Test Yourself Before You Continue

1. Describe how the secretions of insulin and glucagon change during periods of absorption and periods of fasting. How are these changes in hormone secretion produced?

2. Explain how the synthesis of fat in adipose cells is regulated by insulin. Also, explain how fat metabolism is regulated by insulin and glucagon during periods of absorption and fasting.

3. Define the following terms: glycogenolysis, gluconeogenesis, and ketogenesis. How do insulin and glucagon affect each of these processes during periods of absorption and fasting?

4. Describe two pathways used by the liver to produce glucose for secretion into the blood. Why can't skeletal muscles secrete glucose into the blood?

Regulation of Metabolism

Glucose Metabolism Feeding And Fasting

■ Figure 19.10 The effect of feeding and fasting on metabolism.

Metabolic balance is tilted toward anabolism by feeding (absorption of a meal) and toward catabolism by fasting. This occurs because of an inverse relationship between insulin and glucagon secretion. Insulin secretion rises and glucagon secretion falls during food absorption, whereas the opposite occurs during fasting.

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