Digestion and absorbtion of various nutrients present in food
The digestion and absorption of macronutrients (carbohydrates, lipids, proteins) and micronutrients (vitamins and minerals), essential for the requirements of the human body, take place mostly in the small intestine. In most cases digestion occurs both in the gastrointestinal lumen and at the mucosal surface, continuing in some cases within the enterocytes themselves. Substrate-specific enzyme processes exist for polysaccharide carbohydrates and polypeptides, linked with digest-specific active co-transporters to aid absorption at the cellular level.
The hydrophobicity of fats makes lipid handling more complex, requiring the creation of intraluminal emulsions and micelles which present lipid degradation products to the enterocytes. The intestine (small bowel and colon) has additional mechanisms for absorption and re-absorption of water and key electrolytes. In most cases micro-nutrients are liberated from dietary macromolecules by non-specific digestion and then absorbed passively (or as other lipid-soluble moieties in the case of the fat-soluble vitamins) but some specific transporters are also involved as in the case of vitamin B12. Quantitatively minor but important digestion and nutrient absorption also occur in the colon.
Digestion happens in two ways: physical and chemical
- Physical – digestion occurs in the mouth where the teeth break up the food into smaller pieces. The food we eat needs to be broken down into small pieces which we chew up into even smaller ones before swallowing them. Once the food gets to the stomach the food is broken down further by the stomach’s muscular walls.
- Chemical – digestion is caused by digestive enzymes (Enzymes are biological catalysts found in all cells of the body) that are released at various points along the digestive system. Substances which our body needs cannot be absorbed into our blood until they have been broken down further and converted into small soluble chemicals.
Once the food molecules have been digested, they are small enough to diffuse into the bloodstream or lymph vessels. This is called absorption.
- Carbohydrate is turned into glucose, which our bodies need to make energy.
- Protein is turned into amino acids, required for cell growth and repair.
- Fats and oils are turned into fatty acids and glycerol, needed to insulate our bodies and make cell membranes.
- Vitamins and minerals do not have to be digested because they are already small enough to get into our blood.
The digestive process is as follows:
1. Chemical Digestion starts in the mouth through enzymes and saliva. The food is then moved to the stomach
2. The Small Intestine receives the food next and produces protease and lipase, food is absorbed into blood, large surface area by villi
3. Large Intestine (Colon) is where indigestible food is passed to. Any excess water is absorbed before it is excreted from the anus.
The following also play a part in the process
- The Stomach produces protease, HCl and pummels food with muscular walls
- The Gall Bladder stores bile after its been made by the liver
- The Pancreas produces enzymes: carbohydrase, lipase, protease
Villi are small projections covering the inside walls of the small instestine. Food products pass into the blood stream through the villi.
Villi are located in the small intestine, and absorb very small molecules into the blood stream. All other molecules (indigestible) are passed into the large intestine.
Digested molecules of food, as well as water and minerals from the diet, are absorbed from the cavity of the upper small intestine. The absorbed materials cross the mucosa into the blood, mainly, and are carried off in the bloodstream to other parts of the body for storage or further chemical change. This part of the digestive system process varies with different types of nutrients.
Nutrient Absorption in the Digestive System
An average American adult eats about half a pound of carbohydrate each day. Some of our most common foods contain mostly carbohydrates. Examples are bread, potatoes, pastries, candy, rice, spaghetti, fruits, and vegetables. Many of these foods contain both starch, which can be digested and fiber, which the body cannot digest.
The digestible carbohydrates are broken into simpler molecules by enzymes in the saliva, in juice produced by the pancreas, and in the lining of the small intestine. Starch is digested in two steps: First, an enzyme in the saliva and pancreatic juice breaks the starch into molecules called maltose; then an enzyme in the lining of the small intestine (maltase) splits the maltose into glucose molecules that can be absorbed into the blood. Glucose is carried through the bloodstream to the liver, where it is stored or used to provide energy for the work of the body.
