Folate status and health: challenges and opportunities

Background

Folates have a central role in prevention of birth defects and other diseases. Folate deficiency without anemia occurs in a large number of individuals [1]. An estimated 4800 pregnancies are affected by neural tube defects (NTDs) each year in Europe [2]. Between 700 and 1000 of these cases are yearly diagnosed in Germany and at least 50% of these birth defects could be prevented by sufficient periconceptional supplementation of folic acid (FA) [3]. This article aims at reviewing recent evidence on the role of folate mainly during pregnancy and discussing strategies for improving folate intake and status. Efforts to increase folate intake through natural folate-rich foods are unlikely to be effective for health prevention on a population basis and would leave a large portion of the population, particularly women of reproductive age, at increased risk for diseases related to folate insufficiency.

The term dietary folate equivalent (DFE) has been introduced which refers to 1 μg natural folate or 0.5 μg of FA. The recommended dietary allowance (RDA) for folate in German population is 300 μg DFE/day [4] (Table 1), whereas the Institute of Medicine (IOM) of the US National Academy of Sciences set the RDA for folate to 400 μg DFE [5]. Different intake data from the national survey on German population (NVSII 2012) showed a mean folate intake of 200 μg DFE/day [6]. The median intake of dietary folate in young women (19–35 years) is from 170 to 181 μg DFE/day, thus clearly showing that the majority of them are at risk for insufficient intake of folate [6]. To recommend doubling the folate intake from dietary sources [4] (increase by ∼200 μg DFE/day) in the population appears unrealistic. In fact, the German nutrition survey NVSII showed that healthy lifestyle patterns did not predict a difference in dietary folate intake of more than 30 μg DFE/day [6]. In contrast, only people using vitamin supplements with FA were able to obtain a median total folate intake of 500 μg DFE/day [6].

Pteroylglutamic acid [folic acid (FA)] is the synthetic form of folate that is used in supplements and fortification of foods. FA is not biologically active unless reduced to tetrahydrofolate and then converted to methylfolate. FA from supplements or fortified foods is easily absorbed and converted to the active forms that increase serum folate and prevent anemia, hyperhomocysteinemia, and birth defects. FA increases serum and red blood cell (RBC)-folate in a dose-, time-, and baseline-dependent manner [1]. The serum concentration of folate mirrors recent folate status, RBC-folate concentration is a long-term marker that reflects folate storage, and total homocysteine (tHcy) represents a surrogate functional markers for folate status [7]. Serum folate is preferred over RBC-folate because it can be reliably measured using immunological or chromatography methods, shows faster response after supplementation, and reflects the amount available for transfer via the placenta. Polymorphisms in the folate cycle [i.e., methylenetetrahydrofolate reductase (MTHFR)] interfere with folate status, requirements, and diseases susceptibility. Because screening for the polymorphism on a population level is not recommended, the RDA for folate should be adequate to cover the requirements of the most liable genotype (i.e. MTHFR677 TT). A significant association between NTDs and the MTHFR677 TT genotype has been reported [8]. Irrespective of the genotype, a serum folate concentration of at least 18 nmol/L (or RBC-folate > 1000 nmol/L) is necessary to prevent most cases of folate-responsive birth defects. This level must be reached before conception. However, unless women use supplementary FA doses ranging from 400 to 800 μg/day, sufficient folate levels cannot be achieved within 1 month in the majority of women who start from low or intermediate baseline folate (discussed in [9]).

Timely supplementation of folate prevents birth defects

NTDs such as spina bifida and anencephaly are structural malformations of the central nervous system caused by delayed closure of the neural tube in the first few weeks after conception [10]. NTD births constitute considerable social and economic burden for the affected families and the health system [11]. The lifetime hospital admissions because of secondary conditions is markedly higher in patients with spina bifida compared to healthy individuals [12]. The relative risk (RR) of death among children with spina bifida is higher compared to children without birth defects [13]. The higher rate of premature death in adults [14] causes loss of working years and healthy life years. The disease is associated not only with early mortality [14], but also with greater need for health care services, in particular during the first years of life. In general, children aged from 1 to 17 years had 13 times higher medical expenditures than non-spina bifida children [15].

