Nutrition - How The Body Uses Food

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Explaining the process by which the body utilizes the food we eat, to support life and overall health. Knowing how this process works, helps improve our relationship with food.

How The Body Uses Food

In nutrition, the process of food utilization by the body, starts with ingestion. Ingestion is the consumption of a substance by a living being. In humans it normally is accomplished by taking in a substance through the mouth into the gastrointestinal tract; such as through eating or drinking.


Once food has been ingested, it has to be broken down before it is absorbed by the body. This break down of the ingested food into its respective components, occurs in the digestive system.


After food has been broken down into its components and absorbed by the body, it is then subjected to a series of biochemical processes known collectively as metabolism.


Both digestion and metabolism produce useful products but they also produce waste products as well. These waste products are removed from the body through; excretion and egestion.


In other words, digestion, metabolism, excretion, and egestion are the major processes in the body upon which nutrition works.   



The human digestive system consists of the gastrointestinal tract plus the accessory organs of digestion (the tongue, salivary glands, pancreas, liver, and gallbladder). In this system, the process of digestion has many stages, the first of which starts in the mouth. Digestion involves the breakdown of food into smaller and smaller components, until they can be absorbed and assimilated into the body.


Chewing, in which food is mixed with saliva begins the process of digestion. This produces a bolus which can be swallowed down the esophagus and into the stomach. Here it is mixed with gastric juice until it passes into the duodenum where it is mixed with a number of enzymes produced by the pancreas.


Saliva also contains a catalytic enzyme called amylase which starts to act on food in the mouth. Another digestive enzyme called lingual lipase is secreted by some of the lingual papillae on the tongue and also from serous glands in the main salivary glands.


Digestion is helped by the mastication of food by the teeth and also by the muscular actions of peristalsis and segmentation contractions. Gastric juice in the stomach is essential for the continuation of digestion as is the production of mucus in the stomach.


Peristalsis is the rhythmic contraction of muscles that begins in the esophagus and continues along the wall of the stomach and the rest of the gastrointestinal tract. This initially results in the production of chyme which when fully broken down in the small intestine is absorbed as chyle into the lymphatic system.


Most of the digestion of food takes place in the small intestine. Water and some minerals are reabsorbed back into the blood in the colon of the large intestine. The waste products of digestion are passed out via the rectum.



Once the food has been broken down by the digestive system to its respective components and these components absorbed by the body, it enters the metabolic phase. Metabolism is the set of life-sustaining chemical transformations within living cells.


The three main purposes of metabolism are the conversion of food/fuel to energy to run cellular processes, the conversion of food/fuel to building blocks for proteins, lipids, nucleic acids, and some carbohydrates, and the elimination of nitrogenous wastes. These enzyme-catalyzed reactions allow organisms to grow and reproduce, maintain their structures, and respond to their environments.


Metabolism is usually divided into two categories: catabolism, the breaking down of organic matter e.g. the breaking down of glucose to pyruvate, by cellular respiration, and anabolism; the building up of components of cells such as proteins and nucleic acids. Usually, breaking down releases energy and building up consumes energy.


The chemical reactions of metabolism are organized into metabolic pathways, in which one chemical is transformed through a series of steps into another chemical, by a sequence of enzymes.


Enzymes are crucial to metabolism because they allow organisms to drive desirable reactions that require energy that will not occur by themselves, by coupling them to spontaneous reactions that release energy. Enzymes act as catalysts that allow the reactions to proceed more rapidly. Enzymes also allow the regulation of metabolic pathways in response to changes in the cell's environment or to signals from other cells.


Waste removal

The processes of digestion and metabolism that occur in our bodies to break down and utilize the food that we eat for our own health, also produce waste products that have to be removed from the body. These waste products are removed in two main processes that some people tend to get mixed up; excretion and egestion. You really need to know the differences between these two processes which both move different types of waste from the body.



Excretion is concerned with the removal of metabolic waste from the body. The key point to remember here is that, excretory waste is that produced from metabolic reactions.


Metabolic reactions produce useful products but they also produce waste products as well. The waste products need to be removed so as to prevent the buildup of these toxic products that can actually be harmful to the body.


The key excretory organs that remove excretory waste from the body include; the lungs, the kidneys, and the skin.


The lungs are responsible for excreting expired air. This is air that we breathe out and contains carbon dioxide and water vapor. This happens through the process known as respiration.


The kidneys are responsible for excreting urine. Urine is excretory waste composed of mainly water, salts, and urea. It is removed from the body through a process known as urination.


The skin excretes heat and sweat. It’s responsible for getting rid of excess heat that builds up in the body, and sweat. Sweat contains water, salt, and urea, which are pretty similar substances to the content of the urine but in different proportions.



Egestion is the removal of undigested waste form the body. The key point to remember here is that, undigested waste is that from the digestive system. It comes mostly from fiber rich foods.


Fibrous waste is solid in nature and is removed from the body through the colon as solid waste, commonly referred to as stool.


Egestion therefore, is all about removing undigested food material from the colon out of the body as stool.



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