The aim of the study was to assess validity and reproducibility of a self-administered quantitative food frequency questionnaire (FFQ) designed to assess calcium, protein and phosphorus intakes, and to be used in a semilongitudinal study on bone mineralisation in children and adolescents. Validation study: 23 children aged 13-14 years filled in the questionnaire and kept 3-day weighed food records. Reproducibility study: 293 children aged 5-19 years completed the questionnaire twice with a half year interval. Spearman rank correlations between nutrient intake values from the questionnaire and weighed food records were 0.56-0.62 (mean 0.60). There were no significant differences in intake estimated by FFQ and weighed record for either calcium (p=0.07) or protein (p=0.61), while phosphorus intake estimated by FFQ was 10% higher than phosphorus intake estimated by weighed record (p=0.04). On average regarding calcium, protein, and phosphorus intake, 43% of the subjects were classified in the same quartile in the questionnaire and the record, and 1% in the opposite quartile. Spearman rank correlation between nutrient intake from the two questionnaires was 0.72-0.73, with no significant difference between the measurements. It is concluded that this questionnaire is able to rank children according to their intakes of calcium, protein, and phosphorus and to give a reasonable estimate of the quantity of the nutrients in groups of children.
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Mølgaard et al. (1998) studied this question.