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New Perspective on Impact of Folic Acid Supplementation during Pregnancy on Neurodevelopment/Autism in the Offspring Children – A Systematic Review

  • Yunfei Gao ,

    Contributed equally to this work with: Yunfei Gao, Chao Sheng, Ri-hua Xie

    Affiliations Department of Obstetrics and Gynecology, Nanfang Hospital of Southern Medical University, Guangzhou, Guangdong, China, OMNI Research Group, Department of Obstetrics and Gynecology, University of Ottawa Faculty of Medicine, Ottawa, Canada, Clinical Epidemiology Program, Ottawa Hospital Research Institute, Ottawa, Canada

  • Chao Sheng ,

    Contributed equally to this work with: Yunfei Gao, Chao Sheng, Ri-hua Xie

    Affiliation Department of Obstetrics and Gynecology, Nanfang Hospital of Southern Medical University, Guangzhou, Guangdong, China

  • Ri-hua Xie ,

    Contributed equally to this work with: Yunfei Gao, Chao Sheng, Ri-hua Xie

    Affiliations OMNI Research Group, Department of Obstetrics and Gynecology, University of Ottawa Faculty of Medicine, Ottawa, Canada, Hunan University of Medicine Department of Nursing, Huaihua, Hunan, China, McLaughlin Center for Population Risk Assessment, University of Ottawa Faculty of Medicine, Ottawa, Canada

  • Wen Sun,

    Affiliation Department of Obstetrics and Gynecology, Third Affiliated Hospital, Guangzhou Medical University, Guangzhou, China

  • Elizabeth Asztalos,

    Affiliations Centre for Mother, Infant and Child Research, Sunnybrook Health Sciences Centre, Toronto, Ontario, Canada, Departments of Pediatrics and Obstetrics and Gynecology Faculty of Medicine, University of Toronto, Toronto, Canada

  • Diane Moddemann,

    Affiliation Department of Pediatrics and Child Health and Neonatal Follow-up Program, University of Manitoba, Winnipeg, Canada

  • Lonnie Zwaigenbaum,

    Affiliations Departments of Pediatrics and of Psychiatry, University of Alberta, Edmonton, Canada, Autism Research Centre, Glenrose Rehabilitation Hospital, Edmonton, Canada

  • Mark Walker,

    Affiliations OMNI Research Group, Department of Obstetrics and Gynecology, University of Ottawa Faculty of Medicine, Ottawa, Canada, Clinical Epidemiology Program, Ottawa Hospital Research Institute, Ottawa, Canada, School of Epidemiology, Public Health, and Preventive Medicine, University of Ottawa, Ottawa, Canada

  • Shi Wu Wen

    swwen@ohri.ca

    Affiliations OMNI Research Group, Department of Obstetrics and Gynecology, University of Ottawa Faculty of Medicine, Ottawa, Canada, Clinical Epidemiology Program, Ottawa Hospital Research Institute, Ottawa, Canada, School of Epidemiology, Public Health, and Preventive Medicine, University of Ottawa, Ottawa, Canada

New Perspective on Impact of Folic Acid Supplementation during Pregnancy on Neurodevelopment/Autism in the Offspring Children – A Systematic Review

  • Yunfei Gao, 
  • Chao Sheng, 
  • Ri-hua Xie, 
  • Wen Sun, 
  • Elizabeth Asztalos, 
  • Diane Moddemann, 
  • Lonnie Zwaigenbaum, 
  • Mark Walker, 
  • Shi Wu Wen
PLOS
x

Abstract

It has been conclusively established that folic acid supplementation prior to and during early pregnancy (up to 12 weeks of gestation) can prevent neural tube defects (NTDs). We hypothesized that folate effects may extend from neuro-structural defects to alterations in neuro-behavioural and emotional skills including autism spectrum disorders (ASDs) and other developmental disorders. The objective of this review was to comprehensively evaluate evidence on the impact of folic acid on neurodevelopment other than NTDs. We conducted an online search of relevant literature compiled by the National Library of Medicine from Medline and EMBASE (searched on Dec 31, 2014: http://www.ncbi.nlm.nih.gov/entrez/query/fcgi and http://www.elsevier.com/online-tools/embase). We first created 3 files (search restricted to English literature) using the following key words: 1) folate or folic acid (171322 papers identified by this search); 2) maternal or pregnancy or pregnant or gestation or gestational or prenatal or antenatal or periconception or periconceptional (1349219 papers identified by this search); and 3) autism or autism spectrum disorders or developmental delay or development or neurodevelopment or mental or cognitive or language or personal-social or gross motor or fine motor or behaviour or intellectual or intelligence or Bayley Scale (8268145 papers identified by this search). We then merged the 3 files and reviewed the papers that addressed these three issues simultaneously. A total of 22 original papers that examined the association between folic acid supplementation in human pregnancy and neurodevelopment/autism were identified after the screening, with 15 studies showing a beneficial effect of folic acid supplementation on neurodevelopment/autism, 6 studies showed no statistically significant difference, while one study showed a harmful effect in > 5 mg folic acid supplementation/day during pregnancy. Folic acid supplementation in pregnancy may have beneficial effects on the neurodevelopment of children beyond its proven effect on NTDs.

