Dietary supplements, quality scores and missing data in the review of validation studies.

As stated in the foreword (1) to articles by Henriquez-Sanchez et al. (2) and others, this collection presents valuable work that identifies quality features of dietary validation studies and supports the use of FFQ in future research. The authors' substantial effort and results are appreciated; however, inconsistencies in classifying studies as including supplements, assigning quality scores and presenting complete validity data may have limited their findings. The paper by Henriquez-Sanchez et al. (2) will be primarily examined, but the comments may also apply to others in the collection. Henriquez-Sanchez et al. (2) report that 23–31 % of validation studies included dietary supplements in their analyses, but there were no large differences in correlation coefficients between studies with or without dietary supplements. This finding is surprising, given reports that the inclusion of supplements improves intake assessment and validity correlations (3,4). Although Henriquez-Sanchez et al. observed that correlations improve when supplement data from both FFQ and reference methods are included, the inconsistent use of this criterion when reviewing the studies may account for the results just mentioned. Individual studies may describe FFQ as listing supplements, but it cannot be assumed that these data were collected in the reference methods or included in the validity analyses, unless explicitly stated by the original authors. Further, it is erroneous to report a validation study as including supplements if these data were not specifically collected in both dietary methods and also used in the analysis. Four studies in the review of Henriquez-Sanchez et al. serve to illustrate this point. Flagg et al. (5) describe their FFQ as including supplements, but do not specify collecting these data in recalls, and even state that they were excluded from validity correlations. Similarly, Jain et al. (6) describe both the FFQ and records as including supplements, but state that these were not included in their analysis. The study that was assigned the highest quality score (7) describes its FFQ as including supplements, but does not explicitly state that these were included in the recalls or analysis. Although none of the three studies describes supplements in their analyses , all were evaluated by Henriquez-Sanchez et al. as including supplements. In contrast, another study reports including supplements in the FFQ, recalls and validity analysis (3) , yet was classified as excluding these data. Inconsistent assignment of study quality scores by Henriquez-Sanchez et al. and by the research group evaluating minerals (8) was …