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What nutrients can humans absorb in the mouth?

What nutrients can humans absorb in the mouth?


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For instance, I realise being able to absorb simple sugars in the mouth is pivotal in the rapid action of oral glucose gel. Thus I was wondering what nutrients in general can be absorbed directly within the mouth, and at what speed?


Quite a bit can be absorbed through the mouth. Most commonly, starches are broken down to maltose (two glucose molecules formed by a condensation reaction) and are easily absorbed by the bloodstream.

A lot of other factors balance into this, ie pH, lipid solubility, and molecular weight. Generally, if a substance is easily dissolved in saliva, it can be administered buccally or sublingually because the only remaining step is the diffusion into the subepithelial capillaries. In regards to speed, anything properly absorbed via buccal or sublingual administration goes to work much faster than a standard oral medication, and with a higher availability in the bloodstream. You are taking first pass metabolism and enzyme breakdown in the stomach out of the equation by diffusing the drug directly into the bloodstream.

There is quite a laundry list of medications that can be administered transbucally or sublingually, just a few of these: Nitroglycerin Acetylsalicylic acid (aspirin) Glucose gel (as you mentioned) Fentanyl (a narcotic painkiller) buprenorphine (for opioid dependency) Benzodiazepines (alprazolam, clonazepam)


The mouth or the oral cavity is lined by oral mucous membrane. The oral mucous membrane has two parts the epithelium and its supporting connective tissue known as lamina propriya. The charecter of the epithelium and lamina propriya varies to a great extent from the gingiva and hard palate to the floor of the mouth based on functional requirements. However basically the mucous membrane lining the oral cavity is not designed for absorbing nutrients, which was the primary question. As for the oral glucose gel - read the American Red Cross, Scientific Advisory Council recommendation that the buccal absorption of glucose is limited and not recommended. ARC SAC Advisory


Actually, absorption does take place through the mouth.There is a ptyalin enzyme in the saliva which hydrolyzes carbohydrates of the food. These contents are then absorbed in the blood through the facial vein.The facial vein opens into subclavian vein,and it opens into the superior vena cava.


Many vitamins are absorbed in mouth. Even spraying vitamins will help you overcome vitamin deficiency. Even some drugs can be absorbed directly into mouth.


How Does The Human Digestive System Work?

The human digestive system is a continuous long muscular tube starting at your mouth and ending at the anus. The system breaks down food by mechanical and chemical means, and then absorbs the nutrients.

The average amount a person eats per day is 1.5 kilograms, which is about 550 kilograms of food a year. That&rsquos more than the weight of two bears!

All of this food we eat goes to the perpetually hungry cells of our body. However, your cells can&rsquot eat a whole salad, so the body must convert it into smaller molecules that the cells can access and use. This is the primary function of our digestive system. Along with the circulatory system, the digestive system is a &ldquomeals on wheels&rdquo program for the cells of your body.

If it weren&rsquot for digestion, we wouldn&rsquot have the energy to work and play. Not only that, but digestion enables the body to use the nutrients present in food to grow and renew itself.


The Human Digestive System

The process of digestion begins in the mouth with the intake of food (Figure 1). The teeth play an important role in masticating (chewing) or physically breaking food into smaller particles. The enzymes present in saliva also begin to chemically break down food. The food is then swallowed and enters the esophagus—a long tube that connects the mouth to the stomach. Using peristalsis, or wave-like smooth-muscle contractions, the muscles of the esophagus push the food toward the stomach. The stomach contents are extremely acidic, with a pH between 1.5 and 2.5. This acidity kills microorganisms, breaks down food tissues, and activates digestive enzymes. Further breakdown of food takes place in the small intestine where bile produced by the liver, and enzymes produced by the small intestine and the pancreas, continue the process of digestion. The smaller molecules are absorbed into the blood stream through the epithelial cells lining the walls of the small intestine. The waste material travels on to the large intestine where water is absorbed and the drier waste material is compacted into feces it is stored until it is excreted through the anus.

Figure 1. The components of the human digestive system are shown.

Oral Cavity

Both physical and chemical digestion begin in the mouth or oral cavity, which is the point of entry of food into the digestive system. The food is broken into smaller particles by mastication, the chewing action of the teeth. All mammals have teeth and can chew their food to begin the process of physically breaking it down into smaller particles.

The chemical process of digestion begins during chewing as food mixes with saliva, produced by the salivary glands (Figure 2). Saliva contains mucus that moistens food and buffers the pH of the food. Saliva also contains lysozyme, which has antibacterial action. It also contains an enzyme called salivary amylase that begins the process of converting starches in the food into a disaccharide called maltose. Another enzyme called lipase is produced by cells in the tongue to break down fats. The chewing and wetting action provided by the teeth and saliva prepare the food into a mass called the bolus for swallowing. The tongue helps in swallowing—moving the bolus from the mouth into the pharynx. The pharynx opens to two passageways: the esophagus and the trachea. The esophagus leads to the stomach and the trachea leads to the lungs. The epiglottis is a flap of tissue that covers the tracheal opening during swallowing to prevent food from entering the lungs.

