthetic oxidized form of the vitamin that is not found in fresh natural foods and has no physiological function. Supplemental FA must be reduced to dihydrofolate (DHF) and then to tetrahydrofolate (THF) to be able to enter the folate cycle and act as a co-factor and a source for methyl groups in the cell. In contrast to natural forms of folate in the diet, FA is more stable upon exposure to heat. These features have facilitated its use in supplements and fortified foods. The tolerable upper intake level (UL) for FA is 1 mg/day [58]. Supplemental 5-methylTHF is readily absorbed and utilized [13] and has no UL [58].

Further nutrients can support the folate/methionine cycle. For example, vitamin B6 is a cofactor for the serine hydroxymethyltransferase (SHMT) that supports the folate role in thymidylate synthesis. Moreover, vitamin B2 (riboflavin) is the precursor of flavin mononucleotide and flavin adenine dinucleotide that function as cofactors for methionine synthase reductase and MTHFR, respectively. Additionally, the role of 5-methylTHF in delivering methionine and SAM is vitamin B12-dependent. Moreover, choline via its oxidation product, betaine, is also a methyl donor that is required for Hcy methylation to methionine via betaine homocysteine methyl transferase. Therefore, vitamins B6, B12, and B2, and betaine or choline are nutrients that interrelate to folate metabolism and may affect the NTD risk. Many supplements contain a combination of FA, B12, B6, and other nutrients. Multivitamins have the potential to eliminate deficiencies of nutrients that affect folate metabolism. From this point of view, multivitamins [28] can be more effective than FA alone, especially in populations with common deficiencies such as vegetarians or smokers.

Folate homeostasis

The exact mechanism of folate or FA transfer from the small intestine to the blood is poorly understood. Most of the ingested low doses of FA will be metabolized in the liver after absorption. DHFR is expressed in the liver and other tissues [3, 140]. Liver DHFR can bind FA but has a limited capacity for FA reduction [3]. Unmetabolized FA may pass unchanged into the peripheral circulation at doses of FA >200 μg [65].

The hydrolysis of polyglutamate derivatives of THF (mainly 5-methylTHF) into monoglutamate derivatives in the gut by the brush-border enzyme glutamate carboxypeptidase II is a prerequisite for their intestinal absorption. Unlike folate absorption, folate retention in tissues depends on the ability of the cell to form polyglutamate derivatives [40].

There are three folate transporting proteins: the reduced folate carrier (RFC1), the proton-coupled folate transporter (PCFT), and folate receptor (FR). The three systems have distinct tissue distribution and affinities for the different folate forms at different pH levels. The PCFT [93] is highly expressed in the small intestine and favors 5-methylTHF over FA in physiological conditions. After a folate-rich diet, PCFT seems to be the main transporter responsible for folate absorption in the intestine. The folate receptor represents a family of three proteins (FR α, β, and γ) that are expressed on the apical side of some epithelial cells [63]. RFC1, PCFT, and FR are expressed in the placenta. The exact transport pathway of folate from the maternal to the fetal side is not known. FR seems to play a major role in folate homeostasis during early embryogenesis, since disrupting FRα causes NTDs or even lethality in the embryos [108]. The affinity of FRα for FA is much higher than for 5-methylTHF [141], but FRα contribution to tissue folate is not yet known.

Blood biomarkers of folate reflect folate status and intake

The fasting serum concentration of folate is a good marker for folate status, but it is affected by recent folate intake and may fluctuate if folate intake is not constant. Measuring the serum concentration of folate is particularly useful as an early marker that shows folate depletion or repletion (after dietary modification or supplementation). Serum folate is the strongest negative determinant of plasma total homocysteine (tHcy) concentrations in pregnant women [53]. The concentration of whole blood folate (RBC folate) is an indicator for folate storage, and this marker takes longer than serum folate to reach a steady state after folate supplementation (approximately 40 weeks) [92].

The depletion of plasma or erythrocyte folate takes at least 3 months (for plasma folate) to 9 months (for RBC folate). Compared to the levels just after 6 months of supplementation with 400 μg FA/day, 3 months of folate depletion lowered plasma folate by approximately 50% (geometric mean changed from 34.5 to 16.6 nmol/L) [49]. Levels of plasma folate after 3 months of depletion (geometric mean 16.6 nmol/L) remained 73% higher than the starting levels (geometric mean 9.6 nmol/L) where participants received no supplements at baseline [49]. After a 3-month washout period, concentrations of RBC folate remained 20% higher than pre-supplementation levels (725 vs. 603 nmol/L) [49].

A recent meta-analysis quantified the dose-response relationship between folate intake (dietary folate plus FA) and folate biomarkers in young, pregnant, and lactating women [12]. The aim was to establish a recommended intake for optimizing folate biomarkers for women planning for pregnancy. In women aged 20–35 years, Wald et al. [127] estimated that an increase in folate intake of 100 μg/day would increase serum folate concentrations by 0.94 μg/L (95% confidence interval 0.77–1.10) (2.13 nmol/L per 100 μg/day). Berti et al. [12] estimated that a 100 μg/day total intake would increase serum folate level by 3.3 nmol/L. Lamers et al. [69] estimated that, for a 100 μg [6S]-5-MTHF supplementation over 24 weeks, plasma folate concentrations increased by 9.6 nmol/L. A doubling of total folate intake increased the folate concentration in serum and RBC by 47% and 23%, respectively, and lowered plasma tHcy concentration by 7% [12]. Berti et al. [12] observed a weaker dose-response relationship between folate intake and folate markers in pregnant and lactating women, suggesting that during these physiological periods maintaining maternal folate biomarkers at a given level is more difficult. This might be related to maternal folate depletion and higher requirements. In line with this, supplemental FA in folate-deficient women was transported to the infant via the milk in preference even to the maternal hemapoietic system [77].

Folate and B12-related metabolites in cord blood

Maternal vitamin status is the main determinant of the status in neonates, suggesting that improving maternal vitamin status ensures better vitamin status in the newborns. In accordance with this, maternal and cord blood B-vitamins are strongly correlated [84]. 5-MethylTHF is the main folate form in cord blood (mean 89.4% of total folate) (Figure 2) [82]. The concentration of 5-methylTHF in cord serum is approximately two times higher than in maternal serum (mean 35.8 vs. 15.6 nmol/L) [82], suggesting that supplementing with 5-methylTHF during pregnancy can provide an immediate source for folate to be transported to the fetus.

Concentrations of the main folate forms and their percentage of total folate in maternal serum and umbilical cord serum from neonates [82].
Figure 2

Concentrations of the main folate forms and their percentage of total folate in maternal serum and umbilical cord serum from neonates [82].

The risk of neural tube defects is causally related to low maternal folate

Folate deficiency is causally related to NTDs and a few other birth defects [95]. A stepwise dose-response relationship was observed between NTD risk and low plasma or RBC folate in pregnant women [32] or low folate intake [103, 131]. Plasma folate ≥16.0 nmol/L (or RBC folate >906 nmol/L) was related to the lowest risk in one study on Irish pregnant women in their first trimester [32]. However, plasma and RBC folate decrease during pregnancy [53], suggesting that the preconception target level of serum folate should be higher than 16.0 nmol/L. Clinical studies showing pre-pregnancy serum folate levels necessary for NTD prevention are not available. The mean serum folate level reached after supplementing with FA or 5-methylTHF is approximately 50 nmol/L [69]. This level can ensure optimal folate status for optimal prevention [127].

Because of the limited time window for prevention of NTDs, folate supplementation for young women is required to increase serum folate concentrations. Women of childbearing age should ensure a daily intake of at least 400 μg of folate for at least 4 weeks before and 12 weeks after conception to reduce the risk of having a child with NTD [22]. Up to June 2010, 53 countries had regulations regarding FA fortifications of wheat flour. The fortification aims to provide an estimated 200 μg FA/day in addition to food folate. The exposure of the entire population to additional FA is controversially discussed [87, 126]. There is currently an intensive discussion on optimizing the folate status of the target population (young women) without exceeding a certain intake in the population. Targeted supplementation of young women with folate in countries not applying mandatory FA fortification programs (as in Europe) has become extremely important [126].

Protective effects of folate beyond NTD prevention

Improving maternal folate status can protect against other birth defects like congenital heart defects [27, 119] and orofacial clefts [64, 116]. For example, the incidence of severe congenital heart defects decreased by 6.2% yearly after starting FA fortification in Canada [59]. The association between orofacial clefts (cleft lip and cleft palate) and maternal use of FA was not confirmed by all studies [50, 60]. The prevention of orofacial clefts may be dose-dependent, since studies using higher doses of FA (5–10 mg) found a preventive effect [31, 116], whereas studies using lower doses did not find such an effect [27]. A significant relationship between FA intake and higher birth weight was recently reported [30, 37, 88]. However, this relationship is probably dose-dependent [37] or may be related to extension of the gestational age or prevention of preterm birth [30].

