Answer:
A series of events occur, in prolonged fasting; the body undergoes changes and adaptations of its metabolism to continue to meet energy needs and maintain vital functions, despite not receiving food
These changes are divided into three phases according to the elapsed days of fasting and according to the metabolic processes involved.
First phase: main consumption of reserve carbohydrates.
Second phase: main consumption of fats.
Third phase: serious consumption of proteins.
Explanation:
If the fast is prolonged in time, the metabolic processes change in their qualitative and quantitative characteristics, so that the energy products consumed (glucose, free fatty acids and ketone bodies) are modified, their oxidation decreases globally and after the initial depletion of hepatic and muscular glycogen and protein catabolism, the main source of glucose is the liver through gluconeogenesis. The substrates initially come from protein catabolism and lipolysis, but later protein destruction slows down, maximizing lipolysis.
Between the first phase and the second phase of the fast some muscle proteins are lost but not excessively, even with fasts of two or three weeks.
After about 2 weeks of fasting, up to two-thirds of the energy the brain needs is obtained from ketone bodies. These ketone bodies are the breakdown products of fats and are responsible for the typical ketone breath reminiscent of the smell of acid apples; urine also smells like ketone.
In this phase, damage begins to occur in some important organs such as the liver and kidneys. Starting the first week of fasting, acidosis affects the heart's function of the circulation and the brain.
In the third phase of the prolonged fast it happens from 3 weeks, that is, about twenty days without food, although it varies according to the constitution of each person. A peak of muscle protein consumption occurs and a great loss of weight begins to be perceived with great weakness. Edema occurs and albumin concentration in the blood is altered, as a sign of the self-digestion of muscle proteins.
From approximately 30 days of fasting, the state of malnutrition severely affects all body systems. After forty or fifty days the deterioration is noticeable due to physical wear, the person loses his mobility and suffers from unconsciousness. Finally, starvation death can occur from cardiorespiratory arrest or from lack of blood supply to the brain.
