3 Digestion and Absorption of Nutrients

3.1 Overview

To survive, your body must have a system for transforming food and drink into nutrients that it can absorb and use. Digestion begins when you see, smell, feel, or taste foods. The hormonal and nervous systems signal the gastrointestinal tract that food is on the way. Muscles flex and digestive secretions flow. Cooperating organs including the mouth, esophagus, stomach, small and large intestines, pancreas, liver, and gall bladder orchestrate digestion.

To get the nourishment you need, nutrients must successfully traverse the gastrointestinal tract (GIT). The GIT is a long, muscular tube that extends from the mouth to the anus. Foods contain macronutrients that are broken down during digestion into smaller units that are absorbed by cells lining the small intestine. Ultimately, nutrients traverse absorptive cells and are released into the bloodstream or lymph system and transported throughout the body.

Sometimes problems arise such as regurgitation of stomach contents into the esophagus, ulcers in the stomach, a blocked bile duct, or insufficient enzymes. Knowing more about the digestive process helps you avoid these problems and stay healthy.

Objectives

• Describe the role of the mouth, teeth, tongue, epiglottis, and esophagus in chewing, lubricating, and delivering food and drink to the stomach and beyond
• Explain the cause of heartburn or gastroesophageal reflux disease
• Associate the small intestine and villi with their digestive role
• Connect the large intestine to its function

3.2 A System of Muscles, Organs, and Enzymes

Overview

The GIT is a long tube that extends from the mouth to the anus. It consists of longitudinal and circular muscles that contract in waves to propel substances along. Hormones and enzymes assist in the breakdown of food in a process called digestion.

The GIT includes the mouth, esophagus, stomach, small and large intestines, rectum, and anus. Organs that provide substances needed for digestion include the pancreas, gall bladder, and liver. Together, these organs form a system that efficiently transforms the foods that you eat into the nutrients that you need to maintain your body.

Key Concepts

• Hormones and the nervous system and how they express regulatory control
• Mechanisms for breaking down macronutrients during digestion
• Workings of the tongue and its role in taste

Hormones and Nervous System

The body’s hormonal and nervous systems have regulatory control of nutrients. A hormone, such as insulin, is produced by an organ (pancreas) in response to a need. It has a specific site from which it enters the bloodstream, where it begins its journey to target cells that it influences. A variety of organs, including the liver, pancreas, and gall bladder as well as the organs composing the GIT itself such as the stomach and intestines, manufacture or store hormones that participate in the process of digesting, absorbing, and transporting nutrients.

After a meal high in carbohydrates, the pancreas responds to rising levels of blood glucose by increasing its release of insulin. Insulin is a hormone that stimulates body cells to actively absorb glucose. As a result, glucose quickly moves out of the bloodstream and into cells. Insulin, then, is a hormone that lowers blood glucose levels. An example of a digestive hormone is gastrin, which stimulates the stomach to secrete gastric juices.

The enzymes involved in digestion include salivary amylase, which acts on polysaccharides (carbohydrates); pancreatic amylase, also on polysaccharides; maltase, on maltose a disaccharide (short-chain carbohydrate); pepsin, on proteins; trypsin and chymotrypsin, on peptides (short-string amino acids); peptidases, on peptides; and lipase, on lipids (fats). In addition, nursing infants produce lactase, an enzyme that digests lactose, a simple carbohydrate found in milk.

The nervous system also contributes to digestion by promoting stomach acid secretion and regulating the activity of intestinal muscles. Our five senses detect cues in our environment that indicate the availability of food and drink. In response, the nervous system sends a signal to the gastrointestinal tract, telling it of an impending meal.

Nutrients as Raw Materials

Nutrients are provided by the foods that you eat. Nutrients are the raw materials for the chemical processes that take place in all living cells. Your DNA determines how cells in your body use nutrients. Both essential and nonessential nutrients supply materials needed to build and maintain tissues.

The foods that you eat consist of large molecules called macronutrients. Your body must have a mechanism for breaking macronutrients into smaller units that can be absorbed across the lining of the small intestine. The process by which this is done is called digestion.

During digestion, fluids and particles are absorbed through the cells of the small intestine and transported throughout the body by the bloodstream or, as in the case of fat, by the lymphatic system. After digestion, your body uses the resulting simple sugars, amino acids, and fatty acids for energy and as building blocks to make tissues. Absorbed vitamins, minerals, and water are used in various metabolic processes throughout the body.

