Brain Metabolism — FRCA Primary MCQ

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ModeratePhysiology - Endocrine & MetabolicBrain MetabolismFRCA Primary

In a healthy adult following an 8-hour overnight fast, with a normal blood glucose concentration and consuming a usual mixed diet, which metabolic substrate provides the primary energy source for the brain?

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Correct answer: B — Glucose

The correct answer is B, glucose. Following an ordinary overnight fast, hepatic glycogenolysis and gluconeogenesis maintain plasma glucose, which crosses the blood–brain barrier mainly through GLUT1 and is taken up by neurons predominantly through GLUT3. Ketone production increases during fasting, but ketone bodies do not become the principal cerebral fuel until substantially prolonged fasting or starvation. Fatty acids are mobilised systemically but are not a major direct cerebral fuel because brain uptake and mitochondrial fatty-acid oxidation are limited. Lactate can be oxidised by the brain, particularly during increased activity or raised circulating lactate, but is not the principal resting substrate in this scenario. Amino acids make only a minor contribution to cerebral energy production.

Reference: Kapogiannis D, Avgerinos KI. Brain glucose and ketone utilization in brain aging and neurodegenerative diseases. International Review of Neurobiology. 2020. https://pubmed.ncbi.nlm.nih.gov/32739015/