Validation of a food frequency questionnaire for determining calcium and vitamin D intake by adolescent girls with anorexia nervosa.

Assessing calcium and vitamin D intake becomes important in conditions associated with low bone density such as anorexia nervosa. Food records that assess intake over a representative time period are used in research and sometimes clinical settings. However, compliance in adolescents can be suboptimal. This study was undertaken to determine the validity of a food frequency questionnaire (FFQ) for assessing calcium and vitamin D intake in adolescent girls with anorexia nervosa and healthy girls compared to validated food records assessing intake during a 4-day period, the hypothesis being that intake would be adequately predicted by the FFQ. Thirty-six girls with anorexia nervosa and 39 healthy girls aged 12 to 18 years completed both the food record and the FFQ. An additional 31 subjects (20 with anorexia nervosa, 11 controls) completed the FFQ, but not the food record, and one girl with anorexia nervosa completed the food record, but not the FFQ. Subjects demonstrated greater compliance with the FFQ (99%) than the food record (71%). Daily calcium and vitamin D intake calculated using the food record and FFQ did not differ, although the FFQ tended to under-report vitamin D intake corrected for energy intake. Using quartile analysis, no gross misclassification was noted of calcium or vitamin D intake calculated using the food record or FFQ in girls with anorexia nervosa. Strong correlations were observed for daily vitamin D intake derived from the FFQ vs the food record, particularly in girls with anorexia nervosa (r=0.78, P<0.0001). Less robust correlations were observed for calcium intake (r=0.65, P<0.0001). The FFQ used in this study can be effectively used to assess daily calcium and vitamin D intake in adolescent girls with anorexia nervosa.