Introduction

There are major emotional, societal, and economic implications of impaired neurodevelopment and/or autism in children [1, 2]. These children will often require specialized schooling and other community resources. Although the survival/life-span of these infants may not be seriously affected, many of them may need treatments throughout their lifetime, and the cost to the public health care system could be huge. When they reach adulthood, productivity is often lower than those with normal development, indirectly increasing the societal burden.

It has been established that supplementation with folic acid around the time of conception reduces the risk of neural tube defects (NTDs) in the offspring [3, 4, 5, 6]. However, whether folic acid has a similar effect on impaired neurodevelopment and/or autism remains elusive. This article therefore focuses on assessing the role of folic acid supplementation during pregnancy and folate metabolism on neurodevelopmental outcomes including autism spectrum disorders (ASDs), other than NTDs.

Materials and Methods

Search strategy

We conducted an online search of relevant literature compiled by the National Library of Medicine from Medline and EMBASE (searched on December 31, 2014 of the site: http://www.ncbi.nlm.nih.gov/entrez/query/fcgi and http://www.elsevier.com/online-tools/embase), with restriction to human studies. We first created 3 files (restricting our search to English literature) using the following key words: 1) folate or folic acid (171322 papers identified by this search); 2) maternal or pregnancy or pregnant or gestation or gestational or prenatal or antenatal or periconception or periconceptional (1349219 papers identified by this search); and 3) autism or autism spectrum disorders or developmental delay or development or neurodevelopment or mental or cognitive or language or personal-social or gross motor or fine motor or behaviour or intellectual or intelligence or Bayley Scale (8268145 papers identified by this search). We then merged the 3 files. All abstracts of the papers identified by merging the 3 files were screened by two independent reviewers in our group to exclude irrelevant studies (such as those on NTDs); because the causation between folic acid supplementation in pregnancy and NTDs has been established, our review is interested in outcomes other than NTDs.

Study selection

We included randomized controlled trials (RCTs), cohort studies, and case control studies that examined the association between folic acid supplementation during pregnancy and neurodevelopment/autism in the offspring children. Data extraction was conducted independently and screened all records at the title level by two reviewers (Chao Sheng and Ri-hua Xie). To enhance sensitivity, records were only removed if both reviewers excluded at the title level. The second level of review was at the abstract level followed by another round of review at the full-text level. Two independent reviewers abstracted data using a standardized form. When there was a disagreement it was resolved by discussion with a third reviewer (Yunfei Gao). Corresponding authors were contacted via e-mail at least three times to obtain data if the outcome of the neurodevelopment/autism in the offspring children could not be readily abstracted from the publication.

Study quality assessment

The quality of included cohort and case control studies was assessed with the Newcastle Ottawa Scale [7]. Using this checklist, Yunfei Gao evaluated each of the included articles, with additional inputs from Chao Sheng and Ri-hua Xie. The details are shown in Table 1. Divergent views were resolved by consulting a third reviewer.

Data extraction and synthesis

Data extracted from each study included the first author’s last name, publication year, main outcome, sample size, study design, age of children, effect of folic acid, and comments on the study. Because of major heterogeneity in original studies in terms of study design and outcome and exposure measurements, no attempt to summarize the effect by meta-analysis was made. When studies demonstrated conflicting findings on different outcome measures, the study was defined as “no association”.

Results

Literature search

A total of 3,348 papers were identified. Sixty five full-text articles were assessed for eligibility after screening. Most frequently the removed papers were animal studies or reviews or commentary/discussion in the interpretation of the study findings on other pregnancy outcomes (e.g., NTDs) or studies in humans but the effects of folic acid supplementation during pregnancy on neurodevelopment/autism in the offspring children was not examined. See details of selection in Fig 1.