Table sugar is another carbohydrate that must be digested to be useful. An enzyme in the lining of the small intestine digests table sugar into glucose and fructose, each of which can be absorbed from the intestinal cavity into the blood. Milk contains yet another type of sugar, lactose, which is changed into absorbable molecules by an enzyme called lactase, also found in the intestinal lining.
Foods such as meat, eggs, and beans consist of giant molecules of protein that must be digested by enzymes before they can be used to build and repair body tissues. An enzyme in the juice of the stomach starts the digestion of swallowed protein.
Further digestion of the protein is completed in the small intestine. Here, several enzymes from the pancreatic juice and the lining of the intestine carry out the breakdown of huge protein molecules into small molecules called amino acid. These small molecules can be absorbed from the hollow of the small intestine into the blood and then be carried to all parts of the body to build the walls and other parts of cells.
Fat molecules are a rich source of energy for the body. The first step in digestion of a fat such as butter is to dissolve it into the water content of the intestinal cavity. The bile acids produced by the liver act as natural detergents to dissolve fat in water and allow the enzymes to break the large fat molecules into smaller molecules, some of which are fatty acids and cholesterol.
The bile acids combine with the fatty acids and cholesterol and help these molecules to move into the cells of the mucosa. In these cells, the small molecules are formed back into large molecules, most of which pass into vessels (called lymphatics) near the intestine. These small vessels carry the reformed fat to the veins of the chest, and the blood carries the fat to storage depots in different parts of the body.
The large, hollow organs of the digestive system contain muscle that enables their walls to move. The movement of organ walls can propel food and liquid and also can mix the contents within each organ. Typical movement of the esophagus, stomach, and intestine is called peristalsis. The action of peristalsis looks like an ocean wave moving through the muscle. The muscle of the organ produces a narrowing and then propels the narrowed portion slowly down the length of the organ. These waves of narrowing push the food and fluid in front of them through each hollow organ.
Water and Salt
Most of the material absorbed from the cavity of the small intestine is water in which salt is dissolved. The salt and water come from the food and liquid we swallow and the juices secreted by the many digestive glands. In a healthy adult, more than a gallon of water containing over an ounce of salt is absorbed from the intestine every 24 hours.
A fascinating feature of the digestive system is that it contains its own regulators.
The major hormones that control the functions of the digestive system are produced and released by cells in the mucosa of the stomach and small intestine. These hormones are released into the blood of the digestive tract, travel back to the heart and through the arteries, and return to the digestive system, where they stimulate digestive juices and cause organ movement. The hormones that control digestion are gastrin, secretin, and cholecystokinin (CCK):
- Gastrin causes the stomach to produce an acid for dissolving and digesting some foods. It is also necessary for the normal growth of the lining of the stomach, small intestine, and colon.
- Secretin causes the pancreas to send out a digestive juice that is rich in bicarbonate. It stimulates the stomach to produce pepsin, an enzyme that digests protein, and it also stimulates the liver to produce bile.
- CCK causes the pancreas to grow and to produce the enzymes of pancreatic juice, and it causes the gallbladder to empty.
Two types of nerves help to control the action of the digestive system. Extrinsic (outside) nerves come to the digestive organs from the unconscious part of the brain or from the spinal cord. They release a chemical called acetylcholine and another called adrenaline. Acetylcholine causes the muscle of the digestive organs to squeeze with more force and increase the “push” of food and juice through the digestive tract. Acetylcholine also causes the stomach and pancreas to produce more digestive juice. Adrenaline relaxes the muscle of the stomach and intestine and decreases the flow of blood to these organs.
Even more important, though, are the intrinsic (inside) nerves, which make up a very dense network embedded in the walls of the esophagus, stomach, small intestine, and colon. The intrinsic nerves are triggered to act when the walls of the hollow organs are stretched by food. They release many different substances that speed up or delay the movement of food and the production of juices by the digestive organs.