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Physiology and Digestion Education for Fitness and Nutrition Professionals

digestion information for personal trainers

Physiology and Digestion

The primary function of the gastrointesti­nal (or alimentary) tract is to provide the body with nutrients,  water,  and  electrolytes  from  ingested foodstuffs. When food moves through the gastrointestinal tract (see box), it is broken down into small units that can be absorbed in a process called digestion.  Absorption (i.e.,  the  transport of  nutrients  from  the  intestine  into  the  blood  or lymph system) takes place in various parts of the gastrointestinal tract for different nutrients. Here, we give an overview of the anatomy and physiology of the gastrointestinal tract, the various digestion and absorption processes that take place within it, and the changes that occur during exercise.

Anatomy of the Gastrointestinal Tract

The gastrointestinal tract is a long tubular structure that reaches from the mouth to the anus and includes the esophagus, stomach, small intestine, large intestine,  rectum,  anus,  and  several  accessory digestive glands such as the salivary glands, gallbladder, liver, and pancreas. In this six- to eight-meter tube, food is di­gested and nutrients are absorbed. The mouth, stomach, pancreas, and gallbladder have a predominantly digestive function, and most of the absorption occurs in the small and large intestine. After absorption, most nutrients are transported to the  liver,  and from there they enter the main circulation. Mouth chewing (or masticat­ing) of food is the first step of digestion. It is often referred to as mechanical digestion.  The anterior teeth or incisors provide strong cutting action,  and  the  posterior  teeth (molars) are used for grinding. The forces applied to cut and grind the food can be as much as 25 kg on the incisors and 90 kg on the molars.

Chewing  food serves three major purposes:

(1)  It mechanically reduces the size of the food particles, which increases the rate of gastric emptying.

(2) It increases the surface area of  food,  which in  turn  increases  the  contact  area  for  digestive enzymes that are released from the salivary glands and stomach (enzymatic digestion). Increasing the total surface area of the food increases the rate of digestion.

(3) It mixes the food particles with saliva and digestive enzymes. The mouth has three pairs of salivary glands: the parotid glands, the sublingual glands, and the submandibular glands.  Chewing is  especially  important  for plant material (fruits and raw vegetables) because indigestible cellulose cell walls must be mechanically destroyed to release the nutrients.

Esophagus

When the food is small and soft enough to swallow, it moves past the pharynx at the back of the mouth into the esophagus.  The esophagus moves the food particles to the stomach. This transport process is  caused  by  rhythmic  contractions  and relaxations of the esophagus. The esophagus contains an inner layer of smooth muscle consisting of  circular  bands  and  an  outer  layer  of  smooth muscle  that  runs  longitudinally.  Contraction of these muscles causes peristalsis, a squeezing action involving progressive and recurring contractions, which mixes and moves the food to the stomach. This mechanism makes swallowing possible even when a person is hanging upside down or in space with zero gravity. At the end of the esophagus, a valve of smooth muscle called the esophageal sphincter  relaxes to allow food into the stomach. After some food particles have passed the esophageal sphincter, it contracts, preventing reflux of food or fluids from the stomach into the esophagus. If this sphincter does not function properly, people  may experience some acid leaking from the stomach, or  heartburn.  This gastrointestinal problem is common among runners and cyclists.

Stomach

The stomach, which is about 20 to 25 cm long, is divided in three parts: the corpus, or body; the antrum; and the fundus. The corpus and the antrum have different physiological functions. Although the fundus is a different part of the stomach from an anatomical point of view, from a functional point of view it is considered part of the corpus. The end of the stomach is a circular valve called the pyloric sphincter (pylorus), which controls the emptying of food from the stomach into the small intestine. When this muscle relaxes, food leaves the stomach; when it contracts, food stays in the stomach. The functions of the stomach include:

  • storage of large quantities of food until it can be accommodated in the intestine,
  • mixing this  food  with  gastric  secretions  to form a homogeneous, acidic, soup-like liquid, or paste called chyme, and
  • regulation of the emptying of the chyme into the duodenum (upper part of the small intestine) at a rate suitable for proper digestion and absorption.