At least 50% of these birth defects can be prevented by improving maternal folate status before conception [3, 16–19]. The prevention potential of NTDs by providing FA depends on the women’s baseline folate status markers. A dose-response relationship is observed between the risk and maternal serum or RBC-folate even within the reference range [20]. Protective levels of serum and RBC-folate are estimated to be >18 nmol/L and >1000 nmol/L respectively [9, 20, 21]. After introducing a population-wide fortification of staple foods with FA, the prevalence of NTDs in the US, Costa Rica [22], and many other countries declined to <5–6 cases per 10,000 births and thus, it is now only about half as high as the average rate of 9.3 cases per 10,000 births in Europe in 2012 (source EUROCAT, University of Ulster). The pathogenesis of NTDs and the protective mechanisms of FA are still not well understood [23], but it appears that FA cannot prevent all NTD cases.

Lower risks for congenital heart defects (CHDs) [24, 25] were also related to improving maternal folate status starting from the periconceptional period, although the evidence is not as strong as for the NTDs. However, because CHDs are more prevalent [26], the significance of preventing CHD is expected to be even higher than that for NTDs. Approximately, one in four major cardiac defects are predicted to be prevented by timely multivitamin use [24]. For example, a Dutch EUROCAT register-based study has shown that periconceptional use of FA is associated with approximately 20% reduced CHD risk in the infants [27]. The results were confirmed by a recent Chinese study that showed that the use of FA-containing supplements for ≥3 months before pregnancy is associated with approximately 70% reduction in CHD risk (OR=0.31, 95% CI 0.18–0.54) [28]. Ionescu-Ittu et al. showed that the risk of severe CHD (tetralogy of Fallot, endocardial cushion defects, univentricular hearts, truncus arteriosus, or transposition complexes) in Quebec administrative databases declined by approximately 6% in the years following FA fortification in 1998 in Canada (i.e., analyzing birth data between 1990 and 2005) [29]. In contrast, a recent study in Alberta showed that only the prevalence of cases with left ventricular outflow tract obstruction declined after fortification when comparing birth data between 1995 and 1997 and between 1999 and 2002 [30]. A further case-control study on fetuses and infants with conotruncal or limb defects in a Californian birth cohort from 1987 to 1988 showed that consuming FA-containing supplements reduces the risk for conotruncal heart defects [odds ratio 0.53 (0.34–0.85)] [31]. However, when looking at soluble folate markers in maternal blood, no association was observed between one carbon metabolites or B-vitamins in mid-pregnancy and the risk of having a child with conotruncal heart defects in a population sample from California (2002–2007) [32]. The lack of association may be due to the fact that the study was conducted after the start of FA fortification in the US in 1998. In general, the extent of the CHD risk reduction varied, was related to the lesion, and was absent in some studies [32].

Birth weight shows a positive association with FA intake or folate status [33–35] which is explained by extension of the gestational age or prevention of preterm birth. Newborns of Dutch women with serum folate >25.8 nmol/L were on average 167 g heavier than those of women with serum folate <9.3 nmol/L [36]. Considering FA supplementation, birth outcomes were studied in newborns of Hungarian women. Mean gestational age was 0.3 week longer and mean birth weight was by 37 g higher in women taking FA alone (relatively high dose 5.6 mg/day) than in the group without any supplementation [33]. The rate of preterm births was significantly lower in the FA group compared with the reference sample (without vitamin supplement) [7.6% vs. 11.8%: OR (95% CI)=0.68 (0.63–0.73)], but the rate of low-birth-weight newborns was not different [OR (95% CI)=0.88 (0.62–1.14)] [33]. The longest gestational age and lowest rate of preterm birth was found after FA supplementation during 2^nd to 3^rd trimesters (39.9 weeks and 3.8%) and during the entire pregnancy, that is, 1^st to 3^rd trimesters (39.8 weeks and 4.9%), followed by the 3^rd trimester alone (39.5 weeks and 7.6%) [33]. Likewise, the risk for spontaneous preterm birth was lowered by approximately 50% in one large Chinese study using lower doses of FA (that is, 400 μg/day) starting periconceptionally [37]. An elevated plasma tHcy, a folate status marker, has been related to a 25% higher risk in the offspring to be small for gestational age [38]. Hogeveen et al. estimated that a 1.9-μmol/L increase in maternal tHcy would result in a decrease in birth weight of 31 g (95% CI: –13, –51 g) [38]. A significant public health benefit would be expected from reducing the rate of preterm births and births with low birth weight.

