THE DIGESTIVE SYSTEM
The digestive system converts food that you eat into nutrients which are absorbed into the blood and finally assimilated by the cells of the body.
Overview of the Digestive Process
Food is eaten and is chewed ( masticulated ), which partially breaks down the food in the mouth.
Starches in the mouth are partially broken down further by salivary enzymes – known as salivary .
The masticulated food, called a bolus, passes into the esophagus which moves it through muscle contractions of smooth muscle, in a process called peristalsis.
The bolus leaves the esophagus and enters the stomach, which churns and secretes protein-digesting enzymes to break down the bolus into a gooey mass called chyme
The stomach is contained between two smooth muscle valves, called sphincters.
The upper valve, called the cardiac sphincter, lies in the cardiac region between the esophagus and the stomach.
The lower valve, called the pyloric sphincter, lies between the stomach and the duodenum.
The chyme, leaves the stomach via the pyloric sphincter and passes into a finger-length region of the small intestine, called the duodenum
Bile and pancreatic enzymes, which are needed to help break down the chyme, also enter the duodenum.
Into the duodenum are two ducts which originate from the liver and the pancreas.
The liver produces bile, which is a salty-lipid liquid that is used to break-down or emulsify fat into tiny droplets.
The bile is stored in the gall bladder which is connected to the duodenum by a small duct or tube.
The pancreas is a gland which produces
hormones that regulate blood sugar levels
sodium bicarbonate, a base which neutralizes stomach acid
enzymes that break down macromolecules into monomers
amylase – continues the breakdown of carbohydrates
trypsin – continues the breakdown of protein
lipase – breaks down fats
When chyme leaves the stomach, it passes through the pyloric sphincter, which squirts small manageable amounts of chyme into the small intestine.
The small intestine, a lengthly 6 m tube, which consists of three parts.
The first part, the duodenum is about 25 cm in length. The first 8 cm of the duodenum is a rigid tube that is firmly attached to surrounding tissue and receives enzymes from the pancreas and bile from the gallbladder.
These enzymes mix with the chyme and move into the remaining part of duodenum, which is not firmly attached and consists of brush-like lining called micro-villi which extends into the tubular lumen ( cavity ) of the duodenum.
The second part, the jejunum is the thickest part of the small intestine and its inner lining having a brush-like texture due to tiny villi, or hair-like protuberances, called micro-villi that vastly increase the surface area of the small intestine.
Between the villi are glands which work together to absorb water and most of the remaining monomers of food. The epithelial lining of the villi are so active, they are replaced by mitosis about every 72 hours.
The third part of the small intestine is the ileum is a tapered 2 meter section that absorbs the remaining nutrients and water.
Absorption of fats occur in the lacteals, which are vessels at the core of each of the micro-villi, that are surrounded by the blood capillaries.
Before entering the villi, fat molecules are broken down into fatty acids and glycerol and then emulsified by bile ( released by the liver ) into tiny micelles – which are small enough for the pancreatic lipases to do its thing. The products, fatty acids and monoglycerides, diffuse out of the micelle and into the villi. What remains of the micelle remains in the chyme. Once inside the absorptive cells, the monoglycerides and fatty acid chains reassemble to tryglycerides and phospholipids which, along with cholesterol, enter the lacteals and travel via the lymphatic vessels to the thoracic duct and reenter the blood at the subclavian vein. Some of the lipids are eventually stored in the hepatocyte cells of the liver to be used for ATP generation.
The capillaries that carry nutrient rich blood away from the villi converge into the hepatic portal vein which leads directly to the liver.
This is done for two reasons. First: the liver can detoxify the blood by removing foreign molecules and drugs from the blood.
Second: to regulate the blood composition. Since the liver is able to convert many organic molecules from one form to another, such as converting protein to glucose or glucose to glycogen, it is able to regulate the components in the blood before sending it off to the rest of the body. For example, regardless of the glucose concentration of the blood that enters the liver, the blood that leaves it will have a glucose concentration of 90 mg of glucose per 100 mL of blood.
Materials in food that are not digested are passed from the small intestine into the colon or large intestine.
Digestion consists of breaking down food, from macromolecules to monomers, and absorbing the monomers into the blood.
Mechanical Digestion: is where food is masticulated or chewed.
Chemical Digestion: is where digestive enzymes break down macromolecules into monomers which are then absorbed into the blood or lymphatic system.
