Reproducibility and validity of an FFQ to assess usual intake of methyl-group donors

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 (kappa(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; kappa(w)=0.10-0.35), but overestimated the daily intake of folate (280.6 g v. 212.0 g) 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 (kappa(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 methyl-group donor intake (except for a poor agreement for choline (kappa(w)=0.10) and a fair ranking ability for betaine (r=0.32)) in Flemish women of reproductive age.

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