The reliability and relative validity of predefined dietary patterns was higher than of exploratory dietary patterns in the EPIC-Potsdam population.

The aim of this study was to assess the ability of the food frequency questionnaire (FFQ) to describe reliable and valid dietary pattern scores (DPs). In a total of 134 participants of the EPIC-Potsdam study aged 35-67 years, the FFQ was applied twice (baseline and after one year) to assess its reliability. Within this time (November 1995 - March 1997), twelve 24-hour dietary recalls (24HDRs) as reference instrument were applied to assess the validity of the FFQ. Exploratory DPs were derived by principal component analyses. Investigated predefined DPs were the Alternative Healthy Eating Index (AHEI) and two Mediterranean diet indices. From dietary data of each FFQ, two exploratory DPs were retained, but differed in highly loading food groups, resulting in moderate correlations (r = 0.45 - 0.58). The predefined indices showed slightly higher correlations between the FFQs (r(AHEI) = 0.62, r(MedPyr) = 0.62, r(tMDS) = 0.51). From 24HDRs dietary data, one exploratory DP was retained, which differed in composition to the first FFQ-based DP, but showed similarities to the second DP, reflected by a good correlation (r = 0.70). The predefined DPs correlated moderately (r = 0.40 - 0.60). To conclude, long-term analyses on exploratory DPs should be interpreted with caution, due to only moderate reliability. The validity differed extensively for the two exploratory DPs. The investigated predefined DPs showed a better reliability and a moderate validity, comparable to other studies. Within the two Mediterranean diet indices, the MedPyr performed better than the tMDs in this middle-aged, semi-urban German study population.