A total of 22 original papers that looked at the association between folic acid supplementation in pregnancy and neurodevelopment/autism were identified after the screening in Table 2. Because of major heterogeneity in study design, exposure measurement, and outcome measurement, no attempt was made for quantitative synthesis of effect by meta-analysis. The 43 full-text excluded articles with reasons of exclusion were listed as supplement information in S1 Appendix.

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Table 2. Summary of studies of the effects of folic acid supplementation in pregnancy on neurodevelopment and autism in offspring*.

https://doi.org/10.1371/journal.pone.0165626.t002

Study characteristics

The 22 eligible studies include 2 RCTs, 18 cohort studies, and 2 case control studies. The baseline characteristics and further summarized information are outlined in Table 2. The main outcomes include ASDs, autism, developmental delay, cognition, attention function, neurodevelopment, emotional problems, and behavioural problems. The children range in age from 12 months to 11 years. Among the 21studies, 7 studies included more than 1000 children.

Outcomes

Fifteen studies showed a beneficial effect of folic acid supplementation on neurodevelopment/autism, 6 studies found no statistically significant result, while one study found a harmful effect at high dose of folic acid supplementation (see Table 2). There were 3 studies that had ASD as the main outcome measure. The first was a cohort study of 85,176 children aged 3.3 to 10.2 years [8]. The rate of ASD in children whose mothers took folic acid was 0.10%, whereas the rate for mothers who did not take folic acid was 0.21%, with adjusted odds ratio (OR) of folic acid users 0.61 (95% confidence interval (CI), 0.41–0.90). Another study was the Childhood Autism Risks from Genetics and Environment, a case-control study in the United States [9]. In the 837 mother-child pairs, the mean folic acid intake in the first month of pregnancy was significantly greater for mothers of normally developing children than for mothers of children with a confirmed diagnosis of ASD. A mean daily folic acid intake of ≥ 600ug during the first pregnant month was associated with reduced ASD risk (adjusted OR: 0.62; 95% CI: 0.42–0.92; P = 0.02). This finding was consistent with another case-control study by the same author [10], which showed that mean folic acid intake in early pregnancy was significantly higher for mothers of normally developing children than for mothers of children with ASD.

Several studies found similar beneficial effects of folic acid supplementation on other areas of neurodevelopment. For example, a study in Massachusetts [11] showed that for each 600 ug/day increment in total folate intake during the first trimester, Peabody Picture Vocabulary Test-III score at age 3 years was 1.6 points (95% CI 0.1–3.1; p = 0.04) higher. Forns et al found that omission errors (defined as the number of targets to which the individual did not respond) were lower in those whose mothers took dietary supplementation with folic acid and vitamins during pregnancy [12]. In a cohort study [13] involving 553 mother-child pairs in Greece, neurodevelopment at 18 months was assessed using the Bayley Scales of Infant and Toddler Development (3rd edition). Compared with non-users, daily intake of 5 mg supplemental folic acid was associated with a 5-unit increase on the scale of receptive communication and a 3.5-unit increase on the scale of expressive communication. Roth et al assessed severe language delay (defined as minimal expressive language: only 1-word or unintelligible utterances at the age of 3 years) in a cohort of 38,954 children, and found that adjusted ORs for 3 patterns of exposure to maternal dietary supplements (no supplement as the reference) were 1.04 (95% CI, 0.62–1.74) for other supplements but no folic acid; 0.55 (95% CI, 0.35–0.86) for folic acid only; and 0.55 (95% CI, 0.39–0.78) for folic acid in combination with other supplements, demonstrating a clear protective effect of folic acid supplementation during pregnancy [14]. A study by Steenweg-de et al found a higher risk of emotional problems in 3 year old children using the Child Behavior Checklist (CBCL) whose mothers did not use supplements or started folic acid supplements late in pregnancy (OR: 1.45; 95% CI: 1.14, 1.84) compared to children whose mothers started folic acid supplement in early pregnancy [15]. Similarly, in a prospective cohort study, Roza et al examined the association between folic acid supplement use during the first trimester and behavioural and emotional problems identified by the CBCL in 4,214 toddlers at the age of 18 months. This study found that folic acid supplement use protected both from internalizing (OR of no use 1.65; 95% CI 1.24, 2.19) and externalizing problems (OR of no use 1.45; 95% CI 1.17, 1.80), after adjusting for maternal characteristics, birth weight, and fetal head size [16]. In another prospective cohort study, Julvez et al found that folic acid supplement during pregnancy was associated with improved neurodevelopment in children after adjusting for a number of sociodemographic and behavioural factors (results obtained from linear regression models): higher scores on verbal (b (regression slope) = 3.98, SE (standard error of regression slope) = 1.69), motor (b = 4.54, SE = 1.66), verbal-executive function (b = 3.97, SE = 1.68) scores, social competence (b = 3.97, SE = 1.61), and lower rate of inattention symptom [OR = 0.46; 95% CI 0.22, 0.95] [17].