Absorption takes place in the villi of the small intestine. The monomers of carbohydrates and protein are absorbed into the blood and the monomers of lipids are absorbed into the lymphatic system.
After foods have been digested, they are assimilated into the body cells. Assimilation occurs in the capillary beds.
After assimilation, cellular respiration occurs with the cell which further processes the food nutrients.
Components of the Digestive System
The human digestive system, as shown below is a coiled, muscular tube (6-9 meters long when fully extended) stretching from the mouth to the anus.
Several specialized compartments occur along this length: mouth, pharynx, esophagus, stomach, small intestine, large intestine, and anus.
Accessory digestive organs are connected to the main system by a series of ducts: salivary glands, parts of the pancreas, and the liver and gall bladder.
The Mouth and Pharynx
Mechanical breakdown begins in the mouth by chewing (teeth) and actions of the tongue.
Chemical breakdown of starch by production of salivary amylase from the salivary glands.
This mixture of food and saliva is then pushed into the pharynx, which is a tube that leads
to both the air channel and the food channel.
The air channel is called the trachea, and the food channel, the esophagus.
When you swallow, a tissue flap called the epiglottis, covers the trachea and
allows the food to travel into the esophagus.
The esophagus is a muscular tube whose muscular contractions ( peristalsis ) propel food to the stomach.
In the mouth, teeth, jaws and the tongue begin the mechanical breakdown of food into smaller particles.
Most vertebrates, except birds, have teeth for tearing, grinding and chewing food.
The tongue manipulates food during chewing and swallowing; mammals have tastebuds clustered on their tongues.
Salivary glands secrete salivary amylase, an enzyme that begins the breakdown of starch into glucose.
Mucus moistens food and lubricates the esophagus. Bicarbonate ions in saliva neutralize the acids in foods.
Swallowing moves food from the mouth through the pharynx into the esophagus and then to the stomach.
Step 1: A mass of chewed, moistened food, a bolus, is moved to the back of the moth by the tongue. In the pharynx, the bolus triggers an involuntary swallowing reflex that prevents food from entering the lungs, and directs the bolus into the esophagus.
Step 2: Muscles in the esophagus propel the bolus by waves of involuntary muscular contractions (peristalsis) of smooth muscle lining the esophagus.
Step 3: The bolus passes through the gastro-esophogeal sphincter, into the stomach. When gastric juices leak from the stomach back into the esophagus, a condition known as hearburn occurs.
Because this condition can feel like a heart problem, the gastro-esophogeal sphincter is sometimes called the cardiac sphincter
Structure of the throat and the mechanics of swallowing.
Movement of bolus ( food ) and peristalsis
During a meal, the stomach gradually fills to a capacity of 1 liter, from an empty capacity of 50-100 milliliters. At a price of discomfort, the stomach can distend to hold 2 liters or more.
Epithelial cells line inner surface of the stomach, as shown, and secrete about 2 liters of gastric juices per day. Gastric juice contains hydrochloric acid, pepsinogen, and mucus; ingredients important in digestion. Secretions are controlled by nervous (smells, thoughts, and caffeine) and endocrine signals. The stomach secretes hydrochloric acid and pepsin. Hydrochloric acid ( HCl ) lowers pH of the stomach so pepsin is activated. Pepsin is an enzyme that controls the hydrolysis of proteins into peptides. The stomach also mechanically churns the food. Chyme, the mix of acid and food in the stomach, leaves the stomach and enters the small intestine.
Hydrochloric acid does not directly function in digestion: it kills microorganisms, lowers the stomach pH to between 1.5 and 2.5; and activates pepsinogen. Pepsinogen is an enzyme that starts protein digestion. Pepsinogen is produced in cells that line the gastric pits. It is activated by cleaving off a portion of the molecule, producing the enzyme pepsin that splits off fragments of peptides from a protein molecule during digestion in the stomach.
Carbohydrate digestion, begun by salivary amylase in the mouth, continues in the bolus as it passes to the stomach. The bolus is broken down into acid chyme in the lower third of the stomach, allowing the stomach’s acidity to inhibit further carbohydrate breakdown. Protein digestion by pepsin begins.
Alcohol and aspirin are absorbed through the stomach lining into the blood.
