What is the Digestion Process?
Where Does Digestion Occur?
And What Makes It So Important and Complicated?
The whole process of digestion involves many different organs, which are called the digestive system, and include the Mouth, Esophagus, Stomach, Small Intestines, Large Intestines, Rectum and Anus. Other organs are involved in supporting the digestive process as well, but are not technically considered part of the digestive system. These organs are the tongue, the glands in the mouth that produce saliva, the Pancreas, Liver and Gallbladder, and we will address them here so you can get a fuller and more complete understanding of the whole Digestive Process and how it works in synchronicity with its partnered organs.
What happens in the Mouth?
Digestion begins in the mouth with the chewing of food (mastication). Mastication not only breaks down very large aggregates of food molecules into smaller particles and allows saliva and enzymes to enter inside the larger food complexes, but also sets off a signaling message to the body to start the entire digestive process. Research has shown that the activation of taste receptors in your mouth and the physical process of mastication signal the neural (nervous) system. For example, the taste of food can trigger the stomach lining to produce acid, a process called the cephalic phase of digestion; therefore, your stomach begins to respond to food even before any food leaves your mouth.
Saliva is secreted by the salivary glands in your mouth and moistens the food to improve the chewing and grinding. Saliva also contains some enzymes that begin the breakdown of starches and fats. For example, carbohydrate digestion begins with the salivary enzyme alpha-amylase, and fat digestion begins with the secretion of the enzyme lingual lipase by glands under your tongue.
What happens in the Esophagus?
The esophagus, sometimes called the gullet, connects the mouth to the stomach. It delivers the saliva-mixed food from the mouth to the stomach and serves as an air lock between the outside world and the digestive tract. The importance of the esophagus’ ability to separate the mouth and stomach can be seen in the condition known as GERD (gastroesophageal reflux disease), in which the esophageal barrier is not effective, so the acid contents of the stomach can escape into the esophagus. Everyone experiences some gastroesophageal reflux, and the esophagus, with the help of another helpful component of saliva, salivary bicarbonate, has the ability to clear any stomach acid that escapes. In many people, however, this reflux occurs more frequently than it should, causing pain and affecting healthy digestion. This situation is called GERD and is one of the most commonly seen conditions in medicine today.
What happens in the Stomach?
The esophagus opens into the stomach, which is a large chamber consisting of the fundus, the body and then the antrum. The entire involvement of the stomach in digestion is called the gastric phase of digestion. The stomach is the primary place where proteins are disassembled and broken down into small peptides. Due to its acidic environment, the stomach is also a decontamination chamber for bacteria and other potentially toxic microorganisms that may have entered your gastrointestinal system through your mouth.
The fundus and body of the stomach, which are usually referred to together and constitute the majority of the stomach in size, are where the stomach stores food before it is delivered to the intestine. When the food enters the fundus and body of the stomach, the lining of the fundus (called the gastric fundal mucosa) produces hydrocholoric acid (HCl). This acidic environment is critical for destroying toxins in foods, such as bacteria, as well as for untwisting the complex three-dimensional protein chains, a process called denaturation of the proteins.
The gastric fundus mucosa also secretes the enzyme pepsinogen, which is present in the stomach much of the time but is inactive until the acid is present, when it becomes activated as pepsin. Pepsin acts on the denatured proteins by hydrolyzing, or cutting, the bonds between amino acids in the protein chain, resulting in several smaller chains, or peptides.
Fat hydrolysis is very active in the stomach. The fats have already been exposed to lipase in the saliva, which begins the hydrolysis, but it is the gastric lipase, secreted by the stomach, that is primarily responsible for fat hydrolysis in humans.
The antrum, or lower part of the stomach, is the site for the stomach’s grinding action and contains a sensor mechanism, called gastrin, for regulating the level of acid produced in the body of the stomach. The antrum also controls the emptying of food into the intestine through the pyloric sphincter. This way the food can be delivered into the intestine in a controlled manner. Once the food-acid-enzyme mixture leaves the stomach, it is called chyme. The movement of chyme through the pyloric sphincter stimulates the intestine to release the hormones secretin and cholecystokinin, which signal the pancreas to release its contents, the pancreatic juice, inside the lumen (the lining) of the duodenum (the first segment of the small intestine).
