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Digestion and absorption of food.

(revised 17th June 2013)

Our digestive tract is a highly sophisticated system, which uses chewing and churning, enzymes and water, acids and alkalis to break down the complex foods we eat into small, easily absorbable molecules. These molecules are then selectively filtered and absorbed by the body according to its needs. The unabsorbed portion is then passed out in the form of stools, along with other waste products.

When we chew food, the pieces of food are cut, torn and ground into smaller particles, while saliva is secreted and mixed with food in the mouth. The saliva is alkaline and contains an enzyme called amylase to break down starch. It also contains antibiotic like substances called lysozymes and white blood cells which help destroy micro-organisms like bacteria and viruses in the food. Proper chewing of food facilitates digestion, by ensuring that the digestive enzymes are mixed intimately with the food particles. This enables the enzymes to break down the food into an absorbable form. It also allows the lysozymes and white blood cells to seek out invading bacteria and viruses and destroy them before they can attack the body.

Food then passes into the stomach through a tube called the oesophagus. In the stomach, gastric juices containing hydrochloric acid and enzymes are secreted to sterilize the food and to break down protein. This hydrochloric acid has two important functions: It helps breakdown proteins and it destroys micro organisms like bacteria, parasites and viruses which may be present in the food. Hence the use of medication to reduce or neutralize stomach acid secretion, deprives us, of this important protective mechanism and makes us more prone to infections in the stomach and intestines. Heliobacter pylori is a bacteria often found in the gastric secretions of people with peptic ulcers and is often mistakenly blamed as a cause for peptic ulcers. Drugs used to reduce stomach acid secretions allow these bacteria to establish themselves in the stomach lining. An established H. pylori infection may need to be treated with antibiotics for an ulcer to heal.

The stomach also secretes an enzyme called pepsin which helps the digestion of protein. The stomach churns the food into a paste, mixing it, pouring fluid into it and then acts as a reservoir gradually releasing small quantities of food into the duodenum.

The duodenum is the first part of the small intestine where alkaline fluids like bile, pancreatic and intestinal juices are secreted. These juices are alkaline and very rich in enzymes and carry out a major portion of the digestion. They change the pH of the food from the stomach which is highly acidic to highly alkaline. This alkaline fluid is able to destroy any acid resistant bacteria which have survived the transit through the stomach.

The process of digestion is carried out by enzymes which break down complex foods like protein, carbohydrates and fat to amino acids, simple sugars like glucose, glycerine and fatty acids respectively. An enzyme is like a key which unlocks small molecules like glucose from starch, by adding water to it. The digestion of food is accompanied by a constant secretion of fluid. All these secretions aim to breakdown and liquefy the food, into a dilute solution containing glucose, amino acids, fatty acids and ions as the body can only absorb nutrition in this form.

Protein molecules are made up of several thousand to hundreds of thousands of amino acids. An enzyme acts like a key and helps break down the protein for absorption, by unlocking these molecules by adding water to the protein and setting free an amino acid. The liberation of each amino acid requires a molecule of water so the breakdown of dietary proteins for absorption requires the presence of a large amount of water in the digestive system.

A starch molecule is made up of several hundred thousand glucose molecules which have been linked together by removing water in the plant to make it unabsorbable and insoluble in water. An enzyme helps break down starch (contained in Cereals & Potatoes) into glucose, by adding water to the starch molecule and unlocking a glucose molecule for absorption. Each molecule of an enzyme is capable of unlocking several thousands of glucose molecules in a few seconds. This process is carried out by the addition of many thousands of molecules of water to starch to convert it into glucose which is the only form in which the body is able to absorb carbohydrates.

The limiting factors in digestion are the availability of water and that the enzyme has to move from molecule to molecule, unlocking it. Digestive enzymes work better and faster in a liquid medium. Hence liquid foods are easier to digest than solid foods. This fact is made use of in folk wisdom and hospitals where invalids, infants and elderly people are fed liquid diets like soups, porridge, gruel, khichdi etc. as they are easier to digest.

The commonly held belief that one should not drink water with meals is thus a fallacy. Drinking water with meals helps digestion, making the process of digestion quicker and more efficient allowing better digestion and absorption of food. It also helps to prevent over eating.

The body excretes most of the toxic waste products of metabolism and many drugs through the urine. The kidneys also balance the pH of the body by excretion through the urine. An adequate water intake ensures that the body is able to digest food well, and to eliminate waste products of metabolism through the Urine. Adequate removal of waste and toxic products from the body keeps us healthy.

water intake should be between 3 and 4 liters per day increasing with exercise & excessive perspiration. In cooler climates an intake of 2.5 to 3 liters is essential for maintenance of good health. A daily urine output of 1 to 1.5 liters per day indicates that the water intake is adequate.

The digested and liquefied food is then absorbed throughout the small intestine. The food matter which remains is then dehydrated further and solidified to be passed out of the body as Faeces. The process of digestion of food takes between 8-18 hours depending on the type of food which is eaten.

Sweets, fruits and highly processed starch are digested and absorbed within 3-4 hours. High protein & fatty foods take 12-18 hours for digestion and absorption.

Salads & high fibre foods take 12-24 hours for digestion and absorption.

The faeces are an important channel of elimination as the liver eliminates, toxins, heavy metals, pesticides and many waste products from the breakdown and repair of the body through the bile. These waste products give the faeces their brown color. High fibre foods aid this elimination while highly processed foods encourage the reabsorption of these waste products leading to illness.

For more information on digestive disorders and some ways to relieve them click this link.