Maternal folate exposure in pregnancy and childhood asthma and allergy: a systematic review

Authors

  • Susan B Brown,

    1. Division of Biostatistics & Epidemiology, Department of Public Health, School of Public Health & Health Sciences, University of Massachusetts, Amherst, Massachusetts, USA
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  • Katherine W Reeves,

    1. Division of Biostatistics & Epidemiology, Department of Public Health, School of Public Health & Health Sciences, University of Massachusetts, Amherst, Massachusetts, USA
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  • Elizabeth R Bertone-Johnson

    1. Division of Biostatistics & Epidemiology, Department of Public Health, School of Public Health & Health Sciences, University of Massachusetts, Amherst, Massachusetts, USA
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Correspondence: SB Brown, 424 Arnold House, University of Massachusetts, 715 North Pleasant St., Amherst, MA 01003, USA. E-mail: snboyer@schoolph.umass.edu.

Abstract

Adequate folate status in early pregnancy is critical to prevent congenital malformations, yet little is known about whether exposure to folate, specifically folic acid supplementation beyond the recommended dose, influences chronic health outcomes. The link between maternal folate levels and risk of childhood asthma and allergic disease has been investigated in 10 large prospective cohort studies that reported conflicting results. While the majority of studies reported no association, those supporting a positive relationship found a small increase in risk that was generally transient in nature, confined to early childhood, and associated with folic acid supplementation in late pregnancy. This systematic review presents background information on maternal folate exposure and childhood asthma, synthesizes the current epidemiologic evidence in the context of the methodological differences among studies and their potential limitations, and offers direction for future research.

Introduction

While sufficient folate status is essential during early pregnancy, the long-term health effects of supplementation are not well understood, and relatively few studies have investigated potential health risks posed to the mother and fetus due to excessive folate exposure. Over the past decade, the potential link between maternal folate intake and risk of childhood asthma and allergic disease has been investigated in 10 prospective cohort studies with conflicting results. This systematic review presents background information on maternal folate exposure and childhood asthma, summarizes epidemiologic evidence supporting and refuting this relationship, and offers direction for future research.

Folate in Pregnancy

Adequate folate status is especially important 1 to 3 months prior to conception through the first trimester of pregnancy to reduce the risk of major congenital anomalies.[1] Folate (vitamin B9) is a generic term for the water-soluble vitamin naturally present in foods. Synthetic folic acid is rarely found in food, but it is the most oxidized and stable form of folate used in supplements, prenatal vitamins, fortified food products, and multivitamins.[2] To reduce the risk of neural tube defects, the Institute of Medicine recommends that women capable of pregnancy consume 400 μg of folic acid per day from fortified foods, supplements, or both in addition to consuming naturally occurring folate through dietary sources.[2] In 2009, the US Preventive Services Task Force released a recommendation that all women planning or capable of becoming pregnant consume a daily vitamin supplement containing 400–800 μg of folic acid in addition to the amount normally consumed in foods.[3] During pregnancy, women are often advised to increase folate intake to promote fetal developmental processes that include cell division, growth, and differentiation.[3-5] In support of a critical window of susceptibility, a systematic review revealed that periconceptional folic acid supplementation reduced the risk of neural tube defects (e.g., spina bifida, anencephaly) by 72%.[6]

To achieve the recommended dietary allowance (RDA) for folate, women may take supplemental folic acid or obtain folate from foods rich in the vitamin, including leafy green vegetables, citrus fruits and juices, beef liver, beans, and peas.[1, 2, 5] In 1998, dietary folate equivalents (DFEs) were introduced to account for the difference in bioavailability among possible sources of the vitamin.[7] As folic acid from fortified foods and supplements is more completely absorbed by the human body, 1 μg of dietary folate is equivalent to 0.6 μg of supplemental folic acid taken with a meal or 0.5 μg taken on an empty stomach.[2, 7] The upper limit of tolerability for adult intake of synthetic folic acid is established at 1,000 μg/day to prevent neuropathy among individuals with vitamin B12 deficiencies.[7] The safety profile of supplemental folic acid was established with the assumption that individuals will likely acquire the majority of their folic acid through supplements while consuming additional folate through dietary sources.

