The Digestive System Overview
DIGESTIVE SYSTEM ORGANS
Introduction
The digestive system is made up of a series of organs that are involved in digestion, which is the breakdown of food into smaller components called nutrients so they can be absorbed and used by the body. The human digestive system consists of the gastrointestinal tract (also called the alimentary canal or digestive tract) and a number of accessory organs. Organs of the gastrointestinal tract include the mouth, esophagus, stomach, small intestine, and large intestine. The accessory organs include the tongue, salivary glands, liver, gallbladder, and pancreas.
Functions of the Digestive System
The digestive system serves 4 important functions in the human body, as summarized in the table below.
Major Functions of the Digestive System | |
Ingestion | The process of taking in food and liquids into the body |
Digestion | The process of breaking down food molecules into smaller units called nutrients |
Absorption | The process of taking in valuable nutrients into the bloodstream |
Elimination | The process of releasing undigested waste called feces from the body |
Layers of the Gastrointestinal Tract
The gastrointestinal (GI) tract is a series of hollow organs joined in a long, twisting tube from the mouth to the anus. In adults, it is roughly 9 m (30 ft) in length. The walls of the GI tract are composed of 4 distinct layers. From the inner cavity of the GI tract (the lumen) outwards, the layers are called the mucosa, submucosa, muscularis, and serosa.
The innermost layer is the mucosa or mucous membrane. It surrounds the lumen and comes in direct contact with digested food. The mucosa features digestive glands, which secrete digestive enzymes as well as mucus to protect surrounding tissues. The mucosa itself is made up of 3 layers: the epithelium, where most digestive processes occur; the lamina propria, a layer of connective tissue, and the muscularis mucosae, a thin layer of smooth muscle. The second layer is the submucosa. This layer features blood vessels, lymphatic vessels, nerves, and elastic fibers, which allow the GI tract to stretch as food moves through it but still maintain its shape. The third layer is the muscularis. This layer consists of 2 sublayers of smooth muscle. The inner sublayer of circular smooth muscle encircles the GI tract and can contract to decrease its diameter. The outer sublayer of longitudinal smooth muscle runs lengthwise along the GI tract and can contract to shorten its length. The outermost layer is the serosa or serous layer. It is made of loose connective tissue coated in fluid to prevent friction from motions of the GI tract damaging surrounding tissues. The serosa itself is made up of 2 layers. The parietal membrane faces inward and covers the walls of the GI tract, while the visceral membrane faces outward and covers nearby body organs. Glands in the visceral membrane secrete serous fluid to keep the GI tract moist and permit the organs to slide freely past one another.
Organs of the Gastrointestinal Tract
Mouth
At start of the digestive process, food enters the GI tract via an oval-shaped cavity called the mouth. The primary job of the mouth is to take in food, taste it, and break it down both physically and chemically. The main portion of the mouth is a chamber called the oral cavity, which is surrounded by the lips, cheeks, tongue, and palate. The roof of the mouth is made up of 2 parts. The hard palate is made of bone and forms the anterior part of the roof; its job is to help with chewing and protect the nasal cavity above. The soft palate is more flexible as it is made of muscle and connective tissue and forms the posterior part of the roof of the mouth. Its job is to help with speech, breathing, and swallowing. A finger-like projection called the uvula hangs from the soft palate; its job is to block the nasopharynx (nose and throat) during swallowing to prevent food from entering the nasal cavity.
Tongue (Accessory Organ)
The tongue is a muscular accessory organ that is attached to mouth floor. The tongue is involved in speech production, moving food, chewing, swallowing, as well as taste perception. The surface and sides of the tongue are covered with small bumps called papillae that contain taste buds. Tastes buds are sensory structures found at the bottom of papillae which detect different taste molecules in food.