Elevated plasma concentrations of tHcy or low folate concentrations during pregnancy were related to low birth weight and preterm birth [10], pregnancy complications [10, 52], or abortion [81], suggesting that folate may exert a protective effect by lowering tHcy [130]. Since FA supplementation was positively associated with birth weight [102, 115], preterm births may benefit from FA supplementation if they are born with a higher birth weight.

The association between folate status and depression, and the effect of folate administration in the treatment of depression have been addressed by several studies [72, 128]. Supplementation of FA [24] or 5-methylfolate [36] may enhance the antidepressant action of certain medications. Therefore, 5-methylTHF may support the treatment of depression during pregnancy and after delivery, but there is currently no firm evidence on a preventive effect for FA or 5-methylTHF against depression.

Genetic polymorphisms affect folate bioavailability, utilization, and requirements

The 677C→T single nucleotide polymorphism in the MTHFR gene is found in ≈10–22% of the European population. Individuals who are homozygous for this polymorphism have higher tHcy concentrations [104, 107] and lower folate concentrations (plasma or whole blood); they show less response to FA supplementation [25], and women have an increased risk for NTDs [121, 122]. Higher folate status stabilizes the mutated enzyme and increases its activity by increasing the affinity of the enzyme for its flavin adenine dinucleotide coenzyme [136].

Young women with the MTHFR 677TT genotype are more sensitive to folate depletion in short-term studies (7 weeks) [104] and long-term studies (3 months) [25]. Individuals with the TT genotype also show less response to folate repletion compared to those with the CC genotype [25, 104]. A 7-week repletion phase (dietary folate intake 400 μg DFE/day) corrected serum and RBC folate concentrations to the baseline values [104]. The concentrations of tHcy remained higher and plasma folate lower in women with TT compared to those with CC genotype after FA supplementation [25], suggesting that the MTHFR genotype influences the benefit from FA supplementation.

Another important polymorphism that has been studied in relation to folate metabolism and NTD is the dihydrofolate reductase (DHFR) 19-bp deletion polymorphism [a 19-bp deletion of intron 1a (DHFR19bpdel); rs70991108] [90]. The association between this polymorphism and the NTD risk was inconsistent between the studies [61, 90, 120]. The DHFR in human liver-cell homogenates shows a wide range of activity between individual samples [3], suggesting large differences in an individual’s ability to reduce FA. A limited ability of the mutated enzyme to reduce FA (at high FA intake ≥500 μg/day) caused higher unmetabolized FA in blood [62]. The mutated enzyme (del/del) was associated with lower whole blood folate in individuals receiving <250 μg/day FA [62]. The effects of the genotype on plasma or RBC folate or tHcy are currently controversial [42, 109].

The MTHFD1 catalyzes the biosynthesis of 10-formylTHF in the cytoplasm. The 1958G→A polymorphism in MTHFD1 causes lowered enzyme activity and impaired de novo synthesis of purine [23]. The mutation has been linked to several birth defects including NTDs [17, 89]. In an animal model, maternal MTHFD1gt/+ genotype showed a 50% decrease in 10-formylTHF synthesis and impaired fetal growth, disrupted folate metabolism, and purine biosynthesis in the fetus, but did not cause NTDs [8].

Serine hydroxymethyltransferase SHMT1 is involved in thymidylate biosynthesis primarily in the cytoplasm and the nucleus [1]. Beaudin et al. [7] showed that mice embryos with disrupted Shmt1 exhibited failure of neural tube closure especially under maternal folate and choline deficiency. In contrast, maternal disruption of Shmt1 did not cause low RBC folate level, elevated tHcy level, or NTD phenotype in the embryo. In this study, maternal folate and choline-deficient diet rather than maternal Shmt1 genotype were related to NTD [7].

Taken together, polymorphisms in the folate cycle predispose to a higher risk of birth defects when the maternal folate status is limited. The effect of the polymorphisms is probably insignificant at a higher maternal folate status. Available evidence strongly suggests that folate deficiency and FA supplementation may have different metabolic effects in a genetically susceptible subset of the population (for example, MTHFR TT carriers) [25, 104]. This effect can be related to the limited availability of the active folate. The direct administration of (6S)-5-methylTHF offers advantages, since it is directly available and does not need to be metabolized.

Strategies to prevent NTDs by improving maternal folate status

Low dietary folate intake is related to low consumption of folate-rich foods, long storage of the folate-containing foods, and a reduction in vitamin content during food processing [57]. Mean folate intake in European populations ranges from 180 to 280 μg/day [5, 29, 139], which is not sufficient to prevent folate-responsive NTD cases. In addition, factors like smoking, very young age, and lack of knowledge about the importance of folate supplementation before pregnancy can adversely affect the folate status and increase the risk of having a child with a NTD.

It is recommended that all women of childbearing age receive 0.4–0.8 mg of FA per day in order to prevent NTDs. Because of the limited time window for prevention and the low compliance, the recommendation for young women failed to achieve the goal in most countries [6, 35]. Moreover, the optimal dose of FA depends on baseline folate, the folate intake, the MTHFR polymorphism, smoking, and the time window available to reach preventive serum levels. The fortification of grain products with FA (in the USA 100 μg/100 g grain products) was thought to increase folate intake by approximately 200 μg/day [94]. Fortification with FA increased folate status in the whole population, but this effect seems not to be sufficient in the target group [4, 91]. Moreover, increasing FA intake caused the appearance of unmetabolized FA in the blood [83, 113]. Unmetabolized FA was detected in newborns and in 4-day-old formula-fed infants [112]. FA may affect the immune system. For example, unmetabolized FA was associated with reduced natural killer (NK) cell toxicity [117]. NK cells are regarded to be the first line of defense in the prevention of carcinogenesis and viral infections. Furthermore, FA supplementation during pregnancy has also been discussed in relation to an increased risk of respiratory illness in children [132], although this association has not been confirmed by other studies [74, 75].

There is currently no firm evidence indicating that FA is transported or accumulated in the fetus, nor of a firm relationship with disease phenotypes after birth. However, no studies are currently available on concentrations of unmetabolized FA in cord blood from women taking higher doses of FA (4–5 mg/day) for the prevention of recurrent NTDs. The appearance of unmetabolized FA in cord blood can be avoided by supplementing with the natural folate form, 5-methylTHF.

Another concern in people with high intakes of supplemental FA is the masking of vitamin B12 deficiency. This is not the case when supplementing with 5-methylTHF. Since vitamin B12 is required for folate metabolism, vitamin B12 deficiency can cause a folate trap. High doses of FA can correct hematological signs of vitamin B12 deficiency and can delay the diagnosis of B12 deficiency, thus increasing the risk of developing neurological complications. In contrast, 5-methylTHF supplementation given to B12-deficient individuals cannot be utilized for methionine or folate cycles and cannot mask vitamin B12 deficiency.

The purpose of the food fortification programs is to increase folate intake in young women, but the intake has been increased in the whole population [87]. Targeted administration of vitamin supplements is the most effective way to specifically increase folate intake in women of child-bearing age before and during pregnancy when folate requirements are high. This is particularly important in countries that do not apply the fortification.

Natural folate can prevent folate-responsive neural tube defects

Poor maternal diet has been related to the occurrence [96, 106] or recurrence [71] of NTDs. In particular, low dietary folate was related to a higher NTD risk [41]. Folate intake in the first 6 weeks of pregnancy was particularly protective [16]. This relationship has been shown to be dose-dependent [131]. The lowest risk was found in women with total folate intake >350 μg/day [16]. Moreover, the dietary folate intake and supplement usage early in the second trimester were related to the pregnancy outcome in a study of >23,000 women [79]. The risk reduction of NTDs was 0.78 for every 500 μg increase in total folate intake (dietary plus supplemental) [79].

Low dietary folate intake is the main determinant of low serum or RBC folate concentrations and of high plasma concentrations of tHcy. In one prospective study of 56,049 Irish pregnant women, maternal blood samples were available from 81 women with NTD-affected pregnancies and 247 pregnant women as controls [66]. Compared to the controls, lower concentrations of plasma folate (7.9 vs. 10.4 nmol/L; P=0.002) and RBC folate (609 vs. 766 nmol/L; P<0.001) were found in mothers of cases compared with mothers of controls [66]. A continuous dose-response relationship between NTD risk lowering and maternal RBC folate was observed in the following analyses of results from the same population [32].

