Tumor cell metabolism and the whole-body response to tumor metabolic activity produces a complex derangement, including alterations in carbohydrate oxidation with futile cycling and changes in gluconeogenesis and lipid utilization. An interplay between circulating hormones, lymphocyte-mediated cytokines, and tumor growth factors complicates fuel metabolism in these individuals. Adequate attention to these concepts has not been provided in the studies evaluating the effects of parenteral nutrition in cancer patients that have been performed to date. Therefore, it is impossible to determine the validity of the results. Furthermore, it is possible that some of the detrimental effects of TPN observed in earlier studies were secondary to the inappropriate use of large glucose loads in cancer patients.