In addition to poor birth outcome, folate deficiency and hyperhomocysteinemia have been related to maternal health, particularly the risk for preeclampsia. FA supplementation showed no consistent protective effect against preeclampsia [39, 40]. Adequate folate supply is also important during lactation. The concentrations of folate (the main form is 5-methyltetrahydrofolate) in human milk are maintained at levels between 100 and 193 nmol/L even at low maternal serum folate concentrations [41, 42]. Maternal FA supplementation (400 μg/day) after birth increased maternal serum folate (10.9–39.0 nmol/L) and milk folate levels (from a median of 103 nmol/L to 155 nmol/L) postpartum [41]. Maternal serum concentrations of folate was not a determinant of milk folate [41], suggesting that folate is actively transported into the milk. Nonetheless, FA supplementation during lactation is important for preserving maternal folate stores, in particular with regard to subsequent conceptions.

Folate and postnatal health

Adequate maternal folate status before and during pregnancy has been related to better child development [43]. Some studies reported a positive effect of maternal folate on social competence [44], reduced likelihood to develop behavioral problems at 18 month [45], better cognitive ability [43], and less risk of hyperactivity and peer problems in childhood [46]. An association between low maternal serum folate at early pregnancy (≤11 nmol/L) and emotional problems of the child at 3 years [OR 95%CI=1.70 (1.28–2.25)] was observed in a study on Dutch women [47].

Cerebral folate deficiency was observed in a significant number of patients with autistic spectrum disorders [48]. A recent case-control study on 429 children with autism and 278 controls reported that maternal prenatal intake of FA ≥600 μg/day (vs. <600 μg/day) was associated with a lower probability of autism in the offspring [49]. The association was more impressive in mothers with the MTHFR677 TT genotype [49], in accordance with an earlier study showing an increased risk for autism in the offspring of MTHFR677 TT mothers who were not supplemented with FA prenatally [50]. The Norwegian Mother and Child Cohort (MoBa) study, a prospective study on 85,176 children born between 2002 and 2008 in Norway [51], showed that autistic disorders were found in 0.10% of children whose mothers took FA (starting before conception and lasting at least up to the 8^th gestational week) but in 0.21% of those unexposed [adjusted OR 0.61 (95%CI 0.41–0.90] [51]. Potential negative impact of low maternal whole blood folate on behavioral disorders in the children has been reported [46]. Emotional problems in the child at the age of 3 years were related to maternal folate deficiency during early pregnancy [OR 1.57 (95% CI 1.03–2.38, P=0.03] in the Generation R study [47]. Thus, several lines of evidence suggest that improving maternal folate before and early in pregnancy is protective against disorders of psychological development.

Studies showing high asthma risk in children born to mothers supplemented with FA had severe limitations in design because they were either not based on accurate measurements of folate intake or serum folate, lacked a validated definition of asthma, and did not consider child age or relevant environmental factors [52]. Hungarian children born to women supplemented with multivitamin-containing FA or trace elements without FA were examined between postnatal months 8 and 12 [53]. There was no difference in the occurrence of serious or chronic diseases except for atopic dermatitis, asthma, and wheezy bronchitis which were more frequent in the infants of multivitamin-supplemented mothers [53]. In a long-term follow-up study on 6-year-old children of the same Hungarian trial, 147 children born to mothers with FA-containing multivitamin supplementation and 142 children who had mothers with trace element supplementation only were re-examined [54]. There were no differences in the rate and distribution of disorders including allergies or any other adverse effect [54]. Therefore, the previously found association of atopic dermatitis, asthma, or wheezy bronchitis with maternal multivitamin supplementation during gestation was not confirmed [54].

Folate, aging, and population-attributable risk of diseases

An intriguing issue is whether tHcy is a marker for B-vitamin deficiency or a causal risk factor for age-related disorders. Hyperhomocysteinemia is associated with coronary heart disease (CVD) and stroke in case-control studies. A meta-analysis estimated that lowering tHcy by 25% may reduce the risk of coronary heart disease by 11% and that of stroke by 19% [55]. However, analyzing data on the association of tHcy with the MTHFR C677T polymorphism from published and unpublished datasets did not confirm a significant effect of tHcy on CVD [56]. Lowering tHcy failed to reduce cardiovascular events or mortality, but lowered stroke risk [57] which is in line with a moderate reduction of stroke incidence after FA fortification in North America in 1998 [58].

Elevated concentrations of plasma tHcy predicted the risk of future dementia in longitudinal studies [59]. Brain neuroimaging revealed an association between elevated tHcy and lower subcortical brain volumes and cortical thickness, volume, and surface area in elderly people [60]. Vitamin B treatment has been shown to slow brain volume shrinkage [61], prevent atrophy of gray matter [62], and slow cognitive decline in people with mild cognitive impairment [63], an effect that was not confirmed by all studies [64]. The effect of Hcy-lowering was confined to participants with baseline tHcy >11.0 μmol/L.

