A twin analysis of dietary intake: evidence for a need to control for possible environmental differences in MZ and DZ twins.

In the NHLI Twin Study MZ twins are shown to "get together" substantially more often than DZ twins. With this result as an indicator of differences in shared environment, the assumption of equal shared environmental variation for MZ and DZ twins is assessed using nutritional data calculated from a food frequency questionnaire. Six nutrients show significant genetic variance for the total sample. However, when stratified on the basis of how frequently twins see each other, none of the nutrients shows significant genetic variance for both strata. A similar pattern is seen for several individual items from the questionnaire. In addition, four of the nutrients show significant correlation between the absolute difference in the nutrient intake of MZ twin pairs and how often they "get together." These data appear to show that unequal environmental effects may lead to falsely high estimates of genetic variance for nutrient intake.