Over the last 15 years, voluntary and mandatory food fortification programs have been implemented worldwide to increase folic acid levels among women of childbearing age and reduce the incidence of neural tube defects. In 1998, the United States and Canada began mandatory folic acid food fortification of flour, cereal grains, and select wheat and grain products with fortification levels of 140 μg/100 g and 150 μg/100 g, respectively. Costa Rica, Chile, and South Africa implemented similar fortification programs between 1998 and 2003, with fortification doses ranging from 150 μg/100 g to 220 μg/100 g.[8] In Australia, voluntary folic acid food fortification of flour, biscuits, bread, cereal, pasta, rice, and certain juices was recommended in June 1995, with mandatory fortification of wheat flour beginning in September 2009.[9, 10] These initiatives have had significant public health impact, as nearly half of pregnancies are unplanned and women of childbearing age in many countries have limited or irregular folate intake.[11]

Childhood Asthma

Asthma is a public health concern in developed countries because of its high prevalence, interference with daily living activities, and cost of associated medical care.[12] Childhood asthma is a complex and heterogeneous chronic inflammatory disorder characterized by intermittent and recurring symptoms such as airway inflammation and obstruction, wheeze, chest tightness, and shortness of breath.[13-15] The manifestation of childhood asthma ranges from minor periodic episodes of cough and wheeze to chronic disability and death.[12]

The prevalence of self-reported asthma in children has risen in the United States from 3.5% to 7.5% between 1980 and 1995, and from 4.1% to 28.4% between 1964 and 2004 in the United Kingdom.[16] The rapid increase in asthma prevalence has been attributed to environmental and lifestyle factors, and immunizations, obesity, and viral infections may each contribute to an increased risk of respiratory illness.[12, 17] Similarly, the popular but controversial “hygiene hypothesis” proposes that increased incidence of childhood asthma and allergy may result from lower levels of exposure to allergens and pollutants in utero and during early childhood.[18] To date, public health interventions have focused on reducing the prevalence and severity of asthma and allergy-related conditions by targeting modifiable factors related to nutrition, diet, immunizations, and environmental exposures.[17, 19]

Pathophysiology of Maternal Folate Exposure and Childhood Asthma

Ample evidence suggests that maternal folate intake may influence the risk of childhood asthma.[20-23] The etiology of chronic respiratory and dermatological conditions such as asthma, atopy, eczema, allergy, and allergic sensitization is complex and potentially driven by a multitude of factors that interact through inflammatory mechanisms of action.[24] For example, atopy is characterized by immunoglobulin E (IgE)-mediated allergic reactions to environmental allergens or proteins, with associated inflammation of the airway and other target organs.[25] Eczema or atopic dermatitis is a chronic disorder caused by hypersensitivity or allergy in the skin, with symptoms such as swelling, inflammation, rash, or skin lesions.[12, 15] It is common for individuals with atopic dermatitis to suffer from concomitant conditions such as asthma, seasonal or food allergies, and allergic sensitization.[12] Similarly, mild to severe cases of asthma often present with allergic and inflammatory symptoms that include increased pulmonary inflation and airway edema and constriction.[24]

DNA methylation and inflammation have emerged as the two main pathways through which folate may influence development of childhood asthma and allergy. Along the methylation pathway, mechanistic studies have documented the impact of folate exposure on reduced efficiency of DNA repair mechanisms as well as promotion of epigenetic and transcriptional effects through DNA methylation changes.[26-28] Further, experimental studies support the potential role of folate in cell division, nucleotide synthesis, and DNA methylation during fetal development.[21, 28, 29] In both human and animal models, elevated maternal exposure to folic acid in pregnancy was linked to epigenetic changes that include fetal programming and specific cellular modifications associated with an increased risk of childhood asthma and allergic disease.[22, 23]

Folate-induced changes in DNA methylation may affect the expression of T-helper (Th) cells and interrupt healthy immune responses to antigens, though the interaction between immune states and asthmatic inflammation remains unclear.[20, 24, 30] Immunological evidence has linked excess regulatory T cells and Th cells, including those involved in Th1, Th2, and Th17 immunities, with development of allergic asthma.[24] One landmark animal study supported an association between perinatal exposure to a methyl-rich diet consisting of folic acid, choline, betaine, zinc, genistein, cyanocobalamin, and L-methionine and heightened severity of allergic airway conditions in mice.[21] In mice, perinatal exposure to high concentrations of methyl donors was linked to increased airway reactivity, elevated serum IgE concentrations, and histological evidence of allergic airway disease in offspring.[21] As folic acid is recognized as a major source of methyl donors, this study supports gestation as an important time during which offspring have enhanced vulnerability to high concentrations of methyl donor exposures.[21]

Together, human and animal studies implicate inflammatory mechanisms and DNA methylation changes as the primary pathways through which maternal folate exposure may influence development of childhood asthma and allergy. Given the importance of adequate folate status during pregnancy as well as the increasing prevalence of childhood asthma and allergy, it is critical that improved understanding of this relationship is gained.