Digestion Begins

Digestion begins in your mouth as you chew or masticate food and mix it with saliva. Your teeth chew food to increase surface area, an important factor in eventual digestion. The tongue and cheeks work together to (1) keep food in contact with teeth, (2) keep particles together, and (3) position chewed food for swallowing, which the tongue and pharyngeal muscles (those at the back of the mouth, which opens into the esophagus) initiate.

Saliva is secreted to lubricate, moisten, and hold particles together. Saliva also remineralizes teeth. Saliva is low in salt and has a pH of 6.8, which serves as a buffer (resists changes in pH when moderate amounts of food or drink are acidic or alkaline). Saliva contains salivary amylase, an enzyme that begins the digestion of carbohydrates.

Mobility

Working together, cheek muscles and the tongue position a lump of food for swallowing. Food is swallowed in a “lump” called a bolus that passes through the esophagus, where wavelike muscular contractions, called peristalsis, push it to the stomach. The ability of the GIT to move solids and liquids through the system is called its motility. Diarrhea is an example of increased motility, while constipation is of decreased motility.

Tongue and Taste

The tongue is instrumental in the perception of taste. Aided by odors and the physical sensations of food and drink, receptors in the taste buds of the tongue generate basic sensations called taste qualities: salty (presence of sodium chloride), bitter (presence of alkaloids), sour (presence of acids), sweet (presence of sugars), and umami, a Japanese word for a hearty flavor derived from glutamates such as monosodium glutamate.  We each have a complex configuration of taste receptors in our taste buds that together contribute to which flavors we like and which ones we don’t.¹ Taste influences our food choices and ultimately our metabolism.

Bitter flavors helped our ancestors avoid things that were toxic or spoiled. Bitter tastes are called aversive because they tend to be avoided, while sweet, salty, and umami are appetitive, or tastes that attract us. Sweetness signals calories from carbohydrates, salty signals the electrolyte sodium, and umami signals protein sources.^2,3

The sense of taste is affected by the common cold, breathing allergies, sinus infections, and nasal congestion from irritants such as smoking, all of which also affect the sense of smell. Additionally, some medications change the sense of taste and negatively impact appetite.

Summary

Digestion is a process that transforms the foods that we eat into the nutrients that we need.  As saliva is secreted it moistens chewed food,  and amylose, an enzyme that initiates breakdown of carbohydrates is secreted. Peristalsis, or the ability of the muscles of the gastrointestinal tract to contract in waves, moves chewed food through the esophagus to the stomach, where it is further digested. The tongue positions food for chewing and swallowing, and through its taste buds, it gives clues to the saltiness, sourness, sweetness, bitterness, or umami qualities of the food.

3.3 Travel through the Digestive Tract

Overview

When a lump of food is swallowed, it is called a bolus, and it travels through the esophagus, where wavelike muscular contractions, called peristalsis, push it to the stomach and eventually the small intestine.

Key Concepts

• The muscular tube called the epiglottis
• The esophagus and lower esophageal pressure
• Introduction to the stomach

The Epiglottis

The esophagus is a muscular tube that connects the mouth to the stomach. As the esophagus and trachea share a common pathway, a flap of tissue called the epiglottis closes off the trachea when you swallow. Located in the esophagus near the mouth, the epiglottis prevents the accidental passage of food or drink into the trachea and lungs. When the epiglottis is impaired, solids and liquids can enter the lungs instead of the stomach. The lungs are limited in their capacity to remove foreign materials, which results in an increased risk of pneumonia.

The Esophagus

Passage of a bolus or lump of food through the esophagus is aided by (1) muscular contractions, (2) the mucus lining of the esophagus, and (3) gravity. After eating, you can take advantage of the pull of gravity by staying upright in a standing or sitting position. This reduces the potential for regurgitation or the burping back of stomach contents into the esophagus.

At the lower portion of the esophagus is a thick circle of muscles known as the lower esophageal sphincter (LES). After peristalsis forces a bolus of food through the LES and into the stomach, it reverts to its closed position, preventing regurgitation back into the esophagus.

