Final answer:
During the intermittent phase of starvation, the body begins to increase protein catabolism to supply glucose through gluconeogenesis, resulting in increased urinary nitrogen excretion. As the starvation period prolongs, muscle mass and other vital proteins are depleted, and the brain starts utilizing ketone bodies, eventually leading to reduced protein breakdown and nitrogen excretion.
Step-by-step explanation:
During the intermittent phase of starvation, significant metabolic changes occur in the body. Initially, after glycogen stores are depleted, the body begins to mobilize fat reserves and the level of ketone bodies in the blood increases, a condition known as ketoacidosis. This results in a decrease in bicarbonate and the pH of the blood, with respiratory quotient (R.Q) values changing from 0.82 to 0.70. In the quest to provide glucose to glucose-dependent organs, particularly the brain, the body increases the breakdown of proteins through gluconeogenesis, leading to a rise in urinary nitrogen excretion.
The body catabolizes its own muscle and other proteins, liberating amino acids to be used as an energy source. Initially, about 100 grams of protein are metabolized daily but this decreases over time as the brain begins utilizing ketone bodies, reducing the protein catabolism to about 12-15 grams per day. This adaptation, however, comes at the expense of muscle mass and other vital proteins. In the postabsorptive state, as glucose levels drop, insulin levels also fall and glucagon levels increase, promoting glucose production from non-carbohydrate sources, which contributes to further nitrogen excretion from protein breakdown.
In the context of metabolic changes during starvation, starvation represents an extreme form of fasting that involves drastic adaptations. The body prioritizes the energy supply to the brain and conserves amino acids to maintain proteins in cells. Muscles switch from using glucose to fatty acids, sparing glucose for the brain, and as the fasting period extends, a decline in metabolic rate occurs due to the reduction of body mass.