Metabolism of Macronutrients
Metabolism is the series of chemical reactions that usually occur to obtain energy. In other words, the utilization of food in the body is termed as metabolism that it includes all the chemical and biochemical changes that occur in the food components to obtain energy.
Phases of metabolism: There are two phases of metabolism:
Anabolism: It is a process by which the absorbed products of food through digestion become part of the body tissues. It is used to replace body constituents and to provide new cellular materials for growth processes.
Catabolism: Usually oxidation reactions proceed such processes. Moreover, cellular substances are broken down to smaller molecules.
In the body cells, anabolic and catabolic processes occur simultaneously. In this article, the discussion will be about the metabolic processes of macronutrients e.g carbohydrates, proteins and fats that how they are oxidized to obtain energy.
Metabolism of Carbohydrates
Monosacharides are not metabolized being the simplest sugars. However, they are absorbed into the circulatory system as it is. While the digestible polysaccharides ad the disaccharides are metabolized and are converted into the glucose and their respective constituents respectively. Monosaccharides after absorption from small intestine are transported to the liver through portal vein where fructose and galactose are enzymatically converted into glucose. Glucose is obtained from:
Digestion of dietary carbohydrates
Through the breakdown of liver glycogen
By the conversion of other monosaccharides such as glucose and fructose
Deamination of amino acids
Conversion of glycerol
Glucose obtained from above sources is passed to the blood system and then diffused to body cells where they are used for energy purpose. The energy is obtained from glucose through a series of reactions or the catalytic pathway that is Glycolysis. As a result of this process water and carbon dioxide are produced. Glucose that is not utilized may go into the following forms:
Glucose is converted into the glycogen through glycogenesis and then is stored in the liver and skeletal muscles from where it is readily available as energy source. Muscles can store about 110 g of glycogen while the liver can store twice of this amount that is 245 g.
Excess glucose is changed into fatty acids through a process called lipogenesis and stored in the adipose tissue.
Some glucose derivatives are used to produce non-essential amino acids such as alanine by adding amino groups. Heparin, nucleic acids and components of connective tissues are formed from carbohydrates derivatives.
Fats are not metabolized in the whole digestive system except in the small intestine where they after digestion are passed to the lacteals in the microvilli. Before getting into the lacteals the broken products of fats that is glycerol and fatty acids, they are again recombined into triglycerides in the epithelial cells of microvilli. From the lacteals after conjugation with proteins-lipoproteins, then are passed to lymph. They enter the venous blood at the junction of the jugular and sub-clavian veins. In the blood plasma, lipoproteins are hydrolyzed by enzymes and then passed into body cells. Free glycerols are obtained from the digestion of carbohydrates. They are absorbed into the portal system.
The fatty acid molecules are broken down into Acetyl-coA. This enters into the Krebs cycle as does the Acetyl-coA from pyruvic acid. It is then completely oxidized into water and carbon dioxide. Fat are also stored in the adipose tissues by the liver transformation. Of the dietary sterols, only cholesterol is readily absorbed.
Proteins are usually absorbed in the form of amino acids. But some amounts of dipeptides are also absorbed while infants have the ability to absorb some proteins intact. From the small intestine the amino acids are carried to the liver through blood that regulates the body’s amino acid need. The amino acid may pass to:
Amino acids may be stored in the amino acid pool that is present in the extracellular fluid or blood, from where they are used to produce the structural proteins, specific enzymes, antibodies and hormones.
Some amino acids are converted into the other amino acids if needed.
They are also used for the production of the non-protein nitrogenous compounds such as haeme.
Amino acids are converted into the glucose and then are oxidized to obtain energy.
They may be converted into the fats when the amino acid need of the body is fulfilled.
So, in a nutshell the metabolism of macromolecules is mainly meant for the energy provision to keep the body alive and the excess of all macronutrients is stored in the body in the ultimate form of fat.
Metabolism of Macronutrients