Figure 2. (a) Digestion of food begins in the mouth. (b) Food is masticated by teeth and moistened by saliva secreted from the salivary glands. Enzymes in the saliva begin to digest starches and fats. With the help of the tongue, the resulting bolus is moved into the esophagus by swallowing. (credit: modification of work by Mariana Ruiz Villareal)

Esophagus

The esophagus is a tubular organ that connects the mouth to the stomach. The chewed and softened food passes through the esophagus after being swallowed. The smooth muscles of the esophagus undergo peristalsis that pushes the food toward the stomach. The peristaltic wave is unidirectional—it moves food from the mouth the stomach, and reverse movement is not possible, except in the case of the vomit reflex. The peristaltic movement of the esophagus is an involuntary reflex it takes place in response to the act of swallowing.

Ring-like muscles called sphincters form valves in the digestive system. The gastro-esophageal sphincter (or cardiac sphincter) is located at the stomach end of the esophagus. In response to swallowing and the pressure exerted by the bolus of food, this sphincter opens, and the bolus enters the stomach. When there is no swallowing action, this sphincter is shut and prevents the contents of the stomach from traveling up the esophagus. Acid reflux or “heartburn” occurs when the acidic digestive juices escape into the esophagus.

Stomach

A large part of protein digestion occurs in the stomach (Figure 4). The stomach is a saclike organ that secretes gastric digestive juices.

Protein digestion is carried out by an enzyme called pepsin in the stomach chamber. The highly acidic environment kills many microorganisms in the food and, combined with the action of the enzyme pepsin, results in the catabolism of protein in the food. Chemical digestion is facilitated by the churning action of the stomach caused by contraction and relaxation of smooth muscles. The partially digested food and gastric juice mixture is called chyme. Gastric emptying occurs within two to six hours after a meal. Only a small amount of chyme is released into the small intestine at a time. The movement of chyme from the stomach into the small intestine is regulated by hormones, stomach distension and muscular reflexes that influence the pyloric sphincter.

The stomach lining is unaffected by pepsin and the acidity because pepsin is released in an inactive form and the stomach has a thick mucus lining that protects the underlying tissue.

Small Intestine

Chyme moves from the stomach to the small intestine. The small intestine is the organ where the digestion of protein, fats, and carbohydrates is completed. The small intestine is a long tube-like organ with a highly folded surface containing finger-like projections called the villi. The top surface of each villus has many microscopic projections called microvilli. The epithelial cells of these structures absorb nutrients from the digested food and release them to the bloodstream on the other side. The villi and microvilli, with their many folds, increase the surface area of the small intestine and increase absorption efficiency of the nutrients.

The human small intestine is over 6 m (19.6 ft) long and is divided into three parts: the duodenum, the jejunum and the ileum. The duodenum is separated from the stomach by the pyloric sphincter. The chyme is mixed with pancreatic juices, an alkaline solution rich in bicarbonate that neutralizes the acidity of chyme from the stomach. Pancreatic juices contain several digestive enzymes that break down starches, disaccharides, proteins, and fats. Bile is produced in the liver and stored and concentrated in the gallbladder it enters the duodenum through the bile duct. Bile contains bile salts, which make lipids accessible to the water-soluble enzymes. The monosaccharides, amino acids, bile salts, vitamins, and other nutrients are absorbed by the cells of the intestinal lining.

The undigested food is sent to the colon from the ileum via peristaltic movements. The ileum ends and the large intestine begins at the ileocecal valve. The vermiform, “worm-like,” appendix is located at the ileocecal valve. The appendix of humans has a minor role in immunity.

Large Intestine

The large intestine reabsorbs the water from indigestible food material and processes the waste material (Figure 3). The human large intestine is much smaller in length compared to the small intestine but larger in diameter. It has three parts: the cecum, the colon, and the rectum. The cecum joins the ileum to the colon and is the receiving pouch for the waste matter. The colon is home to many bacteria or “intestinal flora” that aid in the digestive processes. The colon has four regions, the ascending colon, the transverse colon, the descending colon and the sigmoid colon. The main functions of the colon are to extract the water and mineral salts from undigested food, and to store waste material.

Figure 3. The large intestine reabsorbs water from undigested food and stores waste until it is eliminated. (credit: modification of work by Mariana Ruiz Villareal)

The rectum (Figure 3) stores feces until defecation. The feces are propelled using peristaltic movements during elimination. The anus is an opening at the far-end of the digestive tract and is the exit point for the waste material. Two sphincters regulate the exit of feces, the inner sphincter is involuntary and the outer sphincter is voluntary.

Accessory Organs

The organs discussed above are the organs of the digestive tract through which food passes. Accessory organs add secretions and enzymes that break down food into nutrients. Accessory organs include the salivary glands, the liver, the pancreas, and the gall bladder. The secretions of the liver, pancreas, and gallbladder are regulated by hormones in response to food consumption.

The liver is the largest internal organ in humans and it plays an important role in digestion of fats and detoxifying blood. The liver produces bile, a digestive juice that is required for the breakdown of fats in the duodenum. The liver also processes the absorbed vitamins and fatty acids and synthesizes many plasma proteins. The gallbladder is a small organ that aids the liver by storing bile and concentrating bile salts.

