One of the subjects that has aroused great attention in hospital nutritional circles since the early 1970s is the relationship between energy and nitrogen metabolism. An enormous amount of confusion has arisen, largely through failure to differentiate between the metabolic changes of starvation and those of injury.Emphasis has largely been placed on N balance, either in terms of making negative balance smaller or achieving positive balance, and the importance of energy supply in its own right has been neglected until recently. This paper will attempt to give a brief overview of amino acid metabolism as a general background and then describe developments that have occurred over the last decade in the application of knowledge in this area. The relationship between energy intake, N intake and N balance in various clinical circumstances will also be described.