The period around bariatric surgery offers a unique opportunity to characterize metabolism responses to dynamic shifts in energy, gut function, and anesthesia. We analyzed plasma acylcarnitines in obese women (n=17) sampled in the overnight fasted/postabsorptive state ca. 1-2 weeks prior to surgery (Condition A), the morning of surgery (prior restriction to a 48 hr. clear liquid diet coupled in some cases a standard polyethylene glycol gut evacuation: Condition B), and following induction of anesthesia (Condition C). Comparisons tested if (a) plasma acylcarnitine derivatives reflective of fatty acid oxidation (FAO) and xenometabolism would be significantly increased and decreased, respectively, by pre-operative gut preparation/negative energy balance (Condition A vs. B), and (b) that anesthesia would acutely depress markers of FAO. Acylcarnitines associated with fat mobilization and FAO were significantly increased in Condition B: long-chain acylcarnitines (i.e., C18:1, ~70%), metabolites from active but incomplete FAO (i.e., C14:1 [161%], C14:2 [102%]) and medium- to short-chain acylcarnitines (i.e., C2 [91%], R-3-hydroxybutyryl- [245%], C6 [45%], cis-3,4-methylene-heptanoyl- [17%], etc.). Branched-chain amino acid markers displayed disparate patterns (i.e. isobutyryl- [40% decreased] vs. isovaleryl-carnitine [51% increased]). Anesthesia reduced virtually every acylcarnitine. These results are consistent with a fasting-type metabolic phenotype coincident with the pre-surgical "gut preparation" phase of bariatric surgery, and a major and rapid alteration of both fat and amino acid metabolism with onset of anesthesia. Whether or not pre-surgical or anesthesia-associated metabolic shifts in carnitine and fuel metabolism impact patient outcomes or surgical risks remain to be evaluated experimentally.