Changes in nutritional status have been traditionally assessed by alterations in body composition. The body's components can be divided into lean body mass and fat mass. Lean body mass is composed of water, potassium, nitrogen, and glycogen. Recent studies have shown that malnutrition and refeeding alter total body potassium earlier than, and differently from, the way in which body nitrogen is altered. These findings suggest that an early effect of nutrition occurs with membrane ion transport. In support of this concept, my colleagues and I, along with other researchers, have shown that nutrition alters muscle performance earlier than it alters body composition. In addition, changes in muscle function correlate with surgical patient outcomes. Loss of body protein alone without altered performance does not correlate with complications in surgical patients.