Chemical digestion in the stomach is done with two components of gastric juice. One is hydrochloric acid ( HCl ) which dissolves the extracellular matrix that binds cells together in meat and plant material. The concentration of HCl is so high that the pH of the gastric juice is about 2, which is strong enough to dissolve iron nails. The acid not only kills most bacteria but also denatures proteins in food, increasing exposure of there peptide bonds. These bonds are attacked by pepsin – which is the second component of the gastric juice. Unlike most enzymes, pepsin works only in a hightly acidic environment. By breaking peptide bonds, it cleaves proteins into smaller polypeptides which are eventually reduced to amino acids in the small intestine.
What prevents the gastric juice from destroying the stomach wall lining? To answer this, we need to look at how gastric juice is produced.
The stomach wall contains gastric glands which contain three types of cells.
Parietal Cells use ATP to drive hydrogen ions into the stomach lumen in very high concentrations. There the hydrogen ions combine with chloride ions, that were secreted by other cells.
Chief Cells, acting at the same time, release pepsinogen ( an inactive form of pepsin ) into the lumen. When the pepsinogen meets hydrochloric acid, it is converted to pepsin – which is the active protein enzyme.
Mucus Cells secrete mucus, a viscous mixture of glycoprotein, salts, water and bicarbonate, which adheres to the epithelial cells of the stomach. The bicarbonate lowers the acidity near the stomach wall, which protects it from the pepsin and HCl, which has accumulated in the stomach lumen.
Another factor which protects the stomach lining it that it has a new epithelial layer of cells is produced every three days, which replaces any cells which were damaged.
Damage to the stomach epithelial linings are called ulcers of which there are three kinds.
Peptic ulcers result when the mucus fails to stop pepsin from attacking the stomach cells.
Bleeding ulcers result when tissue damage is so severe that bleeding occurs.
Perforated ulcers are where a hole has formed in the stomach wall.
At least 90% of all peptic ulcers are caused by Helicobacter pylori.
Other factors, including stress can also produce ulcers.
The Small Intestine
The small intestine is where absorption occurs. The small intestine is a coiled tube over 3 meters long. Coils and folding plus villi give this 3m tube the surface area of a 500-600m long tube. Final digestion of proteins and carbohydrates must occur, and fats have not yet been digested. Villi have cells that produce intestinal enzymes which complete the digestion of peptides and sugars. The absorption process also occurs in the small intestine. Food has been broken down into particles small enough to pass into the small intestine. Sugars and amino acids go into the bloodstream via capillaries in each villus. Glycerol and fatty acids go into the lymphatic system. Absorption is an active transport, requiring cellular energy.
THE SMALL INTESTINE
Food is mixed in the lower part of the stomach by peristaltic waves that also propel the acid-chyme mixture against the pyloric sphincter. Increased contractions of the stomach push the food through the sphincter and into the small intestine as the stomach eempties over a 1 to 2 hour period. High fat diets significantly increase this time period.
The small intestine is the major site for digestion and absorption of nutrients. The small intestine is up to 6 meters long and is 2-3 centimeters wide.
The upper part, the duodenum, is the most active in digestion. Secretions from the liver and pancreas are used for digestion in the duodenum. Epithelial cells of the duodenum secrete a watery mucus. The pancreas secretes digestive enzymes and stomach acid-neutralizing bicarbonate. The liver produces bile, which is stored in the gall bladder before entering the bile duct into the duodenum.
Digestion of carbohydrates, proteins, and fats continues in the small intestine. Starch and glycogen are broken down into maltose by small intestine enzymes. Proteases are enzymes secreted by the pancreas that continue the breakdown of protein into small peptide fragments and amino acids.
Bile emulsifies fats, facilitating their breakdown into progressively smaller fat globules until they can be acted upon by lipases. Bile contains cholesterol, phospholipids, bilirubin, and a mix of salts. Fats are completely digested in the small intestine, unlike carbohydrates and proteins.
Most absorption occurs in the duodenum and jejeunum (second third of the small intestine). The inner surface of the intestine has circular folds that more than triple the surface area for absorption. Villi covered with epithelial cells increase the surface area by another factor of 10. The epithelial cells are lined with microvilli that further increase the surface area; a 6 meter long tube has a surface area of 300 square meters.
Each villus has a surface that is adjacent to the inside of the small intestinal opening covered in microvilli that form on top of an epithelial cell known as a brush border. Each villus has a capillary network supplied by a small arteriole. Absorbed substances pass through the brush border into the capillary, usually by passive transport.
