Is glycine absorbed in critical illness and can it prevent muscle loss?

Intervention 1: single test meal with supplemented glycine - performed after a 2 hour period without continuous feeding - group 1: 0.0667 g/kg glycine via enteral route (nasogastric tube) in 100ml standard enteral nutrition infused over 30 minutes - group 2: 0.1333 g/kg glycine via enteral route (nasogastric tube) in 100ml standard enteral nutrition, infused over 30 minutes - control: 100ml of standard enteral nutrition (without glycine supplementation), infused over 30 minutes - blood will be taken at baseline and thirty minutely for four hours Intervention 2: supplementation of standard enteral nutrition with glycine - Arm 1: 0.2g/kg/day glycine via enteral route (nasogastric tube) in sterile water, infused in two doses per day, each over 1 hour (in addition to standard care) - Arm 2: 0.4g/kg/day glycine via enteral route (nasogastric tube) in sterile water, infused in two doses per day, each over 1 hour (in addition to standard care) - Control: standard care (same volume and type of enteral formula without glycine supplementation) and the same volume of sterile water, infused in two doses, each over 1 hour -Duration: intervention will continue until the day of extubation or for a maximum of 7 days Procedures performed: 1. Quadricep muscle ultrasound to assess quadriceps muscle layer thickness (QMLT) - performed on days 0, 3 and 7 during intensive care unit (ICU) admission, at ICU discharge and 7 days after ICU discharge - performed by a trained practitioner (Drs Ali Abdelhamid, Bellomo or Deane) 2. Percutaneous muscle biopsy (50-150mg of tissue) from the quadriceps (vastus lateralis muscle) - performed under local anaesthetic and sterile technique - performed by a trained practitioner (Drs Ali Abdelhamid, Bellomo or Deane) - performed twice per patient [prior to the commencement of the intervention (day 0) and on the day of extubation (prior to extubation) or on day 7 of the intervention (whichever occurs earlier)] Timing: Intervention 1 will occur once informed consent is obtained and random allocation has occurred. Intervention 2 will begin on the same day, after intervention 1 has been completed. The decision maker may consent only to intervention 1, in which case the patient will be excluded from intervention 2.