Normally, food that enters the stomach forms concentric circles in the corpus and fundus so that the latest food is closest to the esophagus and the oldest food is  nearest  to  the  wall of  the  corpus. The  stomach  volume  is  normally  around  1.5  L, but this volume can change from almost nothing when the stomach is empty to about 6 L when the stomach is full. The muscular tone of the stomach wall decreases as soon as food enters the stomach, which allows the stomach wall to stretch outward to accommodate more food.

The wall of the corpus contains gastric glands that secrete digestive juices. These secretions come in  contact  with  the  food  portions  nearest  the stomach wall.

The stomach can also contract and relax, which mixes the food into chyme. Chyme can be a fluid or a paste, depending on the relative amounts of food and secretions and the degree of digestion. The chyme is passed down to the small intestine by a strictly controlled process of gastric emptying. Little absorption takes place in the stomach (with the exception of some water and alcohol).

Small Intestine

The small intestine is approximately 2 to 3 m in length and 3 to 5 cm in diameter, and it can be divided into the  duodenum  (about  30  cm),  the jejunum (next 1 to 2 m), and the ileum (last 1 m). About  95%  of  all  absorption  takes  place  in  the duodenum  and  jejunum.  The intestinal  mucosa of  the  duodenum  and  jejunum  contains  many folds called the folds of Kerkring.

These folds increase the surface area of the intestine about  three  times  that  of  a  similarly  sized flat  internal  lining.  These  folds  are  covered  by millions of small fingerlike structures called villi, which project about 1 mm from the surface of the mucosa. The villi increase the total surface area of the small intestine 10-fold. The intestinal cells that form the border of the villi are covered by a brush border consisting of about 600  microvilli,  approximately  1 mm  long.  These microvilli increase the total surface area 20-fold further.

The highly specialized construction of the small intestine, including the folds of Kerkring, the villi,  and the microvilli, increases the absorption about 600-fold compared with a simple tube with a flat internal surface. The total surface area of the small intestine may be as large as a regulation tennis court!

Villi

Villi  are  finger-shaped  and  highly . The wall of a villus consists of a layer of epithelial cells, each with its own brush border (brush border is the name for the microvilli-covered surface of the  intestine).  Water, water-soluble particles, and electrolytes require transport or diffusion across the luminal and contra luminal membranes of  the  epithelial  cell  into  the  blood vessels.  These  nutrients  are  then  transported  to the  liver  through  the  hepatic  portal  vein.  Each villus also contains a  lacteal,  located in the central part of a villus. The lacteal transports particles that are not readily water soluble (e.g., long-chain fatty acids) via the lymphatic vessels. These vessels drain into large veins near the heart.

Motility and Transit Time

Food spends one to three days in the gastrointestinal tract before it is eliminated. The time spent in a section of the gastrointestinal tract is the transit time. For instance, the transit time in the small intestine is approximately  3  to  10  hours,  depending  on  the composition of the food and the motility (movement  of  food)  of  the  gastrointestinal  tract.  The small-intestine wall contains two layers of smooth muscle with longitudinal and circular muscle fibers that allow peristalsis and mixing contractions that push  the  chyme  in  the  distal  direction  toward the large intestine (like squeezing toothpaste out of  a  tube).  These  contractions  occur  at  a  rate  of 0.5 to 2.0 cm/s, with the fastest movement in the proximal intestine and the slowest movement in the distal intestine. The average speed of chyme along the small intestine is approximately 1 cm/s. Peristalsis  increases  after  a  meal  and  can increase  greatly  after  intense  irritation  of  the intestinal mucosa, such as during infectious diarrhea. Mixing, or segmentation, contractions differ from  peristalsis.  The  circular  muscles  contract, giving the small intestine the look of linked sausages. These intermittent contractions (8/min to  12/min) cause the chyme to move both forward and backward. The chyme moves backward before it advances. The function of these circular contractions is to mix the chyme with bile from the gallbladder, pancreatic juices, and intestinal juices. The  juices  get  extra  time  to  digest  the  food, and the contact time and area are increased.