Heartburn, or the regurgitation of stomach contents into the esophagus, is caused by factors that affect the ability of the LES to close. Eating or drinking more than the stomach can comfortably handle is one cause. Another is lying down after a large meal. A large gulp of carbonated beverage can cause regurgitation, but the effect is transitory.⁴

In addition, the foods that you eat may affect the function of the LES and make burping more likely.

Foods and Regurgitation

A reduced LES pressure, or tone, reduces its ability to tightly constrict and increases the likelihood that you will regurgitate or burp. Some foods are known to affect tone; for example, foods high in sugars and starches, both carbohydrates, increase the likelihood of regurgitation, while dietary fiber, also a carbohydrate, decreases the frequency of regurgitation and heartburn.⁵

Although people sometimes say that there is a relationship between dietary fats and heartburn, one has yet to be found in a comprehensive study such as the National Health and Nutrition Examination Survey.⁶ A relaxed tone is beneficial when it relieves a distended stomach by allowing air swallowed during eating or drinking to escape through burping. While acidic or spicy foods can irritate the lining of the esophageal, they are not thought to contribute to regurgitation.

Food and beverages that lower pressure include peppermint, spearmint, chocolate, alcohol, and coffee. Consumption of these foods encourages regurgitation because the sphincter does not close tightly enough after swallowing. A small meal size, limiting consumption of sugars and starches, and avoiding late-night eating are recommended practices to reduce the likelihood of regurgitation and heartburn.

Mucus and Stomach Health

The mucus layer lining the esophagus serves to lubricate a passing bolus of food, but the thicker mucus layer that lines the stomach has a different task. It provides a continuous barrier that protects the stomach from the corrosive effects of enzymes and acids that would damage unprotected stomach cells.

An example is the digestion of protein that begins in the stomach as pepsinogen is converted to the active form  pepsin. Without the protection of the mucus layer, stomach cells exposed to pepsin would be damaged, resulting in sores in the stomach lining or an ulcer.

When there is a breakdown in the thick mucus layer protecting the stomach lining from the caustic effects of acid and pepsin, gastric ulcers may result. Stomach pain and bleeding that comes and goes is a sign that underlying tissue is damaged. Genetics, stress, smoking, and the long-term use of nonsteroid anti-inflammatory drugs like aspirin or ibuprofen are among the factors that contribute to ulcer development.⁷

Sometimes a peptic ulcer is caused when the mucous coating of the stomach is damaged by infection by Helicobacter pylori (H. pylori). H. pylori is a bacteria that is transmitted person to person (oral-oral route) through saliva or vomit as well as through water that is contaminated with feces (oral-fecal route).⁸

Antibiotics are effective in treating ulcers where a chronic infection with a bacterial infection is the causative factor. H. pylori bacteria are spread through close contact and exposure to vomit. Help stop the spread of H. pylori by washing your hands! Treatment of ulcers may include stress-reduction techniques and antacids to counteract stomach secretions and reduce pain. It is a good idea to stop smoking and reduce alcohol consumption as well.

The Amazing Stomach

The stomach is a J-shaped pouch positioned between the esophagus and the small intestine. It is grapefruit sized and expands when filled. It churns and mixes food received from the esophagus. When stimulated by the presence of food or drink, the stomach secretes hydrochloric acid, which lowers contents to a pH of less than two, creating an acidic environment. This activates the enzyme pepsinogen, converting it to pepsin, which begins the digestion of protein. It also denatures or uncoils protein molecules, making it easier for pepsin to work.

How acidic are stomach contents? Consider that vinegar has a pH of two; grapefruit juice, three; black coffee, five; distilled water (neutral), seven; and baking soda (alkaline), nine. This highly acidic environment discourages bacterial growth and helps in the prevention of bacterial diseases, such as foodborne illness.

Endocrine cells in the stomach produce gastrin, somatostatin, and ghrelin, which are hormones that help regulate stomach function.⁹ Secretion of gastrin increases in response to eating. Gastrin regulates gastric acid production and stimulates appetite. Conversely, somatostatin counteracts gastrin and reduces its production when a meal is over and eating more food is not imminent. Although ghrelin is sometimes called the hunger hormone, its role goes beyond stimulating appetite.¹⁰ Ghrelin affects a number of organs and is involved in glucose regulation, energy balance, and bone health.

