The experimental protocol used. Mechanically ventilated patients on full nutrition in the ICU (n = 11) were studied. They were randomized to measurement of basal endogenous glutamine Ra before (upper panel) or after (mid panel) exogenous intravenous glutamine dipeptide (Ala-Gln) supplementation. For the glutamine Ra measurements the 13C-glutamine bolus technique was employed. The post-bolus sampling schedule is depicted in the lower panel.