Review of Epidemiologic Evidence and Potential Explanations for Heterogeneous Findings

Over the last decade, the potential link between maternal folate levels and risk of childhood asthma and allergic disease has been investigated in a number of large prospective cohort studies that reported conflicting results (Table 1).[18, 25, 31-38] In addition to asthma, primary endpoints of interest include related conditions such as atopy, eczema, allergy, and allergic sensitization. To date, four prospective studies have provided evidence of a potential relationship between maternal folate exposure and development of childhood asthma and allergic outcomes.[31-34] While the findings of these studies are provocative, six epidemiologic studies found that maternal folate exposure was not associated with childhood asthma and allergy.[18, 25, 35-38]

Table 1. Summary of studies on maternal folate exposure during pregnancy and risk of childhood asthma, allergy, and wheeze
ReferenceSample sizeFA-fortified food supply at exposure assessmentPrevalence of FA supplement use in sampleExposureOutcomeMain results (adjusted OR, 95%CI)
  1. Abbreviations: CI, confidence interval; FA, folic acid; LRTIs, lower respiratory tract infections; OR, odds ratio.
Studies that found a positive association
Whitrow et al. (2009)[31]

490 children (3.5 years of age)

423 children (5.5 years of age)

Voluntary fortification (enrollment was prior to mandatory fortification)

43% prepregnancy;

57% early pregnancy;

38% late pregnancy

FA before 16 weeks and at 30–34 weeks, dietary folate at 30–34 weeksChildhood asthma at 3.5 and 5.5 years of age

Asthma at 3.5 years:

Early pregnancy dietary folate: 1.09 (0.78–1.51)

FA: 0.92 (0.79–1.08); Diet + FA: 0.93 (0.97–1.08)

Late pregnancy dietary folate: 1.03 (0.66–1.60)

FA: 1.26 (1.09–1.47); Diet + FA: 1.26 (1.09–1.47)

Asthma at 5.5 years:

Early pregnancy dietary folate: 0.97 (0.63–1.50)

FA: 0.92 (0.77–1.10); Diet + FA: 0.92 (0.77–1.11)

Late pregnancy dietary folate: 0.86 (0.57–1.28)

FA: 1.16 (0.94–1.43); Diet + FA: 1.16 (0.94–1.43)

Haberg et al. (2009)[32]32,077 childrenNo

79% at some point during pregnancy;

22% first trimester only

Maternal FA supplement use at 0–30 weeksWheeze, LRTIs up to 18 months of age

Wheeze (0–12 weeks): 1.06 (1.03–1.10)

Wheeze (>12 weeks): 1.00 (0.97–1.03)

LRTI (0–12 weeks): 1.09 (1.02–1.15)

LRTI (>12 weeks): 0.98 (0.92–1.04)

LRTI hospitalized (0–12 weeks): 1.24 (1.09–1.41)

LRTI hospitalized (>12 weeks): 0.86 (0.75–0.97)

Bekkers et al. (2012)[33]3,786 newbornsNo53% reported some form of FA useMaternal use of prenatal or multivitamins, or FA supplements in pregnancyAsthma, wheeze, eczema, respiratory infections annually 1–8 years of ageFA supplements and wheeze: 1.20 (1.04–1.39). No association between maternal FA use and frequent asthma symptoms, wheeze, LRTI, or eczema from 1–8 years of age
Dunstan et al. (2012)[34]647 pregnant womenYes76% reported use in third trimesterMaternal FA supplements and dietary folate after 28 weeks, newborn folate levelsAllergic outcomes, eczema, allergic sensitization at 1 year of age

Infant levels >500 μg/day FA vs. <200 μg/day and eczema: 1.85 (1.14–3.02)

Cord blood folate at delivery <50 nmol/L and sensitization: 3.02 (1.16–7.87)

Cord blood folate at delivery >75 nmol/L and sensitization: 3.59 (1.40–9.20)

Studies that found no association
Litonjua et al. (2006)[18]1,290 mother- child pairsYesNot specifiedAntioxidant nutrient intake in pregnancyAny or recurrent wheeze or eczema at 2 years of age

Any wheezing: 0.89 (0.60–1.31)

Recurrent wheezing: 0.68 (0.38–1.22)

Granell et al. (2008)[25]7,356 mothers and 5,364 childrenNo

12% early pregnancy;

30% late pregnancy

Dietary folate at 32 weeks, FA supplements at 18 and 32 weeks of pregnancyChildhood atopy and doctor-diagnosed childhood asthma at age 7–8 years of age

Maternal FA supplement use (18 weeks):

Atopy: 0.99 (0.78–1.25); allergy: 1.16 (0.97–1.37)

Maternal FA supplement use (32 weeks):

Atopy: 1.15 (0.98–1.35); allergy: 1.09 (0.96–1.22)

Maternal dietary folate (32 weeks):

Atopy: 0.98 (0.88–1.10); allergy: 1.03 (0.95–1.12)

Miyake et al. (2011)[35]763 mother-child pairsNoNot specified; study focused on dietary folateDietary folate during pregnancyWheeze and eczema at 16–24 months of age

Folate (Q4 vs. Q1) and wheeze: 1.28 (0.65–2.50)

Folate (Q4 vs. Q1) and eczema: 1.01 (0.51–2.00)

Magdelijns et al. (2011)[36]2,843 mother-child pairsNo

32% early pregnancy;

31% throughout entire pregnancy;