The pancreas secretes bicarbonate that neutralizes the acidic chyme and a variety of enzymes for the digestion of protein and carbohydrates.

ART CONNECTION Figure 4. The stomach has an extremely acidic environment where most of the protein gets digested. (credit: modification of work by Mariana Ruiz Villareal)

With obesity at high rates in the United States, there is a public health focus on reducing obesity and associated health risks, which include diabetes, colon and breast cancer, and cardiovascular disease. How does the food consumed contribute to obesity?

Fatty foods are calorie-dense, meaning that they have more calories per unit mass than carbohydrates or proteins. One gram of carbohydrates has four calories, one gram of protein has four calories, and one gram of fat has nine calories. Animals tend to seek lipid-rich food for their higher energy content. Greater amounts of food energy taken in than the body’s requirements will result in storage of the excess in fat deposits.

Excess carbohydrate is used by the liver to synthesize glycogen. When glycogen stores are full, additional glucose is converted into fatty acids. These fatty acids are stored in adipose tissue cells—the fat cells in the mammalian body whose primary role is to store fat for later use.

The rate of obesity among children is rapidly rising in the United States. To combat childhood obesity and ensure that children get a healthy start in life, in 2010 First Lady Michelle Obama launched the Let’s Move! campaign. The goal of this campaign is to educate parents and caregivers on providing healthy nutrition and encouraging active lifestyles in future generations. This program aims to involve the entire community, including parents, teachers, and healthcare providers to ensure that children have access to healthy foods—more fruits, vegetables, and whole grains—and consume fewer calories from processed foods. Another goal is to ensure that children get physical activity. With the increase in television viewing and stationary pursuits such as video games, sedentary lifestyles have become the norm. Visit www.letsmove.gov to learn more.


17 Chapter 17: Digestive System

All living organisms need nutrients to survive. While plants can obtain nutrients from their roots and the energy molecules required for cellular function through the process of photosynthesis, animals obtain their nutrients by the consumption of other organisms. At the cellular level, the biological molecules necessary for animal function are amino acids, lipid molecules, nucleotides, and simple sugars. However, the food consumed consists of protein, fat, and complex carbohydrates. Animals must convert these macromolecules into the simple molecules required for maintaining cellular function. The conversion of the food consumed to the nutrients required is a multistep process involving digestion and absorption. During digestion, food particles are broken down to smaller components, which are later absorbed by the body. This happens by both physical means, such as chewing, and by chemical means, via enzyme-catalyzed reactions.

One of the challenges in human nutrition is maintaining a balance between food intake, storage, and energy expenditure. Taking in more food energy than is used in activity leads to storage of the excess in the form of fat deposits. The rise in obesity and the resulting diseases like type 2 diabetes makes understanding the role of diet and nutrition in maintaining good health all the more important.

After studying this chapter, you should be able to:

  • With regards to the anatomy of the digestive system
    • a. Locate and recognize the basic function of G.I. tract organs and accessory organs.
    • b. Diagram the path of food as it passes through the digestive system.

    The Digestive System

    The process of digestion begins in the mouth (oral cavity) with the intake of food (Figure). The teeth play an important role in masticating (chewing) or physically breaking food into smaller particles. This action not only decreases the size of the food particles to facilitate swallowing, but also increases surface area for chemical digestion. The enzymes present in saliva (amylase and lipase) also begin to chemically break down food (starch and fats, respectively). The food is then swallowed and enters the esophagus —a long tube that connects the mouth to the stomach. Using peristalsis , or wave-like smooth-muscle contractions, the muscles of the esophagus push the food toward the stomach. The stomach contents are extremely acidic, with a pH between 1.5 and 2.5. This acidity kills microorganisms, breaks down food tissues, and activates digestive enzymes. Further breakdown of food takes place in the small intestine where bile produced by the liver, and enzymes produced by the small intestine and the pancreas, continue the process of digestion. The smaller molecules are absorbed into the blood stream through the epithelial cells lining the walls of the small intestine. The waste material travels on to the large intestine where water is absorbed and the drier waste material is compacted into feces it is stored in the rectum until it is excreted through the anus.

    The components of the human digestive system are shown. The GI tract is the tube that includes the oral cavity, esophagus, stomach, small intestine, large intestine, and rectum. The accessory organs are those that indirectly join to this tube via ducts and include the salivary glands, liver, gall bladder, and pancreas.

    Oral Cavity

    Both physical and chemical digestion begin in the mouth or oral cavity , which is the point of entry of food into the digestive system. The food is broken into smaller particles by mastication, the chewing action of the teeth. All mammals have teeth and can chew their food to begin the process of physically breaking it down into smaller particles.

