Impact of non-dietary nutrients intake on misclassification in the estimation of nutrient intake in epidemiologic study.

BACKGROUND Few previous epidemiologic studies have evaluated the effects of non-dietary nutrient intake, such as supplements, over the counter (OTC) drugs, and prescription drugs containing vitamins or minerals, in examining the relationship between dietary factors and health outcomes. METHODS To examine the influence of the non-dietary intake of vitamins and calcium on the estimation of nutrient intake, we conducted a cross-sectional study with 1,168 community-dwelling Japanese subjects aged 70 years or older in 2002. The subjects were asked to bring their non-dietary nutrient sources to the examining site. The dietary and non-dietary intakes of vitamins B1, C, E and calcium were obtained and the subjects were grouped into quartiles according to their dietary intake and their dietary plus non-dietary intake. The degree of agreement between these two classifications was examined to estimate the degree of misclassification. RESULTS Among the subjects who were classified into the highest intake category for vitamin E with dietary intake plus non-dietary nutrient intake, 34.2 % were misclassified into lower category with dietary intake alone. Similarly, intake of vitamin B(1), vitamin C and calcium were misclassified 28.8%, 18.8 %, 6.2 %, respectively. CONCLUSIONS Our data suggest that estimation of vitamin intake from dietary sources alone would yield a maximum misclassification of one-third, which would lead to misleading conclusions being drawn from epidemiologic studies. In contrast, the degree of misclassification for calcium may be relatively small.