Validation of a food frequency questionnaire for assessment of calcium and bone-related nutrient intake in rural populations

Objective To assess the ability of a semiquantitative food frequency questionnaire (FFQ) to measure calcium and bone-related nutrient intakes in a rural South Dakota population. Design Intake estimates from FFQ were compared with four 24-hour recalls obtained quarterly during the preceding year. Subjects Convenience sample of 100 participants of the South Dakota Rural Bone Health Study were recruited, with 81 completing the FFQ. Main outcome measures Calcium and bone-related nutrient intakes were expressed as milligrams per day, milligrams per 1,000 kcal, or quartiles. Statistical analyses performed Intakes by FFQ and 24-hour recalls were compared using paired t test and quartiles were formed to examine cross-classification. Results Calcium intakes from FFQ and recalls were 1,287 and 1,141 mg/day (P=0.01), but calcium per 1,000 kcal did not differ. Calcium intake by FFQ correlated with intake by recall when expressed as milligrams per day (r=0.49 P<0.001) or milligrams per 1,000 kcal (r=0.59, P<0.001). Bland-Altman graphs indicated fairly good agreement between methods. Seventy-eight percent of subjects fell into the same or within one quartile category when calcium intake was expressed as milligrams per day and 83% when expressed as milligrams per 1,000 kcal. Gross misclassification occurred in 0% to 4% of the nutrients. Conclusions Although FFQ may not be a valid indicator of an individual's intake, it does adequately classify rural populations into quartiles of calcium and bone-related nutrient intakes, making it a useful tool for assessing dietary calcium and bone related intake in rural populations.