Nutrient Homeostasis and Fed/Fasted States - Anatomy and Physiology
- Why can skeletal muscle glycogen not be used to raise blood glucose?
- Muscle lacks glucose-6-phosphatase, so it cannot release free glucose; it uses its glycogen for its own energy.
- After several days of starvation, which fuel supplies a large share of the brain's energy?
- Ketone bodies. The brain adapts to use them, which reduces the need to break down muscle protein for gluconeogenesis.
- What is the main target organ of glucagon during fasting?
- The liver. Glucagon stimulates hepatic glycogenolysis and gluconeogenesis to release glucose into the blood.
- Which pancreatic hormone dominates the absorptive (fed) state?
- Insulin. It promotes glucose uptake and the storage of nutrients as glycogen, triglycerides and protein.
- Where are ketone bodies made, and from what?
- In the liver, from fatty acids, during prolonged fasting or low insulin. They serve as fuel for the brain, heart and muscle.
- About 4 hours after a meal, absorption is complete. Which process mainly maintains blood glucose at this point?
- Liver glycogenolysis. Hepatic glycogen stores supply glucose early in the postabsorptive state, before gluconeogenesis becomes dominant.
- What is glycogenolysis?
- The breakdown of stored glycogen into glucose. Liver glycogenolysis is the first major source of blood glucose after a meal is absorbed.
- What is lipolysis, and which hormonal condition stimulates it during fasting?
- The breakdown of triglycerides in adipose tissue into fatty acids and glycerol. It is stimulated when insulin is low and glucagon and epinephrine are high.
- What is gluconeogenesis, and what are its main substrates?
- The synthesis of new glucose from noncarbohydrate sources, mainly lactate, glycerol and amino acids. It occurs chiefly in the liver.
- Is the absorptive state mainly anabolic or catabolic, and why?
- Anabolic. Insulin drives the building and storage of glycogen, fat and protein from absorbed nutrients.
- Which tissues depend on insulin-stimulated GLUT4 transporters for most of their glucose uptake?
- Skeletal muscle and adipose tissue. Insulin moves GLUT4 transporters to their cell membranes.