Reproducibility and validity of the Diet Quality Index Revised as assessed by use of a food-frequency questionnaire

Background: The Diet Quality Index Revised (DQI-R) is a dietary assessment instrument based on 10 dietary recommendations reflecting dietary guidelines and policy in the United States. Objective: The objective of this study was to assess the reproducibility and validity of the DQI-R as measured by use of food-frequency questionnaires (FFQs). Design: Diet was assessed separately by two FFQs at a 1-y interval and by two 1-wk diet records. DQI-R scores were computed from each method. Venous blood specimens were collected for measurement of dietary biomarkers. Participants (n = 127) were men aged 40-75 y in a validation study of the Health Professionals Follow-up Study. Results: Mean DQI-R scores were 69.5 for FFQ-1, 67.2 for FFQ-2, and 62.0 for the diet records out of a possible score of 100. The reproducibility correlation for the 2 FFQ scores was 0.72. Correlations between scores for each of the 2 FFQs and diet records were 0.66 (FFQ-1) and 0.72 (FFQ-2). DQI-R scores from FFQ-2 were directly correlated with plasma biochemical measurements of α-carotene (r = 0.43, P < 0.0005), β-carotene (r = 0.35, P < 0.005), lutein (r = 0.31, P <0.005), and α-tocopherol (r = 0.25, P < 0.05) and were inversely correlated with plasma total cholesterol (r = -0.22, P < 0.05). Conclusions: These data indicate reasonable reproducibility and validity of the DQI-R as assessed by an FFQ. Future studies are needed to examine whether this index and other instruments of diet quality can reliably predict disease outcomes.