Final answer:
The phases of starvation occur as the body exhausts its glycogen stores and begins breaking down fat and muscle for energy, leading to metabolic changes and hormonal shifts that maintain vital functions at the cost of overall health.
Step-by-step explanation:
The phases of starvation occur as the body transitions through different metabolic states in response to a lack of food intake. Initially, the body enters a postabsorptive state, usually after 12-14 hours without food, utilizing glycogen stores for energy. When these stores are depleted, the body begins to break down adipose tissue and muscle mass to maintain energy production, particularly for vital organs like the brain and heart. This catabolic process can lead to symptoms such as fatigue, irritability, and hyperactivity, while metabolic changes include a shift from glucose to fatty acids and ketone bodies as primary energy sources, signaling a state of starvation.
During starvation, hormones like glucagon play a critical role in mobilizing energy stores and increasing the breakdown of proteins and fats. After 3 to 4 days without food, significant hormonal changes occur due to low glucose levels, triggering adaptations like reduced metabolic rate and muscle wasting. In the absence of adequate glucose, the liver produces ketone bodies, and the blood pH may decrease due to ketoacidosis. Eventually, prolonged starvation can be fatal, with death possibly occurring when an individual loses about 30% of their normal body weight.