Gallbladder

The  gallbladder  stores,  concentrates,  and releases bile. Bile, which is produced by liver cells, consists of water, electrolytes, bile salts, cholesterol, lecithin, and bilirubin. The gallbladder can store approximately 30 to 60 ml, but it secretes as much as 1,200 ml into the duodenum every day, storing up to 12 hours of bile secretion by concentrating  the  bile  constituents.  Bile  facilitates  the digestion and absorption of fat and is released through the hepatic duct, which joins the pancreatic duct just before entering the duodenum. Bile secretion increases after a meal, especially when the meal contains a large amount of fat.

Pancreas

The pancreas is a large gland situated parallel to and just beneath the stomach. It secretes sodium bicarbonate to buffer the hydrochloric  acid  of  the  stomach  and  digestive enzymes to break down carbohydrate, protein, and fat. Pancreatic juice is mainly secreted in response to  chyme  in  the  upper  portions  of  the  small intestine. The regulatory mechanisms for sodium bicarbonate secretion and digestive enzyme secretion  are  different,  and  the  secretion  rates  are highly dependent on the type and amount of food ingested. The concentration of various enzymes in pancreatic juice also depends to some extent on the type of food ingested.

Ileocecal Valve

From the small intestine, the chyme moves into the  large  intestine  through  the  ileocecal  valve. This valve prevents backflow of indigestible fecal material into the small intestine. The valve can resist pressure equal to about 50 to 60 cm of water. The distal end of the small intestine,  or  ileum,  has  a  thicker  muscular  coat  that controls the emptying from the ileum. Contraction of the ileocecal sphincter is regulated by a variety of factors, including:

(1) distension of the cecum (a blind pouch, open only at one end, at the beginning of the large intestine),

(2) irritating substances in  the  cecum,  and

(3)  fluidity  of  the  chyme.

An inflamed  appendix  (a  nonfunctional  part  of  the intestine that is short, thin, and outpouching from the  cecum)  restricts  the  emptying  of  the  ileum. Increased  fluidity  of  chyme,  on  the  other  hand, increases emptying from the ileum.

Large Intestine

In the large intestine, the chyme is called feces. The large intestine consists of the colon, the rectum, and the anal canal. The colon is divided into the ascending colon, the transverse colon, the descending colon, and the sigmoid colon. The functions of the colon include absorption of water and electrolytes from the chyme and storage of feces until they can be expelled. Absorption takes place mainly in the first part of the colon, and storage mainly occurs in the distal parts. The peristaltic movements of the colon are slower than those  of  the  small  intestine.  The  colon  also  has circular  and  longitudinal  smooth  muscle  layers and moves the feces toward the distal parts of the colon by rhythmic contractions. The fecal material is slowly rolled over and mixed so that contact with the surface of the large intestine increases and as much water as possible is absorbed. Normally only 80 to 150 ml of water are present in approximately 300 ml of feces.