Salivary Glands (Accessory Organ)
The mouth features 3 accessory organs called salivary glands, whose job is to lubricate food and produce enzymes that begin the digestive process. There are 3 salivary glands, named according to their location: the parotid gland (base of ear), submaxillary gland (below the jaw), and sublingual gland (below the tongue). The salivary glands play an important role in the chemical digestion of food as they secrete saliva. Saliva is a watery fluid that moistens food so it can be chewed and swallowed easily. In addition, saliva contains amylase, an enzyme that breaks down starches in food.
Teeth
Mechanical digestion in the mouth is performed by the teeth, whose main jobs are to chew or masticate food, aid in the formation of words for speech, and to maintain the shape of the face. In adults, there are 32 permanent teeth, which come in 4 different types. Incisors are the front-most teeth and are used for slicing food. Canines (also called cuspids) are cone-shaped teeth to the side of the incisors and are used for holding and tearing food. Premolars (also called bicuspids) are the flat middle teeth specialized for grinding food. Finally, molars are the larger flat teeth at the back of the mouth and also grind food.
Esophagus
The esophagus is a muscular tube approximately 25 cm (10 inches) in length whose job is to deliver food from the mouth to the stomach. To do this, food must be swallowed, a process also called deglutition. In the mouth, food is chewed and mixed with saliva to form a bolus (rounded mass) which is forced into the pharynx (throat) by movements of the tongue. During deglutition, smooth muscle in the muscularis layer contract in a rhythmic, wavelike pattern called peristalsis to squeeze the bolus downward. At the end of the esophagus is a ring of smooth muscle called the esophageal sphincter, which opens and closes to control the flow of food between the esophagus and the stomach.
Epiglottis
The epiglottis is a specialized flap of cartilage located in the pharynx (throat) behind the tongue and above the trachea. The main job of the epiglottis is to direct the flow of air and food during breathing or eating. When the body is not swallowing, the epiglottis is upright, directing air into the larynx and trachea in order to reach the lungs. When the body is swallowing, the epiglottis folds back and downward. This acts to cover the larynx and trachea and redirect food down the esophagus. The epiglottis is essential to prevent food from entering the lungs and potentially cause choking.
Stomach
The stomach is C-shaped organ lying in the upper left part of the abdomen. Its main job is to break down food physically using muscular movements and chemically through digestive enzymes and acid. The stomach is divided into 3 main regions: the fundus is the rounded upper section, the body is the larger middle section, and the pyloris is the narrow bottom section. The muscularis of the stomach features a third layer of smooth muscle called the oblique layer, which allows the stomach to mechanicaly churn food in multiple directions to break it down into a soupy mixture called chyme. The stomach’s inner lining features thick folds called rugae, which allow the stomach to expand when full and shrink when empty. Within the mucosa layer are millions of gastric glands which secrete a variety of substances to aid in the chemical digestion of food, including hydrochloric acid and enzymes.
Small Intestine
The small intestine is a narrow twisting tube that is the longest section of the GI tract, measuring approximately 6.5 m (18 ft) in length in adults. The main jobs of the small intestine are to complete the chemical digestion of food and to absorb nutrients into the bloodstream. The small intestine is divided into 3 sections. The duodenum is the shorter upper section, measuring only 0.25 m (10 in) long. The jejunum is the middle section, measuring 2.5 m (8 ft) long). The longest section is the lower ileum, measuring 3.7 m (12 ft) long. The final chemical digestion of food is achieved by numerous digestive enzymes produced by the liver, pancreas, and the small intestine itself that are then mixed with the chyme released by the stomach.
Roughly 3-6 hours after eating, large food particles have been fully digested into tiny nutrient molecules. Absorption is the process where nutrients pass through the epithelial cells of the small intestine wall and enter the blood. To maximize absorption, the surface area of the small intestine is greatly increased by millions of tiny folds in its walls. Villi are finger-like folds of the inner lining. Microvilli are even microscopic finger-like projections of the individual epithelial cells. Each villus features capillaries and specialized lymphatic vessels called lacteals that absorb the nutrients rapidly.
