Comparing dietary recall data for mothers and children obtained on two occasions in a case-control study of environmental factors and childhood brain tumours.

BACKGROUND Food frequency questionnaires (FFQ) have been widely used in epidemiological studies of dietary factors, despite persistent questions about the quality of the data they generate. To address concerns about the influence of disease status on dietary recall, we examined the temporal consistency of dietary recall data obtained in a case-control study of environmental factors and childhood brain tumours. METHODS On two occasions separated by approximately 4.5 years, the mothers of children with brain tumours and healthy matched controls were asked about their diets during pregnancy and their children's diets prior to diagnosis. Forty-five case and 60 control mothers completed both FFQ for themselves and/or their children. Daily dietary intakes of substances related to dietary N-nitroso compounds were estimated from the responses to each of the questionnaires and compared. The dietary intakes of the cases were also compared to their matched controls. RESULTS The changes in estimated dietary intakes from the first to the second questionnaire administration were similar in both groups of mothers, indicating negligible case-control differences in healthy individuals recalling their own diets. Control mothers reported for their children a higher intake of the index foods on the second questionnaire than on the first while estimated intakes for the case children were lower the second time compared to the first. Significant case-control differences among the children were seen for the majority of dietary components examined. CONCLUSIONS The results suggest that the magnitude and direction of errors in dietary recall data can vary by disease status, and further suggest that reliance on retrospective reports of children's diets may be inappropriate in a case-control study.