32% other period

FA supplements during pregnancy, and red blood cell folate concentrations in late pregnancyIgE levels, asthma, wheeze, atopic dermatitis up to 6–7 years of age

FA use vs. no use in pregnancy:

Atopic dermatitis (2 years): 1.15 (0.63–2.10)

Eczema (6–7 years): 1.16 (0.90–1.48)

Wheeze (6–7 years): 0.99 (0.80–1.23)

Asthma (6–7 years): 1.09 (0.65–2.20)

Maternal red blood cell folate concentrations:

Asthma (6–7 years; Q5 vs. Q1): 0.31 (0.09–1.10, ptrend = 0.05)

Kiefte-de Jong et al. (2012)[37]8,742 mothersNo69% early pregnancyFA supplement use before conception through first trimester, maternal folate levels in first trimesterAtopic dermatitis, wheeze, shortness of breath up to 4 years of age

FA periconceptional start; FA start in first 10 weeks:

Wheeze: 0.99 (0.89–1.09); 1.02 (0.90–1.16)

Shortness of breath: 1.04 (0.84–1.29); 1.16 (0.85–1.57)

Atopic dermatitis: 1.17 (0.97–1.40); 1.15 (0.90–1.47)

Maternal plasma folate concentrations (Q4 vs. Q1):

Wheeze: 1.02 (0.89–1.18)

Shortness of breath: 0.98 (0.79–1.22)

Atopic dermatitis: 1.18 (1.05–1.33)

Martinussen et al. (2012)[38]1,499 womenFortification program implemented during study enrollment

52% preconception;

61% first month;

81% second month;

88% third month;

92% first trimester

FA use before conception through first trimesterAsthma up to 6 years of age

Asthma per 100 μm increase in daily FA intake: 0.98 (0.93–1.04)

Daily FA intake and childhood asthma (ptrend > 0.05)

Studies conducted to date present conflicting epidemiologic evidence of the effects of maternal folate intake during pregnancy and risk of childhood asthma and allergic outcomes. Differences in study design, study populations, and timing of exposure and outcome assessment likely contribute to the inconsistent findings. These studies raise many important considerations, including the following: 1) whether maternal folate status is influenced in a meaningful way through folic acid food fortification programs; 2) whether variations in sources of folate (e.g., dietary versus supplements) could contribute to conflicting findings; 3) whether the method and timing of exposure assessment affect the direction of findings observed; 4) whether timing of supplementation during pregnancy influences risk of childhood outcomes; 5) whether outcome assessment and duration of follow-up could contribute to variations in study results; and 6) whether the association between folic acid supplementation and childhood asthma is confounded by factors not considered in the analysis.

Influence of folic acid food fortification programs

It is essential to consider differences in folic acid food fortification programs across the various countries in which this association has been evaluated (Table 1). Based on the broad implementation of voluntary and mandatory folic acid food fortification programs worldwide, total folate exposure is likely to be underestimated if the background contribution from folic acid in foods is not considered. Further, possible relationships between high folic acid intake and potential health outcomes may be obscured if the total daily dose of folic acid does not include both supplements and fortified foods.

Among the six studies suggesting no association between maternal folic acid exposure and childhood allergy and asthma, two were conducted in areas without fortification programs and did not measure dietary folate, including fortified or naturally folate-rich foods.[36, 37] One study captured maternal and childhood exposure to dietary folate in the United Kingdom, where fortification is not present,[25] and one examined maternal dietary folate intake in Japan, where foods are not fortified.[35] Of studies based in the United States, one did and one did not assess dietary folate.[18, 38] Only one study examined the impact of folic acid supplementation and dietary folate on childhood development of rash, wheeze, and eczema in a region without folic acid food fortification.[35] In this study, no significant relationships emerged between maternal folate intake and development of infant wheeze and eczema, though average serum folate levels of Japanese pregnant women are frequently below recommended levels due to limited intake from diet and supplements.[39]

In areas where food fortification programs have been implemented, studies varied in their assessment of dietary sources of folate but generally reported no association between maternal folate exposure and risk of childhood asthma, eczema, or wheeze. For example, Litonjua et al.[18] assessed gestational diet, pregnancy outcomes, and offspring health of 1,290 mother-child pairs in Boston, Massachusetts. This study included food frequency questionnaires throughout pregnancy as well as interviews to query vitamin and supplementation intake. Given that the United States introduced mandatory folic acid fortification programs in 1998, this study captured dietary intake of naturally occurring folate as well as folic acid through fortified foods and supplements during pregnancy. Despite the comprehensive assessment of folic acid exposure, no associations emerged between dietary folate or folic acid supplementation during pregnancy and risk of childhood wheeze, eczema, or respiratory infections. In contrast, Martinussen et al.[38] focused on folic acid supplementation alone in the prospective Asthma in Pregnancy study in Connecticut and Massachusetts and found no association between maternal supplemental folic acid taken 1 month before conception or during the first trimester and childhood asthma at 6 years of age. However, this study did not capture dietary sources of folate, and the impact of maternal, fetal, or childhood exposure to folic acid through fortified foods could not be explored.