Regulation of the Gastrointestinal Tract

The gastrointestinal tract is innervated by both the sympathetic and the parasympathetic components of  the  autonomic  nervous  system.  Parasympathetic  stimulation  in  general  stimulates  motility. The vagus nerve is the source of parasympathetic activity  in  the  esophagus,  stomach,  pancreas, gallbladder, small intestine, and upper section of the large intestine. The lower portion of the large intestine  receives  parasympathetic  innervation from spinal nerves in the sacral region (the lower end  of  the  spine).  Autonomic  regulation,  which is “extrinsic” to the gastrointestinal tract, is overruled by “intrinsic” modes of regulation. Sensory neurons  in  various  parts  of  the  gastrointestinal tract  have  their  cell  bodies  in  the  gut  wall  but are not part of the autonomic nervous system. In addition, hormonal regulation plays an important role. Endocrine glands secrete hormones into circulation,  whereas  paracrine  glands  or  cells secrete  products  that  influence  the  secretion  of another product discharged by a local gland or cell. For  example,  gastrin  is  a  hormone  secreted  by the stomach that increases hydrochloric acid and pepsinogen  secretion  in  the  stomach.  Another example is secretin, a hormone produced by the small intestine that increases water and bicarbonate secretion by the pancreas. Substances  within  the  tissues  of  the  gastrointestinal tract and hormones released by organs in the gastrointestinal tract affect secretion and motility.

Digestion

Digestion  starts  the  moment  food  is  ingested and may take four to six hours to complete. Specific enzymes are responsible for the digestion of different macronutrients.

Carbohydrate Digestion

The digestion of carbohydrates starts in the mouth as saliva is added to  food.  Saliva  is  secreted from  the  parotid  glands,  the  sublingual  glands, and the submandibular glands. The daily secretion of saliva normally ranges between 800 and 1,500 ml. In the unstimulated state, saliva secretion rate is about 0.5 ml/min, but this rate can increase up to tenfold during the chewing of food. Saliva consists primarily of water (99.5%) derived from extracellular fluid. In addition, it contains a-amylase (also referred to as ptyalin), an enzyme responsible for the breakdown of  starch  into  smaller  units;  mucoid  proteins; bicarbonate; electrolytes; lysozymes, enzymes that break down proteins and attack bacteria; lingual lipase;  and  protein  antibodies  (the  major  secretory antibody being immunoglobulin, which helps to destroy oral bacteria). Thus, besides a digestive  function,  saliva  has  a  protective  function against invading bacteria. The  mucoid  proteins  give  saliva  its  viscous quality,  which  helps  lubricate  food  and  makes it  easier  to  swallow.  Chewing  mixes  saliva with  the  food  and  increases  the  contact  area  so amylase can start breaking down the glucose chain in starches. Prolonged chewing of a cracker will cause it to taste sweeter because some starch breaks down to disaccharide sugars such as maltose, which tastes much sweeter than starch. When food is swallowed and arrives in the acidic environment  of  the  stomach,  amylase  activity  decreases.  Carbohydrate  digestion  still  takes place but at a much slower rate. In the mouth and stomach, before the stomach content is completely mixed with gastric secretions, approximately 30 to 40% of the carbohydrate may be digested, predominantly  to  maltose,  maltotrioses,  and  small oligosaccharides.  When  the  carbohydrates  are  emptied  from the  stomach  into  the  duodenum  and  the  acid is  neutralized  by  sodium  bicarbonate  from  the pancreas, digestion again proceeds at a high rate. In  the  duodenum,  additional a-amylase  will  be secreted in the pancreatic juice. This a-amylase, like salivary amylase, hydrolyzes the starches into small  glucose  polymers  (dextrins)  and  maltose.  Almost  complete  hydrolysis  of all starches to maltose has taken place when the chyme  enters  the  ileum.  The  disaccharides  and small  polysaccharides  are  further  digested  by specific enzymes located in the brush borders of intestinal epithelial cells. As soon as disaccharides come in contact with the brush border, they are digested by the enzymes lactase, sucrase, and maltase. Lactase breaks lactose down into glucose and galactose, sucrase breaks sucrose down into glucose and fructose, and maltase breaks maltose down into two glucose molecules. Problems with the digestive process can result when a deficiency of one or more of these enzymes exists. Lactose intolerance is caused by an absence or  deficiency  of  the  intestinal  enzyme  lactase. When lactose, the main carbohydrate component of  milk,  is  not  digested,  diarrhea  and  fluid  loss result. In addition, bacteria in the large intestine metabolizes the lactose to produce large quantities of gas, which causes bloating and pain.

