Which energy sources cannot be used by the brain, while others can, and their byproducts are dangerous?

• A. Glucose
• B. Glycerin
• C. Oxygen
• D. Fats and proteins

Answer: (D)

The brain relies on fuels it can access and metabolize inside neural tissue. Glucose is the primary energy source, and during extended fasting the brain can use ketone bodies produced by the liver from fatty acids. Oxygen is required for producing energy, and glycerin can be converted to glucose to support energy supply, but it is not a direct brain fuel.

Fats cannot be used directly by the brain because fatty acids do not cross the blood–brain barrier in meaningful amounts, and neurons lack the enzymes to oxidize fatty acids for quick energy. When fat metabolism occurs, the liver makes ketone bodies as byproducts; while the brain can use ketones, excessive production can lead to ketoacidosis, a dangerous condition.

Proteins cannot be used directly by the brain for energy either. Some amino acids can be converted to glucose in the liver to support brain energy during prolonged fasting, but brain tissue cannot utilize amino acids directly as fuel. The deamination of amino acids produces ammonia, a neurotoxin, which must be detoxified; if ammonia accumulates, it can severely affect brain function.

So fats and proteins are the energy sources that cannot be used by the brain directly, and their typical byproducts can pose serious risks.