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Energy metabolism in the human brain is not fully understood. Classically, glucose is regarded as the major energy substrate. However, lactate (conventionally a product of anaerobic metabolism) has been proposed to act as an energy source, yet whether this occurs in man is not known. Here we show that the human brain can indeed utilize lactate as an energy source via the tricarboxylic acid cycle. We used a novel combination of ¹³C-labelled cerebral microdialysis both to deliver ¹³C substrates into the brain and recover ¹³C metabolites from the brain, and high-resolution ¹³C nuclear magnetic resonance. Microdialysis catheters were placed in the vicinity of focal lesions and in relatively less injured regions of brain, in patients with traumatic brain injury. Infusion with 2-¹³C-acetate or 3-¹³C-lactate produced ¹³C signals for glutamine C4, C3 and C2, indicating tricarboxylic acid cycle operation followed by …