Metabolism refers to all the chemical reactions that occur within the cells of the body.
Fuel metabolism refers to those reactions that involve the degradation, synthesis, and interconversions among the three classes of energy-rich organic molecules (carbohydrate, protein, and fat). Fuel metabolism is also called intermediary metabolism.
This refers to the build up or synthesis of larger organic macromolecules from the small organic molecular subunits.
Examples of anabolic reactions are glycogenesis, protein synthesis, and fat synthesis.
Glycogenesis = conversion of glucose to glycogen
Protein synthesis = formation of protein from amino acid
Fat synthesis (also called lipogenesis or triglyceride synthesis) = formation of triglyceride from fatty acid
Anabolic reactions require energy input in the form of ATP. These reactions lead to either
- the production of materials needed by the cells such as cellular structural proteins or secretory products, or
- Storage of excess ingested nutrients not immediately needed for energy production or needed as cellular building blocks. Excess nutrients are either stored as glycogen (storage form of glucose) of fat reservoirs.
This is the degradation or breakdown of large or energy-rich organic molecules within cells.
Catabolism involves two levels of breakdown:
- hydrolysis of large organic macromolecules into their smaller subunits, identical to the process of digestion except that the reactions occur within the cells of the body instead of within the GI tract lumen (e.g. release of fatty acids and glycerol by the catabolism of triglyceride, or release of glucose by the catabolism of stored glycogen), and
- oxidation of the smaller subunits, such as glucose, to produce energy
In adults, the rate of anabolism and catabolism are usually in balance, so the adult body remains in a dynamic steady state and appears unchanged despite the rapid turnover of the organic molecules that determine its structure and function.
During growth, anabolism exceeds catabolism.
Interconversions among organic molecules
In addition to being able to resynhtesize broken down organic molecules, back into the same type of molecules, many cells of body, especially liver cells, can convert most types of small organic molecules into other types (as in e.g., transforming amino acids into glucose or fatty acids. There are limits however. Essential nutrients, such as the essential amino acids and vitamins, cannot be formed in the body by conversion from another type of organic molecule.
The main fate of ingested carbohydrates and fats is catabolism to yield energy. Amino acids are predominantly used for protein synthesis but can be used to supply energy after being converted to carbohydrate or fat. Therefore, all three classes of foodstuffs can be used as fuel, and excesses of any foodstuff can be deposited as stored fuel.