20 Glucose Homeostasis, Fuel Metabolism, and Insulin
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After reading this chapter you should be able to:
- Characterize the main metabolic fuels.
- Outline the actions of insulin and glucagon.
- Describe the metabolism in the fasting and postprandial state.
- Describe the metabolic response to injury and compare it with metabolism in diabetes.
- Characterize the diabetic syndrome.
- Explain the basis of laboratory tests relevant to fuel metabolism and glucose homeostasis.
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| The existence of an organism depends on the continuous provision of energy to drive metabolic processes. This chapter describes how the body handles energy substrates (metabolic fuels) under different circumstances. It also discusses the most common metabolic disease: diabetes mellitus.
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| The main metabolic fuels are glucose and fatty acids
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| Glucose and fatty acids are the most important metabolic fuels. In normal circumstances, glucose is the only fuel which can be used by the brain. Glucose is also preferentially used by muscle during the initial stages of exercise. The amount of glucose present in the extracellular fluid is minute - only about 20 g (<1 oz) (equivalent of 80 kcal) (335 kJ). Therefore, to ensure the continuous provision of glucose to the brain and other tissues, metabolic fuels are stored for use in times of need. Carbohydrates are stored as glycogen. The amount of available glycogen stored is also not large; approximately 75 g (approx 2.5 oz) in the liver and 400 g (<1 lb) in the muscles (about 1900 kcal (7955 kJ) altogether). Liver glycogen can remain the main supplier of glucose for no longer than 16 h. To safeguard the continuous supply of glucose over longer periods, the body needs to transform noncarbohydrate compounds into glucose. This is done through gluconeogenesis.
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| The esters of glycerol and long-chain fatty acids are the ideal storage fuel
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| The caloric value of fats 9 kcal/g (37 kJ/g) is higher than that of either carbohydrates 4 kcal/g (17 kJ/g) or proteins (4 kcal/g) (see Table 21.4). The body has a virtually un-limited capacity for the accumulation of fats. A 70 kg (154 lb) man will have approximately 15 kg (33 lb) of fat stored as adipose tissue triacylglycerols (triglycerides), equivalent to over 130 000 kcal (544 300 kJ). Fatty acids can support the body's energy needs over prolonged periods of time. In extreme circumstances, people can fast for as long as 60-90 days and obese persons may survive for over a year without food. Fatty acids are stored in the body as esters of glycerol (triacylglycerols; triglycerides).
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| Amino acids can be used as a fuel during fasting, illness, or injury
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| Amino acids normally serve as substrates for the synthesis of the body's own proteins, rather than as a source of energy. However, during a prolonged fast, or after illness or injury, proteins are degraded and the constituent amino acids are converted into glucose. Excess amino acids provided with food are normally converted to carbohydrates either for storage or for energy metabolism. The main metabolic pathways and key metabolites are listed in Table 20.1.
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