What happens in the Small Intestine?
The small intestine, which is specifically designed to maximize the digestion and absorption process, has an expanded surface area with inner folds, called plicae, villi and microvilli, to increase its surface area and enhance its ability to absorb nutrients. All together, this surface is called the brush border of the small intestine. Some enzymes are present on the surface of the brush border, such as disaccharidases like sucrase, maltase, and lactose, which hydrolyze disugars (sugars composed of two monosaccharides) to their two individual sugar molecules.
The duodenum, the part of the small intestine that is closest to the stomach, is a neutralization chamber in which the chyme from the stomach is mixed with bicarbonate, which appears again, this time in the pancreatic juice. Bicarbonate lessens the chyme’s acidity, thus allowing more enzymes to function and furthering the breakdown of macromolecules still present. The pancreatic juice also contains many of the enzymes necessary for digestion of proteins, such as trypsin and chymotrypsin, enzymes that cut proteins and peptides down into one-, two-, and three-amino acid chains; and amylase, an enzyme that continues the hydrolysis of starch.
A few nutrients, like iron and calcium, are taken up most efficiently in the duodenum; however, the jejunum, the middle section of the small intestine, is the place where most nutrients are actively absorbed. The amino acids as well as most vitamins and minerals are absorbed in the jejunum. The process of absorption used by the jejunum is called active absorption since your body uses energy to select the exact nutrients it needs. Protein carriers or channels hook-up to these nutrients and take them through the cell wall of the jejunum and into the portal vein, which carries them to the liver.
Active fat absorption also occurs in the duodenum and the jejunum, and requires that the fat be put into small aggregates that can be transported into your body directly. The body uses bile as a detergent to solubilize the fat. Bile is produced by the liver, stored in the gall bladder, and released into the duodenum and jejunum after a meal. It then can form miscelles, small fat droplets, for fat absorption. This process is particularly important for the absorption of the fat-soluble vitamins (vitamins A, D, E, and K), and for cholesterol absorption.
The majority of starch is also digested in the duodenum and jejunum, the first and second segments of the small intestine. The monosaccharide products of carbohydrate digestion, glucose and galactose, are actively absorbed through the intestine by a process that requires energy. Fructose, another common monosaccharide product of carbohydrate digestion, and also a common sweetener for many processed foods, is absorbed more slowly by a process called facilitated transport. Facilitated transport does not require energy.
The ileum is the final part of the small intestine. The ileum is responsible for completing the digestion of nutrients and for reabsorbing the bile salts that have helped to solubilize (keep in solution), the fats. Although most nutrients are absorbed in the duodenum and jejunum, the first two segments of the small intestine, the ileum is the place where vitamin B12 is selectively absorbed into your body.
At the end of transport through the small intestine, the chyme has been depleted of around 90 percent of its vitamins and minerals and the majority of its other nutrients. In addition, around eight to 10 liters of fluid is also absorbed in the small intestine each day. Complex carbohydrates that resist the enzyme degradation, such as fiber and resistant starch, remain, as do a small amount of other food molecules and nutrients that have escaped the digestion process. For example, about 3-5% of ingested protein normally escapes digestion and continues to the large intestine.
What happens in the Large Intestine?
The large intestine is not designed for enhancing absorption but is particularly specialized to conserve the sodium and water that escape absorption in the small intestine, although it only transports about one liter of fluid per day. The large intestine is about five feet long, including its final segments, the colon and the rectum.
It is interesting, given that most digestion and absorption occurs prior to the large intestine, that food, which at this point is primarily fiber, will spend more time in your large intestine than anywhere else during digestion. On average, food travels through the stomach in 1/2 to two hours, continues through the small intestine over the next two to six hours, and spends six to 72 hours in your large intestine before final removal by defecation.
One reason food stays longer in the large intestine may be that the large intestine is capable of generating nutrients from food. The food that makes it into the large intestine is primarily fiber, and the large intestine contains an ecosystem of bacteria that can ferment much of this fiber, producing many nutrients necessary for the health of the colon cells. Colonic fermentation also produces a series of short-chain fatty acids, including proprionate, acetate, and butyrate, which are required for healthy colonic cell growth and have many other health promoting functions in your body.
