Fasting is going without food for a few days involuntarily while starving refers to food deprivation lasting for weeks or months. During the early stages of fasting, the level of blood glucose begins to drop resulting in a reduction in the secretion of insulin and a rise in glucagon secretion. The pancreas is responsible for secreting glucagon following a drop in sugar levels during fasting. The insulin mobilizes glucagon reserves since there is no intake of glucose. Glucagon stimulates the breakdown of glycogen and inhibits the synthesis of glycogen by triggering the adenosine monophosphate cascade. Glucagon also inhibits the synthesis of fatty acids by reducing the production of pyruvate (Naik, 2012).
The resulting high amount of glucose is released into the blood from the liver. The utilization of glucose in adipose and muscle tissues decreases due to low insulin. Such a reduction in the utilization of glucose helps in the maintenance of the level of blood glucose. All these actions lead to increased glucose release by the liver. When the blood glucose drops, both the liver and the muscles use fatty acids as their main source of fuel.
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If fasting extends for several weeks, the body will be under starvation conditions. In this case, body metabolism prioritizes the production and supply of enough glucose to the brain and red blood cell tissues among others. This is mainly because such tissues depend highly on glucose for fuel. However, the originators of glucose are insufficient, and it is not possible to convert fatty acids into glucose (Naik, 2012). Therefore, metabolic processes preserve proteins. Body metabolism also shifts fuels source to fatty acids, as well as Ketone bodies. The brain begins to take in considerable amounts of acetoacetate instead of glucose. With time, the liver converts fatty acids effectively into ketone bodies for use by the brain. This decreases the need for glucose significantly. The degradation of muscle decreases compared to the initial days of starvation (Naik, 2012).