Maltose, sucrose, and lactose are the main carbohydrates present in the small intestine; they are absorbed by the microvilli. Starch is broken down into two-glucose units (maltose) elsewhere. Enzymes in the cells convert these disaccharides into monosaccharides that then leave the cell and enter the capillary. Lactose intolerance results from the genetic lack of the enzyme lactase produced by the intestinal cells.
Peptide fragments and amino acids cross the epithelial cell membranes by active transport. Inside the cell they are broken into amino acids that then enter the capillary. Gluten enteropathy is the inability to absorb gluten, a protein found in wheat.
Breakdown of Fats
Fat digestion and absorption requires that the complex fat molecules be broken down into smaller more manageable molecules. This is done by mixing the fat with the digestive enzyme lipase, which enters the duodenum from the pancreas – the main source of enzymes for digesting fats and proteins. Lipase chops up lipid molecules into fatty acid molecules and glycerol molecules. However, because fat does not dissolve in water, the fat molecules enter the duodenum in a congealed mass, which makes it impossible for the pancreatic lipase enzymes to attack them, since lipase is a water soluble enzyme and can only attack the surface of the fat molecules. To overcome this problem the digestive system uses a substance called bile, produced in the liver but stored in the gallbladder, which enters the duodenum via the bile duct. Bile emulsifies fats – meaning, it disperses them into small droplets, called micelles, which then become suspended in the watery contents of the digestive tract. Emulsification allows lipase to gain easier access to the fat molecules and thus accelerates their breakdown and digestion. Fat digestion is usually completed by the time the food reaches the ileum, which is the lower third of the small intestine. Bile salts are in turn absorbed in the ileum and are recycled by the liver and gall bladder.
Absorbtion of Fats Into The Bloodstream
Lipase and other digestive juices break down the fat molecules into fatty acids and glycerol. Absorption of fat into the body, which takes 10-15 minutes, occurs in the villi – the millions of finger-like projections which cover the walls of the small intestine. Inside each villus is a series of lymph vessels ( lacteals ) and blood vessels ( capillaries ). The lacteals absorb the fatty acids and glycerol into the lymphatic system which eventually drains into the bloodstream. The fatty acids are transported via the bloodstream to the membranes of adipose cells, where they are either stored or oxidized for energy. Since glucose is the body’s preferred source of energy, and since only about 5 percent of absorbed fat can be converted into glucose, a significant proportion of digested fat is typically stored as body fat in the adipose cells. The glycerol part is absorbed by the liver and is either converted into glucose or is used to help breakdown glucose into energy.
ABSORPTION OF LIPIDS in SMALL INTESTINE
The Liver and Gall Bladder
The liver acts as a filter for the blood that passes through it. The liver removes amino acids that don’t need to be in your body. The excess amino acids are broken down so that they form the urea which is excreted in the urine. The liver can also change hemoglobin from worn-out red blood cells into substances that are useful to the body. In addition, the liver can change any poisonous toxins that have collected in the blood into harmless substances.
The liver produces bile, which is then stored in gallbladder. The gallbladder is connected to the small intestine via the hepatic duct. Bile contains salds and lipids, which emulsify fats, making them susceptible to enzymatic breakdown. In addition to digestive functions, the liver also 1) detoxifies blood, 2) synthesises blood proteins, 3) destroys red blood cells, 4) converts glucose to glycogen, and 5) produces urea from amino groups and ammonia.
THE LIVER and ASSOCIATED ORGANS
The gall bladder stores excess bile for release at a later time. We can live without our gall bladders, in fact many people have had theirs removed. The drawback, however, is a need to be aware of the amount of fats in the food they eat since the stored bile of the gall bladder is no longer available.
In plants starch is the storage form of glucose, while animals use glycogen for the same purpose. Low glucose levels in the blood cause the release of hormones, such as glucagon that travel to the liver and stimulate the breakdown of glycogen into glucose, which is then released into the blood(raising blood glucose levels). When no glucose or glycogen is available, amino acids are converted into glucose in the liver. The process of deamination removes the amino groups from amino acids. Urea is formed and passed through the blood to the kidney for export from the body. Conversely, the hormone insulin promotes the take-up of glusose into liver cells and its formation into glycogen.
Jaundice occurs when the characteristic yellow tint to the skin is caused by excess hemoglobin breakdown products in the blood, a sign that the liver is not properly functioning. Jaundice may occur when liver function has been impaired by obstruction of the bile duct and by damage caused by hepatitis
Hepatitis is a viral disease that can cause liver damage.