Fiber, a form of dietary carbohydrate, contains cellulose, a structural component of plant cells which is resistant to human digestive enzymes. Cellulose can be excreted in the feces. Some of it, however, is fermented by the bacteria present in the  large  intestine.  Similar  to  the  way  in  which yeast ferments the sugars in grape juice to produce wine, the bacteria in the large intestine ferment cellulose to produce hydrogen and carbon dioxide gases, volatile FAs, and, in many instances, methane gas (which has an unpleasant odor). Changes in the diet or in the type of microorganisms can influence the amount of gas produced. Peristaltic  movements  push  undigested  carbohydrates,  including  fibrous  substances,  to  the colon, where more digestion occurs. Indigestible carbohydrates (predominantly cellulose) move to the rectum for expulsion though the anus

Lipid Digestion

Digestion  of  lipids  begins  in  the  mouth  because saliva contains small amounts of lingual  lipase,  the  enzyme  that  splits  triacylglycerols  (triglycerides) into FAs and glycerol. In the stomach, this acid-stable lipase continues to hydrolyze the triacylglycerols. Hydrolysis, however, is  slow  because  triacylglycerols  are  not  soluble in water and therefore do not mix well with the water fraction in which lipase is found. The lingual  and  gastric  lipases  act  together  but  mainly on the short-chain (C4 to C6) and medium-chain (C8 to C10) triacylglycerols, whereas most of the fat (long-chain triacylglycerols, C12 to C24) is digested in the small intestine. Lingual lipase is responsible for 10 to 30% of the triacylglycerol digestion. When chyme enters the duodenum, bile is added and acts on the triacylglycerols, which by this time are organized into large  lipid  globules.  Pancreatic  lipase  is secreted into the duodenum and further hydrolyzes the triacylglycerols. After  initial  hydrolysis  starts  and  the triglycerides  are  converted  into  FAs, monoglycerides and diglycerides organize themselves  into  small  emulsion  droplets. The fat-soluble part of the FA faces inward, and the water-soluble part forms the core of these droplets. When bile salts (bile acids), stored in the gallbladder, are secreted into the duodenum, micelles are formed. Micelles are well-defined structures with a disk-like shape, on which phospholipids and FAs form a bilayer.  The  bile  salts  occupy  the  edge positions, rendering the edge of the disk hydrophilic (i.e., more attractive to water). The bile salts emulsify the lipids into small droplets,  which  increases  the  total  surface  area and thus facilitates the hydrolysis  (breakdown) of triacylglycerols by pancreatic lipase.

Protein Digestion

Protein digestion breaks down the ingested proteins into simple amino acids, dipeptides, and tripeptides for  absorption  across  the  intestinal  mucosa. This process (protein hydrolysis) takes place within the stomach and small intestine and depends  on  specific  protein-digesting  enzymes (proteases) and the acidity of the stomach. Specific cells produce and secrete hydrochloric acid (HCl), a strong acid, into the stomach. These parietal cells secrete an isotonic 160 mM HCl solution with a pH of about 0.8, illustrating its extreme acidity. The pH within the stomach and of the gastric contents is typically around 2.0.

HCl, and thereby the acidic ingested food, has various functions; among other things it:

  • activates the protease enzyme pepsin,
  • kills many pathogenic organisms,
  • increases the absorption of iron and calcium, protein pepsin from stomach
  • inactivates hormones of plant and animal origin, and
  • denatures (breaks down) food proteins, making them more vulnerable to enzyme action.

Proteases are  often  stored  in the form of an inactive precursor, but as soon as it is released into the stomach or small intestine it becomes active. This mechanism prevents the digesting of the cells in which the proteases are produced and stored.

