This background paper considers the extent to which the development of new recommendations for dietary energy requirements needs to account for the macronutrient (fat, carbohydrate, protein and alcohol) profiles of different diets. The issues are discussed from the dual perspectives of avoiding under-nutrition and obesity. It is shown that, in practice, human metabolic processes can adapt to a wide range of fuel supply by altering fuel selection. It is concluded that, at the metabolic level, only diets with the most extreme macronutrient composition would have any consequences by exceeding the natural ability to modify fuel selection. However, diets of different macronutrient composition and energy density can have profound implications for innate appetite regulation and hence overall energy consumption.