Comments on 'adjustment for total energy intake in epidemiologic studies'

Willett et al [Am J Clin Nutr 1997;65(suppl): 1220S-8S] reviewed the case for energy adjustment in the analysis of nutritional studies and argued strongly for basing the main analysis on an energy-adjustment statistical model. They recommended focusing attention on a statistical association that represents the change in disease incidence associated with the substitution of energy from a specific nutrient for energy from other nutrient sources, while keeping total energy intake constant. Although we agree with many of the points made in their paper, we recommend assessing and reporting associations representing not only the substitution but also the addition of energy from the specific nutrient. For these 'addition' associations, it is especially important to check for confounding with measures of body size and physical activity. Restricting analyses to substitution associations will confine investigators to estimating the relative effects of one nutrient to another and will preclude investigating the effects of increased intake of a specific nutrient.