Calculation of energy and protein doses for critically ill patients is still a matter of controversy. For more than 40 years of nutrition support, the total amount of nutrients to be delivered to these patients has been calculated based on expert recommendations, and this calculation is based on the administration of nonprotein calories in one attempt to ameliorate catabolic response and avoid the weight loss. New evidence suggests protein delivery is the most important intervention to improve clinical and metabolic outcomes. This article describes the metabolic rationale and the new evidence supporting a change in the approach of metabolic support of the critically ill, proposing a physiological-based intervention supported by the recognition of ancillary characteristics of the metabolic response to trauma and injury. A moderate dose of calories around 15 kcal/kg/d with a delivery of protein of 1.5 g/kg/d appears to be the new recommendation for many hypercatabolic patients in the first week following injury.