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

Abstract

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 methyl-group 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.