Partitioning macronutrient intake estimates from a food frequency questionnaire

In the 1984-1985 University of Michigan Food Frequency Study, 228 black and white men and women ages 24-51 years completed 16 days of food records over the course of a year and then completed a quantitative food frequency questionnaire. The relative validity of macronutrient (fat, protein, and carbohydrate) estimates from the questionnaire was evaluated with the records used as a standard. The two methods differed both in estimated group mean intake and in the relative ranking of individuals. Partitioning of between-method disagreement showed that frequency differences were the main source of poor agreement in relative ranking; serving size differences contributed mainly to disagreement in group mean intake; and differences in nutrient composition had only minor effects. After adjusting for total calorie intake, agreement in group mean intake improved. Agreement in relative ranking and classification also improved, but substantial disagreement remained. The strategy of adjusting for total calorie intake depends on the assumption that macronutrients are reported in similar proportions on both instruments. This assumption did not appear to hold true. Respondents showed considerable variability in the proportionality of reporting, with 46% of respondents reporting fat intake disproportionately. A summary index of proportionality is described and the relation between disproportionate reporting and misclassification is discussed. Disproportionate reporting of macronutrients limits the ability of food frequency questionnaires to rank individuals similarly to a reference method even if estimates are adjusted for total calorie intake.