Types of nitrogen balance
Depending on the ratio of intake and excretion processes, biochemistry distinguishes three main states of nitrogen balance. Each of them is characteristic of certain physiological or pathological stages in a person's life.
- Positive balance. Occurs when the amount of incoming nitrogen exceeds its excretion. In this case, anabolism processes dominate: the body actively synthesizes new proteins, tissue growth occurs, and catabolism is minimized. Such a state is physiological for growing children, pregnant women, and athletes in the active muscle mass building phase. In addition, a positive balance is characteristic of the convalescence period—recovery from severe illnesses.
- Negative balance. Observed when nitrogen excretion from the body exceeds its dietary intake. This is a clear sign of the predominance of catabolism—the processes of tissue protein breakdown over biosynthesis. Negative values are usually recorded during prolonged fasting, against the background of severe debilitating diseases, and also as a natural process during aging.
- Nitrogen equilibrium (zero balance). A state in which the volume of dietary nitrogen intake strictly equals the volume excreted by the kidneys. This is the reference norm found in fully healthy adults receiving a nutritious and balanced diet.
Influence of essential amino acids on protein metabolism
To maintain a normal nitrogen equilibrium, the intake of certain structural components with food is critically important. These include essential amino acids such as leucine, valine, and phenylalanine. Their main feature is that they cannot be synthesized within the human body at all and must be regularly supplied from the outside.
The exclusion of at least one of these amino acids from the daily diet (e.g., leucine, valine, or phenylalanine) inevitably leads to severe pathological changes. First of all, this instantly affects the indicators of protein metabolism.
Mechanism of pathological changes
When essential amino acids are deficient, a cascade of biochemical reactions is triggered in the body, leading to a sharp shift in the balance:
- Due to the lack of necessary "building material", the biosynthesis of the body's own specific proteins completely stops.
- To compensate for the resulting amino acid deficiency for vital processes, the body begins to destroy its own tissues. The catabolism (breakdown) of existing tissue proteins sharply increases.
- Enhanced protein breakdown leads to the formation of a large amount of byproduct nitrogenous metabolites.
- These metabolic products are filtered by the kidneys, resulting in increased urinary nitrogen excretion.
The outcome of this process is the development of a persistent negative nitrogen balance, as the amount of excreted nitrogen begins to significantly exceed its intake.
Briefly: answers to topical questions
In the form of which specific chemical compounds is nitrogen excreted by the kidneys?
Nitrogen is excreted by the kidneys predominantly as the end products of nitrogen metabolism—urea and ammonium salts. Nitrogen can also be excreted via alternative pathways during metabolic blocks in the form of phenylacetylglutamine and hippuric acid, or as part of citrulline and argininosuccinate.
Which amino acids are classified as essential?
Essential amino acids are those that cannot be synthesized in the body and must be obtained from food. They include:
- Valine (Val)
- Isoleucine (Ile)
- Leucine (Leu)
- Lysine (Lys)
- Methionine (Met)
- Threonine (Thr)
- Tryptophan (Trp)
- Phenylalanine (Phe)
Through what pathways is nitrogen excreted from the body besides the kidneys?
Besides the kidneys, nitrogen is excreted from the body via feces and sweat.
What is Rubner's wear-and-tear coefficient?
Rubner's wear-and-tear coefficient represents the permissible protein loss in the absence of dietary protein, provided there is sufficient intake of other substances. This indicator ranges from 0.025 to 0.075 g of nitrogen per 1 kg of body weight per day.
POSITIVE — the body is building (children, muscle, pregnancy). NEGATIVE — the body is breaking down (disease, starvation, old age). ZERO — stable norm of a healthy adult.
Class and exam questions
In what forms is nitrogen excreted from the body by the kidneys?
Nitrogen is excreted by the kidneys primarily as urea, as well as part of various nitrogenous salts.
Why do athletes exhibit a positive nitrogen balance when gaining muscle mass?
During muscle mass gain in an athlete's body, anabolic processes predominate: the synthesis of new proteins and tissue growth proceed more actively than their breakdown. Therefore, more nitrogen enters than is excreted.
What will the exclusion of leucine from the diet lead to?
The exclusion of leucine will stop the biosynthesis of own proteins, provoke the breakdown of tissue proteins to make up for the deficit, and, consequently, lead to a negative nitrogen balance due to enhanced nitrogen excretion.
Can nitrogen equilibrium be observed in children?
No, for children, the physiological norm is a positive nitrogen balance, as their body is actively growing and needs enhanced protein synthesis. Equilibrium is characteristic of healthy adults.
What else to study on this topic
- Biochemical mechanisms of endogenous protein biosynthesis
- Pathways of tissue protein catabolism during starvation
- The role of leucine, valine, and phenylalanine in metabolism
- The process of urea and nitrogenous salt formation
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