Lactate as a cerebral metabolic fuel for glucose-6-phosphatase deficient children

The main substrates for brain energy metabolism were measured in blood samples taken from the carotid artery and the internal jugular bulb of four children with glycogen storage disease caused by deficiency of glucose-6-phosphatase. Multiple paired arterial and venous blood samples were analyzed for glucose, lactate, pyruvate, D-beta-hydroxybutyrate, acetoacetate, glycerol and O2, and the arteriovenous differences of the concentrations were calculated. In the first three patients the substrates were measured in two successive conditions with lower and higher glucose-intake, respectively, inducing reciprocally higher and lower concentrations of blood lactate. In the fourth patient medium chain triglycerides were administered simultaneously with the glucose-containing gastric drip feeding. Lactate appeared to be taken up significantly. It consumed, if completely oxidized, between 40-50% of the total O2 uptake in most cases. Only once in one patient the uptake of lactate switched to its release, when the blood lactate level decreased to normal. D-beta-hydroxybutyrate and acetoacetate arteriovenous (A-V) differences were small to negligible and these ketone bodies, therefore, did not contribute substantially to the brain's energy expenditure. Glycerol was not metabolized by the brain. Lactate thus appeared to be the second brain fuel next to glucose. It may protect the brain against fuel depletion in case of hypoglycemia.