Objective: To examine putative differences in the quantitative and qualitative performance of a food frequency questionnaire (FFQ) for assessing dietary calcium intake across age and sex in the Greek population.

Materials and methods: A total of 351 children (189 girls and 162 boys, aged 11.9 +/- 1.2 years), 260 adults (192 women and 68 men, aged 29.6 +/- 2.7 years) and 390 elderly individuals (317 women and 73 men, aged 68.6 +/- 4.6 years) were recruited. Estimates of calcium intake from the 30-item FFQ were compared with those from a multi-pass 24-h recall.

Results: The FFQ significantly underestimated mean calcium intake in all age groups and both sexes (P < 0.05). The magnitude of underestimation, however, was greater in adults (-207 +/- 344 mg day^-1), less in the elderly (-137 +/- 310 mg day^-1) and even less in children (-74 +/- 340 mg day^-1; P < 0.025), with no differences between sexes. Calcium intakes by the two methods were positively and significantly correlated in all study groups (r = 0.536-0.739, P < 0.001). Cohen's weighted kappa statistic ranged from 0.39 to 0.57, indicating moderate agreement between the two methods. The 95% limits of agreement were comparably wide across age and sex (boys: -762, 585 mg day^-1; girls: -747, 624 mg day^-1; adult men: -972, 505 mg day^-1; adult women: -867, 412 mg day^-1; elderly men: -858, 486 mg day^-1; elderly women: -732, 480 mg day^-1). A significant association between age, sex and the classification of individuals as true/false positive/negative was detected (P < 0.001), implying that sensitivity, specificity, positive and negative predictive values of the FFQ were not independent of the age and sex of the participants. Gross misclassification by the FFQ ranged from 0% to 4.2%, whereas 75.3-87.3% of the subjects were correctly classified. In this respect, the FFQ performed similarly across the study groups (P = 0.065). Without controlling for age, however, gross misclassification appeared to be higher in females than in males (3.2% versus 0.7%, respectively, P = 0.048).

Conclusions: There may be several significant differences in the quantitative and qualitative performance of a calcium-specific FFQ across age and sex. This should be taken into account when attempting to evaluate dietary calcium intake in men and women or in different age groups, as some of the differences between study groups may actually be due to the different response of these groups to the FFQ.