The differences in the incidence of NTDs and folate-responsive NTDs are partly attributed to differences in dietary folate intake between populations [11]. For example, geographical differences have been reported in the incidence of NTDs in China [11]. After promoting FA supplementation prenatally, the NTD incidence decreased in north and south China by approximately 75% and 40%, respectively [11, 51]. In line with this, strong differences between south and north China in serum folate levels (in women aged 35–44 years: 19.9 vs. 9.7 nmol/L) and RBC folate levels (in women aged 35–44 years: 911 vs. 508 nmol/L) were later reported [48].

Therefore, dietary folate is associated with higher serum folate concentrations and a lower NTD risk. The risk reduction after FA supplementation is probably due to 5-methylTHF, the active natural form and the dominant folate form in plasma or RBCs. The fact that 5-methylTHF is the major folate form in many foods can explain the association between a higher dietary folate supply and the NTD risk reduction [69].

Handling natural folates versus folic acid

Folates from natural sources are polyglutamated derivatives (mainly 5-methylTHF) [18] that must be hydrolyzed before absorption in the ileum can take place. The food matrix affects folate absorption from natural foods [43, 76]. The bioavailability of food folates showed large variability in short-term studies (12–24 h) [44, 85]. Other physiological factors (genetic polymorphisms, baseline folate status, status of the related nutrients) can affect folate bioavailability (both dietary folate and FA) [19, 46, 99, 134].

Consumption of folate-rich foods over 4–6 weeks improves serum and blood folate concentrations (by up to 50%) and decreases plasma concentrations of tHcy by up to 29% [18, 56, 97, 105]. Young women who were consuming 800 DFE/day from natural sources for 12 weeks showed a 67% increase in serum folate concentrations and a 33% increase in RBC folate concentrations compared with women consuming 400 DFE/day [56]. Similarly, Brown et al. [20] found that RBC folate concentrations in the range considered to be protective against NTD (>906 nmol/L) were primarily found in women who took approximately 400 μg FA. Individuals not receiving supplemental FA require 500–600 μg/day of total folate to achieve a plasma tHcy level of 10.0 μmol/L [118], but serum folate level under these conditions (15 nmol/L) seems below the optimal range for NTD prevention [118]. In one study in a Dutch population, the dietary intake in the majority of women (200 μg/day) was not sufficient to reach plasma tHcy concentrations of below 10.0 μmol/L [34], suggesting that current folate intake does not provide maximum protection against NTDs in this European population.

Individuals with the MTHFR 677 TT genotype require a higher intake of folate in order to achieve similar tHcy concentrations as those in individuals with the MTHFR 677 CC genotype [2, 25, 78]. One study has shown that a total folate intake of approximately 660 μg DFE/day derived mainly from fortified cereals was necessary to achieve near-normal plasma tHcy concentrations in adults with the MTHFR 677TT genotype [2].

In a 6-month placebo-controlled study, the effect of FA (100, 200, or 400 μg daily) on RBC folate concentrations was tested in 121 women [33]. RBC folate levels increased in all supplementation groups. The median (95% confidence interval) of post-treatment RBC folate level was 1293 (1098–1481) nmol/L in the group with 400 μg/day of FA and was 1076 (978–1139) nmol/L in the group that received 200 μg/day [33].

Long-term supplementation with 200 μg FA/day, a dose similar to the current intake from fortified foods [94], was as effective as 400 μg in lowering tHcy concentrations [114]. Ashfield-Watt et al. [2] found that supplementation of 400 μg of FA was able to lower tHcy concentrations to a similar extent as those of food folate. After 4 months, plasma concentrations of folate were higher in the FA group compared with the dietary folate group [2]. The effectiveness of increasing RBC folate concentrations by means of increasing folate intake from natural sources versus fortified foods or supplemental FA has been tested in a small study on young women [26]. After 12 weeks, changes in RBC folate concentrations were highest in the supplement and fortified food groups [26], whereas 10 women showed no changes in RBC folate after dietary modifications [26]. Very low doses of FA (50 and 100 μg/day) were tested [98, 123]. In short-term studies (6 weeks) [98], 400 μg FA/day was defined as the minimum dose for adequate tHcy lowering. A meta-analysis of previous dose-finding studies concluded that 800 μg/day was the optimal dose [73]. Studies on genetic modifications in the folate cycle confirmed that the role of folate in preventing NTDs may go beyond a tHcy-lowering effect [8].

The different bioavailability and metabolism of FA and dietary folate cause imprecise estimation of folate requirements especially from foods fortified with FA [100]. The estimated bioavailability for food folate is 50–98% of that for FA [47, 101, 133, 137]. Studies on bioavailability applied different time intervals and depended on different tracing approaches like using isotopes [101] and measuring blood or serum folate, or metabolic markers [18]. Furthermore, unmetabolized FA may cause overestimation of the bioavailability of FA, since many methods cannot distinguish between folate forms [18]. The recovery of FA by some methods is higher than that for 5-methylTHF, thus causing overestimation of active folate [138]. Recently, the validity of the conventional approach used in bioavailability studies comparing FA with dietary food folate has been questioned [135]. Using whole diets rather than single foods has been suggested in order to test the post-absorptive metabolism of the different folate forms (including free FA) [100].

In summary, the bioavailability of food folates depends on pre-absorptive, absorptive, and post-absorptive processes. The estimated relative bioavailability of food folate is lower when compared with the supplemental FA. However, some evidence suggests that FA is not a proper reference material for the bioavailability studies and should be replaced by 5-methylTHF. After all, increasing food folate consumption is currently not an effective strategy for optimizing folate status in young women [21].

Handling methyl folate versus folic acid

The dose, form, and duration of folate intake for NTD prevention have been central topics for several years. For example, the US FDA has recently approved oral contraceptives combined with (6S)-5-methylTHF in order to reduce the risk of NTDs in women who conceive while using the pill or shortly thereafter (reviewed in Ref. [54]).

The response of folate status parameters (plasma and RBC folate) was tested after supplementing with 453 nmol/day of dietary folate and an equimolar dose of supplemental FA or the bioactive diastereoisomer (6S)-5-methylTHF for 16 weeks [135]. The increase in serum and RBC folate concentrations observed after increasing dietary folate was less than that observed after FA or (6S)-5-methylTHF supplementation over 16 weeks [135]. Unmetabolized FA was detected in the plasma of subjects who received FA (mean 0.2 nmol/L) but not in those who received 5-methylTHF [39]. Using (6S)-5-methylTHF rather than FA was recommended as the reference folate to estimate dietary (food) folate bioavailability [135]. Accordingly, (6S)-5-methylTHF (416 μg/day) can improve [69] or maintain [55] RBC folate concentrations to a significantly greater extent than FA.

Lamers et al. [70] conducted a trial in 144 young females using 400 μg FA, 416 μg (6S)-5-methylTHF, 208 μg (6S)-5-methylTHF, or placebo for 24 weeks. Concentrations of plasma tHcy and folate were tested in 4-week intervals. An increase in plasma total folate levels and a decrease in plasma tHcy levels were seen 4 weeks after starting the trial [70]. Plasma folate continued to increase at 8 weeks, but tHcy did not continue to decline. The decline in plasma tHcy in the groups receiving 208 μg (6S)-5-methylTHF or 400 μg FA was similar. The percentage changes of tHcy after supplementation of 400 μg of FA or 416 μg of (6S)-5-methylTHF relative to the placebo group after adjustment for baseline concentration were –15% vs. –19%, respectively, (P>0.05) [70]. However, the study used an immunological assay that cannot rule out unmetabolized FA measured as total folate.

In one study on lactating women, (6S)-5-methylTHF (416 μg/day, 906 nmol/day), FA (400 μg/day, 906 nmol/day), or placebo was administered for 16 weeks [55]. The mean RBC folate concentration after supplementation with (6S)-5-methylTHF (2178 nmol/L) was higher than after supplementation with FA (1967 nmol/L; P<0.05) or placebo (1390 nmol/L; P<0.002). Women in this study were pre-saturated with 1 mg FA/day during pregnancy [55], thus explaining the high concentrations of RBC. Studies using the plasma area under the curve (AUC) after pre-saturation of the participants may show a higher AUC [85]. In addition, after delivery, women in all three study arms seemed to be in the depletion phase [55]. The extent of the depletion as shown by lowered RBC folate after 4 weeks (8.2% vs. 11.1%) and 16 weeks (21% vs. 39%) seemed to be approximately 50% lower in the (6S)-5-methylTHF compared to the FA group [55]. Plasma tHcy was relatively stable in the three groups over 16 weeks. However, the mean plasma folate concentrations seemed to be maintained over 16 weeks only in the (6S)-5-methylTHF group. The mean plasma folate concentrations declined slightly over 16 weeks in the FA group [55]. (6S)-5-MethylTHF seemed to be slightly better than FA in maintaining RBC and plasma folate after delivery.