Discussion

A total of 22 original papers that looked at the association between folic acid supplementation in pregnancy and neurodevelopment/autism were identified after the screening, with 15 studies showing a beneficial effect of folic acid supplementation on neurodevelopment/autism, 6 studies found no statistically significant effect, while one study found a harmful effect at high dose of folic acid supplementation [18]. Two papers that suggested an adverse effect of folic acid on ASDs were not included in our review because no data on individual subjects were available in these two studies [19, 20]. Both papers used ecological data to support their hypothesis: prenatal folic acid supplementation and autism diagnoses in the United States since the 1980s in King’s study, and published autism incidence rates and prescriptions for folic acid in Rochester, Minnesota from 1976 to 1997 in Beard’s study. Beard’s study found a correlation coefficient of 0.87 (95% CI 0.19–0.99) between autism rates and the prescription prenatal vitamins containing folic acid and a correlation coefficient of 0.62 (95% CI 0.38–0.95) between autism rates and pediatric folic acid. However, during the same period, major changes in other factors such as diagnostic criteria, public awareness, disease surveillance, and screening efforts have all played an important role in the increased rates of diagnosed ASDs, so ecological data may not be suitable to analyze the association between folate and ASDs. Data from recent ASDs surveillance in the United States revealed a major increase in ASDs prevalence during a period with no change in policies regarding prenatal folic acid supplementation or folic acid food fortification (2002 to 2008), suggesting that ecological analyses are seriously flawed [21]. On the other hand, in a small sample of children (77) born to mothers used folic acid supplementation >5 mg/day during pregnancy had a statistically significantly lower mean psychomotor scale score (difference, -4.35 points; 95% CI, -8.34 to -0.36) than children whose mothers used a recommended dosage of folic acid supplements (0.4–1.0 mg/day) [18]. The finding from a single study with small sample needs to be replicated. Castro et al conducted a systematic review of studies involving on relationship between folic acid and ASD. All 11 papers included in Castro’s review met the inclusion criteria in our review. It concluded that although lower folate levels were associated with increased risk of ASD, the effects of folate-enhancing interventions on the clinical symptoms of ASD have yet to be confirmed [22].

Folic acid, or folate (vitamin B9) is an essential nutrient that is required for DNA replication and as a substrate for a range of enzymatic reactions involved in amino acid synthesis and vitamin metabolism. Demands for folate increase during pregnancy because of increased demands from the fetus. It has been conclusively established that folic acid deficiency prior to and during early pregnancy (up to 12 weeks of gestation) causes increased risk of NTDs, and periconceptional supplementation of folic acid can dramatically lower this risk (as much as 70%) [3, 4, 5, 6]. If folic acid deficiency prior to and during early pregnancy can cause NTDs, it may also cause milder forms of fetal brain damage that could be expressed as impaired neurodevelopment/autism in early childhood, and this effect may not be restricted to pre-conception and early gestation (<12 weeks of gestation, as the neural tube closes at that time so no NTDs after that). Laboratory investigations in animals and humans have shown that folate plays an important role in early brain development. In humans, there are active placental transports of folate and fetal brain folate levels are higher than adult levels [6]. In rats, the concentrations of many folate-dependent enzymes were substantially higher during early development than adult levels [23]. Dams and developing pups fed with diets eliminating folic acid 1 week prior to birth were less viable and had lower brain weights, lower activity level, and lower level of S-adenosyl-L-methionine concentrations in brain tissue of surviving offspring than animals reared on normal diets [24]. Ferguson et al examined whether gestational dietary folate deficiency not producing severe NTDs could lead to other functional impairments in mice [25]. They found that prenatal folate deficiency in mice led to an increase in anxiety-related behaviours. Worthy of our attention is that Padmanabhan et al found a hypomorphic mutation of the mouse MTRR gene, which results in developmental delay, as well as congenital malformations, including neural tube, heart, and placental defects, showing that folate metabolism has distinct transgenerational epigenetic functions that are responsible for specific developmental processes [26]. Even with normal dietary folate, the hypomorphic mutations in the MTRR gene associated with reduced expression may still lead to congenital abnormalities.