The ability of your stomach to expand, or its capacity, is related to the amount of food that you routinely eat at one sitting. In most cases, stomach capacity is about thirty-two to forty-six ounces. People who habitually overeat have larger stomach capacities than they would if they ate smaller portions. While the stomach does not shrink, making a habit of eating smaller amounts tightens stomach muscles and reduces the overall ability to stretch. As a result, stretching sensors that signal that the stomach is full are activated at a smaller capacity when fewer calories have been consumed.

Workings of the Stomach

After mixing is complete, the stomach moves food and gastric secretions to the small intestine in a watery solution called chyme. Stomach muscles contract in waves to squirt chyme through the pyloric sphincter, separating the stomach from the small intestine at a rate of one to five milliliters per thirty seconds, or about one to two teaspoons per minute.

It takes two to four hours for a typical meal to pass completely into the small intestine. The type of food or drink affects the rate of passage. Isotonic liquids, which have the same solute concentration as body cells, leave the stomach more quickly than hypertonic liquids or solids, which tend to spend the most time in the stomach. A hypertonic liquid has a higher solute concentration than body cells or blood, while hypotonic liquid has a lower one. An example of an isotonic liquid is Gatorade or Powerade. Sweetened, carbonated beverages are hypertonic, and water is hypotonic.

Foods that are high in fat leave the stomach more slowly than foods high in either protein or carbohydrates. Fiber also reduces the rate at which gastric contents empty into the small intestine. As a result, meals with adequate fiber depress the rate at which carbohydrates elevate blood glucose levels as well as prolong the sense of satisfaction or satiety generated by a full stomach. How? By moderating the rate at which chyme passes into the small intestine, where carbohydrates are digested and absorbed.

Overall, an additional three to ten hours is needed for your meal to traverse the large intestine and complete its journey. An additional one to two days may pass before residues that are mostly fiber leave your body.

Summary

Chewed food is swallowed as a lump, or bolus, which the muscles of the gastrointestinal tract push in a wavelike motion past the epiglottis, through the esophagus, and into the stomach. Swallowing causes a temporary relaxation of the LES, which returns to a contracted state after the bolus passes into the stomach. Gastroesophageal reflux disease (GERD) happens when stomach contents pass back through the LES into the esophagus, causing heartburn and regurgitation. GERD treatment includes behavioral modification and medications that reduce stomach acid content.

The stomach continues the breakdown of foods that started with chewing. Hydrochloric acid in the stomach denatures food proteins, making them more digestible, and inhibits bacterial growth, which reduces the risk of foodborne illness. Gastrin, somatostatin, and ghrelin manage stomach function, while pepsinogen is activated to make pepsin, which begins the enzymatic breakdown of protein. Stomach contractions move the mixture of food and gastric juices into the small intestine, where further digestion takes place.

3.4 Workings of the Small Intestine

Overview

The vast majority of the nutrients that we get from our food and drink are absorbed in the small intestine. An amazing list of hormones, enzymes, emulsifiers, and carrier molecules makes this possible. Even though fat, carbohydrates, and protein are absorbed in the small intestine, much work remains for the large intestine, where fiber supports beneficial bacteria, water is conserved through absorption, and digestive residues are prepared for excretion.

Key Concepts

• Functions of the small intestine
• Role of liver, gall bladder, and pancreas in digestion
• Actions of enzymes, hormones, and emulsifiers
• Functions of the large intestine
• Gut microflora and breastfeeding

The Small Intestine

The small intestine is the primary site for the digestion and eventual absorption of nutrients. In fact, over 95 percent of the nutrients gained from a meal, including protein, fat, and carbohydrate, are absorbed in the small intestine. Alcohol, an additional source of energy, is largely absorbed in the small intestine, although some absorption takes place in the mouth and stomach as well.

Liver, Gall Bladder, Pancreas

Three organs of the body assist in digestion: the liver, the gall bladder, and the pancreas. The liver produces bile, a substance that is crucial to the digestion and absorption of fat, and the gall bladder stores it. The pancreas provides bicarbonate and enzymes that help digest carbohydrates and fat. The liver, gall bladder, and pancreas share a common duct into the small intestine, and their secretions are blended. If the common duct becomes blocked, as with a gall stone, adequate bile is not available, and the digestion of fat is seriously reduced, leading to cramping and diarrhea.