The effects of daily FA (100 μg) and (6S)-5-methylTHF (113 μg) were compared in a 24-week supplementation trial in middle-aged male and female subjects [124]. Plasma concentrations of tHcy decreased by 9.3% and 14.6% in the FA and the (6S)-5-methylTHF groups, respectively, compared to the placebo group [124]. In contrast, FA supplementation increased the concentrations of plasma folate to 34.5 nmol/L (52% higher than baseline) and RBC folate to 1137 nmol/L (31% higher than baseline). This increase was higher than that caused by (6S)-5-methylTHF (plasma folate 25.6 nmol/L, 34% increase; RBC folate 984 nmol/L, 23% increase) [124]. At 24 weeks, the increases in plasma and RBC folate concentrations did not differ significantly between the two supplemented groups [124]. In a study of 104 young female subjects (18–49 years), FA and (6S)-5-methylTHF increased plasma and blood folate concentrations to a similar extent [125]. Neither supplement group showed differences in the curves and did not reach a steady state after 24 weeks [125].

The estimate of the dose-response relationship showed that the association between folate status biomarkers and 5-methylTHF intake was stronger than for FA intake [12], suggesting that 5-methylTHF may be more effective in improving folate biomarkers. Despite the fact that the administered doses, sampling intervals, number of sampling occasions, and pre-saturation of volunteers vary between trials, equimolar doses of FA or reduced folate concentrations resulted in at least equivalent metabolic response (lowering tHcy) or increasing plasma or RBC folate concentrations. The advantages and limitations of food folate, 5-methylTHF, and FA are shown in Table 1. Efficacy studies on the effect of 5-methylTHF in improving folate blood markers are available and encouraging. However, the role of 5-methylTHF in preventing NTDs or other birth defects has not been tested in clinical studies.

Table 1

Summary of advantages and limitations of food folate, 5-methylTHF, and folic acid.


A mean serum folate level of approximately 50 nmol/L was achieved by supplementing with 400 μg of 5-methylTHF or FA for 12 weeks [69]. An optimal serum folate level for NTD prevention should be reached before conception. Dietary folate intake is low in the general population, and dietary modifications are unlikely to improve blood folate status in the target group within a short time. Moreover, women with polymorphisms in the folate cycle have higher requirements that cannot be achieved by increasing dietary folate intake over a few weeks. Despite mandatory FA fortification, the optimal folate level for prevention of NTDs could not be achieved, especially in low-income and less educated women [14]. Supplementation in the preconceptional period seems to be the best effective way to improve folate status within a short time (4–12 weeks). This is particularly difficult for unplanned pregnancies.

Supplementation studies showed comparable effects for 5-methylTHF and FA in increasing serum or RBC folate concentrations. The natural form of folate, 5-methylTHF, offers several advantages compared to FA (Table 2): it does not mask B12 deficiency, it is already a biologically active form, it does not cause unmetabolized FA in blood, and it is absorbed and utilized at least as well as FA. Although there are no clinical trials on the effectiveness of 5-methylTHF in preventing NTDs, metabolic studies have shown that 5-methylTHF is a biologically active form of the vitamin and it seems to be at least as effective as FA in improving folate biomarkers. The literature clearly shows that a better food folate intake is associated with better folate markers and that food folate can prevent NTDs (by increasing folate status). FA can prevent NTDs by increasing serum or blood folate level. 5-MethyTHF can effectively increase serum or blood folate markers. Therefore, supplementing with 5-methylTHF for NTD prevention seems to be rational. In contrast to the MRC study [80], testing the efficacy of 5-methylTHF against a placebo would be unethical. Comparing the preventive effect of FA with a proposed better alternative (5-methylTHF) will require following several thousands of pregnancies over a long time. The costs of testing 5-methylTHF against FA would be extremely high. We do not have any reason to assume that a randomized controlled trial is justified before recommending 5-methylTHF.

Table 2

Why 5-methyltetrahydrofolate is an alternative to FA.


  • [1]

    Anderson DD, Stover PJ. SHMT1 and SHMT2 are functionally redundant in nuclear de novo thymidylate biosynthesis. PLoS ONE 2009;4:e5839.Google Scholar

  • [2]

    Ashfield-Watt PA, Pullin CH, Whiting JM, Clark ZE, Moat SJ, Newcombe RG, et al. Methylenetetrahydrofolate reductase 677C→T genotype modulates homocysteine responses to a folate-rich diet or a low-dose folic acid supplement: a randomized controlled trial. Am J Clin Nutr. 2002;76:180–6.Google Scholar

  • [3]

    Bailey SW, Ayling JE. The extremely slow and variable activity of dihydrofolate reductase in human liver and its implications for high folic acid intake. Proc Natl Acad Sci USA. 2009;106:15424–9.CrossrefGoogle Scholar

  • [4]

    Bailey RL, Dodd KW, Gahche JJ, Dwyer JT, McDowell MA, Yetley EA, et al. Total folate and folic acid intake from foods and dietary supplements in the USA: 2003–2006. Am J Clin Nutr. 2010;91:231–7.Google Scholar

  • [5]

    Baker PN, Wheeler SJ, Sanders TA, Thomas JE, Hutchinson CJ, Clarke K, et al. A prospective study of micronutrient status in adolescent pregnancy. Am J Clin Nutr. 2009;89:1114–24.CrossrefGoogle Scholar

  • [6]

    Barbour RS, Macleod M, Mires G, Anderson AS. Uptake of folic acid supplements before and during pregnancy: focus group analysis of women’s views and experiences. J Hum Nutr Diet. 2012;25:140–7.CrossrefGoogle Scholar

  • [7]

    Beaudin AE, Abarinov EV, Noden DM, Perry CA, Chu S, Stabler SP, et al. Shmt1 and de novo thymidylate biosynthesis underlie folate-responsive neural tube defects in mice. Am J Clin Nutr. 2011;93:789–98.CrossrefGoogle Scholar

  • [8]

    Beaudin AE, Perry CA, Stabler SP, Allen RH, Stover PJ. Maternal Mthfd1 disruption impairs fetal growth but does not cause neural tube defects in mice. Am J Clin Nutr. 2012;95:882–91.Google Scholar

  • [9]

    Bentley S, Hermes A, Phillips D, Daoud YA, Hanna S. Comparative effectiveness of a prenatal medical food to prenatal vitamins on hemoglobin levels and adverse outcomes: a retrospective analysis. Clin Ther. 2011;33:204–10.CrossrefGoogle Scholar

  • [10]

    Bergen NE, Jaddoe VW, Timmermans S, Hofman A, Lindemans J, Russcher H, et al. Homocysteine and folate concentrations in early pregnancy and the risk of adverse pregnancy outcomes: the Generation R Study. BJOG. 2012;119:739–51.CrossrefGoogle Scholar

  • [11]

    Berry RJ, Li Z, Erickson JD, Li S, Moore CA, Wang H, et al. Prevention of neural-tube defects with folic acid in China. China – U.S. Collaborative Project for Neural Tube Defect Prevention. N Engl J Med. 1999;341:1485–90.Google Scholar

  • [12]

    Berti C, Fekete K, Dullemeijer C, Trovato M, Souverein OW, Cavelaars A, et al. Folate intake and markers of folate status in women of reproductive age, pregnant and lactating women: a meta-analysis. J Nutr Metab. 2012;2012:470656.Google Scholar

  • [13]

    Bhandari SD, Gregory JF III. Folic acid, 5-methyl-tetrahydrofolate and 5-formyl-tetrahydrofolate exhibit equivalent intestinal absorption, metabolism and in vivo kinetics in rats. J Nutr. 1992;122:1847–54.Google Scholar

  • [14]

    Bodnar LM, Himes KP, Venkataramanan R, Chen JY, Evans RW, Meyer JL, et al. Maternal serum folate species in early pregnancy and risk of preterm birth. Am J Clin Nutr. 2010;92:864–71.CrossrefGoogle Scholar

  • [15]

    Bostom AG, Shemin D, Bagley P, Massy ZA, Zanabli A, Christopher K, et al. Controlled comparison of L-5-methyltetrahydrofolate versus folic acid for the treatment of hyperhomocysteinemia in hemodialysis patients. Circulation. 2000;101:2829–32.CrossrefGoogle Scholar

  • [16]

