Effects of Fasting:
Effect of fasting on human metabolism. As blood glucose levels decrease, insulin levels decrease and glucagon levels increase, stimulating the release of stored fuels into the blood.
A. The liver during fasting
The liver produces glucose and ketone bodies, which are released into the blood and serve as sources of energy for other tissues.
1. Production of glucose by the liver:
a. Glycogenolysis: About 2 to 3 hours, liver begins to break down its glycogen stores by the process of glycogenolysis.
(i) After about 4 to 6 hours of fasting, the liver begins the process of gluconeogenesis. Within 30 hours, liver glycogen stores are depleted, leaving gluconeogenesis as the major process, responsible for maintaining blood glucose levels.
(ii) Carbon sources for gluconeogenesis are as follows:
(a) Lactate produced by tissues like red blood cells or exercising muscle
(b) Glycerol from breakdown of triacylglycerols in adipose tissue
(c) Amino acids, particularly alanine, from muscle protein
(d) Propionate from oxidation of odd-chain fatty acids (minor source)
2. Production of ketone bodies by the liver
Increased glucagon levels –> adipose tissue breaks down its triacylglycerol stores into fatty acids and glycerol –> beta-oxidation by the liver –> converts the fatty acids to acetyl CoA –> Acetyl CoA is used by the liver for the synthesis of the ketone bodies, acetoacetate and ahydroxybutyrate.
The liver cannot oxidize ketone bodies, and hence releases them into the blood.
B. Adipose tissue during fasting
1. As glucagon levels rise, adipose triacylglycerol stores are mobilized. The triacylglycerol is degraded to three free fatty acids and glycerol, which enter the circulation. The liver converts the fatty acids to ketone bodies and the glycerol to glucose.
2. Tissues such as muscle oxidize the fatty acids to CO2 and H2O.
C. Muscle during fasting
1. Degradation of muscle protein
a. During fasting, muscle protein is degraded, producing amino acids, which are partially metabolized by muscle and released into the blood, mainly as alanine and glutamine.
b. Tissues, such as gut and kidney, metabolize the glutamine.
c. The products (mainly alanine and glutamine) travel to the liver, where the carbons are converted to glucose or ketone bodies and the nitrogen is converted to urea.
2. Oxidation of fatty acids and ketone bodies
a. During fasting, muscle oxidizes fatty acids released from adipose tissue and ketone bodies produced by the liver.
b. During exercise, muscle can also use its own glycogen stores as well as glucose, fatty acids, and ketone bodies from the blood.