Final answer:
In late-stage starvation, the body undergoes muscle wasting, gluconeogenesis, and a transition to ketogenic metabolism, leading to significant muscle atrophy, ketosis, and eventually the use of proteins as a last-resort energy source, which can be fatal.
Step-by-step explanation:
During the late stages of starvation, significant changes occur in muscle metabolism to meet the body's energy needs. Initially, when nutritional intake is insufficient, the body enters a state of gluconeogenesis to produce glucose from non-carbohydrate sources, including amino acids from muscle proteins. Over time, depicted by the spindly arms and legs of starving individuals, muscle wasting becomes evident as muscle proteins are increasingly catabolized.
Once liver glycogen reserves are depleted and gluconeogenesis cannot meet the energy demands of vital organs like the brain, ketogenic fat metabolism is upregulated to produce ketone bodies that can be used as fuel. This leads to ketosis, often discernible by 'acetone breath', as fat breakdown becomes the primary energy source. Notably, a drop in the respiratory quotient (R.Q) from 0.82 to 0.70 can indicate this shift to fat as a primary energy substrate.
If the situation exacerbates and fat reserves are exhausted, the body then resorts to breaking down proteins as its last source of energy, which can severely affect organ function and potentially lead to death through mechanisms such as cardiac arrhythmia or electrolyte imbalances.