A few clinical studies have compared metabolites or cofactors of the folate-methionine pathway in children with autism [2737]. While results from these studies have not been consistent a dysfunctional folate-methionine pathway has been identified that may have an impact on developmental delays including autism [38]. This pathway is crucial for DNA synthesis, DNA methylation, and cellular redox balance.

Clinical case series have also linked folic acid deficiency to other types of fetal brain damage such as intracranial calcification [39]. Del Campo et al reported several cases of patients who, in addition to the structural anomalies typical of maternal methotrexate exposure, have significant developmental delay, and suggested that prenatal exposure to folic acid antagonists increases the risk of mental retardation [40]. Arakawa et al observed that the EEG maturation was delayed in children born to mothers with low serum folate [41]. A recent study evaluated the nutritional intake in 111 Chinese children with autism (aged 2 to 9 years) and compared with the national Dietary Reference Intakes (DRI) [42]. They found that the children with autism had adequate or exceeded intakes in energy, protein, vitamins B1, B2, E, niacin, magnesium, and iron, but had inadequate intakes in folic acid, vitamins A, B6, C, calcium, and zinc, nutrients known to be important for brain development and function [42].

Strengths

To our knowledge, this is the first systematic review examining the impact of folic acid supplementation during pregnancy on neurodevelopment/autism in the offspring children. We did an extensive search of relevant literature and selected studies strictly. After merging and analyzing the selected studies, we provided a preliminary conclusion.

Limitations

Our study has some limitations. First, the magnitude of the protective effect of folic acid supplementation observed in most of the included studies was quite small as compared with the known effect of folic acid supplementation on NTDs. We speculate that compared with NTDs, the diagnosis of neurodevelopmental disorders is more subtle and requires a much longer observation period. As a result, the potential effect of folic acid supplementation may have been offset by measurement errors or loss of follow ups. Second, most of the included studies were observational. However, one RCT [43] showed beneficial effects of folic acid supplementation, consistent with a majority of the observational studies, which adds weight to the evidence. Third, because of major heterogeneity in original studies in terms of study design and outcome and exposure measurements, no attempt to summarize the effect by meta-analysis was made. Finally, there may be studies with negative results that were not published in the searchable databases because of potential publication bias. Although the major heterogeneity of the included studies prevented us from a formal assessment of publication bias, global inspection of all included studies did not find any systematic trends in terms of positive/negative findings.

Implications for research

The limited data identified suggests that folic acid supplementation in pregnancy protects against impaired neurodevelopment including ASDs in children, and may improve cognitive function and intellectual and motor function. However, it is hard to draw a conclusion due to the limitations of the identified studies. Large scale RCTs with validated diagnosis and high follow up rate are needed in order to produce robust evidence regarding the effects of folic acid supplementation in pregnancy on fetal neurodevelopment.

Conclusion

In summary, our review of the literature suggests that folic acid supplementation in pregnancy may protect against impaired neurodevelopment including ASDs in children, and may improve cognitive function, intellectual, and motor function.

Supporting Information

S1 Appendix. List of full-text excluded articles with reasons of exclusion.

https://doi.org/10.1371/journal.pone.0165626.s002

(DOCX)

Acknowledgments

This study was funded by the Canadian Institutes for Health Research (CIHR) (MOP-142723).

Author Contributions

  1. Conceptualization: SWW YFG.
  2. Data curation: YFG CS RHX WS.
  3. Formal analysis: YFG CS RHX WS.
  4. Funding acquisition: SWW.
  5. Investigation: YFG CS RHX WS EA DM LZ MW SWW.
  6. Methodology: SWW YFG CS RHX WS.
  7. Project administration: SWW.
  8. Resources: SWW.
  9. Supervision: SWW.
  10. Validation: YFG CS RHX WS.
  11. Visualization: YFG CS RHX WS EA DM LZ MW SWW.
  12. Writing – original draft: YFG CS.
  13. Writing – review & editing: SWW WS EA DM LZ MW.

