The relationship between physical activity, nutritional requirements and protein-energy metabolism is well-established. However, and contrary to bedside practice at home, non-ambulatory techniques of enteral or parenteral nutrition are often used in hospitalized patients, even those who are able to move about. The purpose of the present study is: (a) to review the relationship, as we know it at present, between physical activity and protein metabolism in both normal subjects and patients with denutrition, and (b) to demonstrate that using from the start, or as early as possible, ambulatory techniques of enteral or parenteral nutrition does represent, both physiologically and psychologically, a decisive step forward in the care of hospital patients.