Metabolism and nutritional adaptation to altered intakes of energy substrates

Adaptive changes in energy expenditure to changes in energy intake are far less than previously believed once changes in body weight and physical activity are taken into account. Alterations in metabolic efficiency of +/- 10% in total energy expenditure are the limits of what can normally be expected on prolonged under- or overfeeding. The supposed twofold variation in energy requirements of adults is based on measurements of food intake, which, despite involving weighted intakes over a week, cannot be considered realistic. Subtle differences in the energetic responses to fat, protein, and carbohydrate signify the different effective energetic contribution of isoenergetic substrates and may account for the propensity to obesity in individuals and societies on a high-fat diet.