This study explored the relationship between maternal consumption of dietary nitrates, total nitrites, nitrites (from both animal and plant sources) and nitrosamines and specific NTDs, orofacial clefts and limb malformations in their offspring. The primary strength of the study is its large and very well-characterized sample. It is the largest study to date to investigate the relation between estimated maternal intake of dietary nitrate, nitrite, and nitrosamines and neural tube defects and examines several other types of birth defects in relation to these exposures that have not been examined before. Overall, estimated dietary intake of these compounds did not appear to be significant risk factors for neural tube, oral cleft, or limb deficiency defects.
Croen et al.
 also found no compelling associations between maternal dietary intake of nitrates, nitrites, and nitrosamines and neural tube defects in a California study population, with most odds ratios for neural tube defects slightly below 1.00 in the second, third, and fourth quartiles compared with the first quartile of intake. Using tertiles instead of quartiles of intake, Brender et al.
 noted odds ratios of 0.8 and 0.9 for the upper two tertiles of dietary nitrite and for total nitrite, 0.9 and 0.8. Neither study reported findings of the relation between dietary intake of these compounds and neural tube defects by specific phenotype.
Of interest to this study, three studies found dietary nitrite and total nitrite to modify the association between nitrosatable drug use and birth defects in offspring. In a study of Mexican American women who resided in Texas counties bordering Mexico, nitrosatable drug use was associated with these defects in the upper two tertiles of nitrite and total nitrite intake, but not in the lowest tertile of intake
. These findings were corroborated in the National Birth Defects Prevention Study population
 in which the strongest associations between secondary/tertiary amine drug exposure and anencephaly and spina bifida were noted in the upper two tertiles of dietary nitrite and total nitrite intake. Associations for cleft palate and several types of heart defects were also stronger in offspring of NBDPS participants who had the highest estimated total nitrite intake
. These findings are consistent with results of an experimental study with mice exposed to ethylenethiourea (a nitrosatable compound) and nitrite
. Malformations were observed when these compounds were administered together, but not separately. With the dose of the nitrosatable compound held constant, the percentage of malformations increased as the dose of nitrite increased, indicating that the combined effects of these compounds might be due to the nitrosation products formed within the stomach.
One possible limitation of our study is the potential for measurement error in the self-reported food frequency questionnaire. Because data from food frequency questionnaires are known to be measured with error
[19, 20], logistic regression models were estimated using the SIMEX algorithm. Since there was no “gold standard” available to quantify the amount of measurement error in the data, extra error was considered by adding 0% to 60% additional variance in increments of 10%. There was no evidence that even the highest levels of measurement error made any substantive different in the results so maximum likelihood estimation was used for all subsequent modeling. It is possible that the inability to explicitly quantify the degree of measurement error in the exposure variables could bias the results towards the null. However, any bias due to measurement error should be non-differential because the mothers were not aware of the nitrate, nitrite and nitrosamine content in the foods they consumed when they completed the dietary recall questionnaire.
In this study, we focused on dietary contributions to daily intake of nitrate, nitrite, and nitrosamines, although drinking water is another potential source of nitrate intake. On the other hand, the World Health Organization noted, in a recent review, that the contribution of drinking water to nitrate intake is usually less than 14%