Dietary folate was not assessed in two studies in which follow-up was mainly completed before folic acid food fortification initiatives were implemented.[33, 37] Bekkers et al.[33] recruited 4,146 pregnant Dutch women from the general population between 1996 and 1997 and conducted follow-up for approximately 8 years after delivery. Besides a small increase in risk of early childhood wheeze, no association was found between maternal folic acid use and respiratory tract infections, eczema, wheeze at later ages, or presentation or frequency of childhood asthma symptoms. Similarly, Kiefte-de Jong et al.[37] conducted a population-based prospective cohort study that included 8,742 mothers with delivery dates between April 2002 and January 2006 and found no association between folic acid use before and during the first 10 weeks of pregnancy and risk of wheeze, shortness of breath, or atopic dermatitis during childhood. In the Netherlands, folic acid food fortification was prohibited until 2007, when an exemption was passed to allow maximum fortification of 100 μg/100 kcal. Although these studies did not examine sources of dietary folate, the recent implementation of folic acid fortification is unlikely to have materially changed the study findings.

Given the variability in food fortification programs and assessment of dietary folate across studies, it remains to be seen if folic acid through food fortification has a meaningful impact on development of childhood asthma and allergy. Evaluation of dietary folate or the presence of a food fortification program did not result in a specific trend among the null studies; however, the positive findings reported by Whitrow et al.[31] and Dunstan et al.[34] may be in part due to the authors' comprehensive evaluation of dietary folate and folic acid supplements and the presence of either voluntary or mandatory food fortification programs in the respective study regions. It is important that exposure to folic acid in fortified foods is considered in future studies to better capture the true total dose from supplements and food sources combined.

Sources of folate

The studies highlighted in this review differed in their examination of dietary folate compared with supplemental folic acid throughout pregnancy (Table 2).[18, 25, 31-38] Of the 10 studies focusing on childhood respiratory outcomes, four considered maternal exposure to both dietary folate and folic acid supplements,[18, 25, 31, 34] five examined folic acid supplementation,[32, 33, 36, 38] and one assessed the effect of dietary folate exposure alone.[35]

Table 2. Summary of main study exposures and timing of assessment
ReferenceFolic acidDietary folateBiomarkers
PreconceptionEarly pregnancyaLate pregnancyPreconceptionEarly pregnancyaLate pregnancyMaternal plasmaCord bloodMTHFR C677T
  1. a Early pregnancy mainly captures assessments in the first trimester for all studies but includes measurements up to 20 weeks, after which the period is considered late pregnancy.
  2. b 80% of participants completed a questionnaire between 30 and 36 weeks (median, 33 weeks) of gestation.
Litonjua et al. (2006)[18] XX XX   
Granell et al. (2008)[25] XX  X  X
Whitrow et al. (2009)[31] XX XX   
Haberg et al. (2009)[32] XX      
Miyake et al. (2011)[35]    Between weeks 5 and 39   
Magdelijns et al. (2011)[36] XX   X  
Bekkers et al. (2012)[33]b  X      
Dunstan et al. (2012)[34]  X  XXX 
Kiefte-de Jong et al. (2012)[37]XX    X X
Martinussen et al. (2012)[38]XX       

Studies that captured both dietary folate and supplemental folic acid intake may better approximate overall maternal and fetal exposure, though synthetic folic acid is specifically of interest in relation to childhood respiratory outcomes. Women are actively encouraged to increase folic acid intake before and during pregnancy, and most women achieve this through consumption of folic acid via fortified foods and supplements. Folic acid in fortified foods and supplements has greater bioavailability (70–85%) than folate present in naturally folate-rich foods (50%).[27]

Results from the prospective cohort study conducted by Whitrow et al.[31] support the importance of folic acid over dietary folate alone, as the same 26% increase in childhood asthma risk at 3.5 years of age was observed among women who reported folic acid supplement use alone (odds ratio [OR] = 1.26, 95% confidence interval [CI]: 1.09–1.47) and those who reported both dietary folate intake and folic acid use in late pregnancy (OR = 1.26, 95%CI 1.09–1.47). Similarly, a consistent 32% increase in risk of persistent childhood asthma was detected among children of women who consumed folic acid supplements in late pregnancy (OR = 1.32, 95%CI: 1.03–1.69) and children of women consuming supplements and dietary folate in late pregnancy (OR = 1.32, 95%CI: 1.02–1.69).[31] These results suggest that consumption of dietary folate does not substantially influence the risk of childhood asthma beyond the increase in risk observed with folic acid supplementation alone, though it is unclear if this applies to countries with comprehensive folic acid food fortification programs.

Method of exposure assessment

Maternal and fetal folate exposure was examined through a variety of methods, including food frequency questionnaires and diet records to assess dietary folate intake, self-administered questionnaires to capture folic acid supplement use, blood measurements such as plasma or cord blood concentrations, and identification of specific genetic polymorphisms (Table 2).

