After a traumatic brain injury, brain cells may struggle to produce energy normally and may start relying more on less efficient, oxygen-limited processes. Doctors often monitor this using the balance between two chemicals, lactate and pyruvate. In this study, researchers tested a different measure that looks at how lactate changes in relation to pyruvate over periods of time. They analysed monitoring data from 591 patients treated in Cambridge and Uppsala, and also tested the measure in human nerve cells grown in the laboratory. The new measure was linked to lower glucose levels in the brain and, in the Cambridge group, to poorer recovery six months after injury. It also increased when energy production was deliberately disrupted in laboratory-grown nerve cells. These findings suggest that this time-based measure may identify abnormal, oxygen-limited energy use in the injured brain more accurately than the commonly used lactate-to-pyruvate ratio. It could therefore help clinicians better understand changes in brain metabolism after traumatic brain injury.