References

  1. 1. Baker BL, McIntyre LL, Blacher J, Crnic K, Edelbrock C, Low C. Pre-school children with and without developmental delay: behaviour problems and parenting stress over time. J Intellect Disabil Res. 2003; 47: 217–230. pmid:12787154
  2. 2. Zwaigenbaum L. Screening, risk and early identification of Autism Spectrum Disorders, D. 1 st Eds. New York: Oxford University Press; 2011.
  3. 3. 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; 341:1485–1490. pmid:10559448
  4. 4. Czeizel AE, Dudás I, Paput L, Bánhidy F. Prevention of neural-tube defects with periconceptional folic acid, methylfolate, or multivitamins? Ann Nutr Metab. 2011;58: 263–271. pmid:21865678
  5. 5. Prevention of neural tube defects: results of the Medical Research Council Vitamin Study. MRC Vitamin Study Research Group. Lancet. 1991, 338:131–137. pmid:1677062
  6. 6. Greenblatt JM, Huffman LC, Reiss AL. Folic acid in neurodevelopment and child psychiatry. Prog Neuropsychopharmacol Biol Psychiatry. 1994, 18:647–60. pmid:7938557
  7. 7. Wells GA, Shea B, O'Connell D, Peterson J, Welch V, Losos M, et al. The Newcastle-Ottawa Scale (NOS) for assessing the quality of nonrandomised studies in meta-analyses. Available from: http://www.ohri.ca/programs/clinical_epidemiology/oxford.asp.
  8. 8. Surén P, Roth C, Bresnahan M, Haugen M, Hornig M, Hirtz D,et al. Association between maternal use of folic acid supplements and risk of autism spectrum disorders in children. JAMA. 2013; 309:570–577. pmid:23403681
  9. 9. Schmidt RJ, Hansen RL, Hartiala J, Allayee H, Schmidt LC, Tancredi DJ, et al. Prenatal vitamins, one-carbon metabolism gene variants, and risk for autism. Epidemiology. 2011, 22: 476–485. pmid:21610500
  10. 10. Schmidt RJ, Tancredi DJ, Ozonoff S, Hansen RL, Hartiala J, Allayee H, et al. Maternal periconceptional folic acid intake and risk of autism spectrum disorders and developmental delay in the CHARGE (CHildhood Autism Risks from Genetics and Environment) case-control study. Am J Clin Nutr. 2012; 96: 80–89. pmid:22648721
  11. 11. Villamor E, Rifas-Shiman SL, Gillman MW, Oken E. Maternal intake of methyl-donor nutrients and child cognition at 3 years of age. Paediatr Perinat Epidemiol. 2012; 26:328–335. pmid:22686384
  12. 12. Forns J, Torrent M, Garcia-Esteban R, Cáceres A, Pilar Gomila M, Martinez D, et al. Longitudinal association between early life socio-environmental factors and attention function at the age 11 years. Environ Res. 2012;117:54–59. pmid:22608140
  13. 13. Chatzi L, Papadopoulou E, Koutra K, Roumeliotaki T, Georgiou V, Stratakis N, et al. Effect of high doses of folic acid supplementation in early pregnancy on child neurodevelopment at 18 months of age: the mother-child cohort 'Rhea' study in Crete, Greece. Public Health Nutr. 2012;15:1728–1736. pmid:22314109
  14. 14. Roth C, Magnus P, Schjølberg S, Stoltenberg C, Surén P, McKeague IW, et al. Folic acid supplements in pregnancy and severe language delay in children. JAMA. 2011; 306:1566–73. pmid:21990300
  15. 15. Steenweg-de Graaff J, Roza SJ, Steegers EA, Hofman A, Verhulst FC, Jaddoe VW, et al. Maternal folate status in early pregnancy and child emotional and behavioral problems: the Generation R Study. Am J Clin Nutr. 2012;95:1413–21. pmid:22572645
  16. 16. Roza SJ, van Batenburg-Eddes T, Steegers EA, Jaddoe VW, Mackenbach JP, Hofman A, et al. Maternal folic acid supplement use in early pregnancy and child behavioural problems: The Generation R Study. Br J Nutr. 2010;103:445–452. pmid:19772683
  17. 17. Julvez J, Fortuny J, Mendez M, Torrent M, Ribas-Fitó N, Sunyer J. Maternal use of folic acid supplements during pregnancy and four-year-old neurodevelopment in a population-based birth cohort. Paediatr Perinat Epidemiol. 2009;23:199–206. pmid:19775381
