Posted: May 22nd, 2023
The fed-fast cycle provides a very concise description of the interrelationship of the different metabolic pathways. During the fed state, glucose is readily available in various tissues and is usually absorbed directly to provide the required energy. In this state, enzymes in the liver convert excess glucose into fatty acids and glycogen (Gropper and Smith 252). The transformation of glucose into fatty acids only takes place when energy use is far lower than the intake.
During the post-absorptive phase, the tissues are unable to obtain energy from consumed glucose and the other nutrients but must rely on other energy sources. During this phase, glycogenolysis occurs in the liver where the synthesis of triacylglycerol and glycogen occurs to maintain the glucose levels in the blood (Grisham and Garrett 761). In the muscles, lactate is stored and released as required and becomes an essential energy source. The amino group alanine from the liver enters the glucose-alanine cycle and glycogenolysis occurs to transform this amino group into pyruvate.
In the fasting state, the hydrolysis of proteins in the muscles to create glucogenic amino acids increases. In this state, amino acids from protein breakdown form the bulk of the substrates needed for gluconeogenesis (Stoker 613). Through lipolysis, glycerol is also a chief provider of energy in this phase. Apart from this, anaerobic metabolism occurs in the muscles to create lactate that also offers a means for energy provision. In this phase, all the macronutrients offer a means of energy through various reactions in the body.
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