Liver and Gallbladder (Accessory Organ)
The liver is large, cone-shaped accessory organ located in the upper right portion of the abdomen. Its main job in the digestive system is to produce bile, a fluid which breaks down larger lipids into smaller droplets called micelles. Bile produced by the liver is released into a pear-shaped sac called the gallbladder, located just below the liver. The gall bladder stores and concentrates bile until it is needed, releasing it into the small intestine when chyme enters the passage from the stomach.
Pancreas (Accessory Organ)
The pancreas is a leaf-shaped accessory organ located below the liver and behind the stomach. Its main job is to secrete pancreatic juice into the small intestine as well as produce hormones for the regulation of blood glucose, namely insulin and glucagon. Pancreatic juice is a clear, colorless fluid containing water, salts, bicarbonate ions, and digestive enzymes. The bicarbonate ions help neutralize the acidity of digestive materials entering the small intestine from the stomach.
Large Intestine
The large intestine, also known as the colon, connects to the ileum and continues to the anus. It is so named because its circumference is much larger than the small intestine. The main job of the large intestine is to reabsorb water, ions, vitamins, as well as compact the remaining undigested material to form a waste product called feces. The large intestine is divided into 6 sections, as summarized below.
Sections of the Large Intestine | |
Cecum | Small pouch; connects to small intestine |
Ascending Colon | Extends upward to the liver’s lower edge |
Transverse Colon | Extends laterally across the abdomen |
Descending Colon | Extends downward to the sigmoid colon |
Sigmoid Colon | S-shaped bend that joins with the rectum |
Rectum | Final section; storage area for feces |
Rectum
The rectum forms the last 20 cm (8 in) of the GI tract. Its main job is to store feces, a solid material which consists of undigested food, excess water and ions, bacteria, as well as dead cells shed from the GI tract lining. When the rectum is full, the brain stimulates the release of feces, a process known as defecation.
The rectum terminates at the anus, an opening through which feces is eliminated from the body. Defecation is a reflex action aided by contractions of smooth muscle in the muscularis. It is controlled by 2 anal sphincters, which are ringlike muscles that act as valves to open or close body passages. The internal anal sphincter is an involuntary muscle while the external anal sphincter is a voluntary muscle.
DIGESTION, ABSORPTION AND ELIMINATION
The digestive system consists of the gastrointestinal (GI) tract and accessory organs. The GI tract is a series of hollow organs joined in a long, twisting tube. The hollow organs include the mouth, esophagus, stomach, small intestine, large intestine, and anus. The accessory organs include the tongue, salivary glands, liver, gallbladder, and pancreas. The main role of the digestive system is to process food into smaller molecules that can be utilized by the cells of the body. To do this, food undergoes 3 processes in the body, digestion, absorption, and elimination, as summarized below.
3 Processes in the Digestive System | ||
Digestion | Absorption | Elimination |
The process of breaking down food particles into smaller units called nutrients | The process of taking in valuable nutrients into the bloodstream | The process of releasing undigested food and waste products from the body |
Digestion
Digestion is the process of breaking down large food particles into smaller units called nutrients that can pass into the bloodstream. Large carbohydrate molecules (such as starch) are digested into smaller monomers of glucose. Large lipid molecules (such as fats) are digested into fatty acids and glycerol. Large protein molecules (such as collagen) are digested into amino acids.
There are 2 types of digestion in the body: mechanical and chemical. Mechanical digestion involves physically breaking food down into smaller pieces by cutting, tearing, squeezing, and grinding. Chemical digestion involves chemically breaking food down into smaller units by enzymes and hydrochloric acid.
