Investigation of factors determining the optimal glucose infusion rate in total parenteral nutrition.

We have used the primed constant infusion of U-13C-glucose to study glucose metabolism during conventional total parenteral nutrition (TPN) in five postoperative surgical patients. Glucose production from nonrecycled carbon sources was suppressed to 17% of the basal level at the lowest rate of glucose infusion tested (4 mg/kg x min). Subsequent increases in glucose infusion rate had minimal effect in further suppressing glucose production. Additional nitrogen-sparing effects of glucose when glucose is infused at rates in excess of 4 mg/kg x min must therefore be derived from oxidation of the infused glucose. An increase in the infusion rate from 4 mg/kg x min to 7 mg/kg x min was associated with an increased rate of glucose oxidation, but a further increase in glucose infusion rate (9 mg/kg x min) was without significant effect on glucose oxidation. As the rate of TPN administration (glucose and amino acids) increased, both metabolic rate and RQ rose significantly. Our calculations indicate that the high RQ's observed during the highest glucose infusion rate (X = 1.13) could be attributed to the synthesis of fat from infused glucose, and that about 30% of the increase in VO2 above the basal level could also be attributed to fat synthesis. The progressive increase in the ability to clear glucose from the blood that occurred as TPN progressed was not due to an increase in the rate of oxidation of glucose; we found no correlation between glucose clearance and glucose oxidation.