Pepsin   (an  important  group  of  proteases), secreted  as  its  precursor pepsinogen from  the cells of the stomach wall, is initially inactive. As soon as pepsinogen comes in contact with the HCl of the stomach, it is automatically converted into the active pepsin that breaks down protein. Pepsin degrades the collagenous connective tissue fibers of  meat.  After  dismantling  these  fibers,  other proteases  can  effectively  digest  the  remaining animal protein.

Stomach enzymes and acids attack the long, complex protein strands and hydrolyze approximately 10 to 20% of the ingested proteins.  The  low  pH  causes  denaturation  of  the protein,  meaning  that  the  three-dimensional structure of protein is uncoiled and breaks into smaller  polypeptide  and  peptide  units.  When the chyme passes into the small intestine, pepsin becomes inactivated by the relatively high pH in the duodenum.

Other  proteases  (alkaline  enzymes),  including trypsin, are released and become active to digest the  remaining  proteins  and  polypeptides.  The pancreatic  juice  is  rich  in  precursors  of  endopeptidases,  carboxypeptidases,  enteropeptidases, trypsinogen, and trypsin. These proteases digest the polypeptides into tripeptides, dipeptides, and single amino acids. Amino acids, dipeptides,  and  tripeptides  can  be  transported across the enterocyte.

Absorption

Absorption of nutrients across the intestinal walls occurs either by active transport or by simple diffusion. Active transport requires energy and usually takes place against a concentration gradient or  an  electrical  potential.  Active  transport  often requires specialized carrier proteins. Diffusion is the movement of substances across a membrane along, rather than against, an electrochemical gradient. Simple diffusion does not require transport proteins or energy in the form of ATP, but many nutrients are transported by facilitated diffusion, which requires a protein transporter or channel.

Absorption of Carbohydrates

The  major  monosaccharides  that  result  from digestion of polysaccharides and disaccharides are glucose, fructose, and galactose. These monosaccharides are absorbed by carrier-mediated transport processes. The transporters that mediate the uptake  of  monosaccharides  in  the  epithelial  cell are  a  sodium  monosaccharide  cotransporter  (most  commonly  the  sodium dependent  glucose  transporter  [SGLT1])  and  a sodium-independent facilitated-diffusion transporter with specificity for fructose (GLUT5). For each molecule of glucose, two sodium ions will be transported into the epithelial cell. The sodium is then actively transported back into the gut lumen through  a  Na +-K +-ATPase  pump.  Galactose  also uses SGLT1. A separate monosaccharide transporter on the contraluminal side of the epithelial cell accepts all three monosaccharides (GLUT 2). The monosaccharides then enter the circulation in the hepatic portal vein, which will transport them to the liver.

Absorption of Fats

The monoacylglycerols and FAs incorporated into micelles are transported to the villi and move into the spaces between the microvilli. Here FAs diffuse across the membrane of the epithelium and enter the epithelial cell. The micelles then move away from the villi, incorporate new FAs, and transport them to the villi. Micelles  formed  within  the  intestinal  lumen, therefore, perform an important ferrying function. In the presence of bile salts  (and, thus, micelles), fat absorption is almost complete (97%), whereas in the absence of bile, only about 50% of the FAs are absorbed. The  absorption  of  FAs  through  the  epithelial membranes is by diffusion (because they are highly soluble in the lipid membranes). In the epithelial cell, FAs are altered to triacylglycerols in the endoplasmatic reticulum.

Once formed, triacylglycerols combine with cholesterol and phospholipids to form chylomicrons.  In  this  chylomicron,  the  fatty  sides  of  the phospholipids face toward the center, and the polar parts form the surface. Chylomicrons make possible the transport of fat in the aqueous environment of the lymph and blood plasma. These large molecules move toward the central lacteal of the villi and are slowly transported through the lymphatic system, reaching the circulation in the subclavian veins.

