Protein‐Sparing Effect of Substrate Infusion in Surgical Patients is Governed by the Clinical State, and Not by the Individual Substrate Infused

We have determined iostopically the effect of either glucose or lipid infusion on rates of glucose turnover, glucose oxidation, and net protein catabolism (NPC) in three groups of surgical patients. Kinetic measurements were performed using the primed constant infusion of 3 H‐glucose, and either 14 C‐glucose or “C‐urea. The three groups included patients with: (1) sepsis and/or trauma (ST); (2) upper gastrointestinal cancer (UGI); (3) lower gastrointestinal cancer (LGI). In each patient group the effect of either glucose infusion (approximately 4 mg/kg·min) or lipid infusion (20% Intralipid lipid emulsion infused to provide calories approximately iso‐caloric to the glucose infusion) was assessed. The infusion of calories as either glucose or lipid was equally effective as a means of suppressing NPC in each individual patient group, and the degree of response was governed by the clinical disease state. In the LGI and ST patients the infusion of either glucose or fat resulted in a significant suppression of NPC (p < 0.005) of approximately 15%. However, the ongoing rate of NPC that occurred despite substrate infusion was more than twice as great in the ST patients as in the LGI patients. In contrast to what was seen in the ST and LGI patients, in the UGI patients neither glucose infusion nor lipid infusion significantly decreased NPC. Glucose infusion resulted in at least a 55% suppression of endogenous glucose production in all three groups studied. In contrast, lipid infusion had only a minor effect on glucose metabolism: lipid infusion did not significantly decrease the rate of glucose oxidation in any of the patient groups, and although lipid infusion was associated with a decrease in glucose production in all groups, this effect was minor (<9% suppression). The major hormonal response following glucose infusion was a significant increase in the plasma insulin level (plasma cortisol levels did not change), although in contrast lipid infusion did not significantly alter the plasma insulin level, but serum cortisol decreased significantly. We conclude from these studies the following: (1) “calorie for calorie,” glucose and lipid are equally effective in decreasing NPC in severely ill patients; (2) following glucose infusion, the plasma insulin level increased, and serum cortisol levels did not change, whereas in response to lipid infusion plasma insulin levels did not change and the plasma cortisol level decreased. (3) In contrast to the profound effect of glucose infusion to suppress free fatty acid appearance and utilization, lipid infusion has a minor inhibitory effect on both glucose production and glucose oxidation. ( Journal of Parenteral and Enteral Nutrition 12: 433–440, 1988)