Question

Within a few days after a fast begins, nitrogen excretion accelerates to a higher-than-normal level. After a few weeks, the rate of nitrogen excretion falls to a lower level and continues at this low rate. However, after the fat stores have been depleted, nitrogen excretion rises to a high level. (a) What events trigger the initial surge of nitrogen excretion? (b) Why does nitrogen excretion fall after several weeks of fasting? (c) Explain the increase in nitrogen excretion when the lipid stores have been depleted.

Short answer

(a) Muscle protein breakdown for energy. (b) Adaptation to using fat for energy. (c) Increased protein breakdown after fat depletion.

Step-by-step solution

Understanding Nitrogen Excretion and Fasting

Nitrogen excretion in the body largely comes from the breakdown of amino acids, which occurs when proteins are catabolized. During fasting, the body initially breaks down muscle protein to supply energy, leading to increased nitrogen excretion.

Analyzing the Initial Surge

(a) The initial surge in nitrogen excretion is triggered by the body's increased breakdown of muscle protein for energy. As glycogen stores become depleted, the body shifts to using proteins to maintain blood glucose levels.

Understanding Why Nitrogen Excretion Falls

(b) After several weeks, the body adapts to fasting by reducing protein breakdown. It shifts towards using fat reserves for energy, which conserves muscle tissue and lowers nitrogen excretion rates.

Explaining the Increase After Fat Depletion

(c) Once fat stores are depleted, the body has limited energy sources and thus increases the breakdown of body proteins for energy, again leading to a rise in nitrogen excretion.

Key concepts

Amino Acid Catabolism

During fasting, amino acid catabolism plays a crucial role in the body's survival strategy. When food intake is cut off, the body must find alternative ways to supply energy, especially to vital organs like the brain, which primarily relies on glucose for fuel. Initially, the body turns to glycogen stores for energy, but these deplete quickly. Following this, the breakdown of muscle protein commences to provide amino acids.

When proteins are catabolized, their amino acids undergo deamination, a process where the amino group is removed. This results in the formation of ammonia, which the liver then converts into urea for excretion. This increased production of urea is evident in the rise of nitrogen excretion.

• The rapid breakdown of proteins supplies not just energy, but also essential building blocks for other metabolic pathways.
• This response is crucial, especially during the initial phase of fasting, to maintain blood glucose levels for energy.

Muscle Protein Breakdown

In the early stages of fasting, one of the body's main responses is the breakdown of muscle protein. This process is both a necessary and temporary measure to replenish the body's energy needs. Initially, as carbohydrate stores dwindle, the body shifts toward breaking down muscle proteins to release amino acids. These amino acids are then transformed into glucose through gluconeogenesis in the liver.

Muscle protein breakdown is a significant cause of the initial surge in nitrogen excretion during fasting. Each breakdown cycle releases nitrogen, which becomes part of the urea output measured in urine. Over time, however, the body aims to conserve muscle to preserve strength and function:

• The body decreases the reliance on amino acids once fat metabolism becomes more efficient.
• This change helps in adapting to longer durations of food scarcity, thus reducing the nitrogen excretion after several weeks.

Energy Metabolism in Fasting

Fasting challenges the body’s usual energy metabolism pathways, necessitating various adaptive mechanisms. Initially, energy is derived from glycogen; however, these reserves are limited. As fasting progresses, the body transitions primarily to lipid catabolism, utilizing fat stores as the main energy source. This shift conserves protein and minimizes nitrogen excretion.

But what happens once these fat stores are depleted? At this juncture, the body undergoes significant metabolic changes. Without fat available, it reverts to intense protein catabolism as its last resort energy source, leading again to an increase in nitrogen excretion. Key aspects include:

• Lipolysis, the breakdown of fats into fatty acids, becomes the primary energy process during fasting.
• Fatty acids are turned into ketone bodies in the liver, which many tissues can use, preserving muscle mass.
• As energy reserves continue to dwindle, the body shifts focus back to proteins, increasing nitrogen waste.