Which statement correctly contrasts energy yield and substrate use in fed versus fasting states?

Which statement correctly contrasts energy yield and substrate use in fed versus fasting states?

• A. Fed state favors glycolysis and lipid synthesis; fasting state increases gluconeogenesis, glycogenolysis, and ketogenesis.
• B. Fed state favors gluconeogenesis and ketogenesis; fasting favors glycolysis.
• C. In both states, ATP yields are identical.
• D. The liver does not adjust metabolic flux with feeding status.

Answer: (B)

The statement tests how metabolic flux shifts with feeding status and how that affects the main fuel pathways. When you’re in the fed state, there’s plenty of glucose and insulin is high, so the body preferentially uses glycolysis to extract energy and channels excess acetyl-CoA into lipid synthesis. Ketone production is kept low because the liver isn’t in a fasting mode. In contrast, during fasting insulin falls and glucagon rises, so the liver shifts toward maintaining blood glucose through gluconeogenesis and glycogenolysis. As fatty acids are released, ketogenesis increases, providing ketone bodies that can fuel the brain and other tissues when glucose is scarce. So the best contrast is that the fed state favors glycolysis and lipid synthesis, while the fasting state favors gluconeogenesis, glycogenolysis, and ketogenesis. ATP yield and fuel efficiency differ between these routes: glycolysis provides rapid ATP generation, gluconeogenesis consumes ATP, and ketone bodies become an efficient alternate fuel during prolonged fasting. The other statements don’t fit this regulated shift in metabolic flux.

The statement tests how metabolic flux shifts with feeding status and how that affects the main fuel pathways. When you’re in the fed state, there’s plenty of glucose and insulin is high, so the body preferentially uses glycolysis to extract energy and channels excess acetyl-CoA into lipid synthesis. Ketone production is kept low because the liver isn’t in a fasting mode. In contrast, during fasting insulin falls and glucagon rises, so the liver shifts toward maintaining blood glucose through gluconeogenesis and glycogenolysis. As fatty acids are released, ketogenesis increases, providing ketone bodies that can fuel the brain and other tissues when glucose is scarce. So the best contrast is that the fed state favors glycolysis and lipid synthesis, while the fasting state favors gluconeogenesis, glycogenolysis, and ketogenesis. ATP yield and fuel efficiency differ between these routes: glycolysis provides rapid ATP generation, gluconeogenesis consumes ATP, and ketone bodies become an efficient alternate fuel during prolonged fasting. The other statements don’t fit this regulated shift in metabolic flux.