To assess the accuracy of three self-administered food frequency questionnaires (FFQs) to measure dietary calcium intake in healthy adults.
Design
Estimates of dietary calcium intake from one previously validated and two recently developed FFQs were compared with those from 7-day food records.
Subjects/setting
Healthy adults enrolled in an outpatient study of calcium supplementation completed the 36-page Dietary History Questionnaire (DHQ), a 3-page Calcium Questionnaire, and a 1-page Short Calcium Questionnaire. Subjects then completed a 7-day food record.
Main outcome measures
Differences between calcium intake reported on FFQs and calcium intake from food records were compared.
Statistical analyses
Spearman correlations were used to measure associations among variables; Bland-Altman pairwise comparisons were conducted to assess systematic and magnitude biases.
Results
We studied 341 subjects, 74.5 % female, mean (±standard deviation) age of 38±11 years and body mass index (calculated as kg/m2) of 31.8±7.1. Mean (±standard deviation) food record calcium intake was 896±380 mg/day; data from all three FFQs were positively related to food record calcium intake, but accounted for <40 % of the variance in food record dietary calcium intake (DHQ: r2=0.21; Calcium Questionnaire: r2=0.33; Short Calcium Questionnaire: r2=0.37; all P<0.001). The DHQ underestimated daily calcium intake (systematic bias: −94 mg/day, P<0.001; magnitude bias r=−0.40; P<0.001), whereas the Calcium Questionnaire overestimated calcium intake (systematic bias +177 mg/day, P<0.001), but had no significant magnitude bias (r=−0.09;P=0.11). The Short Calcium Questionnaire showed minimal systematic bias (+34 mg/day, P=0.09), but had magnitude bias (r=−0.33; P<0.001).
Conclusions
All three FFQs performed reasonably well at estimating dietary calcium intake compared to food records; each may be appropriate for use in select clinical and research settings.