    Bower C, Stanley FJ. Periconceptional vitamin supplementation and neural tube defects; evidence from a case-control study in Western Australia and a review of recent publications. J Epidemiol Community Health. 1992;46:157–61.CrossrefGoogle Scholar

  • [17]

    Brody LC, Conley M, Cox C, Kirke PN, McKeever MP, Mills JL, et al. A polymorphism, R653Q, in the trifunctional enzyme methylenetetrahydrofolate dehydrogenase/methenyltetrahydrofolate cyclohydrolase/formyltetrahydrofolate synthetase is a maternal genetic risk factor for neural tube defects: report of the Birth Defects Research Group. Am J Hum Genet. 2002;71:1207–15.Google Scholar

  • [18]

    Brouwer IA, van Dusseldorp M, West CE, Meyboom S, Thomas CM, Duran M, et al. Dietary folate from vegetables and citrus fruit decreases plasma homocysteine concentrations in humans in a dietary controlled trial. J Nutr. 1999;129:1135–9.Google Scholar

  • [19]

    Brouwer IA, Van Dusseldorp M, West CE, Steegers-Theunissen RP. Bioavailability and bioefficacy of folate and folic acid in man. Nutr Res Rev. 2001;14:267–94.CrossrefGoogle Scholar

  • [20]

    Brown JE, Jacobs DR Jr., Hartman TJ, Barosso GM, Stang JS, Gross MD, et al. Predictors of red cell folate level in women attempting pregnancy. J Am Med Assoc. 1997;277:548–52.Google Scholar

  • [21]

    Caudill MA. Folate bioavailability: implications for establishing dietary recommendations and optimizing status. Am J Clin Nutr. 2010;91:1455S-60S.CrossrefGoogle Scholar

  • [22]

    Centers for Disease Control and Prevention. Recommendations for the use of folic acid to reduce the number of cases of spina bifida and other neural tube defects. MMWR Recomm Rep. 1992;41:1–7.Google Scholar

  • [23]

    Christensen KE, Rohlicek CV, Andelfinger GU, Michaud J, Bigras JL, Richter A, et al. The MTHFD1 p.Arg653Gln variant alters enzyme function and increases risk for congenital heart defects. Hum Mutat. 2009;30:212–20.Google Scholar

  • [24]

    Coppen A, Bailey J. Enhancement of the antidepressant action of fluoxetine by folic acid: a randomised, placebo controlled trial. J Affect Disord. 2000;60:121–30.CrossrefGoogle Scholar

  • [25]

    Crider KS, Zhu JH, Hao L, Yang QH, Yang TP, Gindler J, et al. MTHFR 677C->T genotype is associated with folate and homocysteine concentrations in a large, population-based, double-blind trial of folic acid supplementation. Am J Clin Nutr. 2011;93:1365–72.Google Scholar

  • [26]

    Cuskelly GJ, McNulty H, Scott JM. Effect of increasing dietary folate on red-cell folate: implications for prevention of neural tube defects. Lancet. 1996;347:657–9.Google Scholar

  • [27]

    Czeizel AE. Reduction of urinary tract and cardiovascular defects by periconceptional multivitamin supplementation. Am J Med Genet. 1996;62:179–83.CrossrefGoogle Scholar

  • [28]

    Czeizel AE, Dudas I. Prevention of the first occurrence of neural-tube defects by periconceptional vitamin supplementation. N Engl J Med. 1992;327:1832–5.Google Scholar

  • [29]

    Czeizel AE, Susanszky E. Diet intake and vitamin supplement use of Hungarian women during the preconceptional period. Int J Vitam Nutr Res. 1994;64:300–5.Google Scholar

  • [30]

    Czeizel AE, Puho EH, Langmar Z, Acs N, Banhidy F. Possible association of folic acid supplementation during pregnancy with reduction of preterm birth: a population-based study. Eur J Obstet Gynecol Reprod Biol. 2010;148:135–40.CrossrefGoogle Scholar

  • [31]

    Czeizel AE, Timar L, Sarkozi A. Dose-dependent effect of folic acid on the prevention of orofacial clefts. Pediatrics. 1999;104:e66.CrossrefGoogle Scholar

  • [32]

    Daly LE, Kirke PN, Molloy A, Weir DG, Scott JM. Folate levels and neural tube defects. Implications for prevention. J Am Med Assoc. 1995;274:1698–702.Google Scholar

  • [33]

    Daly S, Mills JL, Molloy AM, Conley M, Lee YJ, Kirke PN, et al. Minimum effective dose of folic acid for food fortification to prevent neural-tube defects. Lancet. 1997;350:1666–9.Google Scholar

  • [34]

    de Bree A, Verschuren WM, Blom HJ, Kromhout D. Association between B vitamin intake and plasma homocysteine concentration in the general Dutch population aged 20–65 years. Am J Clin Nutr. 2001;73:1027–33.Google Scholar

  • [35]

    de Walle HE, de Jong-van den Berg LT. Ten years after the Dutch public health campaign on folic acid: the continuing challenge. Eur J Clin Pharmacol. 2008;64:539–43.Google Scholar

  • [36]

    Fava M, Shelton RC, Zajecka JM. Evidence for the use of l-methylfolate combined with antidepressants in MDD. J Clin Psychiatry. 2011;72:e25.CrossrefGoogle Scholar

  • [37]

    Fekete K, Berti C, Trovato M, Lohner S, Dullemeijer C, Souverein OW, et al. Effect of folate intake on health outcomes in pregnancy: a systematic review and meta-analysis on birth weight, placental weight and length of gestation. Nutr J. 2012;11:75.CrossrefGoogle Scholar

  • [38]

    Finglas PM, Witthoft CM, Vahteristo L, Wright AJ, Southon S, Mellon FA, et al. Use of an oral/intravenous dual-label stable-isotope protocol to determine folic acid bioavailability from fortified cereal grain foods in women. J Nutr. 2002;132:936–9.Google Scholar

  • [39]

    Fohr IP, Prinz-Langenohl R, Bronstrup A, Bohlmann AM, Nau H, Berthold HK, et al. 5,10-Methylenetetrahydrofolate reductase genotype determines the plasma homocysteine-lowering effect of supplementation with 5-methyltetrahydrofolate or folic acid in healthy young women. Am J Clin Nutr. 2002;75:275–82.Google Scholar

  • [40]

    Fox JT, Stover PJ. Folate-mediated one-carbon metabolism. Vitam Horm. 2008;79:1–44.Google Scholar

  • [41]

    Friel JK, Frecker M, Fraser FC. Nutritional patterns of mothers of children with neural tube defects in Newfoundland. Am J Med Genet. 1995;55:195–9.CrossrefGoogle Scholar

  • [42]

    Gellekink H, Blom HJ, van dL I, den HM. Molecular genetic analysis of the human dihydrofolate reductase gene: relation with plasma total homocysteine, serum and red blood cell folate levels. Eur J Hum Genet. 2007;15:103–9.CrossrefGoogle Scholar

  • [43]

    Gregory JF III. Case study: folate bioavailability. J Nutr. 2001;131:1376S–82S.Google Scholar

  • [44]

    Gregory JF III, Bhandari SD, Bailey LB, Toth JP, Baumgartner TG, Cerda JJ. Relative bioavailability of deuterium-labeled monoglutamyl and hexaglutamyl folates in human subjects. Am J Clin Nutr. 1991;53:736–40.Google Scholar

  • [45]

    Grosse SD, Collins JS. Folic acid supplementation and neural tube defect recurrence prevention. Birth Defects Res A Clin Mol Teratol. 2007;79:737–42.CrossrefGoogle Scholar

  • [46]

    Guinotte CL, Burns MG, Axume JA, Hata H, Urrutia TF, Alamilla A, et al. Methylenetetrahydrofolate reductase 677C-->T variant modulates folate status response to controlled folate intakes in young women. J Nutr. 2003;133:1272–80.Google Scholar

  • [47]

    Hannon-Fletcher MP, Armstrong NC, Scott JM, Pentieva K, Bradbury I, Ward M, et al. Determining bioavailability of food folates in a controlled intervention study. Am J Clin Nutr. 2004;80:911–8.Google Scholar

  • [48]

    Hao L, Ma J, Stampfer MJ, Ren A, Tian Y, Tang Y, et al. Geographical, seasonal and gender differences in folate status among Chinese adults. J Nutr. 2003;133:3630–5.Google Scholar

  • [49]

    Hao L, Yang QH, Li Z, Bailey LB, Zhu JH, Hu DJ, et al. Folate status and homocysteine response to folic acid doses and withdrawal among young Chinese women in a large-scale randomized double-blind trial. Am J Clin Nutr. 2008;88:448–57.Google Scholar

