The brain usually depends almost exclusively on glucose for its energy requirements. During hypoglycemia associated with prolonged fasting or strenuous exercise, circulating ketone-body and lactate levels increase severalfold; in both situations, certain signs and symptoms of hypoglycemia are diminished. Therefore, to test the hypothesis that hyperketonemia or hyperlacticacidemia of the magnitude seen during certain clinical situations can substitute for glucose as an energy source for the brain and alter physiological responses to hypoglycemia, we assessed autonomic and neuroglycopenic symptoms, counterregulatory hormone responses, and cognitive function during standardized insulin-induced hypoglycemia in normal volunteers with and without infusion of (β-hydroxybutyrate (BOHB) or lactate designed to reproduce circulating levels of these substrates seen during prolonged fasting and strenuous exercise. Compared with paired control experiments, infusion of BOHB and lactate increased the glycemic threshold (required greater hypoglycemia for initiation) and reduced the magnitude of autonomic and neuroglycopenic symptoms, counterregulatory hormone responses, and cognitive dysfunction (all P < 0.05). The hypoglycemic threshold for autonomic symptoms increased from 3.8 ± 0.1 to 3.1 ± 0.2 mmoM during BOHB infusion and from 3.7 ± 0.1 to 2.8 ± 0 . 1 mmol<1 during lactate infusion, and the threshold for neuroglycopenic symptoms increased from 2.8 ± 0.1 to 2.4 ± 0.1 and 2.3 ± 0.1 mmol/1, respectively. The magnitude for autonomic symptoms decreased from 12 ± 2 and 11 ± 1 to 6 ± 2 and 4 ± 1 during BOHB and lactate infusion, respectively. Neuroglycopenic symptoms decreased from 11 ± 2 to 3 ± 1 during both series of experiments. Infusion of BOHB and lactate-reduced responses for all counterregulatory hormones, the reduction being the greatest for epinephrine (∼57 and 73%, during BOHB and lactate infusion, respectively) and least for cortisol (7sim;28 and 29%, respectively). These results indicate that under certain clinical conditions, BOHB and lactate may substitute for glucose as a fuel for the brain and alter physiological responses to hypoglycemia.