The friendly bacteria that are responsible for the primary amount of healthy colonic fermentation are called the probiotics (pro-life) and include the Bifidobacteria and Lactobaccillus genuses. Along with providing beneficial fermentation products, probiotic bacteria keep pathogenic, or disease-promoting bacteria, from colonizing your colon. Certain fibers in food, called prebiotics, specifically support these probiotic bacteria. Prebiotics include such molecules as inulin and fructooligosaccharides, which are found in chicory and Jerusalem artichoke, and may include some other carbohydrates such as galactooligosaccharides, arabinogalactans, and arabinoxylans, which are found in soy and rice fibers, and in larch tree extracts.
Some fiber isn’t fermented, but it is also important because it provides bulk for stool excretion, and can bind toxins and waste products for their removal through the stool. Finally, the rectum and the anus allow for controlled elimination of stool.
What happens in the Pancreas?
The pancreas can be thought of as a protein factory. It produces and secretes many of the enzymes necessary for digestion, which include the enzymes that digest protein (trypsin, chymotryosin, carboxypeptidase, and elastase), enzymes that digest fat (lipase and phospholipase), and the enzyme that digests carbohydrate (alpha-amylase). The pancreas releases these enzymes in a pancreatic juice, which is enriched with bicarbonate. The bicarbonate is used to neutralize the acid in chyme. More than a liter of pancreatic juice is released per day in response to signals from eating a meal.
Since your body’s tissues are made of protein, the pancreatic enzymes that digest protein have the ability to digest your own tissues. Your body has an intricate protection from self-digestion by these enzymes. The stomach and intestinal tract lining have a mucous layer protecting the tissue from direct digestion by these enzymes. The pancreas uses other mechanisms for protection. Primarily, it produces the enzymes in an inactive form, called zymogens or proenzymes. For example, trypsin is produced as the inactive proenzyme trypsinogen. Trypsinogen is transported to the intestine where it is activated to trypsin by a protease enzyme on the brush border of the intestinal cells. All pancreatic enzymes except lipase and alpha-amylase are secreted as proenzymes, and are therefore inactive within the pancreas.
What happens in the Liver?
The liver is one of the most active organs in your body. The liver is the clearinghouse for all nutrient absorption through the gastrointestinal system. The liver reviews the compounds that have been taken in and has the ability to distinguish toxins and other molecules. It has a detoxification system, in which drugs and toxins are chemically converted to molecules that can be eliminated through the kidneys (urine) or the intestine (stool). The liver is also responsible for synthesizing most of the proteins that circulate in your blood, and it produces bile, which is important for the digestion of fats and is used for the excretion of cholesterol and other fat-soluble molecules.
The liver is the major organ involved in maintaining healthy blood sugar (glucose) levels. It monitors your body’s glucose needs and provides glucose from digestion, or obtains glucose by breaking down glycogen, the form in which glucose is stored in your liver. The liver has only about a 24-hour supply of glycogen. In prolonged fasting, when glucose is not provided in the diet and glycogen stores have been used, your liver will synthesize glucose from amino acids and other molecules.
The liver is also the primary organ in which fats are metabolized. The liver can make cholesterol and is the primary place where cholesterol is removed from the blood. The liver eliminates cholesterol in the form of bile acids. Every day, your liver secretes about 500 milliliters of bile acids, which are used during digestion to solubilize fats.
What happens in the Gallbladder?
The gallbladder is the storage site for the bile acids produced by the liver. After a meal is consumed, the gallbladder is signaled to release its contents into the duodenum and jejunum, where they are available for fat digestion.
Ways to Support Healthy Digestion
Healthy digestion requires support for all the different components of digestion:
- Chew thoroughly. Chewing is the physical process of breaking the food down into smaller fragments. Thorough chewing mixes food well with saliva, which moistens the food particles and provides a means for enzymes, like amylase and lipase, to get to the pieces of food and begin the process of starch and fat digestion. Chewing also signals the body to begin the digestion process, alerting the stomach to prepare to make stomach acid, and signaling the pancreas to prepare to secrete its contents into the lumen of the small intestinal tract.When a meal is not well chewed, the food fragments are too big. Since the digestive enzymes can only work on the surface of the food fragments, inadequate chewing results in incomplete digestion. This means not only nutrients being left in the food and unabsorbed, but also extra food for bacteria in the colon. This extra bacterial food results in bacterial overgrowth, gas and symptoms of indigestion.Eating should always begin with thorough chewing of food to allow for complete digestion to occur.
