Final answer:
The body's glycogen stores are typically depleted within 1 day of fasting or starvation, as liver glycogen reserves last for around 24 hours. Subsequently, the body begins to break down proteins and lipids to maintain blood glucose levels, which can lead to muscle wasting and ketoacidosis.
Step-by-step explanation:
During fasting or starvation, the body's glycogen stores are depleted relatively quickly. After 12-14 hours from the last meal, the individual enters a postabsorptive state of fasting, following which starvation gradually sets in. During the starvation phase, the liver glycogen is broken down and falls to about 10% of its normal concentration. This occurs quickly and remains at that level throughout the duration of the starvation. The most accurate answer to the question of how quickly the body's glycogen stores are depleted during fasting or starvation would be 1 day, as the liver glycogen reserves typically last 24 hours or less.
As the glycogen stores are depleted, blood glucose levels remain maintained at about 80 mg/100 ml, mainly through the continuous process of gluconeogenesis, which involves the conversion of proteins and lipids into glucose. This is a critical adaptive response to prevent low blood glucose levels. However, this process leads to an increase in the excretion of urinary nitrogen, reflecting the breakdown of proteins for gluconeogenesis, even when adequate fat stores are present. This increased protein catabolism can eventually lead to muscle wasting as the body tries to provide glucose to the brain and other organs.
After the initial days of fasting, the body begins to use alternative fuel sources such as ketone bodies, derived from fats. Ketoacidosis can develop due to increased levels of ketone bodies. As the starvation state progresses and fat stores are eventually depleted, the brain adapts to utilizing ketone bodies, reducing the rate of protein metabolism and sparing muscle tissue to some extent. Nevertheless, once the fat reserves are exhausted, the body's condition can deteriorate rapidly.