  18. 18. Valera-Gran D, García de la Hera M, Navarrete-Muñoz EM, Fernandez-Somoano A, Tardón A, Julvez J, et al. Folic acid supplements during pregnancy and child psychomotor development after the first year of life. JAMA Pediatr. 2014 Nov;168(11):e142611 pmid:25365251
  19. 19. Beard CM, Panser LA, Katusic SK. Is excess folic acid supplementation a risk factor for autism? Med Hypotheses. 2011;77:15–7. pmid:21454018
  20. 20. King CR. A novel embryological theory of autism causation involving endogenous biochemicals capable of initiating cellular gene transcription: a possible link between twelve autism risk factors and the autism 'epidemic'. Med Hypotheses. 2011;76:653–660. pmid:21388746
  21. 21. Autism and Developmental Disabilities Monitoring Network Surveillance Year 2008 Principal Investigators; Centers for Disease Control and Prevention: Prevalence of autism spectrum disorders—Autism and Developmental Disabilities Monitoring Network, 14 sites, United States, 2008. MMWR Surveill Summ. 2012; 61:1–19.
  22. 22. Castro K, Klein LD, Baronio D, Gottfried C, Riesgo R, Perry IS. Folic acid and autism: What do we know? Nutr Neurosci. 2014 Aug 2.
  23. 23. Boetz MI, Reynolds EH. Biochemical functions in the CNS in folic acid. In: Folic Acid In Neurology, Psychiatry and Internal Medicine, 1 st ed. New York: Raven Press; 1979.
  24. 24. Middaugh LD, Grover TA, Blackwell LA, Zemp JW. Neurochemical and behavioral effects of diet related perinatal folic acid restriction. Pharmacol Biochem Behav. 1976; 5:129–134.
  25. 25. Ferguson SA, Berry KJ, Hansen DK, Wall KS, White G, Antony AC. Behavioral effects of prenatal folate deficiency in mice. Birth Defects Res A Clin Mol Teratol. 2005; 73:249–252. pmid:15744731
  26. 26. Padmanabhan N, Jia D, Geary-Joo C, Wu X, Ferguson-Smith AC, Fung E, et al. Mutation in folate metabolism causes epigenetic instability and transgenerational effects on development. Cell. 2013; 155:81–93. pmid:24074862
  27. 27. Adams JB, Holloway C. Pilot study of a moderate dose multivitamin/mineral supplement for children with autistic spectrum disorder. J Altern Complement Med. 2004; 10:1033–1039. pmid:15673999
  28. 28. Adams JB, George F, Audhya T. Abnormally high plasma levels of vitamin B6 in children with autism not taking supplements compared to controls not taking supplements. J Altern Complement Med. 2006; 12:59–63. pmid:16494569
  29. 29. Arnold GL, Hyman SL, Mooney RA, Kirby RS. Plasma amino acids profiles in children with autism: potential risk of nutritional deficiencies. J Autism Dev Disord. 2003; 33: 449–54. pmid:12959424
  30. 30. D'Eufemia P, Finocchiaro R, Celli M, Viozzi L, Monteleone D, Giardini O. Low serum tryptophan to large neutral amino acids ratio in idiopathic infantile autism. Biomed Pharmacother.1995; 49:288–292. pmid:7579010
  31. 31. James SJ, Melnyk S, Jernigan S, Cleves MA, Halsted CH, Wong DH, et al. Metabolic endophenotype and related genotypes are associated with oxidative stress in children with autism. Am J Med Genet B Neuropsychiatr Genet. 2006; 141B:947–956. pmid:16917939
  32. 32. Lowe TL, Cohen DJ, Miller S, Young JG. Folic acid and B12 in autism and neuropsychiatric disturbances of childhood. J Am Acad Child Psychiatry. 1981; 20:104–11. pmid:7217543
  33. 33. Moretti P, Sahoo T, Hyland K, Bottiglieri T, Peters S, del Gaudio D, et al. Cerebral folate deficiency with developmental delay, autism, and response to folinic acid. Neurology. 2005; 64:1088–90. pmid:15781839
  34. 34. Moretti P, Peters SU, Del Gaudio D, Sahoo T, Hyland K, Bottiglieri T, et al. Brief report: autistic symptoms, developmental regression, mental retardation, epilepsy, and dyskinesias in CNS folate deficiency. J Autism Dev Disord. 2008; 38:1170–1177. pmid:18027081
  35. 35. Paşca SP, Nemeş B, Vlase L, Gagyi CE, Dronca E, Miu AC, et al. High levels of homocysteine and low serum paraoxonase 1 arylesterase activity in children with autism. Life Sci. 2006; 78:2244–2248. pmid:16297937