    The chemical process of digestion begins during chewing as food mixes with saliva, produced by the salivary glands (Figure). Saliva contains mucus that moistens food and buffers the pH of the food. Saliva also contains lysozyme, which has antibacterial action. It also contains an enzyme called salivary amylase that begins the process of converting starches in the food into a disaccharide called maltose. Another enzyme called lipase is produced by cells in the tongue to break down fats. The chewing and wetting action provided by the teeth and saliva prepare the food into a mass called the bolus for swallowing. The tongue helps in swallowing—moving the bolus from the mouth into the pharynx. The pharynx opens to two passageways: the esophagus and the trachea. The esophagus leads to the stomach and the trachea leads to the lungs. The epiglottis is a flap of tissue that covers the tracheal opening during swallowing to prevent food from entering the lungs.

    (a) Digestion of food begins in the mouth. (b) Food is masticated by teeth and moistened by saliva secreted from the salivary glands. Enzymes in the saliva begin to digest starches and fats. With the help of the tongue, the resulting bolus is moved into the esophagus by swallowing. (credit: modification of work by Mariana Ruiz Villareal)

    Esophagus

    The esophagus is a tubular organ that connects the mouth to the stomach. The chewed and softened food (i.e. the bolus) passes through the esophagus after being swallowed. The smooth muscles of the esophagus undergo peristalsis (contractions) that pushes the food toward the stomach. The peristaltic wave is unidirectional—it moves food from the mouth the stomach, and reverse movement is not possible, except in the case of the vomit reflex. The peristaltic movement of the esophagus is an involuntary reflex it takes place in response to the act of swallowing and you don’t exert conscious control over it.

    Ring-like muscles called sphincters form valves in the digestive system. The gastro-esophageal sphincter (a.k.a. lower esophageal or cardiac sphincter) is located at the stomach end of the esophagus. In response to swallowing and the pressure exerted by the bolus of food, this sphincter opens, and the bolus enters the stomach. When there is no swallowing action, this sphincter is shut and prevents the contents of the stomach from traveling up the esophagus. Acid reflux or “heartburn” occurs when the acidic digestive juices escape back into the esophagus and the low pH irritates the unprotected surface. Prolonged and repeated exposure of the esophagus to this acidity can cause physical damage.

    Stomach

    A large part of protein digestion occurs in the stomach (Figure). The stomach is a saclike organ that secretes gastric digestive juices.

    Protein digestion is carried out by an enzyme called pepsin in the stomach chamber. The highly acidic environment kills many microorganisms in the food and, combined with the action of the enzyme pepsin, results in the catabolism of protein in the food. Chemical digestion is facilitated by the churning action of the stomach caused by contraction and relaxation of smooth muscles. The partially digested food and gastric juice mixture is called chyme . Gastric emptying occurs within two to six hours after a meal. Only a small amount of chyme is released into the small intestine at a time. The movement of chyme from the stomach into the small intestine is regulated by hormones, stomach distension and muscular reflexes that influence the pyloric sphincter. The low pH of the stomach will denature the amylase and lipase that were secreted in the mouth. Therefore, over time, chemical digestion of starches and fats will decrease in the stomach.

    The stomach lining is unaffected by pepsin and the acidity because pepsin is released in an inactive form (pepsinogen) that is activated by the low pH. The stomach also has a thick mucus lining that protects the underlying tissue.

    Small Intestine

    Chyme moves from the stomach to the small intestine. The small intestine is the organ where the digestion of protein, fats, and carbohydrates is completed. The small intestine is a long tube-like organ with a highly folded surface containing finger-like projections called the villi. The top surface of each villus has many microscopic projections called microvilli. The epithelial cells at the surface of these structures absorb nutrients from the digested food and release them to the bloodstream on the other side. Methods of transport previously discussed (e.g.active transport)are used during this movement. The villi and microvilli, with their many folds, increase the surface area of the small intestine and increase absorption efficiency of the nutrients.

    The human small intestine is over 6 m (19.6 ft) long and is divided into three parts: the duodenum, the jejunum and the ileum. The duodenum is separated from the stomach by the pyloric sphincter. The chyme is mixed with pancreatic juices, an alkaline/basic solution rich in bicarbonate that neutralizes the acidity of chyme from the stomach. This result raises the pH and creates an environment that is appropriate for enzymes. Pancreatic juices contain several digestive enzymes (amylase, trypsin, and lipase) that break down starches, proteins, and fats, respectively. Bile is produced in the liver and stored and concentrated in the gallbladder it enters the duodenum through the bile duct. Bile contains bile salts, which make lipids accessible to the water-soluble enzymes. This is accomplished via a process called emulsification, a type of physical digestion. Bile keeps fat droplets from coming back together again, thus increasing the surface area available to lipase. The wall of the small intestines secrete disaccharidases, which faciltate digestion of disaccharides (e.g. maltose, sucrose, and lactose) into their respective monosaccharides. The monosaccharides, amino acids, bile salts, vitamins, and other nutrients are absorbed by the cells of the intestinal lining.

    The undigested food is sent to the colon from the ileum via peristaltic movements. The ileum ends and the large intestine begins at the ileocecal valve. The vermiform, “worm-like,” appendix is located at the ileocecal valve. The appendix of humans has a minor role in immunity.

