To fast or not to fast? Metabolic preparation for elective surgery

Fasting overnight has been the routine before elective surgery for the past century. Fasting guidelines in many countries have recently been modernized and it is now recommended that most patients drink clear fluids up until 2_3 h before anaesthesia. Several studies have shown that if the overnight fasted state is changed into a carbohydrate fed state before the onset of the operation, several further benefits can be gained. In particular, patients treated with carbohydrates at sufficient concentrations (12.5%) in a specially designed drink for preoperative use have less preoperative discomfort, and fewer metabolic derangements postoperatively. Of special interest is the finding that this treatment (or a 30% glucose infusion given at a fairly high rate) results in substantially less postoperative insulin resistance. This is important, since insulin resistance is the underlying mechanism behind hyperglycaemia, which has been shown to be detrimental in postoperative patients. Recent studies show that preoperative carbohydrates counteract the key disturbances that cause hyperglycaemia, primarily increased endogenous glucose production and reduced glucose uptake in peripheral tissues. This paper reviews the concept of preoperative carbohydrates as opposed to overnight fasting, the mechanisms of action and clinical outcomes for this treatment.