Short-chain  and  medium-chain  FAs  are  more water soluble than long-chain FAs and therefore follow  a  slightly  different  route  of  absorption. They enter the epithelial cell, and, without being reesterified to triacylglycerols, they directly diffuse through  the  contraluminal  membrane  into  the portal blood, where they are bound to the plasma protein  albumin  and  passed  to  the  liver  via  the hepatic portal vein. The bile salts are reabsorbed again in the intestinal mucosa of the distal ileum. They enter the portal blood and pass to the liver. In the liver, they are resecreted into the bile. In this way, 94% of bile salts are reutilized. The recirculation of bile salts is called the enterohepatic circulation.

Absorption of Amino Acids
Amino  acids,  dipeptides,  and  tripeptides  are absorbed by active transport (coupled to the transport of sodium) in the small intestine and delivered to the liver via the hepatic portal vein. Dipeptides and tripeptides that have been transported across the epithelial membrane are broken down inside the  cell  into  their  amino  acid  constituents  by specific dipeptidases and tripeptidases.

Most  amino  acids  are  transported  across  the epithelium against a concentration gradient, and, therefore, carrier-mediated transport is needed. At least seven brush border-specific transport proteins have been identified. The luminal membrane usually contains sodium-dependent transport  systems,  whereas  the  contraluminal membrane transport does not require sodium. The small intestine has a large and effective capacity to  absorb  amino  acids  and  small  peptides.  Most amino acids can use more than one transporter for absorption. Less than 1% of the ingested protein is usually found in feces. After amino acids have passed the epithelium, they are transported to the liver where they can be converted to glucose, fat, or protein, or they can be released into the bloodstream as free amino acids.

Absorption of Water

Most water absorption (99%) take place in the small  intestine,  mainly  in  the  duodenum  (72%), entirely by simple diffusion. This absorption obeys the laws of osmosis. A membrane that is impermeable to solutes but permeable to water separates two compartments with the same amount of fluid but different  numbers  of  solute particles.

Water  diffuses  across  this  membrane  in  both directions, but relatively more water flows toward the compartment with the lower water concentration (higher solute concentration). This net movement of water eventually results in a similar solute concentration on both sides of the membrane. But the amount of water in the compartment with the lower water concentration increases.

Absorption of Vitamins

Most vitamin absorption takes place in the jejunum and ileum and is usually a passive process (diffusion). Fat-soluble vitamins (A, D, E, and K) are absorbed along with FAs. They are also incorporated into chylomicrons and transported through the  lymph  system  into  the  systemic  circulation to liver and other tissues. Most of the absorption of  fat-soluble  vitamins  takes  place  in  the  small intestine.

Water-soluble vitamins are also mostly absorbed in  the  small  intestine  by  diffusion.  The  water-soluble  vitamins  are  not retained to any great extent by different tissues, and when large amounts are ingested, they are mostly excreted in urine. Most vitamin C is absorbed in the distal portion of the small intestine. Excess intake of vitamin C (above approximately 1,200 mg/day) decreases the efficiency of renal reabsorption of vitamin C, and much of the excess intake appears in the urine. B vitamins are often ingested as part of coenzymes in food; digestion liberates the vitamins. For example, pantothenic acid is usually present in food as part of coenzyme A. Digestion releases the vitamin from its coenzyme, and absorption takes place. Thiamin and vitamin B6 are mainly absorbed in the jejunum. Biotin and riboflavin are mainly absorbed in the proximal part of the small intestine. Niacin is partly absorbed in the stomach but mostly in the small intestine.  Vitamin B12 is mainly absorbed in the ileum. Its absorption is more complex, involving binding to a specific protein (called intrinsic factor, which is secreted by the parietal cells of the gastric mucosa). Absorption of folic acid depends on the presence of the intestinal enzyme conjugase, which facilitates the absorption of folic acid in the small intestine.