  36. 36. Ramaekers VT, Blau N, Sequeira JM, Nassogne MC, Quadros EV. Folate receptor autoimmunity and cerebral folate deficiency in low-functioning autism with neurological deficits. Neuropediatrics. 2007; 38:276–281. pmid:18461502
  37. 37. Ramaekers VT, Rothenberg SP, Sequeira JM, Opladen T, Blau N, Quadros EV. Autoantibodies to folate receptors in the cerebral folate deficiency syndrome. N Engl J Med. 2005; 352:1985–1991. pmid:15888699
  38. 38. Main PA, Angley MT, Thomas P, O'Doherty CE, Fenech M. Folate and methionine metabolism in autism: a systematic review. Am J Clin Nutr. 2010; 91:1598–1620. pmid:20410097
  39. 39. Garwicz S, Mortensson W. Intracranial calcification mimicking the Sturge-Weber syndrome: a consequence of cerebral folic acid deficiency? Pediatr Radiol. 1976; 5:5–9. pmid:1012795
  40. 40. Del Campo M, Kosaki K, Bennett FC, Jones KL. Developmental delay in fetal aminopterin/methotrexate syndrome. Teratology. 1999; 60:10–2. pmid:10413333
  41. 41. Arakawa T, Mizuno T, Honda Y, Tamura T, Sakai K. Brain function of infants fed on milk from mothers with low serum folate levels. Tohoku J Exp Med. 1969; 97:391–397. pmid:5800583
  42. 42. Xia W, Zhou Y, Sun C, Wang J, Wu L. A preliminary study on nutritional status and intake in Chinese children with autism. Eur J Pediatr. 2010; 169:1201–1206. pmid:20422215
  43. 43. Christian P, Murray-Kolb LE, Khatry SK, Katz J, Schaefer BA, Cole PM, et al. Prenatal micronutrient supplementation and intellectual and motor function in early school-aged children in Nepal. JAMA. 2010; 304:2716–2723. pmid:21177506
  44. 44. Tamura T, Goldenberg RL, Chapman VR, Johnston KE, Ramey SL, Nelson KG. Folate status of mothers during pregnancy and mental and psychomotor development of their children at five years of age. Pediatrics. 2005; 116:703–708. pmid:16140711
  45. 45. Wehby GL, Murray JC. The effects of prenatal use of folic acid and other dietary supplements on early child development. Matern Child Health J. 2008; 12:180–187. pmid:17554612
  46. 46. del Río Garcia C, Torres-Sánchez L, Chen J, Schnaas L, Hernández C, Osorio E, et al. Maternal MTHFR 677C>T genotype and dietary intake of folate and vitamin B(12): their impact on child neurodevelopment. Nutr Neurosci. 2009; 12:13–20. pmid:19178787
  47. 47. Glaser B, Ades AE, Lewis S, Emmet P, Lewis G, Smith GD, et al. Perinatal folate-related exposures and risk of psychotic symptoms in the ALSPAC birth cohort. Schizophr Res. 2010; 120:177–83. pmid:20418067
  48. 48. Schlotz W, Jones A, Phillips DI, Gale CR, Robinson SM, Godfrey KM. Lower maternal folate status in early pregnancy is associated with childhood hyperactivity and peer problems in offspring. J Child Psychol Psychiatry. 2010; 51:594–602. pmid:19874428
  49. 49. Veena SR, Krishnaveni GV, Srinivasan K, Wills AK, Muthayya S, Kurpad AV, et al. Higher maternal plasma folate but not vitamin B-12 concentrations during pregnancy are associated with better cognitive function scores in 9- to 10- year-old children in South India. J Nutr. 2010;140:1014–1022. pmid:20335637
  50. 50. Campoy C1, Escolano-Margarit MV, Ramos R, Parrilla-Roure M, Csábi G, Beyer J, et al. Effects of prenatal fish-oil and 5-methyltetrahydrofolate supplementation on cognitive development of children at 6.5 y of age. Am J Clin Nutr. 2011; 94:1880S–1888S. pmid:21849596
  51. 51. Wu BT, Dyer RA, King DJ, Richardson KJ, Innis SM. Early second trimester maternal plasma choline and betaine are related to measures of early cognitive development in term infants. PLoS One. 2012; 7: e43448. pmid:22916264
  52. 52. Steenweg-de Graaff J, Ghassabian A, Jaddoe VW, Tiemeier H, Roza SJ. Folate concentrations during pregnancy and autistic traits in the offspring. The Generation R Study. Eur J Public Health. 2014 Jul 31. pii: cku126.
  53. 53. Dobó M, Czeizel AE. Long-term somatic and mental development of children after periconceptional multivitamin supplementation. Eur J Pediatr. 1998; 157:719–723. pmid:9776529