    Large Intestine

    The large intestine reabsorbs the water from indigestible food material and processes the waste material (Figure). The human large intestine is much smaller in length compared to the small intestine but larger in diameter. It has three parts: the cecum, the colon, and the rectum. The cecum joins the ileum to the colon and is the receiving pouch for the waste matter. The colon is home to many bacteria or “intestinal flora” that aid in the digestive processes. The colon has four regions, the ascending colon, the transverse colon, the descending colon and the sigmoid colon. The main functions of the colon are to extract the water and mineral salts from undigested food, and to store waste material.

    The large intestine reabsorbs water from undigested food and stores waste until it is eliminated. (credit: modification of work by Mariana Ruiz Villareal)

    The rectum (Figure) stores feces until defecation. The feces are propelled using peristaltic movements during elimination. The anus is an opening at the far-end of the digestive tract and is the exit point for the waste material. Two sphincters regulate the exit of feces, the inner sphincter is involuntary and the outer sphincter is voluntary.

    Accessory Organs

    The organs discussed above are the organs of the digestive tract through which food passes. Accessory organs add secretions and enzymes that break down food into nutrients. Accessory organs include the salivary glands, the liver, the pancreas, and the gall bladder. The secretions of the liver, pancreas, and gallbladder are regulated by hormones in response to food consumption.

    The liver is the largest internal organ in humans and it plays an important role in digestion of fats and detoxifying blood. The liver produces bile, a digestive juice that is required for the breakdown of fats in the duodenum. The liver also processes the absorbed vitamins and fatty acids and synthesizes many plasma proteins. The gallbladder is a small organ that aids the liver by storing bile and concentrating bile salts.

    The pancreas secretes bicarbonate that neutralizes the acidic chyme and a variety of enzymes (trypsin, amylase, and lipase) for the digestion of proteins, carbohydrates, and fats, respectively.

    The stomach has an extremely acidic environment where most of the protein gets digested. (credit: modification of work by Mariana Ruiz Villareal)

    Nutrition

    The human diet should be well balanced to provide nutrients required for bodily function and the minerals and vitamins required for maintaining structure and regulation necessary for good health and reproductive capability (Figure).

    For humans, a balanced diet includes fruits, vegetables, grains, protein, and dairy. (credit: USDA)

    Explore this interactive United States Department of Agriculture website to learn more about each food group and the recommended daily amounts.

    The organic molecules required for building cellular material and tissues must come from food. During digestion, digestible carbohydrates are ultimately broken down into glucose and used to provide energy within the cells of the body. Complex carbohydrates, including polysaccharides, can be broken down into glucose through biochemical modification however, humans do not produce the enzyme necessary to digest cellulose (fiber). The intestinal flora in the human gut are able to extract some nutrition from these plant fibers. These plant fibers are known as dietary fiber and are an important component of the diet. The excess sugars in the body are converted into glycogen and stored for later use in the liver and muscle tissue. Glycogen stores are used to fuel prolonged exertions, such as long-distance running, and to provide energy during food shortage. Fats are stored under the skin of mammals for insulation and energy reserves.

    Proteins in food are broken down during digestion and the resulting amino acids are absorbed. All of the proteins in the body must be formed from these amino-acid constituents no proteins are obtained directly from food.

    Fats add flavor to food and promote a sense of satiety or fullness. Fatty foods are also significant sources of energy, and fatty acids are required for the construction of lipid membranes. Fats are also required in the diet to aid the absorption of fat-soluble vitamins and the production of fat-soluble hormones.

    While the animal body can synthesize many of the molecules required for function from precursors, there are some nutrients that must be obtained from food. These nutrients are termed essential nutrients , meaning they must be eaten, because the body cannot produce them. Essential nutrients include some fatty acids, some amino acids, vitamins, and minerals.

    Section Summary

    There are many organs that work together to digest food and absorb nutrients. The mouth is the point of ingestion and the location where both mechanical and chemical breakdown of food begins. Saliva contains an enzyme called amylase that breaks down carbohydrates and an enxyme lipase that breaks down triglycerides. The food bolus travels through the esophagus by peristaltic movements to the stomach. The stomach has an extremely acidic environment. The enzyme pepsin digests protein in the stomach. Further digestion and absorption take place in the small intestine. The large intestine reabsorbs water from the undigested food and stores waste until elimination.

    Carbohydrates, proteins, and fats are the primary components of food. Some essential nutrients are required for cellular function but cannot be produced by the animal body. These include vitamins (both fat and water soluble) , minerals, some fatty acids, and some amino acids. Food intake in more than necessary amounts is stored as glycogen in the liver and muscle cells, and in adipose tissue. Excess adipose storage can lead to obesity and serious health problems.

    Adapted from Openstax Human Biology


    From the Small Intestine to the Large Intestine

    The process of digestion is fairly efficient. Any food that is still incompletely broken down (usually less than ten percent of food consumed) and the food’s indigestible fiber content move from the small intestine to the large intestine (colon) through a connecting valve. A main task of the large intestine is to absorb much of the remaining water. Remember, water is present not only in solid foods and beverages, but also the stomach releases a few hundred milliliters of gastric juice, and the pancreas adds approximately 500 milliliters during the digestion of the meal. For the body to conserve water, it is important that excessive water is not lost in fecal matter. In the large intestine, no further chemical or mechanical breakdown of food takes place unless it is accomplished by the bacteria that inhabit this portion of the intestinal tract. The number of bacteria residing in the large intestine is estimated to be greater than 1014, which is more than the total number of cells in the human body (1013). This may seem rather unpleasant, but the great majority of bacteria in the large intestine are harmless and many are even beneficial.


