The effect of standard versus energy dense feeds on gastric emptying and glucose metabolism in critically ill patients

Patients will be studied on two consecutive days, in which they will receive either standard feed (1 kcal/ml) or energy dense feed (2 kcal/ml), in a randomised, double blind fashion. On each day, enteral feeding will be ceased four hours prior to the commencement of the study. Blood glucose levels will be managed according to the Royal Adelaide Hospital Intensive Care Unit protocol i.e. <10 mmol/l. However, due to patients not receiving feeds following the test meal, exogenous insulin infusions will be ceased prior to the commencement of the test meal. Computer randomisation will be performed by a nuclear medicine technician, who will measure out the study feed and add the radioactive marker. This nuclear medicine technician will not be involved in the conduct of the study or interpretation of the results and will instill the study feed to ensure allocation concealment is maintained. At t = -5, an intragastric ‘meal’ will be infused over 5 minutes. The meal will contain 200 ml of ‘Standard’ feed (Nutrison 1kcal/ml, Nutricia Australia, containing carbohydrate 49%, fat 35%, and protein 16%, osmolality 305 mOsm/kg H2O) or 100 ml of ‘Energy dense’ feed (TwoCal 2kcal/ml, Abbott Nutrition, containing carbohydrate 43%, fat 40%, and protein 17%, osmolality 690 mOsm/kg H2O). The order in which standard or energy dense feeds are given will depend on randomisation. The meal will also contain 3 g of 3-O-Methyl-D-gluco-pyranose (3-OMG) (Sigma-Aldrich) dissolved in 5 ml H2O and 20MBq 99mTc-calcium phytate colloid. Radioisotopic data will be acquired in dynamic mode every minute for the first 60 minutes and every 3 minutes thereafter for a total of 240 minutes using a gamma camera (DigiRad 2020tc, Gammasonics) with the patient lying in the semi-recumbent position. An arterial blood sample will be obtained every 15 minutes for the first hour and every 30 minutes thereafter for measurement of blood glucose concentration, plasma GLP-1, CCK, PYY and Glucagon, and serum Insulin, C-peptide and 3-OMG. Enteral nutrition will be commenced immediately on study completion (4 hours following meal administration).