2 Types of Digestion | |
Mechanical Digestion | Chemical Digestion |
Physically breaking down food molecules | Chemically breaking down food molecules |
Examples: teeth, stomach movements | Examples: stomach acid, digestive enzymes |
Digestion in the Mouth
The primary job of the mouth is to take in food and break it down both physically and chemically. In the mouth, mechanical digestion begins with the teeth, which slice, tear, and grind food into smaller pieces. This is aided by motions of the tongue and the upper palate. Chemical digestion begins with saliva, a watery fluid secreted by the salivary glands. Saliva helps moistens food, bind it together, and initiates the breakdown of carbohydrates through an enzyme called amylase. Amylase chemically digests large starch molecules into smaller 2-sugar units (disaccharides) called maltose. Since food is only in the mouth for a short period of time, only a small amount of starch is digested; the remainder is digested in the small intestine.
Digestion in the Stomach
The stomach is C-shaped organ lying in the upper left quadrant of the abdomen. The stomach is a major site of digestion. Physical digestion is accomplished by muscular movements of the stomach wall, which squeeze and churn food into smaller pieces. Chemical digestion is accomplished by mixing food with liquids in the stomach. Within the mucosa layer of the stomach are millions of pits that end in tubular gastric glands. These glands produce gastric juice, a complex fluid that contains a variety of substances. There are 4 types of secretory cells in each gastric gland, each of which release their own digestive product.
Secretory Cell | Product | Description |
Mucus cells | Mucus | Viscous liquid; protects stomach lining from self-digestion by acid |
Parietal cells | Hydrochloric acid | Acid; breaks down numerous foods; activates pepsin; kills pathogens |
Chief cells | Pepsinogen | Precursor molecule; converted to pepsin by acid; breaks down proteins |
| Lipase | Digestive enzyme; breaks down fats into fatty acids and glycerol |
G cells | Gastrin | Regulatory hormone; stimulates gastric glands after ingesting food |
The stomach is also the site of initial absorption of handful of nutrients. These include small amounts of water, glucose, and ions. After spending roughly 0.5 - 4 hours in the stomach, food had been turned into a pulpy liquid called chyme, which passes into the small intestine for further processing.
Digestion in the Small Intestine
The small intestine is the longest section of the gastrointestinal tract, averaging 5 - 6 m (17 - 20 ft) in length. It is divided into 3 main sections. The duodenum is the short upper section, measuring 0.25 m (10 in.) long and is where most chemical digestion of food occurs. The jejunum is the middle section, measuring 2.5 m (8 ft) long and is where most absorption takes place. Finally, the ileum is the longest lower section, measuring 3.7 m (12 ft) in length and is where the absorption of important vitamins, such as A, B12, D, E, and K occurs.
The duodenum receives chyme and gastric juice from the stomach through a ring of smooth muscle called the pyloric sphincter. Chemical digestion in the duodenum is achieved by a variety of substances released by the liver, pancreas, and the duodenum itself.
Firstly, tubular glands in the mucosa layer of the duodenum, known as intestinal glands or crypts of Lieberkühn, secrete intestinal juice, a watery fluid composed of hormones, alkaline mucus, and digestive enzymes. The alkaline mucus helps neutralize the hydrochloric acid from the stomach. There are numerous digestive enzymes that break down large food molecules into smaller nutrients. Proteases such as peptidase, dipeptidases, and aminopeptidase complete protein digestion and release free amino acids. Carbohydrases, including lactase, sucrase, and maltase break down their complementary disaccharides (lactose, sucrose, maltose) into monosaccharides. Finally, nucleases break down large RNA and DNA molecules into smaller nucleotides.
Secondly, the pancreas, a leaf-shaped accessory organ located just behind the stomach, contributes pancreatic juice, a colorless, alkaline fluid which enters the duodenum through the pancreatic duct. Pancreatic juice contains a number of digestive enzymes which break down proteins, fats, and starches. Pancreatic proteases such as trypsin, chymotrypsin, and carboxypeptidase break down proteins into amino acids. Pancreatic lipase breaks down fats into fatty acids and glycerol. Pancreatic amylase breaks down starch into the disaccharide maltose. In addition, pancreatic juice contains a large amount of bicarbonate ions (OH-) which further neutralizes stomach acid. This is important as most digestive enzymes (besides pepsin in the stomach) are only functional in a neutral pH environment.