    When the digestive system has broken down food to its nutrient components, the body eagerly awaits delivery. Water soluble nutrients absorbed into the blood travel directly to the liver via a major blood vessel called the portal vein. One of the liver&rsquos primary functions is to regulate metabolic homeostasis. Metabolic homeostasis is achieved when the nutrients consumed and absorbed match the energy required to carry out life&rsquos biological processes. Simply put, nutrient energy intake equals energy output. Whereas glucose and amino acids are directly transported from the small intestine to the liver, lipids are transported to the liver by a more circuitous route involving the lymphatic system. The lymphatic system is a one-way system of vessels that transports lymph, a fluid rich in white blood cells, and lipid soluble substances after a meal containing lipids. The lymphatic system slowly moves its contents through the lymphatic vessels and empties into blood vessels in the upper chest area. Now, the absorbed lipid soluble components are in the blood where they can be distributed throughout the body and utilized by cells (see Figure 2.9 &ldquoThe Absorption of Nutrients&rdquo).

    Figure 2.9 The Absorption of Nutrients

    Maintaining the body&rsquos energy status quo is crucial because when metabolic homeostasis is disturbed by an eating disorder or disease, bodily function suffers. This will be discussed in more depth in the last section of this chapter. The liver is the only organ in the human body that is capable of exporting nutrients for energy production to other tissues. Therefore, when a person is in between meals (fasted state) the liver exports nutrients, and when a person has just eaten (fed state) the liver stores nutrients within itself. Nutrient levels and the hormones that respond to their levels in the blood provide the input so that the liver can distinguish between the fasted and fed states and distribute nutrients appropriately. Although not considered to be an organ, adipose tissue stores fat in the fed state and mobilizes fat components to supply energy to other parts of the body when energy is needed.

    All eleven organ systems in the human body require nutrient input to perform their specific biological functions. Overall health and the ability to carry out all of life&rsquos basic processes is fueled by energy-supplying nutrients (carbohydrate, fat, and protein). Without them, organ systems would fail, humans would not reproduce, and the race would disappear. In this section, we will discuss some of the critical nutrients that support specific organ system functions.


    From the Mouth to the Stomach

    There are four steps in the digestion process (Figure 2.5 “The Human Digestive System”). The first step is ingestion, which is the intake of food into the digestive tract. It may seem a simple process, but ingestion involves smelling food, thinking about food, and the involuntary release of saliva in the mouth to prepare for food entry. In the mouth, where the second step of digestion starts, the mechanical and chemical breakdown of food begins. The chemical breakdown of food involves enzymes, such as salivary amylase that starts the breakdown of large starch molecules into smaller components.

    Mechanical breakdown starts with mastication (chewing) in the mouth. Teeth crush and grind large food particles, while saliva provides lubrication and enables food movement downward. The slippery mass of partially broken-down food is called a bolus , which moves down the digestive tract as you swallow. Swallowing may seem voluntary at first because it requires conscious effort to push the food with the tongue back toward the throat, but after this, swallowing proceeds involuntarily, meaning it cannot be stopped once it begins. As you swallow, the bolus is pushed from the mouth through the pharynx and into a muscular tube called the esophagus. As the bolus travels through the pharynx, a small flap called the epiglottis closes to prevent choking by keeping food from going into the trachea. Peristaltic contractions also known as peristalsis in the esophagus propel the food bolus down to the stomach (Figure 3.6 “Peristalsis in the Esophagus”). At the junction between the esophagus and stomach there is a sphincter muscle that remains closed until the food bolus approaches. The pressure of the food bolus stimulates the lower esophageal sphincter to relax and open and food then moves from the esophagus into the stomach. The mechanical breakdown of food is accentuated by the muscular contractions of the stomach and small intestine that mash, mix, slosh, and propel food down the alimentary canal. Solid food takes between four and eight seconds to travel down the esophagus, and liquids take about one second.

    Figure 2.6 Peristalsis in the Esophagus

    Image by Allison Calabrese / CC BY 4.0


    Chemical Digestion

    Chemical digestion is the biochemical process in which macromolecule s in food are changed into smaller molecules that can be absorbed into body fluids and transported to cells throughout the body. Substances in food that must be chemically digested include carbohydrates , protein s , lipid s , and nucleic acids . Carbohydrates must be broken down into simple sugar s , proteins into amino acid s , lipids into fatty acids and glycerol, and nucleic acids into nitrogen bases and sugars. Some chemical digestion takes place in the mouth and stomach, but most of it occurs in the first part of the small intestine ( duodenum ).

