Energy Source During Early Fasting/Starvation

During early fasting/starvation stage, the energy is supplied by:

Correct Answer: Glycogen

During the early stage of fasting, the body's first response is to mobilize stored glycogen from the liver and muscles (glycogenolysis). Liver glycogen is broken down to glucose to maintain blood glucose (normoglycemia) for the brain and other glucose-dependent tissues.

Sequence of Fuel Utilization During Starvation

1. Glycogen: First few hours — liver and muscle glycogen broken down (glycogenolysis).
2. Fatty acids: After glycogen depletion (~24 hours) — fat stores mobilized via lipolysis; fatty acids undergo β-oxidation; liver converts acetyl-CoA to ketone bodies.
3. Ketone bodies: Brain adapts to use ketone bodies as glycogen stores deplete.
4. Protein: Last resort — muscle protein catabolized to amino acids for gluconeogenesis; indicates severe, prolonged starvation.

Why Other Options Are Wrong

• Fatty acids → Mobilized after glycogen depletion in early starvation, not at the very beginning.
• Creatine phosphate → Used only for immediate short-burst exercise energy, not for fasting metabolism.
• Protein → Used only in severe, prolonged starvation as a last resort to prevent death.