Thirdly, the liver, a cone-shaped accessory organ located in the upper right quadrant of the abdomen, produces bile. Bile is a dark green to yellow fluid secreted by the liver and stored in the gall bladder. Bile enters the lumen of the duodenum via the common bile duct. Bile helps with digestion by breaking down large fat globules into smaller droplets known as micelles in a process called emulsification. The emulsified fats are further broken down by pancreatic lipases into fatty acids and glycerol.
Digestive Enzymes and Other Products | ||||
Location | Fluid | Enzyme / Product | Substrate | Product |
Mouth | Saliva | Amylase | Starches | Maltose |
Stomach | Gastric juice | Pepsin | Proteins | Peptides |
Hydrochloric acid | Many foods | Smaller units | ||
Pancreas | Pancreatic juice | Amylase | Starches | Maltose |
Lipase | Fats | Fatty acids, glycerol | ||
Trypsin | Proteins | Peptides | ||
Chymotrypsin | Proteins | Peptides | ||
Carboxypeptidase | Proteins | Peptides | ||
Small intestine | Intestinal juice | Maltase | Maltose sugar | Glucose |
Lactase | Lactose | Glucose | ||
Sucrase | Sucrose | Glucose | ||
Aminopeptidase | Peptides | Amino Acids | ||
Dipeptidase | Peptides | Amino Acids | ||
Nuclease | DNA & RNA | Nucleotides | ||
Liver | Bile | Bile salts | Large fat droplets | Small fat droplets |
Absorption in the Small Intestine
In the jejunum and ileum, nutrients from digested food are absorbed. To maximize the rate of absorption, the surface area of the intestinal wall is greatly increased by millions of tiny projections called villi and microvilli. Villi are finger-like folds of the mucosa, while microvilli are microscopic projections of the membranes of individual mucosal cells. Within each villus are capillaries that absorb monosaccharides, amino acids, as well as water, minerals, and vitamins into the bloodstream. Each villus also features specialized lymphatic vessels called lacteals, which absorb the products of fat digestion, mainly fatty acids and glycerol.
Absorption in the Large Intestine
The large intestine, also known as the colon, is shorter and wider than the small intestine, measuring 1.5 m (5 ft) in length with an average diameter of 6 cm (2.5 in). It is divided into 6 main sections: the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, and rectum. The large intestine is subdivided into small pouches called haustra, which increase the surface area to maximize absorption and give the organ its characteristic segmented appearance.
Since the chemical digestion of food and absorption of nutrients occurs primarily in the small intestine, the main role of the large intestine is to reabsorb any remaining water, ions, and vitamins, as well and compact undigested material to form a solid waste product called feces. Feces consists of undigested food, water, inorganic salts, bacteria, and epithelial cells that are continually shed by the GI tract.
Elimination in the Rectum
The rectum is a straight tube measuring 18 - 20 cm (7 - 8 in) in length that connects the large intestine to an opening called the anus. The rectum functions as a storage area for feces. To do this, it continually receives fecal material from the large intestine. As the rectal walls expand due to filling from within, stretch receptors in the walls are activated. When the rectum is full, the receptors send a signal to the brain which stimulates the need to pass feces from the body in a process called defecation. Defecation is a reflex action aided by contractions of smooth muscle in the muscularis. In addition, 2 anal sphincters, which are ringlike muscles that open or close body passages, work together to control the release of feces from the body.
Phases of Gastric Control and Hormones
The brain is the control center for the processes of digestion, absorption, and elimination, as well as the sensations of hunger and satiety (feeling of fullness). The actions of digestive system organs are regulated through nerve signals and hormones. There are 3 main phases of gastric control.
The first phase, called the cephalic phase, is the body’s response to food before it even enters the mouth. It is triggered by nervous system responses to any stimulus from food, including the sight, smell, taste, and mere thought of eating food. Neural responses trigger the secretion of saliva by salivary glands in the mouth and production of gastric juice in the stomach to prepare for ingested food.