  • [50]

    Hayes C, Werler MM, Willett WC, Mitchell AA. Case-control study of periconceptional folic acid supplementation and oral clefts. Am J Epidemiol. 1996;143:1229–34.Google Scholar

  • [51]

    Heseker HB, Mason JB, Selhub J, Rosenberg IH, Jacques PF. Not all cases of neural-tube defect can be prevented by increasing the intake of folic acid. Br J Nutr. 2009;102:173–80.CrossrefGoogle Scholar

  • [52]

    Hogg BB, Tamura T, Johnston KE, Dubard MB, Goldenberg RL. Second-trimester plasma homocysteine levels and pregnancy-induced hypertension, preeclampsia, and intrauterine growth restriction. Am J Obstet Gynecol. 2000;183:805–9.Google Scholar

  • [53]

    Holmes VA, Wallace JM, Alexander HD, Gilmore WS, Bradbury I, Ward M, et al. Homocysteine is lower in the third trimester of pregnancy in women with enhanced folate status from continued folic acid supplementation. Clin Chem. 2005;51:629–34.CrossrefGoogle Scholar

  • [54]

    Holzgreve W, Pietrzik K, Koletzko B, Eckmann-Scholz C. Adding folate to the contraceptive pill: a new concept for the prevention of neural tube defects. J Matern Fetal Neonatal Med. 2012;25:1529–36.CrossrefGoogle Scholar

  • [55]

    Houghton LA, Sherwood KL, Pawlosky R, Ito S, O’Connor DL. [6S]-5-Methyltetrahydrofolate is at least as effective as folic acid in preventing a decline in blood folate concentrations during lactation. Am J Clin Nutr. 2006;83:842–50.Google Scholar

  • [56]

    Hung J, Yang TL, Urrutia TF, Li R, Perry CA, Hata H, et al. Additional food folate derived exclusively from natural sources improves folate status in young women with the MTHFR 677 CC or TT genotype. J Nutr Biochem. 2006;17:728–34.Google Scholar

  • [57]

    Iniesta MD, Perez-Conesa D, Garcia-Alonso J, Ros G, Periago MJ. Folate content in tomato (Lycopersicon esculentum) influence of cultivar, ripeness, year of harvest, and pasteurization and storage temperatures. J Agric Food Chem. 2009;57:4739–45.CrossrefPubMedGoogle Scholar

  • [58]

    Institute of Medicine. Dietary reference intakes for thiamin, riboflavin, niacin, vitamin B6, folate, vitamin B12, pantothenic acid, biotin, and choline. Washington, DC, USA: National Academy Press; 1998:390–422.Google Scholar

  • [59]

    Ionescu-Ittu R, Marelli AJ, Mackie AS, Pilote L. Prevalence of severe congenital heart disease after folic acid fortification of grain products: time trend analysis in Quebec, Canada. Br Med J. 2009;338:b1673.Google Scholar

  • [60]

    Johnson CY, Little J. Folate intake, markers of folate status and oral clefts: is the evidence converging? Int J Epidemiol. 2008;37:1041–58.CrossrefGoogle Scholar

  • [61]

    Johnson WG, Stenroos ES, Spychala JR, Chatkupt S, Ming SX, Buyske S. New 19 bp deletion polymorphism in intron-1 of dihydrofolate reductase (DHFR): a risk factor for spina bifida acting in mothers during pregnancy? Am J Med Genet A. 2004;124A:339–45.Google Scholar

  • [62]

    Kalmbach RD, Choumenkovitch SF, Troen AP, Jacques PF, D’Agostino R, Selhub J. A 19-base pair deletion polymorphism in dihydrofolate reductase is associated with increased unmetabolized folic acid in plasma and decreased red blood cell folate. J Nutr. 2008;138:2323–7.CrossrefGoogle Scholar

  • [63]

    Kamen BA, Smith AK. A review of folate receptor alpha cycling and 5-methyltetrahydrofolate accumulation with an emphasis on cell models in vitro. Adv Drug Deliv Rev. 2004;56:1085–97.Google Scholar

  • [64]

    Kelly D, O’Dowd T, Reulbach U. Use of folic acid supplements and risk of cleft lip and palate in infants: a population-based cohort study. Br J Gen Pract. 2012;62:466–72.CrossrefGoogle Scholar

  • [65]

    Kelly P, McPartlin J, Goggins M, Weir DG, Scott JM. Unmetabolized folic acid in serum: acute studies in subjects consuming fortified food and supplements. Am J Clin Nutr. 1997;65:1790–5.PubMedGoogle Scholar

  • [66]

    Kirke PN, Molloy AM, Daly LE, Burke H, Weir DG, Scott JM. Maternal plasma folate and vitamin B12 are independent risk factors for neural tube defects. Q J Med. 1993;86:703–8.Google Scholar

  • [67]

    Kirsch SH, Herrmann W, Geisel J, Obeid R. Assay of whole blood (6S)-5-CH(3)-H(4) folate using ultra performance liquid chromatography tandem mass spectrometry. Anal Bioanal Chem. 2012;404:895–902.Google Scholar

  • [68]

    Krauss-Etschmann S, Shadid R, Campoy C, Hoster E, Demmelmair H, Jimenez M, et al. Effects of fish-oil and folate supplementation of pregnant women on maternal and fetal plasma concentrations of docosahexaenoic acid and eicosapentaenoic acid: a European randomized multicenter trial. Am J Clin Nutr. 2007;85:1392–400.Google Scholar

  • [69]

    Lamers Y, Prinz-Langenohl R, Bramswig S, Pietrzik K. Red blood cell folate concentrations increase more after supplementation with [6S]-5-methyltetrahydrofolate than with folic acid in women of childbearing age. Am J Clin Nutr. 2006;84:156–61.Google Scholar

  • [70]

    Lamers Y, Prinz-Langenohl R, Moser R, Pietrzik K. Supplementation with [6S]-5-methyltetrahydrofolate or folic acid equally reduces plasma total homocysteine concentrations in healthy women. Am J Clin Nutr. 2004;79:473–8.Google Scholar

  • [71]

    Laurence KM, James N, Miller M, Campbell H. Increased risk of recurrence of pregnancies complicated by fetal neural tube defects in mothers receiving poor diets, and possible benefit of dietary counselling. Br Med J. 1980;281:1592–4.Google Scholar

  • [72]

    Lewis SJ, Araya R, Leary S, Smith GD, Ness A. Folic acid supplementation during pregnancy may protect against depression 21 months after pregnancy, an effect modified by MTHFR C677T genotype. Eur J Clin Nutr. 2012;66:97–103.Google Scholar

  • [73]

    Lowering blood homocysteine with folic acid based supplements: meta-analysis of randomised trials. Homocysteine Lowering Trialists’ Collaboration. Br Med J. 1998;316:894–8.Google Scholar

  • [74]

    Magdelijns FJ, Mommers M, Penders J, Smits L, Thijs C. Folic acid use in pregnancy and the development of atopy, asthma, and lung function in childhood. Pediatrics. 2011;128:e135–44.CrossrefGoogle Scholar

  • [75]

    Martinussen MP, Risnes KR, Jacobsen GW, Bracken MB. Folic acid supplementation in early pregnancy and asthma in children aged 6 years. Am J Obstet Gynecol. 2012;206:72e1–7.Google Scholar

  • [76]

    McKillop DJ, McNulty H, Scott JM, McPartlin JM, Strain JJ, Bradbury I, et al. The rate of intestinal absorption of natural food folates is not related to the extent of folate conjugation. Am J Clin Nutr. 2006;84:167–73.Google Scholar

  • [77]

    Metz J, Zalusky R, Herbert V. Folic acid binding by serum and milk. Am J Clin Nutr. 1968;21:289–97.Google Scholar

  • [78]

    Molloy AM, Daly S, Mills JL, Kirke PN, Whitehead AS, Ramsbottom D, et al. Thermolabile variant of 5,10-methylenetetrahydrofolate reductase associated with low red-cell folates: implications for folate intake recommendations. Lancet. 1997;349:1591–3.Google Scholar

  • [79]

    Moore LL, Bradlee ML, Singer MR, Rothman KJ, Milunsky A. Folate intake and the risk of neural tube defects: an estimation of dose-response. Epidemiology. 2003;14:200–5.CrossrefGoogle Scholar

  • [80]

    MRC Vitamin Study Research Group. Prevention of neural tube defects: results of the Medical Research Council Vitamin Study. Lancet. 1991;338:131–7.Google Scholar

  • [81]

