[Adjustment for energy intake in the assessment of dietary risk factors]

Epidemiologic studies assessing the association between health status and nutritional factors raise the issue of adjusting for energy intake. Indeed, as most nutrients are highly correlated with energy intake which can itself be associated with disease risk, energy intake needs to be adjusted for upon assessing the effect of a specific nutrient. To avoid problems of estimation and interpretation incurred by the use of the standard method which rests on directly adjusting for energy intake, several other methods have been suggested. Namely, the density method uses the ratio of nutrient intake over total energy intake, the residual method relies on the residuals from the regression of nutrient intake on total energy intake, and the partition method fits energy from the nutrient and energy from other sources. These methods yield estimates of different effects but do not allow direct estimation of specific nutrient effects. Estimated effects combine specific and generic energy effects of nutrients and reflect effects of adding or substituting one nutrient for another. We review and apply these methods to the assessment of the association between protein intake and colorectal adenoma occurrence in the E3N-EPIC cohort. This example illustrates how considering findings from all of these methods rather than one single method can lead to a more in-depth understanding of such associations and provide useful guidance for nutritional recommendations.