To determine if objective biochemical measurements of maternal plasma folate concentrations would support or refute the reported positive associations observed in their earlier study, Haberg et al.[29] conducted a case-control study of 507 cases and 1,455 controls in the Norwegian Mother and Child Cohort Study. All eligible women in the study had plasma folate concentrations measured at 18 weeks of pregnancy, and concentrations were found to be elevated among women reporting folic acid use, nonsmoking status, nulliparity, age over 30 years, normal weight, and higher levels of education. A dose-dependent increase in risk of childhood asthma was observed as maternal plasma folate concentrations rose (highest quintile >17.84 nmol/L compared with lowest quintile <5.54 nmol/L, OR = 1.66, 95%CI: 1.16–2.37, ptrend = 0.006). While biologic sampling provides increased accuracy in estimating true exposure to folate, this study included specimens from a single collection during the second trimester, which may not represent folate levels throughout pregnancy.

Only one study measured maternal red blood cell folate concentrations during late pregnancy, which provided an objective measurement of folic acid status over the prior 3 months.[36] While maternal red blood cell folate concentrations were not associated with most atopic outcomes, a dose-response relationship was observed through which childhood asthma risk at 6 to 7 years of age decreased as folic acid concentrations increased (ptrend = 0.05).

Two studies examined the C677T single nucleotide polymorphism of the maternal MTHFR gene to investigate developmental and degenerative conditions that may be modified by adequate folate status.[25, 27, 37] Prior studies have suggested that those with homozygous recessive (TT) C677T MTHFR genotype are more likely to have diminished folate concentrations compared with wildtype (CC) or heterozygous (CT) individuals.[25, 40] However, neither study identified significant associations between maternal MTHFR C677T genotype and childhood development of asthma, allergy, atopy, and related outcomes.[25, 37] As both studies applied principles of Mendelian randomization, these findings suggest that folate status is not associated with childhood respiratory outcomes, as genetic determinants of folate metabolism are unlikely to be confounded by other factors.[25, 37] Given that MTHFR is an important enzyme in folate metabolism, it is important to consider the limitations of this exposure assessment approach because it utilizes a proxy of maternal folate exposure instead of a direct assessment of folate intake during pregnancy.

While many studies in this review utilized subjective exposure assessment methods that rely mainly on participant self-report, more objective methods such as DNA genotyping or maternal red blood cell folate concentrations provide additional lines of evidence that are less prone to measurement error and bias. However, many of these methods measure surrogates of the true exposures of interest, which poses significant issues in the interpretation of study results. Questionnaire-based methods and dietary records collect comprehensive information on folate intake throughout pregnancy, which more broadly reflects true maternal and fetal exposures. While questionnaire-based methods are commonly used for data collection among large samples of women over multiple time points during pregnancy in order to limit logistical and financial burden, self-reported questionnaire data are prone to nondifferential misclassification, which may contribute to the null findings observed in several studies. Overall, no specific trends emerged among studies utilizing questionnaire-based versus biomarker methods of maternal folate exposure during pregnancy. Thus, results from studies using biochemical measurements should be taken together with those utilizing more subjective and comprehensive tools, such as food frequency questionnaires, which typically represent extended reference periods and reflect usual intake instead of a single point in time.

Timing of folate intake during pregnancy

Despite the prospective nature of studies included in this review, the timing of exposure assessment varied from preconception, early pregnancy, and late pregnancy windows versus overall use of folic acid supplements throughout pregnancy (Table 2). The critical window for folic acid supplementation to reduce the risk of neural tube defects falls between preconception and the first trimester; however, given the various permutations of timing of assessment and source of folate, it is unclear if maternal folate exposure during a specific point during pregnancy is most relevant to the development of asthma or allergy in offspring.

To better understand if associations differed in important ways based on timing of folate intake during pregnancy, those studies reporting positive associations were compared with studies finding no association. The timing of exposure assessment varied substantially across the six studies that found no or weak associations. One study focused on folic acid supplement use during the first trimester, which was later classified as no exposure, periconceptional exposure, or folic acid use in the 10 weeks after conception, and another focused on supplement use in the first 24 weeks of pregnancy.[37, 38] Two studies included a comprehensive assessment of dietary folate and supplement use in both early and late pregnancy.[18, 36] One study included supplement use questionnaires in early and late pregnancy, with dietary folate assessment in late pregnancy only,[25] and one study broadly recruited women between weeks 5 and 39 of pregnancy and assessed maternal folate exposure at baseline through dietary habits and food frequency questionnaires.[35] Therefore, it is likely that nondifferential misclassification attenuated findings in studies without measurements in the first trimester and throughout pregnancy to reflect true maternal and fetal exposure.

