Objective: To develop and validate a novel FFQ to assess the daily intake of four methyl-group donors (methionine, choline, betaine and folate). Design: The relative validity of the FFQ was assessed by comparison with 7 d estimated diet records (n 80) and its reproducibility was evaluated by repeated administrations 6 weeks apart (n 92). Paired Student t tests were used to compare group means and de-attenuated intra-class correlations to investigate the ability of the FFQ to rank individuals according to their methyl-group donor intake. De-attenuated intra-class correlation coefficients were calculated between the test and reference method for methionine, choline, betaine, folate and the sum of methyl-group donors. The weighted kappa (κ w ) was calculated as a measure of tertile agreement. Setting: Flanders, Belgium. Subjects: The FFQ was validated among Flemish women of reproductive age (18-35 years). Results: The questionnaire had an acceptable ranking ability (r = 0·32-0·68; κ w = 0·10-0·35), but overestimated the daily intake of folate (280·6 μg v. 212·0 μg) and betaine (179·1 mg v. 147·0 mg) compared with the 7 d estimated diet record. Cross-classification analysis indicated that 20 % (choline) of the participants were grossly misclassified in the validation study. The correlation between repeated administrations was good (r = 0·62-0·83) with a maximal misclassification of 7 % for betaine (κ w = 0·44-0·66). Conclusions: These results indicate that this newly developed FFQ is a reliable instrument with acceptable validity for ranking individuals according to methylgroup donor intake (except for a poor agreement for choline (κ w = 0·10) and a fair ranking ability for betaine (r = 0·32)) in Flemish women of reproductive age.

In this study, the intake of methyl-group donors (methionine, folate, betaine, and choline) among Flemish women of reproductive age (n = 30) assessed by a 7-d estimated diet record (7 d EDR) and food-frequency questionnaire (FFQ) was compared with plasma S-adenosylmethionine (SAM), S-adenosylhomocysteine (SAH), and SAH:SAM ratio. Pearson correlation coefficients were calculated between each of the dietary methods and the validity coefficient was calculated using the method of triads. Correlations were higher between intake assessed by the FFQ and biomarkers than between 7 d EDR and biomarkers. The validity coefficients of the FFQ, when using SAH as a biomarker, were high (0.86 for methionine to 0.94 for folate), when the SAH:SAM ratio was used as a biomarker the validity coefficients ranged from 0.63 to 1.00. These data indicate that the FFQ is a reliable tool to estimate the intake of the methyl-group donors in women of reproductive age.