Indexes for Assessing Adherence to a Mediterranean Diet from Data Measured through Brief Questionnaires: Issues Raised from the Analysis of a Greek Population Study

Background: Adherence to a Mediterranean diet (MD) has been quantified through various indexes that rely on full-length questionnaires, but their application in data collected with brief questionnaires has not been systematically investigated. Objective: We aimed to evaluate the ability of the commonly used MD score (MDS) to classify individuals according to their adherence to an MD when applied to data collected with brief questionnaires. Methods: We assessed the diet of 200 participants from a Greek national health survey with the use of 2 instruments: 1) a validated, detailed food-frequency questionnaire (FFQ) estimating grams per day of intake of individual foods and 2) a selection of 19 questions from the Baseline Nutrition Credits4Health (BNC4H) electronic platform questionnaire that assessed servings with subjective serving sizes. We calculated the MDS_FFQ (referent) and MDS_BNC4H indexes from each questionnaire and estimated their correlation and the percentage of study participants who were ranked in an identical tertile in both indexes. We repeated the analyses for 2 additional indexes defined with criteria different from the MDS [Mediterranean Diet Index (MDI)_BNC4H and Mediterranean Diet Assessment Score (MEDAS)_ BNC4H]. Results: Spearman correlation coefficients for the MDS_FFQ were 0.31 with the MDS_BNC4H, 0.24 with the MDI_BNC4H, and 0.23 with the MEDAS_BNC4H. The proportion of participants ranked into the same adherence level as the referent MDS_FFQ was 50% for the MDS_BNC4H (weighted κ = 0.27) and lower for the other indexes. The use of medians as cutoffs (as in the MDS_FFQ) had low discriminative ability when applied to servings per day (MDS_BNC4H) in some MDS components, leading to comparability problems in the range of values between the 2 indexes. Conclusions: Our findings highlight certain issues that need to be considered when applying pre-existing MD indexes in settings with different dietary assessments. Given the widespread use of electronic platforms for dietary assessment, our results may offer further insight into designing brief, simplified questionnaires that aim to estimate MD adherence with easily quantifiable scores.