Among the studies with positive findings, two included early and late assessment, which better estimate maternal folate status throughout pregnancy, and two focused on folic acid use in the third trimester.[31-34] Generally, a greater increase in risk of asthma or allergic conditions was observed in relation to supplementation in late pregnancy compared with supplementation in early pregnancy. For example, Whitrow et al.[31] reported a 26% increase in risk of childhood asthma among women with exposure to folic acid and/or dietary folic acid during late pregnancy. Similarly, Dunstan et al.[34] found that infants exposed in utero during the last trimester of pregnancy to >500 μg/day folic acid compared with <200 μg/day had an 85% higher risk of eczema.

Therefore, future research should consider the impact of folic acid exposure in late pregnancy and the potential for increased risk of childhood asthma and allergy. While this possibility is intriguing, the timing, source, and method of exposure assessment varied, and associations with asthma and allergic outcomes were inconsistent (Tables 2 and 3).[18, 25, 31-38] It will be important to better understand the effect of timing on the relationship between maternal folic acid exposure and childhood asthma and allergy to gain insight on the precise biologic mechanisms involved.

Outcome assessment and duration of follow-up

Variations in study design as well as differential assessment of respiratory outcomes may be associated with inconsistencies in study findings, as primary outcomes of interest and length of follow-up were not consistent across cohorts (Table 3). It is also important to consider the impact of sample size of each cohort, as this may influence the feasibility of frequent and longer follow-up duration as well as the level of invasiveness associated with outcome measurements. Of the 10 prospective studies included in this review, seven assessed wheezing or whistling in the chest,[18, 29, 33, 35-38] five collected information on atopy, allergic sensitization, or total or specific IgE levels to specific allergens,[25, 31, 34, 36, 37] four collected data on eczema,[33-36] four focused on parental report of physician-diagnosed asthma or persistent asthma,[25, 31, 36, 38] and three collected information on lower or upper respiratory tract infections.[18, 32, 33]

Table 3. Summary of major study findings by outcome and timing of assessment
ReferenceOutcomeTiming of outcome assessment
 Atopy, IgE, allergic sensitizationPhysician-diagnosed asthmaEczema or atopic dermatitisUpper or lower RTIWhistling, wheeze, or recurrent wheeze 
  1. Abbreviations: IgE, immunoglobulin E; RTI, respiratory tract infection; +, positive association.
Litonjua et al. (2006)[18]  NullNullNull6 months, 1 year, and 2 years of age
Granell et al. (2008)[25]NullNull   Annually through 7–8 years of age
Whitrow et al. (2009)[31]Null+   3.5 years and 5.5 years of age
Haberg et al. (2009)[32]   ++18 months of age
Miyake et al. (2011)[35]  Null Null16–24 months of age
Magdelijns et al. (2011)[36]NullNullNull  3 and 7 months of age; 1, 2, 4–5, and 6–7 years of age
Bekkers et al. (2012)[33]  NullNull+/NullAnnually through 8 years of age
Dunstan et al. (2012)[34]+ +  1 year of age
Kiefte-de Jong et al. (2012)[37]  + NullAnnually through 4 years of age
Martinussen et al. (2012)[38] Null   At 6 years of age

Despite the range of conditions considered, most studies used standard validated items from the International Study of Asthma and Allergies in Childhood (ISAAC) questionnaire to assess physician-diagnosed asthma, wheezing or whistling in the chest, tightness of the chest or shortness of breath, and symptoms of eczema or development of an itchy rash, which improved the ability to compare findings across studies. High agreement was found between physician assessment and the validated ISAAC questionnaire, with 75% agreement for wheeze, 81% agreement for shortness of breath, and 79% agreement for eczema or atopic dermatitis.[37] Elevated maternal folate levels were associated with an increase in childhood asthma, wheeze, lower respiratory tract infections, and allergic sensitization among the studies reporting positive relationships. Eczema and atopic dermatitis were not strongly associated with maternal folate status, perhaps due in part to the ISAAC core question, which inquires about the presence of an itchy rash that comes and goes instead of physician-diagnosed eczema. In contrast, the ISAAC questions for asthma and respiratory tract infections include only conditions that have been physician diagnosed, thereby reducing the potential for nondifferential misclassification.

Across studies, length of follow-up ranged from 1 to 8 years after birth (Table 3). Four studies included follow-up periods of 2 years or less,[18, 29, 34, 35] and the remaining six studies followed children for between 4 and 8 years.[25, 31, 33, 36-38] After delivery, most studies included one to three assessments before 12 months of age, with later assessments administered annually until the completion of the study. Additionally, most cohorts utilized questionnaire-based methods for outcome assessment, including parental self-report of symptoms and incident diagnoses related to childhood asthma or allergy, with a final clinic or laboratory visit to gather blood samples or conduct a skin prick test. The studies reporting positive associations between maternal folic acid exposure and childhood allergy and asthma had relatively shorter periods of follow-up that ranged from 12 months to 5.5 years, while several null studies had follow-up periods through 7 or 8 years after delivery.[25, 31, 32, 34, 36, 38] While additional studies are needed to further understand this relationship, the positive associations observed in early childhood might be reflective of transient rather than long-term increases in risk of childhood respiratory outcomes.