Hepatitis A is usually mild malady indicated by a sudden fever, malaise, nausea, anorexia, and abdominal discomfort. Jaundice follows up for several days. The virus causing Hepatitis A is primarilly transmitted by fecal contamination, although contaminated food and water also can promote transmission. A rare disease in the United States, hepatitis B is endemic in parts of Asia where hundreds of millions of individuals are possibly infected.
Hepatitis B may be transmitted by blood and blood products as well as sexual contact. The blood supply in developed countries has been screened for the virus that causes this disease for many years and transmission by blood transfusion is rare. The risk of HBV infection is high among promiscuous homosexual men although it is also transmitted hetereosexually. Correct use of condoms is thought to reduce or eliminate the risk of transmission. Effective vaccines are available for the prevention of Hepatitis B infection. Some individuals with chronic hepatitis B may develop cirrhosis of the liver. Individuals with chronic hepatitis B are at an increased risk of developing primary liver cancer. Although this type of cancer is relatively rare in the United States, it is the leading cause of cancer death in the world, primarily because the virus causing it is endemic in eastern Asia.
Hepatitis C affects approximately 170 million people worldwide and 4 million in the United States. The virus is transmitted primarily by blood and blood products. Most infected individuals have either received blood transfusions prior to 1990 (when screening of the blood supply for the Hepatitis C virus began) or have used intravenous drugs. Sexual transmission can occur between monogamous couples (rare) but infection is far more common in those who are promiscuous. In rare cases, Hepatitis C causes acute disease and even liver failure. About twenty percent of individuals with Hepatitis C who develop cirrhosis of the liver will also develop severe liver disease. Cirrhosis caused by Hepatitis C is presently the leading cause of the need for liver transplants in the United States. Individuals with cirrhosis from Hepatitis C also bear increased chances of developing primary liver cancer. All current treatments for Hepatitis C employ of various preparations of the potent antiviral interferon alpha. However, not all patients who have the disease are good candidates for treatment, so infected individuals are urged to regularly consult their physician.
Cirrhosis is a liver disorder that is most commonly caused by excessive alcohol consumption.
It places the liver in a stress situation due to the amount of alcohol to be broken down. Cirrhosis can cause the liver to become unable to perform its biochemical functions. Chemicals responsible for blood clotting are synthesized in the liver, as is albumin, the major protein in blood. The liver also makes or modifies bile components. Blood from the circulatory system passes through the liver, so many of the body’s metabolic functions occur primarily there including the metabolism of cholesterol and the conversion of proteins and fats into glucose. Cirrhosis is a disease resulting from damage to liver cells due to toxins, inflammation, and other causes. Liver cells regenerate in an abnormal pattern primarily forming nodules that are surrounded by fibrous tissue. Changes in the structure of the liver can decrease blood flow, leading to secondary complications. Cirrhosis has many cuses, including alcoholic liver disease, severe forms of some viral hepatitis, congestive heart failure, parasitic infections (for example schistosomiasis), and long term exposure to toxins or drugs.
The pancreas sends pancreatic juice, which neutralizes the chyme, to the small intestive through the pancreatic duct. In addition to this digestive function, the pancrease is the site of production of several hormones, such as glucagon and insulin.
The pancreas contains exocrine cells that secrete digestive enzymes into the small intestine and clusters of endocrine cells (the pancreatic islets).
The islets secrete the hormones insulin and glucagon, which regulate blood glucose levels.
After a meal, blood glucose levels rise, prompting the release of insulin, which causes cells to take up glucose, and liver and skeletal muscle cells to form the carbohydrate glycogen. As glucose levels in the blood fall, further insulin production is inhibited. Glucagon causes the breakdown of glycogen into glucose, which in turn is released into the blood to maintain glucose levels within a homeostatic range. Glucagon production is stimulated when blood glucose levels fall, and inhibited when they rise.
Diabetes results from inadequate levels of insulin. Type I diabetes is characterized by inadequate levels of insulin secretion, often due to a genetic cause. Type II usually develops in adults from both genetic and environmental causes. Loss of response of targets to insulin rather than lack of insulin causes this type of diabetes. Diabetes may cause impairment in the functioning of the eyes, circulatory system, nervous system, and failure of the kidneys. Diabetes is the second leading cause of blindness in the United States. Treatments might involve daily injections of insulin, oral medications such as metformin, monitoring of blood glucose levels, and a controlled diet. Type I diabetes may one day be cured by advances in gene therapy/stem cell research. On recently recognized condition is known as prediabetes, in which the body gradually loses its sensitivity to insulin, leading eventually to Type II diabetes. Ora; medications, diet and behavior (in other words EXERCISE!!!) changes are thought to delay if not outright postpone the onset of diabetes if corrected soon enough.
