Development and validation of a food frequency questionnaire for assessing dietary calcium intake in the general population

The aim of this study was to develop and evaluate a food frequency questionnaire (FFQ) for assessing dietary calcium intake in the general population, since all available questionnaires at present are age-and/or gender-specific. A total of 1001 individuals (including children, adults, and elderly people of both genders) were randomly recruited throughout Greece. Estimates of calcium intake from the 30-item FFQ were compared with those from a multi-pass 24-h recall. The FFQ underestimated mean calcium intake compared to the 24-h recall by (mean ± SD) -133 ± 333 mg/day or -5.4 ± 47.6% (P < 0.001). The two methods were strongly correlated (r = 0.639, P < 0.001), but the 95% limits of agreement for individual assessment were rather wide, as the FFQ could provide estimates of calcium intake from 533 mg/day above to 799 mg/day below the 24-h recall. Actual values for surrogate FFQ quartiles manifested a progressive increase, with significant differences between mean calcium intakes (P < 0.001). The FFQ could identify individuals who consumed less calcium than 800 mg/day or less than the age-specific adequate intake with a relatively high sensitivity (82.8 and 95.5%, respectively), but low specificity (54.9 and 34.1%, respectively). Cross-classification analysis indicated that only 17 subjects (1.7%) were grossly misclassified (lowest quartile for one method and highest quartile for the other), while 827 subjects (82.6%) were correctly classified (into the same or adjacent quartiles). The FFQ could be used in population-based epidemiological studies or screening programs involving individuals of all ages and both genders, where the discrimination of subjects with relatively low (< 500 mg/day) and relatively high (> 1000 mg/day) calcium intakes is of primary interest. Results, however, do not support its use for the quantitative assessment of individual calcium intakes.