The second phase, called the gastric phase, is the body’s response to food arriving in the stomach. It is triggered by expansion of the stomach, the presence of undigested food in the stomach, and a decrease in stomach pH as hydrochloric acid levels increase. The gastric phase uses both neural signals and hormones to stimulate gastric secretions and stomach contractions in order to digest food.
The third phase, called the intestinal phase, it the body’s response to food exiting the stomach. It is triggered by the arrival of chyme in the small intestine. When this occurs, neural signals and hormones act to reduce gastric secretions and stomach contractions since digestion there is slowing down, as well as coordinate the activity of the small intestine, pancreas, liver, and gallbladder.
Gastrointestinal or gut hormones consist of a group of hormones secreted by endocrine cells in the stomach, pancreas, and small intestine to help coordinate the activity of digestive organs. While there are many more gut hormones involved with digestion, key hormones are described in the summary table.
Major Gut Hormones and Their Action | |||
Hormone | Source | Target | Action |
Gastrin | Stomach | Stomach | · Stimulates parietal cells to secrete gastric acid · Stimulates chief cells to secrete pepsinogen |
Secretin | Duodenum | Pancreas | · Stimulates pancreas cells to secrete bicarbonate ions |
Cholecystokinin (CCK) | Duodenum | Pancreas Gallbladder | · Stimulates pancreas to secrete enzymes · Stimulates gallbladder to release bile |
Motilin | Duodenum | Stomach Intestines | · Stimulates stomach to release chyme · Stimulates motility in small and large intestines |
Gastric inhibitory peptide (GIP) | Duodenum Jejunum | Stomach Pancreas | · Inhibits stomach to reduce gastric acid · Stimulates pancreas to release insulin |
THE SIX ESSENTIAL NUTRIENTS
Introduction
Humans need to consume food each day because food provides nutrients. Nutrients are substances that the body needs to carry out its normal functions. Specifically, nutrients provide the body with 2 main things: energy for daily activity and raw materials for growth, repair, and defense. There are 6 essential nutrients: carbohydrates, lipids, proteins, vitamins, minerals, and water.
Two Types of Nutrients
There are 2 main types of nutrients. Macronutrients are nutrients the body needs in larger amounts to grow and function normally and include carbohydrates, lipids, and proteins. Micronutrients are nutrients the body needs in smaller amounts and include vitamins and minerals.
Macronutrients | Micronutrients |
· Nutrients required in larger amounts · Includes: carbohydrates, lipids, proteins | · Nutrients required in smaller amounts · Includes: vitamins, minerals |
Carbohydrates
Carbohydrates are organic molecules (made by living things) built from units of glucose to form sugars, starches, and fiber. Carbohydrates serve 2 main functions in the body. First, they provide quick energy since they can be broken down easily into glucose, a smaller carbohydrate that muscles and the brain use to function. Second, they provide long-term energy storage since they can be combined into larger molecules such as glycogen. Foods rich in carbohydrates include grains such as wheat, rice and oats, legumes such as beans and lentils, fruits, vegetables, and sugar.
There are 2 main types of carbohydrates. Simple carbohydrates are small molecules that release sugar quickly and do not contain vitamins, minerals or fiber. They are commonly called “bad carbs”. Simple carbohydrates are found in processed grains such as white flour, white rice, corn syrup, and table sugar. Complex carbohydrates are larger molecules that release sugar slowly and contain other nutrients such as vitamins and minerals. They are commonly called “good carbs”. Complex carbohydrates are found in whole grains, legumes, fruits, and vegetables.
