Pyruvate recycling is the metabolic pathway that generates pyruvate, lactate and alanine from the tricarboxylic acid (TCA) cycle intermediates, oxaloacetate (OAA) and malate. It is active in the liver and the kidney. Although existence and origin of pyruvate recycling mechanism in human brain has been shown to be active, it still remains controversial. Here, we demonstrate that pyruvate recycling mechanism is active in human GBM patients by 13C isotopomer analysis of resected tumor tissues. We have developed a simple method to determine the relative flux of pyruvate recycling with respect to glycolysis using C2 lactate 13C isotopomers.