Minerals

Minerals  are  not  well  absorbed  in  the  human intestine. The intake in food is therefore usually far in excess of actual requirements. Mineral absorption  often  depends  on  its  chemical  form. The  best-known  example  of  this  requirement  is probably  the  difference  in  absorption  between nonheme  and  heme  iron.  (Heme  iron  is  found in meat; nonheme iron is obtained from plants.) About 15% of all ingested heme iron is absorbed in the small intestine, whereas only 2% to 10% of nonheme  iron  is  absorbed.  Absorption  of  other minerals  is  also  relatively  poor.

A  maximum  of only 35% of ingested calcium is absorbed, 20 to 30% of ingested magnesium is absorbed, 14 to 41% of ingested zinc is absorbed, and less than 2% of  ingested  chromium  is  absorbed.  Besides  poor absorption, excretion rates by urine are also high. About 65% of absorbed phosphorus and 50% of absorbed calcium is excreted by urine. When daily mineral  intake  is  insufficient,  increased  intake may  result  in  increased  retention.  For  example, many  women  in  Western  countries  have  insufficient iron and calcium intake; increasing intake  generally  increases  storage  of  these minerals.

Sodium is actively transported out of the epithelial  cell  into  the  portal  circulation,  a  process requiring ATPase carrier enzymes and energy (in the form of ATP). The transport of sodium out of the epithelial cell creates a low sodium concentration in the cell, which increases diffusion of sodium from the gut lumen into the epithelial cell. About 30 g of sodium is secreted in intestinal secretions every  day.  Daily  sodium  ingestion  is  about  5  to 8  g.  Thus,  about  25  to  35  g  of  sodium  must  be reabsorbed each day, representing a large percentage of the body sodium stores.

This requirement explains  why  extreme  diarrhea  results  in  large sodium losses, which can be dangerous and even life threatening.

Function of Bacteria in the Colon

The adult human gut contains about 1 kg of various  bacteria  (colon  bacilli).  The  gastrointestinal tract contains an immensely complex ecology of microorganisms.  A  typical  person  harbors  more than 500 distinct species of bacteria. The composition and distribution of these microorganisms vary with age, state of health, and diet.

The number and type of bacteria in the gastrointestinal  tract  vary  dramatically  by  region. In healthy people the stomach and proximal  small intestine  contain few microorganisms, largely a result of the bacteriocidal activity of gastric acid. One interesting testimony to the ability of gastric acid to suppress  bacterial populations is seen in patients with achlorhydria, a genetic condition that prevents  secretion  of  gastric  acid.  Patients  with achlorhydria who are otherwise healthy may have as many as 10,000 to 100 million microorganisms per milliliter of stomach contents.

In sharp contrast to the stomach and small intestine, the colon literally teems with bacteria, predominantly strict anaerobes (bacteria that survive only in environments virtually devoid of oxygen).  Between  these  two  extremes  is a  transitional  zone,  usually  in  the  ileum,  where moderate numbers of both aerobic and anaerobic bacteria  are  found.  The  gastrointestinal  tract  is sterile  at  birth,  but  colonization  typically  begins within a few hours of birth, starting in the small intestine and progressing caudally over a period of several days. In most circumstances, a mature microbial flora is established by three to four weeks of age.

Bacterial populations in the large intestine digest carbohydrates,  proteins,  and  lipids  that  escape digestion  and  absorption  in  the  small  intestine. The bacteria are responsible for the fermentation of  small  amounts  of  cellulose.  More  important, however, is the production of vitamin K, vitamin B12,  thiamine,  riboflavin,  and  other  substances. Vitamin  K  is  especially  important  because  most people’s daily  vitamin  K  intake  in  food  is  insufficient.

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NESTA Fitness Nutrition Coach Certification

The NESTA Sports Nutrition Specialist course is designed for personal fitness trainers, strength coaches and nutrition experts who want to learn cutting-edge techniques for increasing sports performance, reducing recovery time, and enhancing the overall well-being of your clients and athletes.

Learn more about becoming a Sports Nutrition Specialist by clicking the link below!

>>> https://www.nestacertified.com/sports-nutrition-training-course/

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