The risk of osteoporosis, hip fractures, bone mineral density [65, 66], or age-related macular degeneration has been also related to hyperhomocysteinemia, but the response to tHcy was not conclusive. Therefore, considering the current evidence, we conclude that tHcy lowering will not reduce secondary CVD or already manifested cognitive dysfunction but is likely to reduce the risk of stroke.

Folic acid fortification has been shown to be safe for the entire population. Large trials of FA supplementation with doses up to 10 times higher than the average intake confirms that such a policy is safe [18, 56, 67, 68]. Reports on increased colon cancer risk that appeared in 1996 almost simultaneously with the approval of food fortification by the FDA [69] were not confirmed later [70] or even by controlled trials using FA supplementation [68, 71–73]. Considering results from controlled trials and data from countries applying fortification programs since 1998, there is no evidence that cancer risk can be attributed to or altered by (i.e., increased or decreased) low or moderate intake of FA.

Improving population folate status

Currently, there is no consensus on folate intake recommendations for the population in several parts of the world. Although, there is a firm evidence that periconceptional supplementation is crucial, the recommendations vary even for women of reproductive age or those planning for pregnancy (Table 1). Fortification strategies provide approximately 150 μg/day FA, an amount which is lower than all current RDAs. This amount is thought to achieve optimal folate markers for preventing NTDs. The traditional “reference range” for serum folate was based on experiments conducted by Victor Herbert who induced experimental folate depletion and defined the cut-off for serum folate where anemia becomes expressed [74]. However, recent evidence suggests that anemia may be a late manifestation of severe folate deficiency [75] and NTD prevention by FA occur in women without frank anemia or folate deficiency [21].

One strategy to improve maternal folate status is targeted supplementation of young women. In spite of strategies targeting the public as well as health care professionals, the recommendation to supplement FA periconceptionally was not successful in 60–70% of the women [76] because of the short-time interval available for NTD prevention. The importance of periconceptional services as an obligatory medical infrastructure of effective FA/multivitamin supplementation is supported by the Hungarian experience [77]. Periconceptional services are not offered by the current European health care structure. In Germany and in other parts of Europe, supplementation recommendations failed to enhance vitamin use and to reduce the incidence of NTDs during the last 10 years [78]. Therefore, integrating antenatal centers within the health care system, which can provide personalized care for all young women, is urgently needed in countries not applying mandatory fortification.

The most important factors associated with maternal folate deficiency or lack of supplementation before pregnancy were young age, low education, low income, smoking before pregnancy, obesity, and immigration background. Educational programs showed a significant increase in the awareness of women; however, the effect on supplement usage was insufficient [79]. The use of FA supplements is highly dependent on the educational and economic status of the individuals. For prepregnancy supplementation of FA to be effective, long-term and repeated educational and monitoring programs must be implemented. Obviously, engagement of health care providers and policy makers is required to reach adequate preventive measures on a population level.

The mandatory fortification of staple foods with FA is an alternative strategy that has been practiced in the US and Canada since 1998 and meanwhile has been implemented in over 70 countries with the aim of increasing the daily folate intake by approximately 200 μg DFE. The fortification has shown to be effective in terms of lowering tHcy, increasing serum, and RBC-folate [80, 81], eliminating anemia [82], and reducing NTD [22]. Fortification of staple foods requires supportive legislations or regulations, and its implementation and effects should be monitored. Compared to targeted supplementation, fortification is far more efficient and cost-effective [83].

Conclusions

The low dietary folate intake in European women explains endemic spina bifida and anencephaly. Mandatory fortification of staple foods with FA has proven to be efficient and cost-effective in more than 70 countries worldwide, but it requires legislation and continuous monitoring. Optimal folate status should be defined based on serum or RBC folate concentrations. An optimal status cannot be achieved on a population level by only recommending increased intake of natural folates. Targeted supplementation currently recommended in Europe can improve folate status. However, this theoretical option has been ineffective and failed to reduce NTDs in Europe in the last 10 years. Although voluntary fortification can offer an option, consumer’s health awareness, education, and economic status determine its efficacy. There are currently no nutritional policies in Europe to monitor the results. Population folate status as a public health issue requires more awareness from health care providers and authorities.