________ ________ is the primary fuel for the nervous system.

• A. Plasma glucose
• B. Fatty acids
• C. Amino acids
• D. Ketone bodies

Answer: (A)

Glucose is the primary fuel for the nervous system because neurons depend on a steady, readily available source of energy to meet their high metabolic demands. The brain consumes a large portion of resting energy and relies on glucose because it can be rapidly transported into brain cells via specialized transporters and oxidized efficiently through glycolysis and oxidative phosphorylation. The blood-brain barrier limits the direct use of fatty acids by neurons, so fatty acids aren’t the brain’s main energy source. Amino acids can be used for energy only in limited, non-preferred ways and are primarily needed for protein synthesis. Ketone bodies can supply energy during prolonged fasting or carbohydrate restriction, but they do not replace glucose as the brain’s main fuel under normal conditions. Therefore, circulating plasma glucose best explains the brain’s primary energy source.