Vigorous exercise increases lactate and glucose uptake by the brain in excess of the increase in brain oxygen uptake. The metabolic fate of this non-oxidized carbohydrate entering the brain is poorly understood, but accumulation of lactate in the brain and/or increased net synthesis of amino acid neurotransmitters are possible explanations. Previous proton magnetic resonance spectroscopy (1H-MRS) studies using conventional pulse sequences have not detected changes in brain lactate following exercise. This contrasts with 1H-MRS studies showing increased brain lactate when blood lactate levels are raised by an intravenous infusion of sodium lactate. Using a J-editing 1H-MRS technique for measuring lactate, we demonstrated a significant 19% increase in lactate in the visual cortex following graded exercise to approximately 85% of predicted maximum heart rate. However, the magnitude of the increase was insufficient to account for more than a small fraction of the non-oxidized carbohydrate entering the brain with exercise. We also report a significant 18% increase in Glx (combined signal from glutamate and glutamine) in visual cortex following exercise, which may represent an activity-dependent increase in glutamate. Future studies will be necessary to test the hypothesis that non-oxidized carbohydrate entering the brain during vigorous exercise is directed, in part, toward increased net synthesis of amino acid neurotransmitters. The possible relevance of these findings to panic disorder and major depression is discussed.
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