Folic acid supplementation patterns and potential for residual confounding

Patterns of folic acid supplementation and the potential for residual confounding based on characteristics inherent in women who use folic acid supplements warrant further discussion. Specific sociodemographic and lifestyle factors (e.g., nonsmoker, physically active, higher education) related to healthy behaviors in general may be associated with greater adherence to folic acid supplementation recommendations during pregnancy.[5, 32] Further, women planning their pregnancies (e.g., indicate intention to become pregnant, use of fertility treatments) may be more likely to engage in folic acid supplementation before and during pregnancy.[41] For example, in the Norwegian Mother and Child study, women with the highest level of education in the study were six times more likely to take folic acid during the preconception window than women with the lowest education levels, and married women were twice as likely as single women to use folic acid supplements prior to conception.[41]

Of the prospective studies that found a positive relationship between maternal folate exposure and childhood asthma or allergy, women using folic acid supplements were generally older, achieved higher levels of education, engaged in longer durations of breastfeeding, and were less likely to smoke.[29, 34] Among studies suggesting no association, women who were older, reported higher levels of education, earned higher income, and were married were had higher intakes of folate or were more likely to supplement during pregnancy.[25, 38] In general, studies using questionnaire-based methods were better able to achieve adequate control for confounders in comparison with studies that focused on biochemical folate measurements without gathering the same breadth of demographic and lifestyle information, though this did not differ in a meaningful way between studies reporting a positive association versus no association.

Folic acid supplementation may be associated with other healthy lifestyle characteristics, including health consciousness during pregnancy and awareness of developing health problems in the child. Thus, residual confounding related to other aspects of health consciousness may persist, which would likely overestimate the association between maternal folate exposure and childhood asthma and could contribute to positive associations.[37] Therefore, it is important that findings are interpreted in light of the confounding factors that were considered as well as the unknown or unmeasured factors not accounted for in the analysis. In addition, the findings of this review support potential lifestyle and demographic correlates associated with supplementation patterns and suggest opportunities for public health interventions to encourage women to consume recommended levels of folic acid supplementation before and during pregnancy.

Conclusion

The majority of the studies conducted to date suggest that maternal folate exposure is not associated with the development of childhood asthma and allergy. Further, there is limited evidence of a consistent dose-response relationship between maternal folate exposure during pregnancy and risk of childhood asthma or allergy. Conflicting findings may be due, in part, to the various study designs and exposure and outcome assessment strategies employed in the epidemiologic studies conducted to date. Among the studies reporting a positive association between maternal folate exposure and childhood asthma and allergic outcomes, the increase in risk was generally transient in nature and confined to the early childhood years. Further, among studies with statistically significant positive associations, the increase in risk was generally small, ranging from approximately a 6% increase in risk of wheeze to up to a 26% increase in childhood asthma. Lastly, among the studies with positive associations, the small increase in risk was consistently associated only with maternal folate exposure during late pregnancy. This is important, as folic acid supplementation is critical prior to conception and during the first trimester to reduce the risk of congenital anomalies such as spina bifida. Thus, these collective findings should reassure women who are concerned about the potential relationship between folic acid intake during pregnancy and childhood asthma and allergy later in life.

Comprehensive evaluation of maternal folate exposure throughout pregnancy is recommended to reveal the potential critical window of susceptibility that may influence risk of childhood asthma and allergy. Future studies may reveal the maximum dose of folic acid at which the risk of developing neural tube defects and childhood asthma is reduced without potential excess risk for development of childhood conditions later in life. As this review examines the collective literature specific to folate exposure, further research is warranted to elucidate if other donors or cofactors involved in DNA methylation may directly or indirectly contribute to childhood asthma and allergy. Additional follow-up into later childhood is recommended to elucidate potential long-term effects of maternal folic acid supplementation on risk of childhood asthma and allergy. The use of objective biochemical measurements of fetal exposure to folate as well as postnatal assessments of folate exposure throughout childhood will expand current understanding of the transient or chronic nature of the outcomes observed. Finally, given the breadth of outcomes and methods of assessment covered in this review, future studies may consider physician-diagnosed asthma or allergic outcomes over self-report of such conditions to reduce nondifferential misclassification.

This systematic review is intended to present findings related to the development of early childhood asthma, allergy, and wheeze based on maternal exposure to folate in pregnancy. As folic acid supplementation during preconception and pregnancy has been shown to substantially reduce risk of neural tube defects, findings related to potential development of childhood asthma are considered secondary to the importance of folic acid in fetal neurodevelopment.

Acknowledgments

Funding

No external funding was received for this work.

Declaration of interest

The authors have no relevant interests to declare.

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