It is recommended that the period of fasting before elective surgery should be shortened to facilitate a rapid recovery by preventing catabolism. We examined the effects of a short period of fasting on metabolism by performing indirect calorimetry (IC) under general anesthesia. A prospective observational study involving 26 consecutive patients who underwent elective surgery and whose metabolism was evaluated using IC during anesthesia was conducted. The patients were divided into two groups, those who fasted for < 8 h (group S) and those who fasted for > 10 h (group L). Oxygen consumption, the volume of carbon dioxide emissions (VCO2), the respiratory quotient (RQ), resting energy expenditure (REE), and basal energy expenditure (BEE) were compared. The REE, VCO2, and RQ of group L (17.7 +/- A 2.3 kcal/kg/day, 118.5 +/- A 20.8 ml/min, and 0.71 +/- A 0.12, respectively) were significantly lower than those of group S (19.7 +/- A 2.3 kcal/kg/day, 143.6 +/- A 30.9 ml/min, and 0.81 +/- A 0.09, respectively) (P < 0.05). In group L, the relationship between REE and BEE was weaker (r (2) = 0.501) and the BEE-REE slope was less steep (REE = 0.419BEE + 509.477) than those seen in group S (r (2) = 0.749 and REE = 1.113BEE - 376.111, respectively). Our findings suggest that a short period of fasting (< 8 h) before surgery is more strongly associated with the conservation of basal metabolism.