We observed several significant associations between NTDs and maternal dietary micronutrient intake. Our results suggest maternal diets higher in folate, betaine, thiamin, iron, and vitamin A may contribute to lowering anencephaly risk among some women. For spina bifida, diets higher in thiamin, riboflavin, vitamin B6, vitamin C, vitamin E, niacin, and retinol appear to decrease risk in certain women. Thiamin had associations for both anencephaly and spina bifida. Interestingly, analyses revealed inverse associations for higher intakes of alanine related to anencephaly-affected pregnancies in normal-weight women.
Similarities in folic acid supplement use among cases and controls were reported in previous NBDPS data on folate and NTDs (Mosley and others, 2009
). In a larger sample including two additional years of enrollment, we did not observe any difference in folic acid supplement use between cases and controls. Current analysis reveals an association between increased dietary folate intake and decreased risk of anencephaly among Hispanic women, (Shaw and others, 1995
) and perhaps among supplement users and overweight women. The previous NBDPS report (Mosley and others, 2009
) shows similar reduced occurrence of anencephaly with higher dietary folate. Specifically, for highest quartile of dietary folate intake, Hispanic women were 54% less likely to have offspring with anencephaly compared to Hispanic women with lowest dietary folate intake. Similarly, overweight women with high folate intake were 62% less likely to have offspring with anencephaly when compared to overweight women with lowest dietary folate intake. In the post-fortification era, these results suggest additional periconceptional folate intake may be necessary to further prevent anencephaly. Folate intake did not appear to significantly influence spina bifida risk whether stratified for folic acid supplementation, race/ethnicity, or BMI.
Recently, researchers are focusing on folate-related micronutrients involved in one-carbon metabolism including methionine, choline, betaine, thiamin, riboflavin, vitamin B6
, and vitamin B12
. Shaw et al. (Shaw and others, 1999
) reported decreased NTD risk among women with increased thiamin intake who did not take periconceptional vitamin supplements. Our study supports an association between increased thiamin intake and decreased anencephaly risk among non-users of folic acid supplements, as well as Hispanic women. Additionally, overweight women with the highest intakes of thiamin were 71% less likely to have a pregnancy affected by anencephaly and 55% less likely to have a pregnancy affected by spina bifida. Higher levels of choline (Shaw and others, 2009
) and vitamin B12
(Suarez and others, 2003
) in maternal serum have been suggested to be protective against NTDs. Diets high in choline, betaine, methionine, and vitamin B12
have also been reported to decrease NTD risk (Shaw and others, 2004
; Shaw and others, 1997
; Suarez and others, 2003
). This study did not observe an association between higher choline, methionine, or vitamin B12
intake and NTD risk. We observed that increased betaine intake was associated with decreased risk of anencephaly among Hispanics. Black, non-Hispanic women and overweight women with the highest riboflavin intakes were approximately 67% and 61%, less likely to have a spina bifida affected pregnancy than women with lowest intakes of these micronutrients. Factors related to genetics or metabolic activity could explain differences among subpopulations or this could be due to chance.
Despite an overall decrease in NTD prevalence in the US, NTD prevalence among Hispanic women continues to remain higher than other racial/ethnic groups (2009
; Canfield and others, 2005
; Canfield and others, 2006
; Williams and others, 2005
). Participant demographics in the current study support this finding. Our results suggest increased intake of several micronutrients related to one-carbon metabolism, including folate, betaine, thiamin, riboflavin, and vitamin B6
may be associated with decreased occurrence of NTDs among Hispanics. Additionally, higher intakes of antioxidants vitamin C and E appear to be associated with decreased likelihood of having a pregnancy affected by spina bifida. In fact, higher intakes of many micronutrients within this study were associated with decreased risk of anencephaly or spina bifida among Hispanic women. While the answer may simply be inadequate nutrition among Hispanic populations due to lower socioeconomic status, additional explanations include potential existence of an underlying genetic component affecting the folate-related metabolic pathway or the propensity towards a state of increased oxidative stress.
Other studies found associations between maternal obesity and occurrence of birth defects (Gilboa and others, 2010
; Rasmussen and others, 2008
; Waller and others, 2007
; Watkins and others, 2003
). The 2007 NBDPS report on pre-pregnancy obesity noted obese women were twice as likely to have offspring affected by spina bifida (Waller and others, 2007
). Our data indicate obese women are more likely to have offspring affected with spina bifida. Higher vitamin E intake among overweight and obese women was associated with decreased risk of spina bifida by almost half compared to the lowest intake of vitamin E. A 2008 review suggests obesity and obesity-related metabolic alterations result in a state of oxidative stress similar to diabetes, a known condition leading to increased risk of birth defects (Reece, 2008
). Thus, increased dietary intake of antioxidant-rich foods may decrease the risk of NTDs particularly among overweight and obese women.
When taking a broader approach than restricting discussion of odds ratio to those excluding 1, i.e. using an a priori threshold of 30% reduction or increase in risk, even more associations are possible between maternal dietary intake of micronutrients and NTD-risk. This view supports the need for additional research into these micronutrients contributing towards reduced NTD occurrence.
Primary limitations include potential for recall bias and small numbers of cases in certain subgroups. The validity of food questionnaires has been published (Willett and others, 1987
), although we cannot discount limitations of this type of dietary assessment. Food items which are either over- or under-reported could result in false presence or absence of association of micronutrients with NTDs. Available data on subgroups based on race/ethnicity or BMI may be too limited to observe potential factors affecting NTD risk within these populations. The number of black, non-Hispanic case women was only 76, representing 8% of all NTD cases, while obese women comprised 21.1% of all NTD cases. Furthermore, small sample sizes limited our ability to stratify analyses by or adjust for multivitamin supplement use when examining the association between NTD risk and race/ethnicity or BMI. Lack of daily micronutrient intake from supplements prevented including these data in calculating the quartiles of intake. Increased numbers in these subgroups could help unearth important differences among subpopulations. We made 816 comparisons testing associations between NTDs and maternal micronutrient intake and some may insist that adjustment is made for multiple comparisons. However, adjustment of P
-values for multiple comparisons remains controversial (Rothman and others, 2008
). We did not correct for multiple comparisons; if we had, the significance level would have been affected. With 816 comparisons, many of the observed results were consistent with random variation. This trade-off is acceptable in this analysis to identify other micronutrients involved in one-carbon metabolism or antioxidant activity that might influence NTD risk for future research. Additionally, for many statistically significant odds ratios where a lower intake of a micronutrient (2nd
quartile and/or 3rd
quartile) appeared protective but not higher levels (3rd
quartiles), speculations regarding biological plausibility are challenging. Our findings consistent with other studies provide greater confidence in results. Many of the associations between specific micronutrients, phenotypes, and covariates have been evaluated for the first time and will require replication within independent populations.
Even with such limitations, the NBDPS has provided the most extensive study base to date to explore important questions between nutrients and risk of NTDs. Strengths of this study are large number of cases and amount of detailed dietary information and potential cofactors examined. This allowed for epidemiologic study of NTDs and dietary intake evaluating these relationships for potential confounders and by population subgroups. We observed that, in addition to folate, several micronutrients involved in either one-carbon metabolism or antioxidant activity may influence NTD risk and these effects might be different in certain subpopulations. Further investigation should be directed towards determining which factors are important for certain cohorts of women, particularly based upon their racial/ethnic background and pre-pregnancy body habitus.