Question:medium
A man survives 5 days without food. What is the brain's primary energy source during this prolonged fasting period?
A man survives 5 days without food. What is the brain's primary energy source during this prolonged fasting period?
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In prolonged fasting, the brain shifts from glucose dependence to using ketone bodies.
Updated On: May 14, 2026
- Gluconeogenesis
- Glycogenolysis
- Ketosis
- Lipolysis
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The Correct Option is C
Solution and Explanation
Step 1: Understanding the Question:
We must identify the primary fuel source for the brain during late-stage starvation (>3 days of fasting).
Step 2: Detailed Explanation:
Early Fasting (0-24 hours): The body relies on hepatic Glycogenolysis to maintain blood glucose levels. Glycogen stores are usually exhausted within 24 hours.
Intermediate Starvation (1-3 days): Hepatic Gluconeogenesis (using lactate, glycerol, and amino acids) becomes the dominant source of glucose for the brain.
Prolonged Starvation (3+ days): To preserve muscle protein (which would otherwise be broken down for gluconeogenesis), the liver begins massive production of Ketone bodies (acetoacetate and beta-hydroxybutyrate).
Brain Adaptation: Under normal conditions, the brain can only use glucose. However, during starvation, the brain upregulates enzymes that allow it to metabolize ketone bodies.
Metabolic Shift at Day 5: By the 5th day, nearly 60-70% of the brain's energy requirements are met by the oxidation of ketone bodies (Ketosis). This "protein-sparing" effect allows humans to survive for weeks without food.
Why not Lipolysis (D)? While lipolysis provides the fatty acids needed to make ketones, fatty acids themselves cannot cross the blood-brain barrier. They must be converted into ketones first.
Step 3: Final Answer:
In prolonged starvation (5 days), the brain shifts its primary metabolic reliance from glucose to ketone bodies (Ketosis).
We must identify the primary fuel source for the brain during late-stage starvation (>3 days of fasting).
Step 2: Detailed Explanation:
Early Fasting (0-24 hours): The body relies on hepatic Glycogenolysis to maintain blood glucose levels. Glycogen stores are usually exhausted within 24 hours.
Intermediate Starvation (1-3 days): Hepatic Gluconeogenesis (using lactate, glycerol, and amino acids) becomes the dominant source of glucose for the brain.
Prolonged Starvation (3+ days): To preserve muscle protein (which would otherwise be broken down for gluconeogenesis), the liver begins massive production of Ketone bodies (acetoacetate and beta-hydroxybutyrate).
Brain Adaptation: Under normal conditions, the brain can only use glucose. However, during starvation, the brain upregulates enzymes that allow it to metabolize ketone bodies.
Metabolic Shift at Day 5: By the 5th day, nearly 60-70% of the brain's energy requirements are met by the oxidation of ketone bodies (Ketosis). This "protein-sparing" effect allows humans to survive for weeks without food.
Why not Lipolysis (D)? While lipolysis provides the fatty acids needed to make ketones, fatty acids themselves cannot cross the blood-brain barrier. They must be converted into ketones first.
Step 3: Final Answer:
In prolonged starvation (5 days), the brain shifts its primary metabolic reliance from glucose to ketone bodies (Ketosis).
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