Collection of dietary-supplement data and implications for analysis.

The role of nutrient intake in disease outcome is often examined in epidemiologic studies. Most such studies conducted in the United States, however, have not included fortified foods or vitamin supplements. In the United States, these are important sources of vitamins C and E, but not of beta-carotene. In addition, the importance of these nutrient sources varies by race, sex, and age. Failure to include these sources produces errors in nutrient estimates, notable misclassification of individuals with regard to their total intake, and rankings of intake that bear little or no relationship to blood concentrations of those nutrients. Implications for statistical analysis are also considered. Risk analyses in which nutrients from supplements are handled as control variables or are analyzed separately may impair the ability to detect associations between total nutrient intake and disease risk. The additional source of misclassification of nutrient status for vitamins C and E and other nutrients derived from fortified foods or supplements would make it more difficult to obtain significant and consistent results in etiologic studies of these nutrients; this has not been a factor for beta-carotene, which is derived almost exclusively from fruits and vegetables.