- Ensure adequate amounts of digestive factors. After chewing, the food’s next stop is the stomach, where an adequate amount of stomach acid (hydrochloric acid) is the next necessity. Stomach acid is required for adequate breakdown of proteins. Without adequate stomach acid, not only is protein digestion ineffective, but also digestion of vitamin B12 is seriously affected. Vitamin B12 digestion and absorption requires that it be liberated from protein. In addition, intrinsic factor, the protein that is necessary for vitamin B12 absorption, is low when stomach acid is low.Low stomach acid (hypochlorhydria) is common, especially in older people since as we age, we make less stomach acid. Research suggests that as many as half of the people over 60 years old have hypochlorhydria. A variety of factors can inhibit sufficient stomach acid production including the pathogenic bacteria, Helicobacter pylori, and frequent use of antacids. Hypochlorhydria is also associated with many diseases, such as asthma, celiac sprue, hepatitis, rheumatoid arthritis, osteoporosis, and diabetes mellitus. Signs of hypochlorhydria include a sense of fullness after eating, bloating, excessive belching, indigestion, multiple food allergies, undigested food in the stool, and peeling and cracked fingernails.In addition to hydrochloric acid, the production of pancreatic enzymes and bicarbonate is also compromised in some people. If necessary, these digestive factors can be replaced with appropriate supplementation. Digestive enzyme support can also be obtained from fresh pineapple or papaya, which contain the enzyme bromelain, and other fresh vegetables and herbs. Processed foods, like canned pineapple, contain little enzyme activity since digestive enzymes are proteins, which are destroyed by heating, such as in the sterilization process. So beginning a meal with fresh fruits or salad can provide support for healthy digestion.
- Identify and eliminate food allergens. The intestinal brush border (the absorptive surface of the small intestine) can be negatively affected by food allergies, which cause inflammation along the intestinal tract wall. When a food allergic reaction occurs, the immune system perceives specific food molecules as hostile invaders, and forms antibodies, which latch on to these allergens to assist in their removal. As part of the immune system’s defensive action against food allergens, inflammation can occur along the intestinal tract lining, interrupting the absorption process and causing damage to the lining. Gastrointestinal inflammatory diseases-such as diverticulosis or inflammatory bowel disease-and celiac sprue (intolerance of gluten found in wheat products) also result in damage to the intestinal wall. Most common food allergens include milk proteins, wheat, soy, some shellfish, and peanuts.
- Support the gastrointestinal barrier. The gastrointestinal cell wall is the barrier between what you ingest and the inside of your body; therefore, the integrity of this barrier is vital to your health. Support for the mucus that covers the cells in the gastrointestinal tract is very important, especially in the stomach. The mucus layer is one way the stomach and upper small intestine protect themselves against the damaging effects of stomach acid. Alcohol, over-the-counter anti-inflammatory drugs, called NSAIDS (e.g. aspirin), and the pathogenic bacteria, Helicobacter pylori can reduce the mucous layer, leading to lesions in the stomach and small intestinal tract walls.Choline provides nutritional support for a healthy mucous layer and is found in vegetables such as cauliflower and lettuce. Choline can be obtained from lecithin (phosphatidylcholine) as well, which is high in eggs and soybeans. Some foods also help combat or protect against the damage of Helicobacter pylori, and these include catechins found in green tea, some spices such as cinnamon, carotenoids found in vegetables, and vitamin C, found in citrus foods.
