DEVELOPMENT OF AN APPROACH FOR ESTIMATING USUAL NUTRIENT INTAKE DISTRIBUTIONS AT THE POPULATION-LEVEL

Assessment of the dietary intake of a population must consider the lar ge within-person variation in daily intakes. A 1986 report by the Nati onal Academy of Sciences (NAS), commissioned by the U.S. Department of Agriculture (USDA), marked an important milestone in the history of t his issue. Since that time, USDA has been working cooperatively with s tatisticians at Iowa State University (ISU), who have further develope d the measurement error model approach proposed by NAS. The method dev eloped by the ISU statisticians Can be used to estimate usual dietary intake distributions for a population but not for specific individuals . It is based on the assumption that an individual can more accurately recall and describe the foods eaten yesterday than foods eaten at an earlier time. The method requires as few as two independent days of nu trient intake information or three consecutive days for at least a sub sample of the individuals. It removes biases of subsequent reporting d ays compared with the first day, and temporal effects such as day-of-t he-week and seasonal effects can be easily removed. The method develop ed at ISU is described conceptually and applied to data collected in t he 1989-91 USDA Continuing Survey of Food Intakes by Individuals to es timate the proportion of men and women age 20 y and older having ''usu al'' (long-run average) intakes below 30% of energy from fat, below th e 1989 Recommended Dietary Allowances for vitamin A and folate, and ab ove 1000 mu g for folate. These results were compared with the results from the distributions of 1-d intakes and of 3-d mean intakes to demo nstrate the effect of within-person variation and asymmetry on usual n utrient intakes in a population.