Neutralizing Chyme

Bicarbonate secreted by the pancreas neutralizes chyme (makes it less acidic) and helps create an environment favorable to enzymatic activity. The pancreas provides lipase, an enzyme for digesting fat, and amylase for digesting polysaccharides (carbohydrate). The small intestine produces intermediate enzymes, such as maltase, that digest maltose and peptidase to break down proteins further into amino acids.

Wonders of the Villi

The villi are fingerlike projections from the walls of the small intestine. They are a key part of the inner surface and significantly increase the absorptive area. A large surface area is important to the speed and effectiveness of digestion. Some medical treatments, such as radiation therapy, can damage villi and impair the function of the small intestine.

Diseases also affect villi health. One sign of chronic alcoholism is blunted villi that lack adequate surface area, resulting in poor absorption of nutrients. Someone in the advanced stages of alcoholism often experiences diarrhea due to reduced water and sodium absorption, poor eating habits that limit vitamin C intake coupled with an increased loss in urine, and zinc deficiency due to poor absorption.¹¹

Villi Cells

Cells in the villi are continuously exposed to a harsh environment and, as a result, have a short life-span of about three days. Adequate nutrition is required for optimal health and to ensure that new cells are ready to replace aging ones. Insufficient protein in the diet depresses cell replacement and reduces the efficiency of absorption, thereby further compromising overall health. This is a significant issue for people who have experienced starvation. A quick introduction of large amounts of food can result in cramping and diarrhea, further threatening survival.

The Enzymes of Digestion

Enzymes are biological catalysts that speed up reactions without being changed themselves. Enzymes produced by the stomach, pancreas, and small intestine are critical to digestion. For example, carbohydrates are large molecules that must be broken into smaller units before absorption can take place.

Enzymes such as amylase, lactase, and maltase catalyze the breakdown of starches (polysaccharides) and sugars (disaccharides) into the monosaccharides, glucose, galactose, and fructose. Proteases such as pepsin and trypsin digest protein into peptides and subsequently into amino acids, and lipase digests a triglyceride into a monoglyceride and two fatty acids.

Digestion of Fat

The digestion of fat poses a special problem because fat will not disperse, or go into solution, in water. The lumen of the small intestine is a liquid or watery environment. This problem is solved by churning, the action of enzymes, and bile salts secreted by the liver and gall bladder. Bile acts as an emulsifier, or a substance that allows fat to remain in suspension in a watery medium. The resulting micelle, or a droplet with fat at the center and hydrophilic or water-loving phospholipid on the exterior, expedites digestion of fats and transportation to the intestinal epithelial cell for absorption.

Rate of Absorption

Nutrients truly enter the body through the absorptive cells of the small intestine. Absorption of nutrients takes place throughout the small intestine, leaving only water, some minerals, and indigestible fiber for transit into the large intestine. There are three mechanisms that move nutrients from the lumen, or interior of the intestine, across the cell membrane and into the absorptive cell itself. They are passive, facilitated, and active absorption.

In passive absorption, a nutrient moves down a gradient from an area of higher concentration to one of lower concentration. For this downhill flow, no energy is required. Fat is an example of a nutrient that is passively absorbed. In facilitated absorption, a carrier protein is needed to transport a nutrient across the membrane of the absorptive cell. For this type of absorption, no energy is required. Fructose is an example of a nutrient that undergoes facilitated absorption.

In active absorption, both a carrier protein and energy are needed. Active absorption rapidly moves a nutrient from an area of low concentration in the lumen to an area of high concentration in the cell and eventually into the blood. Glucose and galactose are examples of nutrients that require active absorption.

The Large Intestine

The large intestine completes the process of absorption. In the upper large intestine, most of the remaining water and minerals are absorbed. Fiber becomes a food source for resident bacteria that generate gas and acids as by-products as well as some vitamins. Over four hundred different bacteria colonize the colon, or large intestine, and provide the body with vitamin K and vitamin B12 as by-products of their life processes. The normal flora, or bacteria, that reside in the intestine also resist colonization efforts of other, unfamiliar bacteria.

Finally, the residues of a meal move into the rectum and are further concentrated and prepared for expulsion from the body as feces.

