METABOLIC WATER: PRODUCTION AND IMPORTANCE - CHEMISTRY - 2025

The metabolic water is the water generated in an organism or living being as a product of the oxidative metabolism of nutrients. Through catabolism, the degradation of nutrients occurs, with the production of energy, carbon dioxide and metabolic water.

Metabolic water is also called combustion, oxidation water or water produced endogenously by the body. It represents a small portion, just 8 to 10%, of the total water that the body needs.

Metabolic water generated within the mitochondria in the electron transport chain. Source: Mitochondrial_electron_transport_chain-Etc4.svg: Fvasconcellos 22:35, 9 September 2007 (UTC) derivative work: Masur

An average adult produces about 300 to 350 mL of metabolic water per day. This amount of water generated in metabolism constitutes only a small fraction of the water that the body needs to live.

The production of metabolic water is vital for the subsistence of some animals in the desert, such as camels. It has been described as crucial for insects and other animals that live in dry environments.

It is an indicator of the body's metabolic rate; however, its determination is not easy. It is easier to measure the CO ₂ expired or exhaled as a result of oxidative metabolism, than the amount of metabolic water generated.

Metabolic water production

Metabolic water is generated in the body during the enzymatic breakdown of organic substances such as fats, carbohydrates and proteins. Complete oxidation of these nutrients occurs by cellular metabolism carried out under aerobic conditions or in the presence of oxygen.

Nutrient oxidation is a complex and slow process that includes various chemical reactions that occur in catabolic stages or pathways. Several of these routes at the beginning are specific for each type of nutrient, ending the process with routes or reactions that are common.

This oxidation ends with cellular respiration in the inner membrane of the mitochondria with the production of energy or ATP (Adenosine triphosphate).

Simultaneously with oxidative phosphorylation (ATP production), CO ₂ and metabolic water are formed. There are four enzymes in the membrane: NADH dehydrogenase, succinic dehydrogenase, cytochrome C, and cytochrome oxidase (also known as the flavoprotein-cytochrome system).

In this system, the electrons and hydrogens of NADH and FADH are captured, product of the reactions of catabolism or oxidation of nutrients. Finally, in this enzymatic complex is where these hydrogens join oxygen to produce metabolic water.

From fats

The oxidation of fats or lipids occurs with the oxidation of free fatty acids, such as tripalmitate, for example. This catabolic process includes beta-oxidation, by which the fatty acid is oxidized to form acetyl-CoA that goes to the Krebs cycle.

Once acetyl-CoA is incorporated into the cycle, the reducing equivalents NADH and FADH _{2 are formed,} which pass into the respiratory chain. Finally, the electrons from the hydrogens are transported to the enzymes of the chain originating ATP, CO ₂ and metabolic water.

The formation of metabolic water from the oxidation of the fatty acid tripalmitate can be summarized as follows:

2C ₅₁ H ₉₈ O ₆ + 145O ₂ → 102CO ₂ + 98H ₂ O

The catabolism of the fat stored in the hump of camels provides them with the water they need to survive in desert areas.

From carbohydrates

The oxidative pathway for carbohydrates includes the reactions of glycolysis with the production of pyruvic acid and a water molecule. In the presence of oxygen, pyruvic acid enters the mitochondrial matrix, where it is transformed into acetyl-CoA, joining the Krebs cycle.

This cycle is the common pathway of nutrient metabolism, the reducing equivalents produced are oxidized in the respiratory chain.

The following equation can be used to summarize the metabolic water production from the complete oxidation of glucose:

C ₆ H ₁₂ O ₆ + 6O ₂ → 6CO ₂ + 6H ₂ O

When glycogen, which is a complex carbohydrate, is oxidized through a process called glycogenolysis, metabolic water and glucose are released.

From proteins

Protein catabolism is more complex than that described for fats and carbohydrates, since proteins are not totally oxidized. Among the end products of protein catabolism are urea, some nitrogenous compounds, as well as CO ₂ and metabolic water.

Production balance

The approximate balance of metabolic water production can be expressed by the oxidation of 100 g of each nutrient. It can also be considered an approximate or average of the amount of water produced in 24 hours or a day.

The production balance is close to 110 g of water for every 100 g of oxidized fat. The amount of metabolic water produced in 24 hours from the oxidation of fatty acids is 107 mL.

Approximately 60 g of metabolic water are produced for every 100 g of metabolically oxidized carbohydrates in the body. The amount produced from carbohydrates on average during a day is close to 55 mL.

And with proteins, less water is generated, just about 42 g for every 100 g of protein. The protein oxidation water generated during one day on average is equal to 41 mL.

It was mentioned earlier that an adult produces only 8 to 10% of metabolic water, of the total water he needs. Your body, in good health, provides approximately 300 to 350 mL of metabolic water each day.

Importance

As mentioned, its contribution to the daily amount of water that the body needs is considered little. However, its contribution is significant in meeting the athlete's need for fluids during prolonged exercise.

By oxidation of nutrients, approximately 300 to 350 mL of metabolic water are produced per day. However, its production also increases in those cases in which the water intake decreases.

Although the physiological mechanisms are not well defined, the production of metabolic water constitutes a compensatory mechanism for the loss of body fluids. Although its contribution to the body's water homeostasis tends to be ignored, it is important to consider.

There are living things that depend exclusively on metabolic water for their subsistence, such as camels that live in the desert. Migratory birds that make long nonstop flights also depend exclusively on it for survival, and so do several species of insects.

References

1. Diaz, OG (1987). Biochemistry and Physiology. Mexico: Interamerican.
2. Edney EB (1977) Metabolic Water. In: Water Balance in Land Arthropods. Zoophysiology and Ecology, vol 9. Springer, Berlin, Heidelberg.
3. Ganong, WF (2004). Medical Physiology. (19 ^a Edition). Mexico: The Modern Manual.
4. Murray, RK, Granner, DK Mayes, PA and Rodwell, VW (1992). Harper's Biochemistry. (12 ^th Edition). Mexico: The Modern Manual.
5. Wikipedia. (2019). Metabolic water. Recovered from: en.wikipedia.org