Simple Carbohydrates | Complex Carbohydrates |
· Foods containing refined sugar · More processed, digested quickly, fast-release energy, lower nutritional value · Foods: white bread, white pasta, white rice, table sugar, cakes, cookies, sweetened drinks | · Foods containing unrefined sugar · Less processed, digested slowly, slow-release energy, higher nutritional value · Foods: whole grain bread and pasta, brown rice, fruit, vegetables, beans, lentils, oats |
Special Topic: Fiber
Fiber is a special type of complex carbohydrate that makes up the tough, stringy part of raw fruits and vegetables, whole grains, and legumes. Since fiber is not digested or absorbed by the body it is technically not a nutrient, however it is still vital for proper health. Fiber absorbs harmful chemicals, makes feces soft and bulky to improve overall gut function, slows digestion and absorption which helps prevent obesity.
Lipids
Lipids, also called fats, are organic molecules made of 3 smaller units joined together in a structure called a triglyceride. Fats can either be solid at room temperature (called fats) or liquid (called oils). Fats are used by the body to store energy, form cell membranes, insulation, as well as aiding the absorption of certain vitamins like vitamin A. Foods rich in fats include butter, oil, cheese, meat, and dairy.
There are 3 types of fats found in foods. Unsaturated fats are liquids since they have a low density and are derived from plant and fish products. They are known as “healthy fats” as they regulate metabolism, improve blood flow, and reduce cholesterol. Saturated fats are solids since they have a high density and are derived mainly from animal products. While the body doesn’t need saturated fats, they do provide it with cholesterol, which plays an important role in cell membranes and hormones production. Trans fats are chemically processed fats made by converting unsaturated fats from plant oils into saturated fats to make them thicker or harder. Trans fats are unhealthy and should generally be avoided in the diet.
Unsaturated Fats | Saturated Fats | Trans Fats |
· Liquids, derived from plants products, generally healthy · Foods: most vegetable oils, nuts, seeds, oily fish | · Solids, derived from animal products, generally unhealthy · Foods: butter, lard, cheese, coconut oil, red meat | · Solids, commercially made, generally unhealthy · Foods: margarine, fried foods, prepared baked goods |
Proteins
Proteins are organic molecules made up of smaller units called amino acids. Proteins are used by the body as building blocks for growth and repair. As such, proteins make up nearly every body tissue, including muscle, bone, skin, hair. In addition, some proteins function as hormones, enzymes, and antibodies. Unlike carbohydrates, proteins are not a direct source of energy, but work as materials for other structures in the body. Foods rich in proteins include meat, fish, eggs, cheese, beans, and dairy.
There are 20 different amino acids that make up proteins, which can be divided into 2 groups. Non-essential amino acids are made by the body either from scratch or by modifying others; there are 11 in total. Essential amino acids cannot be made from scratch and must come from food; there are 9 in total.
Essential Amino Acids (9) | Non-Essential Amino Acids (11) |
· Amino acids that must come from food · Examples: lysine, methionine, tryptophan | · Amino acids that the body produces naturally · Examples: alanine, cysteine, tyrosine |
Vitamins and Minerals
Vitamins are organic micronutrients essential for normal body functions such as digestion, repair, and defense. There are 13 vitamins in total, classified into 2 groups. Fat-soluble vitamins (vitamins A, D, E, K) dissolve in fat and tend to accumulate in the body. Water-soluble vitamins (vitamins C, B-complex) dissolve in water and are lost in urine if unused. Foods rich in vitamins include fruit, vegetables, and dairy.
Minerals are inorganic micronutrients found in soil and water that are essential for normal body functions such as controlling chemical reactions, maintaining fluid balance, promoting healthy bones and teeth, and producing enzymes. There are 17 essential minerals the body needs daily, including calcium, sodium, iron, and iodine. Foods rich in minerals include milk, fruits, vegetables, and salt.
Water
Water is an essential nutrient that is vital for proper health. Water is used in virtually all body processes, including digestion, temperature regulation, and maintaining fluid balance. The body needs 1.8 L of water a day, equal to 8 medium-sized glasses. Besides liquid water, foods rich in water include fruits, vegetables, soups, juices, and dairy products.
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