Reproducibility and biomarker-based validity and responsiveness of a food frequency questionnaire to estimate protein intake

This study, conducted in the Netherlands in 1994-1996, assessed the reprodu cibility, validity, and responsiveness of a food frequency questionnaire (F FQ) used with older subjects with type 2 diabetes. During a period of 6 mon ths, 93 subjects had a stable protein intake according to their urinary ure a excretion (UUE). An FFQ was completed before and after this period. Repro ducibility was evaluated on the basis of the correlation (0.80) between dup licate FFQ estimates of protein intake. At the group level, the smallest de tectable difference was 3 g. For 354 subjects, average protein intake accor ding to the FFQ was similar to that from UUE estimates. Validity was evalua ted on the basis of the crude and deattenuated correlations, which were 0.3 1 and 0.39, respectively. The crude correlation was highest for former smok ers (r = 0.39) and for patients with a higher body mass index (r = 0.37), a nd it was very low for current smokers (r = 0.14, p > 0.20). The correlatio n did not vary with age or gender. Responsiveness was judged on the crude c orrelation between changes in intake according to the FFQ and UUE; the corr elation was 0.22 (n = 134). The responsiveness ratio was 0.73. The authors concluded that FFQ estimates are well reproducible. At the group level, pro tein intake was estimated correctly, whereas individual protein intake and changes in intake were reflected to a moderate extent only.