In epidemiologic studies, total energy intake is often related to disease risk because
of associations between physical activity or body size and the probability of disease.
In theory, differences in disease incidence may also be related to metabolic efficiency
and therefore to total energy intake. Because intakes of most specific nutrients,
particularly macronutrients, are correlated with total energy intake, they may be
noncausally associated with disease as a result of confounding by total energy intake.
In addition, extraneous variation in nutrient intake resulting from variation in total
energy intake that is unrelated to disease risk may weaken associations. Furthermore,
individuals or populations must alter their intake of specific nutrients primarily
by altering the composition of their diets rather than by changing their total energy
intake, unless physical activity or body weight are changed substantially. Thus, adjustment
for total energy intake is usually appropriate in epidemiologic studies to control
for confounding, reduce extraneous variation, and predict the effect of dietary interventions.
Failure to account for total energy intake can obscure associations between nutrient
intakes and disease risk or even reverse the direction of association. Several disease-risk
models and formulations of these models are available to account for energy intake
in epidemiologic analyses, including adjustment of nutrient intakes for total energy
intake by regression analysis and addition of total energy to a model with the nutrient
density (nutrient divided by energy).