Brain lactate may be involved in the development of impaired awareness of hypoglycemia (IAH), a condition that affects approximately 25\% of patients with type 1 diabetes and increases the risk of severe hypoglycemia. The aim of this study was to investigate the effect of acute hypoglycemia on brain lactate concentration in patients with IAH, as compared to those with normal awareness of hypoglycemia (NAH) and healthy controls (n=7 per group). After an overnight fast, all subjects underwent a two-step hyperinsulinemic euglycemic (5.0 mmol/L) hypoglycemic (2.8 mmol/L) glucose clamp. Brain lactate concentrations were measured continuously with (1)H-MRS, using a specific lactate detection method. Hypoglycemia generated symptoms in patients with NAH and healthy controls, but not in patients with IAH. Brain lactate fell significantly by ?20\% in response to hypoglycemia in type 1 diabetes patients with IAH, but remained stable in both healthy controls and in patients with NAH. The fall in brain lactate is compatible with increased brain lactate oxidation providing an alternative fuel source during hypoglycemia, which may contribute to impaired detection of hypoglycemia.NCT02146404, ClinicalTrials.gov.