The fifth leading cause of cancer death in the United States is from pancreatic cancer, which is nearly always fatal. Scientists estimate that 25,000 people may die from this disease each year. Standard treatments are ineffective, although some promising avenues may open with advances in genomics and molecular biology of cancer cells.
The Large Intestine
The large intestine is made up by the colon, cecum, appendix, and rectum. Material in the large intestine is mostly indigestible residue and liquid. Movements are due to involuntary contractions that shuffle contents back and forth and propulsive contractions that move material through the large intestine. The large intestine performs three basic functions in vertebrates: 1) recovery of water and electrolytes from digested food; 2) formation and storage of feces; and 3) microbial fermentation: The large intestine supports an amazing flora of microbes. Those microbes produce enzymes that can digest many of molecules indigestible by vertebrates.
Secretions in the large intestine are an alkaline mucus that protects epithelial tissues and neutralizes acids produced by bacterial metabolism. Water, salts, and vitamins are absorbed, the remaining contents in the lumen form feces (mostly cellulose, bacteria, bilirubin). Bacteria in the large intestine, such as E. coli, produce vitamins (including vitamin K) that are absorbed.
Regulation of Appetite
The hypothalamus in the brain has two centers controlling hunger. One is the appetite center, the other the satiety center.
Gastrin , secretin, and cholecystokinin are hormones that regulate various stages of digestion. The presence of protein in the stomach stimulates secretion of gastrin, which in turn will cause increased stomach acid secretion and mobility of the digestive tract to move food. Food passing into the duodenum causes the production of secretin, which in turn promotes release of alkaline secretions from the pancreas, stops further passage of food into the intestine until the acid is neutralized. Cholecystokinin (CCK) is released from intestinal epithelium in response to fats, and causes the release of bile from the gall bladder and lipase (a fat digesting enzyme) from the pancreas.
Nutrition deals with the composition of food, its energy content, and slowly (or not at all) synthesized organic molecules. Chemotrophs are organisms (mostly bacteria) deriving their energy from inorganic chemical reactions. Phototrophs convert sunlight energy into sugar or other organic molecules. Heterotrophs eat to obtain energy from the breakdown of organic molecules in their food.
Macronutrients are foods required on a large scale each day. These include carbohydrates, lipids, and amino acids.
Water is essential and a correct water balance is a must for proper functioning of the body.
About 60% of the diet should be carbohydrates, obtained from foods such as milk, meat, vegetables, grains and grain products. The diet should contain at least 100 grams of carbohydrate every day. Recently, however, new recommendations have been developed that suggest a lowering of the amount of carbohydrate.
Proteins are polymers composed of amino acids. Proteins are found in meat, milk, poultry, fish, cereal grains and beans. They are needed for cellular growth and repair. Twenty amino acids are found in proteins, of which humans can make eleven. The remaining nine are the essential amino acids which must be supplied in the diet. Normally proteins are not used for energy, however during starvation (or a low-carb diet) muscle proteins are broken down for energy. Excess protein can be used for energy or converted to fats.
Lipids and fats generate the greatest energy yield, so a large number of plants and animals store excess food energy as fats. Lipids and fats are present in oils, meats, butter, and plants (such as avocado and peanuts). Some fatty acids, such as linoleic acid, are essential and must be included in the diet. When present in the intestine, lipids promote the uptake of vitamins A, D, E, and K.
Vitamins are organic molecules required for metabolic reactions. They usually cannot be made by the body and are needed in trace amounts. Vitamins may act as enzyme cofactors or coenzymes. Some vitamins are soluble in fats, some in water.
Minerals are trace elements required for normal metabolism, as components of cells and tissues, and for nerve conduction and muscle contraction. They can only be obtained from the diet. Iron (for hemoglobin), iodine (for thyroxin), calcium (for bones), and sodium (nerve message transmission) are examples of minerals.
There is a quantitative relationship between nutrients and health. Imbalances can cause disease. Many studies have concluded nutrition is a major factor in cardiovascular disease, hypertension, and cancer.