Food frequency questionnaires require updating over time, due to population changes in diet, posing analytical challenges in consistently measuring diet in prospective studies. We compared reliability and agreement between nutrients in two versions of the National Cancer Institute's web-based Diet History Questionnaire (DHQ, III vs. II) in an ongoing North American preconception study. We invited 51 consecutively-enrolled U.S. female participants aged 21-45 years to complete both DHQ versions within a 2-week period, in a randomized order. We compared 30 nutrients from both DHQ versions and calculated within-person reliability using intraclass correlation coefficients (ICCs). Bland-Altman plots and 95% limits of agreement (LOA) were generated to assess nutrient agreement between DHQ versions. We observed highest reliability in percent energy from carbohydrates and cholesterol (ICCs: 0.88; 95% CI: 0.80-0.93) and lowest for percent energy from protein and vitamin D (ICCs: 0.56; 95% CI: 0.34-0.72). At the group level, all nutrients had most observations within the LOA. Bland-Altman plots showed assessment differences between DHQs for protein, fat, monounsaturated fat, and vitamin D. The remaining nutrients showed good agreement and good-to-moderate reliability. Some nutrients may require adjustment and calibration analysis before using them interchangeably across DHQ versions.
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