    Nelen WL, Blom HJ, Steegers EA, den Heijer M, Thomas CM, Eskes TK. Homocysteine and folate levels as risk factors for recurrent early pregnancy loss. Obstet Gynecol. 2000;95:519–24.CrossrefGoogle Scholar

  • [82]

    Obeid R, Kasoha M, Kirsch SH, Munz W, Herrmann W. Concentrations of unmetabolized folic acid and primary folate forms in pregnant women at delivery and in umbilical cord blood. Am J Clin Nutr. 2010;92:1416–22.CrossrefGoogle Scholar

  • [83]

    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;60:673–8.Google Scholar

  • [84]

    Obeid R, Munz W, Jager M, Schmidt W, Herrmann W. Biochemical indexes of the B vitamins in cord serum are predicted by maternal B vitamin status. Am J Clin Nutr. 2005;82:133–9.Google Scholar

  • [85]

    Ohrvik VE, Witthoft CM. Human folate bioavailability. Nutrients. 2011;3:475–90.CrossrefGoogle Scholar

  • [86]

    O’Keefe CA, Bailey LB, Thomas EA, Hofler SA, Davis BA, Cerda JJ, et al. Controlled dietary folate affects folate status in nonpregnant women. J Nutr. 1995;125:2717–25.Google Scholar

  • [87]

    Osterhues A, Holzgreve W, Michels KB. Shall we put the world on folate? Lancet. 2009;374:959–61.Google Scholar

  • [88]

    Papadopoulou E, Stratakis N, Roumeliotaki T, Sarri K, Merlo DF, Kogevinas M, et al. The effect of high doses of folic acid and iron supplementation in early-to-mid pregnancy on prematurity and fetal growth retardation: the mother-child cohort study in Crete, Greece (Rhea study). Eur J Nutr. 2013;52:327–36.CrossrefGoogle Scholar

  • [89]

    Parle-McDermott A, Kirke PN, Mills JL, Molloy AM, Cox C, O’Leary VB, et al. Confirmation of the R653Q polymorphism of the trifunctional C1-synthase enzyme as a maternal risk for neural tube defects in the Irish population. Eur J Hum Genet. 2006;14:768–72.Google Scholar

  • [90]

    Parle-McDermott A, Pangilinan F, Mills JL, Kirke PN, Gibney ER, Troendle J, et al. The 19-bp deletion polymorphism in intron-1 of dihydrofolate reductase (DHFR) may decrease rather than increase risk for spina bifida in the Irish population. Am J Med Genet A. 2007;143A:1174–80.Google Scholar

  • [91]

    Pfeiffer CM, Caudill SP, Gunter EW, Osterloh J, Sampson EJ. Biochemical indicators of B vitamin status in the US population after folic acid fortification: results from the National Health and Nutrition Examination Survey 1999–2000. Am J Clin Nutr. 2005;82:442–50.Google Scholar

  • [92]

    Pietrzik K, Lamers Y, Bramswig S, Prinz-Langenohl R. Calculation of red blood cell folate steady state conditions and elimination kinetics after daily supplementation with various folate forms and doses in women of childbearing age. Am J Clin Nutr. 2007;86:1414–9.Google Scholar

  • [93]

    Qiu A, Jansen M, Sakaris A, Min SH, Chattopadhyay S, Tsai E, et al. Identification of an intestinal folate transporter and the molecular basis for hereditary folate malabsorption. Cell. 2006;127:917–28.CrossrefGoogle Scholar

  • [94]

    Quinlivan EP, Gregory JF III. Effect of food fortification on folic acid intake in the United States. Am J Clin Nutr. 2003;77:221–5.Google Scholar

  • [95]

    Ramakrishnan U, Grant F, Goldenberg T, Zongrone A, Martorell R. Effect of women’s nutrition before and during early pregnancy on maternal and infant outcomes: a systematic review. Paediatr Perinat Epidemiol. 2012;26:285–301.CrossrefGoogle Scholar

  • [96]

    Renwick JH. Spina bifida, anencephaly, and potato blight. Lancet. 1972;2:967–8.CrossrefGoogle Scholar

  • [97]

    Riddell LJ, Chisholm A, Williams S, Mann JI. Dietary strategies for lowering homocysteine concentrations. Am J Clin Nutr. 2000;71:1448–54.Google Scholar

  • [98]

    Rydlewicz A, Simpson JA, Taylor RJ, Bond CM, Golden MH. The effect of folic acid supplementation on plasma homocysteine in an elderly population. QJM. 2002;95:27–35.CrossrefGoogle Scholar

  • [99]

    Said HM, Chatterjee N, Haq RU, Subramanian VS, Ortiz A, Matherly LH, et al. Adaptive regulation of intestinal folate uptake: effect of dietary folate deficiency. Am J Physiol Cell Physiol. 2000;279:C1889–95.Google Scholar

  • [100]

    Sanderson P, McNulty H, Mastroiacovo P, McDowell IF, Melse-Boonstra A, Finglas PM, et al. Folate bioavailability: UK Food Standards Agency workshop report. Br J Nutr. 2003;90:473–9.CrossrefGoogle Scholar

  • [101]

    Sauberlich HE, Kretsch MJ, Skala JH, Johnson HL, Taylor PC. Folate requirement and metabolism in nonpregnant women. Am J Clin Nutr. 1987;46:1016–28.Google Scholar

  • [102]

    Shaw GM, Carmichael SL, Nelson V, Selvin S, Schaffer DM. Occurrence of low birthweight and preterm delivery among California infants before and after compulsory food fortification with folic acid. Public Health Rep. 2004;119:170–3.Google Scholar

  • [103]

    Shaw GM, Schaffer D, Velie EM, Morland K, Harris JA. Periconceptional vitamin use, dietary folate, and the occurrence of neural tube defects. Epidemiology. 1995;6:219–26.CrossrefGoogle Scholar

  • [104]

    Shelnutt KP, Kauwell GP, Chapman CM, Gregory JF III, Maneval DR, Browdy AA, et al. Folate status response to controlled folate intake is affected by the methylenetetrahydrofolate reductase 677C-->T polymorphism in young women. J Nutr. 2003;133:4107–11.Google Scholar

  • [105]

    Silaste ML, Rantala M, Sampi M, Alfthan G, Aro A, Kesaniemi YA. Polymorphisms of key enzymes in homocysteine metabolism affect diet responsiveness of plasma homocysteine in healthy women. J Nutr. 2001;131:2643–7.Google Scholar

  • [106]

    Smithells RW, Sheppard S, Schorah CJ. Vitamin deficiencies and neural tube defects. Arch Dis Child. 1976;51:944–50.CrossrefGoogle Scholar

  • [107]

    Solis C, Veenema K, Ivanov AA, Tran S, Li R, Wang W, et al. Folate intake at RDA levels is inadequate for Mexican American men with the methylenetetrahydrofolate reductase 677TT genotype. J Nutr. 2008;138:67–72.Google Scholar

  • [108]

    Spiegelstein O, Mitchell LE, Merriweather MY, Wicker NJ, Zhang Q, Lammer EJ, et al. Embryonic development of folate binding protein-1 (Folbp1) knockout mice: effects of the chemical form, dose, and timing of maternal folate supplementation. Dev Dyn. 2004;231:221–31.Google Scholar

  • [109]

    Stanislawska-Sachadyn A, Brown KS, Mitchell LE, Woodside JV, Young IS, Scott JM, et al. An insertion/deletion polymorphism of the dihydrofolate reductase (DHFR) gene is associated with serum and red blood cell folate concentrations in women. Hum Genet. 2008;123:289–95.CrossrefGoogle Scholar

  • [110]

    Storozhenko S, De B V, Volckaert M, Navarrete O, Blancquaert D, Zhang GF, et al. Folate fortification of rice by metabolic engineering. Nat Biotechnol. 2007;25:1277–9.CrossrefGoogle Scholar

  • [111]

    Suitor CW, Bailey LB. Dietary folate equivalents: interpretation and application. J Am Diet Assoc. 2000;100:88–94.CrossrefGoogle Scholar

  • [112]

    Sweeney MR, McPartlin J, Weir DG, Daly S, Pentieva K, Daly L, et al. Evidence of unmetabolised folic acid in cord blood of newborn and serum of 4-day-old infants. Br J Nutr. 2005;94:727–30.CrossrefGoogle Scholar

  • [113]

    Sweeney MR, Staines A, Daly L, Traynor A, Daly S, Bailey SW, et al. Persistent circulating unmetabolised folic acid in a setting of liberal voluntary folic acid fortification. Implications for further mandatory fortification? BMC Public Health. 2009;9:295.CrossrefGoogle Scholar