    Digestive Enzymes

    Chemical digestion could not occur without the help of many different digestive enzymes. Enzymes are proteins that catalyze, or speed up, biochemical reactions. Digestive enzymes are secreted by exocrine gland s or by the mucosal layer of epithelium lining the gastrointestinal tract. In the mouth , digestive enzymes are secreted by salivary gland s . The lining of the stomach secretes enzymes, as does the lining of the small intestine . Many more digestive enzymes are secreted by exocrine cells in the pancreas and carried by ducts to the small intestine. The following table lists several important digestive enzymes, the organs and/or glands that secrete them, the compounds they digest, and the pH necessary for optimal functioning. You can read more about them below.

    Table 15.3.1: Digestive Enzymes
    Digestive Enzyme Source Organ Site of Action Reactant and Product Optimal pH
    Salivary Amylase Salivary Glands Mouth starch + water ⇒ maltose Neutral
    Pepsin Stomach Stomach protein + water ⇒ peptides Acidic
    Pancreatic Amylase Pancreas Duodenum starch + water ⇒ maltose Basic
    Maltase Small intestine Small intestine maltose + water ⇒ glucose Basic
    Sucrase Small intestine Small intestine sucrose + water ⇒ glucose + fructose Basic
    Lactase Small intestine Small intestine lactose + water ⇒ glucose + galactose Basic
    Lipase Pancreas Duodenum fat droplet and water ⇒ glycerol and fatty acids Basic
    Trypsin Pancreas Duodenum protein + water ⇒ peptides Basic
    Chymotrypsin Pancreas Duodenum protein + water ⇒ peptides Basic
    Peptidases Small intestine Small intestine peptides + water ⇒ Basic
    Deoxyribonuclease Pancreas Duodenum DNA + water ⇒ nucleotide fragments Basic
    Ribonuclease Pancreas Duodenum RNA + water ⇒ nucleotide fragments Basic
    Nuclease Small intestine Small intestine nucleic acids + water ⇒ nucleotide fragments Basic
    Nucleosidases Small intestine Small intestine nucleotides + water ⇒ nitrogen base + phosphate sugar Basic

    Chemical Digestion of Carbohydrates

    About 80% of digestible carbohydrates in a typical Western diet are in the form of the plant polysaccharide amylose, which consists mainly of long chains of glucose and is one of two major components of starch . Additional dietary carbohydrates include the animal polysaccharide glycogen , along with some sugars, which are mainly disaccharide s .

    The process of chemical digestion for some carbohydrates is illustrated Figure 15.3.4. To chemically digest amylose and glycogen, the enzyme amylase is required. The chemical digestion of these polysaccharides begins in the mouth, aided by amylase in saliva. Saliva also contains mucus — which lubricates the food — and hydrogen carbonate, which provides the ideal alkaline conditions for amylase to work. Carbohydrate digestion is completed in the small intestine, with the help of amylase secreted by the pancreas. In the digestive process, polysaccharides are reduced in length by the breaking of bonds between glucose monomers. The macromolecules are broken down to shorter polysaccharides and disaccharides, resulting in progressively shorter chains of glucose. The end result is molecules of the simple sugars glucose and maltose (which consists of two glucose molecules), both of which can be absorbed by the small intestine.

    Other sugars are digested with the help of different enzymes produced by the small intestine. Sucrose (or table sugar), for example, is a disaccharide that is broken down by the enzyme sucrase to form glucose and fructose, which are readily absorbed by the small intestine. Digestion of the sugar lactose, which is found in milk, requires the enzyme lactase, which breaks down lactose into glucose and galactose. Glucose and galactose are then absorbed by the small intestine. Fewer than half of all adults produce sufficient lactase to be able to digest lactose. Those who cannot are said to be lactose intolerant.

    Figure 15.3.4 The process of chemical digestion for some carbohydrates.

    Chemical Digestion of Proteins

    Proteinsno post consist of polypeptides, which must be broken down into their constituent amino acid s before they can be absorbed. An overview of this process is shown in Figure 15.3.5. Protein digestion occurs in the stomach and small intestine through the action of three primary enzymes: pepsin (secreted by the stomach), and trypsin and chymotrypsin (secreted by the pancreas). The stomach also secretes hydrochloric acid (HCl), making the contents highly acidic, which is a required condition for pepsin to work. Trypsin and chymotrypsin in the small intestine require an alkaline (basic) environment to work. Bile from the liver and bicarbonate from the pancreas neutralize the acidic chyme as it empties into the small intestine. After pepsin, trypsin, and chymotrypsin break down proteins into peptides, these are further broken down into amino acids by other enzymes called peptidase s , also secreted by the pancreas.

    Figure 15.3.5 Chemical digestion of proteins.

    Chemical Digestion of Lipids

    The chemical digestion of lipids begins in the mouth. The salivary glands secrete the digestive enzyme lipase , which breaks down short-chain lipids into molecules consisting of two fatty acids. A tiny amount of lipid digestion may take place in the stomach, but most lipid digestion occurs in the small intestine.

