Validity and reproducibility of a dietary history method estimating the usual food intake during one month.

The validity and reproducibility of a dietary history method with a time of reference of one month was assessed with 44 young adults (aged 19-32 yr). The concurrent validity of the method was assessed by means of the 24-h urine nitrogen excretion. The mean difference between N-intake and N-excretion (24-h urine N-excretion plus 2 g for extra renal nitrogen losses) was 0.0 g with 95%-confidence limits of +/- 1.1 g. These limits for the mean difference between excretion and intake indicate a valid assessment of the protein intake of this group. The reproducibility was evaluated in the same group through a test-retest design. The intraclass correlation coefficients were high over a weighted average of weekdays and for an average workday with regard to the intakes of energy and selected nutrients. For the Saturday and Sunday intakes, the intraclass correlation coefficients were lower for the energy intakes and most of the nutrients (except alcohol), indicating a poorer reproducibility for weekend assessments.