The term malnutrition is a continuum that progresses from only a disequilibrium of intake in relation to needs, to gross structural and functional changes. Furthermore, these changes may be mutually exclusive. How then do we define malnutrition? Strictly, malnutrition starts, as we have shown, when the patient fails to eat enough to meet needs and progresses through a series of functional changes that precede any changes in body composition. These functional changes are related to the duration of reduced intake and its severity. Finally, body wasting occurs, and biochemical indices become abnormal. To base the definition of malnutrition on any one of these changes is inappropriate. Thus, a patient's nutritional status should be defined by the following criteria: nutrient intake in relation to needs; associated changes in organ function; associated changes in body composition; and associated changes in biochemistry. Only by recognizing the different facets of malnutrition can we define its various manifestations in relation to our clinical objectives. In attempting to achieve this goal, we have found muscle function tests a most useful and objective investigational tool.