  • [114]

    Tighe P, Ward M, McNulty H, Finnegan O, Dunne A, Strain J, et al. A dose-finding trial of the effect of long-term folic acid intervention: implications for food fortification policy. Am J Clin Nutr. 2011;93:11–8.CrossrefGoogle Scholar

  • [115]

    Timmermans S, Jaddoe VW, Hofman A, Steegers-Theunissen RP, Steegers EA. Periconception folic acid supplementation, fetal growth and the risks of low birth weight and preterm birth: the Generation R Study. Br J Nutr. 2009;1–9.Google Scholar

  • [116]

    Tolarova M. Periconceptional supplementation with vitamins and folic acid to prevent recurrence of cleft lip. Lancet. 1982;2:217.CrossrefGoogle Scholar

  • [117]

    Troen AM, Mitchell B, Sorensen B, Wener MH, Johnston A, Wood B, et al. Unmetabolized folic acid in plasma is associated with reduced natural killer cell cytotoxicity among postmenopausal women. J Nutr. 2006;136:189–94.Google Scholar

  • [118]

    Tucker KL, Mahnken B, Wilson PW, Jacques P, Selhub J. Folic acid fortification of the food supply. Potential benefits and risks for the elderly population. J Am Med Assoc. 1996;276:1879–85.CrossrefGoogle Scholar

  • [119]

    van Beynum I, Kapusta L, Bakker MK, den Heijer M, Blom HJ, de Walle HE. Protective effect of periconceptional folic acid supplements on the risk of congenital heart defects: a registry-based case-control study in the northern Netherlands. Eur Heart J. 2010;31:464–71.CrossrefGoogle Scholar

  • [120]

    van der Linden IJ, Nguyen U, Heil SG, Franke B, Vloet S, Gellekink H, et al. Variation and expression of dihydrofolate reductase (DHFR) in relation to spina bifida. Mol Genet Metab. 2007;91:98–103.Google Scholar

  • [121]

    Van Der Put NM, Eskes TK, Blom HJ. Is the common 677C-->T mutation in the methylenetetrahydrofolate reductase gene a risk factor for neural tube defects? A meta-analysis. QJM. 1997;90:111–5.Google Scholar

  • [122]

    Van Der Put NM, Steegers-Theunissen RP, Frosst P, Trijbels FJ, Eskes TK, van den Heuvel LP, et al. Mutated methylenetetrahydrofolate reductase as a risk factor for spina bifida. Lancet. 1995;346:1070–1.Google Scholar

  • [123]

    van Oort FV, Melse-Boonstra A, Brouwer IA, Clarke R, West CE, Katan MB, et al. Folic acid and reduction of plasma homocysteine concentrations in older adults: a dose-response study. Am J Clin Nutr. 2003;77:1318–23.Google Scholar

  • [124]

    Venn BJ, Green TJ, Moser R, Mann JI. Comparison of the effect of low-dose supplementation with L-5-methyltetrahydrofolate or folic acid on plasma homocysteine: a randomized placebo-controlled study. Am J Clin Nutr. 2003;77:658–62.Google Scholar

  • [125]

    Venn BJ, Green TJ, Moser R, McKenzie JE, Skeaff CM, Mann J. Increases in blood folate indices are similar in women of childbearing age supplemented with [6S]-5-methyltetrahydrofolate and folic acid. J Nutr. 2002;132:3353–5.Google Scholar

  • [126]

    Verhoef P. New insights on the lowest dose for mandatory folic acid fortification? Am J Clin Nutr. 2011;93:1–2.CrossrefGoogle Scholar

  • [127]

    Wald NJ, Law MR, Morris JK, Wald DS. Quantifying the effect of folic acid. Lancet. 2001;358:2069–73.Google Scholar

  • [128]

    Watanabe H, Ishida S, Konno Y, Matsumoto M, Nomachi S, Masaki K, et al. Impact of dietary folate intake on depressive symptoms in young women of reproductive age. J Midwifery Womens Health. 2012;57:43–8.CrossrefGoogle Scholar

  • [129]

    Wei MM, Bailey LB, Toth JP, Gregory JF III. Bioavailability for humans of deuterium-labeled monoglutamyl and polyglutamyl folates is affected by selected foods. J Nutr. 1996;126:3100–8.Google Scholar

  • [130]

    Wen SW, Chen XK, Rodger M, White RR, Yang Q, Smith GN, et al. Folic acid supplementation in early second trimester and the risk of preeclampsia. Am J Obstet Gynecol. 2008;198:45–7.Google Scholar

  • [131]

    Werler MM, Shapiro S, Mitchell AA. Periconceptional folic acid exposure and risk of occurrent neural tube defects. J Am Med Assoc. 1993;269:1257–61.Google Scholar

  • [132]

    Whitrow MJ, Moore VM, Rumbold AR, Davies MJ. Effect of supplemental folic acid in pregnancy on childhood asthma: a prospective birth cohort study. Am J Epidemiol. 2009;170:1486–93.Google Scholar

  • [133]

    Winkels RM, Brouwer IA, Siebelink E, Katan MB, Verhoef P. Bioavailability of food folates is 80% of that of folic acid. Am J Clin Nutr. 2007;85:465–73.Google Scholar

  • [134]

    Winkels RM, Brouwer IA, Verhoef P, van Oort FV, Durga J, Katan MB. Gender and body size affect the response of erythrocyte folate to folic acid treatment. J Nutr. 2008;138:1456–61.Google Scholar

  • [135]

    Wright AJ, King MJ, Wolfe CA, Powers HJ, Finglas PM. Comparison of (6S)-5-methyltetrahydrofolic acid v. folic acid as the reference folate in longer-term human dietary intervention studies assessing the relative bioavailability of natural food folates: comparative changes in folate status following a 16-week placebo-controlled study in healthy adults. Br J Nutr. 2010;103:724–9.Google Scholar

  • [136]

    Yamada K, Chen Z, Rozen R, Matthews RG. Effects of common polymorphisms on the properties of recombinant human methylenetetrahydrofolate reductase. Proc Natl Acad Sci USA 2001;98:14853–8.CrossrefGoogle Scholar

  • [137]

    Yang TL, Hung J, Caudill MA, Urrutia TF, Alamilla A, Perry CA, et al. A long-term controlled folate feeding study in young women supports the validity of the 1.7 multiplier in the dietary folate equivalency equation. J Nutr. 2005;135:1139–45.Google Scholar

  • [138]

    Yetley EA, Pfeiffer CM, Phinney KW, Fazili Z, Lacher DA, Bailey RL, et al. Biomarkers of folate status in NHANES: a roundtable summary. Am J Clin Nutr. 2011;94:303S-12S.CrossrefGoogle Scholar

  • [139]

    Young PJ, Nicolas G, Freisling H, Biessy C, Scalbert A, Romieu I, et al. Comparison of standardised dietary folate intake across ten countries participating in the European Prospective Investigation into Cancer and Nutrition. Br J Nutr. 2011;1–18.Google Scholar

  • [140]

    Zhang D, Ochi N, Takigawa N, Tanimoto Y, Chen Y, Ichihara E, et al. Establishment of pemetrexed-resistant non-small cell lung cancer cell lines. Cancer Lett. 2011;309:228–35.Google Scholar

  • [141]

    Zhao R, Matherly LH, Goldman ID. Membrane transporters and folate homeostasis: intestinal absorption and transport into systemic compartments and tissues. Expert Rev Mol Med. 2009;11:e4.CrossrefGoogle Scholar

  • [142]

    Zhao W, Mosley BS, Cleves MA, Melnyk S, James SJ, Hobbs CA. Neural tube defects and maternal biomarkers of folate, homocysteine, and glutathione metabolism. Birth Defects Res A Clin Mol Teratol. 2006;76:230–6.CrossrefGoogle Scholar

  • [143]

    Zhu L, Ling H. National neural tube defects prevention program in China. Food Nutr Bull. 2008;29:S196–204.Google Scholar

The authors stated that there are no conflicts of interest regarding the publication of this article.

About the article

Corresponding author: Prof. Dr. rer. med Rima Obeid, Department of Clinical Chemistry and Laboratory Medicine, Medical School, Saarland University, Building 57, D-66421 Homburg, Germany, Tel.: +49 68411630711, Fax: +49 68411630703

Received: 2012-10-30

Accepted: 2013-01-29

Published Online: 2013-03-13

Published in Print: 2013-09-01

Citation Information: Journal of Perinatal Medicine, ISSN (Online) 1619-3997, ISSN (Print) 0300-5577, DOI: https://doi.org/10.1515/jpm-2012-0256.

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