GIT and Breastfeeding

Did you know that the gastrointestinal tract of a newborn baby is sterile? It’s true; there are no bacteria present at birth. Exposure to the world and the first swallow of milk changes everything by introducing bacteria. A breastfed baby tends to have a more stable and uniform microbiota than a formula-fed infant, and this is advantageous.¹²

The protective influence of breastfeeding reduces the incidence of diarrhea and modifies the risk of allergic diseases during childhood. Exclusive breastfeeding during the first six months of life is recommended by the World Health Organization followed by supplemental breastfeeding throughout the first two years of life.¹³

Summary

Getting the energy and nutrients that we need from our food and drink is a complex process that involves multiple organs and an array of substances. The small intestine is a muscular tube with villi projecting into the lumen that vastly increase its absorptive surface area. The liver produces bile, which the gall bladder stores and secretes into to small intestine via a common duct. Bile is an emulsifier that suspends fats in the watery chyme, making enzymatic breakdown possible. The pancreas produces lipase and secretes it into a common duct, where it is delivered to the small intestine. Lipase breaks down large fat molecules into manageable parts. The large intestine plays an important part in concentrating the residues of digestion and conserving water through absorption. It also is a home for beneficial bacteria that are nourished by fiber that is indigestible for humans.

References

1. Kuhn ME. Decoding the science of taste. Food Technology. 2016;70(5). https://www.ift.org/news-and-publications/food-technology-magazine/issues/2016/may/features/science-of-taste. Accessed January 16, 2021.
2. Dando R. Food Facts on Taste. Department of Food Science, Cornell University. https://youtu.be/eVtXDFTMU9Q. Published August 2015. Accessed January 16, 2021.
3. Chaudhari N, Roper SD. The cell biology of taste. J Cell Biol. 2010;191(2):429. doi:10.1083/jcb.20100314420100927c.
4. Johnson T, Gerson L, Herschcovici T, Stave C, Fass R. Systematic review: The effects of carbonated beverages on gastro-oesophageal reflux disease. Aliment Pharmacol Ther. 2010;31(6):607-614. doi:10.1111/j.1365-2036.2010.04232.x.
5. Newberry C, Lynch K. The role of diet in the development and management of gastroesophageal reflux disease: Why we feel the burn. J Thorac Dis. 2019;11(Suppl. 12):S1594-S1601. doi:10.21037/jtd.2019.06.42.
6. Ruhl CE, Everhart JE. Overweight, but not high dietary fat intake, increases risk of gastroesophageal reflux disease hospitalization: The NHANES I epidemiologic follow-up study. Ann Epidemiol. 1999;9:424-435. doi:10.1016/S1047-2797(99)00020-4.
7. Symptoms and Causes of Peptic Ulcers (Stomach Ulcers). National Institute of Diabetes and Digestive and Kidney Diseases. National Institute of Health. https://www.niddk.nih.gov/health-information/digestive-diseases/peptic-ulcers-stomach-ulcers/symptoms-causes. Published November 2014. Accessed January 16, 2021.
8. Brown LM. Helicobacter pylori: Epidemiology and routes of transmission. Epidemiol Rev. 2000;22(2):283-297. doi:10.1093/oxfordjournals.epirev.a018040.
9. Pradhan G, Samson SL, Sun Y. Ghrelin: Much more than a hunger hormone. Curr Opin Clin Nutr Metab Care. 2013;16(6):619-624. doi:10.1097/MCO.0b013e328365b9be.
10. Hunt RH, Camilleri M, Crowe SE, El-Omar EM, Fox JG, Kuipers EJ, Malfertheiner P, McColl KEL, Pritchard DM, Rugge M, Sonnenberg A, Sugano K, Tack T. The stomach in health and disease. Gut. 2015;64(10):1650-1668. Published September 2015. doi:10.1136/gutjnl-2014-307595.
11. Bode C, Bode CJ. Effect of alcohol consumption on the gut. Clinical Gastroenterology. 2003;17(4):575-592. doi:10.1016/S1521-6918(03)00034-9.
12. Guaraldi F, Salvatori G. Effect of breast and formula feeding on gut microbiota shaping in newborns. Front Cell Infect Microbiol. 2012;2:94. Published October 16, 2012. doi:10.3389/fcimb.2012.00094.
13. Infant and Young Child Feeding . World Health Organization. https://www.who.int/news-room/fact-sheets/detail/infant-and-young-child-feedingAccessed August 1, 2021.