Overview of biochemical markers used for nutrition support.

Altered metabolism has been shown to exist in the settings of surgical stress, cancer, cirrhosis, sepsis, and trauma. Each condition is characterized by varying degrees of alteration in metabolic processes, and within a given patient, these metabolic alterations will change as the patient's status changes. Nutrition support is an integral part of the metabolic management of critically ill patients. Metabolic changes impact nutritional substrate requirements and utilization. As the patient's clinical condition deteriorates, clinical signs and symptoms become less reliable in predicting or assessing the existing physiologic state. Objective measurements are needed to define the metabolic status during these physiologic changes. The purpose of this article is to review selected indices that have been used to identify abnormalities in nutritional substrate metabolism. Although some of these tests are readily available and inexpensive, many have not been used outside of the research setting and, therefore, their clinical utility has yet to be determined. However, their use as research tools for defining metabolism warrants their inclusion in order to assist the clinician in interpreting research studies. The biochemical markers discussed include glucose, lactate, pyruvate, triglycerides, beta-hydroxybutyrate, acetoacetate, urinary nitrogen, acute phase proteins, visceral proteins, 3-methylhistidine, plasma amino acids, oxygen consumption, and resting energy expenditure. Each marker is defined in terms of its biochemical significance, and the literature describing changes that occur in various stress states is cited.