- Provide a healing environment for the small intestine. Research studies have shown that the small intestinal tract barrier can become leaky under some conditions. That is, the cells loose their attachments to each other, resulting in a wall with holes between the cells instead of the cells forming a strong, connected and continuous surface. When this “leaky gut” happens, molecules can get inside the body that normally wouldn’t be transported through the intestinal cell wall. Furthermore, studies have shown that this leaky gut can also cause problems in the normal transport of nutrients. This is probably because most nutrients are taken into the body through the cells in the intestinal wall by the selective process of active transport, and they may need to go through the cells and not around them to get to the right transport systems in your body. Therefore, with leaky gut, the things that shouldn’t get in do, and those that should can’t get where they need to be for adequate transport through the body. The result is the body doesn’t get the nutrition it needs.Anything that irritates the lining of the gastrointestinal tract can cause leaky gut, but a major contributor is inflammation (e.g., food allergies). Leaky gut occurs under stress (see below), and is found after radiation treatments for cancer, after some chemotherapy, with diseases such as inflammatory bowel disease, and with bacterial infections, which can result in bacterial overgrowth in the small intestine.Eliminating foods to which you are intolerant or allergic can help provide a healing environment in the small intestine. Carotenoids, (a precursor to vitamin A), may be particularly important since vitamin A supports the maturation of epithelial cells, which are the type of cell that line the intestinal tract, and it is the mature epithelial cells that form the strongest barrier in the intestinal tract. Carotenoids are found at high levels in vegetables, especially the orange- and red-colored vegetables.Glutathione, a small peptide found in the highest concentrations in fresh vegetables, fruits, and lean meats is also beneficial to the small intestine, since it can directly act as an antioxidant in the intestinal tract and help decrease damaging molecules that may be produced during inflammation. Vitamin C, from citrus fruits, and vitamin E, found in whole grain cereals and nut oils, are important antioxidants for the small intestine and work with glutathione to support intestinal healing.The cells that line the intestinal tract need fuel to continue their process of nutrient uptake. The preferred fuel for these cells is the amino acid glutamine, which can be obtained from proteins. Some studies have shown that short-chain fatty acids may also support the small intestinal tract barrier because they can serve as an alternate fuel for the cells that make up the intestinal lining. The small intestinal tract cells also require energy to maintain integrity of the cell wall, and production of energy requires healthy levels of vitamin B5. Mushrooms, cauliflower, sunflower seeds, corn, broccoli, and yogurt are concentrated sources of vitamin B5. The intestinal tract cells also require a number of vitamins, so adequate overall nutrition is necessary.
- Support the growth of probiotic bacteria. When a good balance of probiotic bacteria have colonized the colon, they crowd out pathogenic bacteria and other microorganisms that compromise your health, preventing them from growing. By fermenting the fiber your body couldn’t directly digest, these healthy colonic bacteria also produce short-chain fatty acids that the cells of the colon use for their own nourishment. In addition, these short-chain fatty acids are absorbed into the body and have beneficial effects on the small intestine and the system in general. For example, they may help maintain healthy blood sugar and lipid (fat) levels, and may also increase the amount of calcium taken in by the small intestine, and promote the movement of food through the intestinal tract. Foods that will supply probiotic bacteria include some yogurts, kefir, and other foods that have been fermented with Lactobacillus or contain Bifidobacteria, the beneficial types of bacteria. Foods that will nourish probiotic bacteria include foods that contain soy fiber, inulin (from chicory or Jerusalem artichoke), and rice fiber.
- Provide for healthy intestinal transit. The movement of the food, or chyme, through the digestive tract is very important. Healthy intestinal transit is supported, in part, by the short-chain fatty acids produced by fermentation of prebiotic fibers in the colon. Fiber, in general, supports overall transit of the chyme and healthy elimination. Some fibers, like those found in rye, wheat and flax, also can bind to environmental toxins, such as pesticides, and carry them through the digestive tract for direct elimination, decreasing the amount that is absorbed into your body.
- Learn how to deal with stress effectively. Research has shown that the intestine responds negatively to stress, during which the intestinal lining becomes leaky, absorption is less effective, and your body is unable to selectively take up the nutrients it needs. The reasons for these effects of stress on the intestinal tract are not entirely known, however many neurotransmitters (brain-produced signaling molecules) are found surrounding the intestinal tract. Furthermore, neurotransmitter receptors, which can bind and respond to these signaling molecules, are located along the intestinal tract. Therefore, it is known that brain signaling molecules can affect the intestinal tract. Foods with a calming effect include herb teas, like chamomile. Alcohol, caffeine, and refined carbohydrates, like table sugar, should be avoided. Eating meals at regular times and in a relaxed environment can also help decrease stress.
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