    Digestion of lipids in the small intestine occurs with the help of another lipase enzyme from the pancreas, as well as bile secreted by the liver . As shown in the diagram below (Figure 15.3.6), bile is required for the digestion of lipids, because lipids are oily and do not dissolve in the watery chyme. Bile emulsifies (or breaks up) large globules of food lipids into much smaller ones, called micelles, much as dish detergent breaks up grease. The micelles provide a great deal more surface area to be acted upon by lipase, and also point the hydrophilic (“water-loving”) heads of the fatty acids outward into the watery chyme. Lipase can then access and break down the micelles into individual fatty acid molecules.

    Figure 15.3.6 Bile from the liver and lipase from the pancreas help digest lipids in the small intestine.

    Chemical Digestion of Nucleic Acids

    Nucleic acids (DNA and RNA) in foods are digested in the small intestine with the help of both pancreatic enzymes and enzymes produced by the small intestine itself. Pancreatic enzymes called ribonuclease and deoxyribonuclease break down RNA and DNA, respectively, into smaller nucleic acids. These, in turn, are further broken down into nitrogen bases and sugars by small intestine enzymes called nucleases.

    Bacteria in the Digestive System

    Your large intestine is not just made up of cells. It is also an ecosystem , home to trillions of bacteria known as the “gut flora” (Figure 15.3.7). But don’t worry, most of these bacteria are helpful. Friendly bacteria live mostly in the large intestine and part of the small intestine. The acidic environment of the stomach does not allow bacterial growth.

    Gut bacteria have several roles in the body. For example, intestinal bacteria:

    • Produce vitamin B12 and vitamin K.
    • Control the growth of harmful bacteria.
    • Break down poisons in the large intestine.
    • Break down some substances in food that cannot be digested, such as fibre and some starches and sugars. Bacteria produce enzymes that digest carbohydrates in plant cell walls. Most of the nutritional value of plant material would be wasted without these bacteria. These help us digest plant foods like spinach.

    Figure 15.3.7 Commensal (good) bacteria (shown in red) reside among the mucus (green) and epithelial cells (blue) of a small intestine.

    A wide range of friendly bacteria live in the gut. Bacteria begin to populate the human digestive system right after birth. Gut bacteria include Lactobacillus, the bacteria commonly used in probiotic foods such as yogurt, and E. coli bacteria. About a third of all bacteria in the gut are members of the Bacteroides species. Bacteroides are key in helping us digest plant food.

    It is estimated that 100 trillion bacteria live in the gut. This is more than the human cells that make up you. It has also been estimated that there are more bacteria in your mouth than people on the planet — there are over 7 billion people on the planet!

    The bacteria in your digestive system are from anywhere between 300 and 1,000 species. As these bacteria are helpful, your body does not attack them. They actually appear to the body’s immune system as cells of the digestive system, not foreign invaders. The bacteria actually cover themselves with sugar molecules removed from the actual cells of the digestive system. This disguises the bacteria and protects them from the immune system.

    As the bacteria that live in the human gut are beneficial to us, and as the bacteria enjoy a safe environment to live, the relationship that we have with these tiny organisms is described as mutualism, a type of symbiotic relationship.

    Lastly, keep in mind the small size of bacteria. Together, all the bacteria in your gut may weigh just about two pounds.


    Can You Absorb Nutrients Through Your Skin?

    Nicotine, hormones and certain medications can all be delivered through the skin through medicinal patches or creams. Why not vitamins and minerals?

    When we think about taking nutrients into our bodies, we usually think about swallowing them, in the form of pills, powders, or that radical format known as food. For that matter, when we talk about nutrient absorption, we&rsquore usually talking about the absorption of nutrients from the digestive system into the bloodstream.

    But a handful of companies are trying to change the way we think about nutritional supplementation. Instead of swallowing a handful of pills and worrying about whether or not they are being absorbed, why not bypass the digestive tract altogether and apply them directly to your skin?

    Nicotine, estrogen, testosterone, and certain pain medications can all be delivered through the skin through medicinal patches, gels, or creams. Why not vitamins and minerals?

    ABOUT THE AUTHOR(S)

    Monica Reinagel, MS,LD/N, CNS, is a board-certified, licensed nutritionist and professionally trained chef, author of Nutrition Diva&rsquos Secrets for a Healthy Diet, and host of the Nutrition Diva podcast on Quick and Dirty Tips.


    Absorbing Vitamins and Minerals

    Virtually any food you consume provides vitamins and minerals, although processed junk foods tend to have lower amounts than produce, low-fat milk and other nutritious foods. Most vitamins and minerals separate from other food components and absorb into your bloodstream through the small intestine. Some nutrients have additional steps that further delays absorption. For example, vitamins A, D, E and K are fat-soluble, meaning they absorb and are stored alongside fat. If you take a multivitamin with these nutrients but do not eat something with fat when you take it, your system may not pick up these vitamins. Vitamin B-12 absorbs differently than any other nutrient. This vitamin attaches to a protein called intrinsic factor in your stomach. Once B-12 and intrinsic factor combine, your small intestine is able to pick it up and send it to your bloodstream.

    Melodie Anne Coffman specializes in overall wellness, with particular interests in women's health and personal defense. She holds a master's degree in food science and human nutrition and is a certified instructor through the NRA. Coffman is pursuing her personal trainer certification in 2015.