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         Giuseppe Archimboldo, The Four Seasons, 1573

Appetite                      Salivary glands              Large intestine
Hunger                        Enzymes                      Heartburn
Hypothalamus                  Esophagus                    Gastroesophageal reflux
Hormone                       Stomach                      disease (GERD)
Cell                          Chime
Digestion                     Small intestine              Food allergy
Absorption                    Gall bladder                 Food intolerance
Elimination                   Bile                         Celiac disease
Gastrointestinal (GI) tract   Pancreas                     Diarrhea
Saliva                        Liver                        Constipation

    • Distinguish between appetite and hunger, describing the mechanisms that
      stimulate each.
   •   Describe the contribution of each organ of the gastrointestinal system to the
       digestion, absorption, and elimination of food.
   •   Describe the causes, symptoms, and treatments of GERD,
   •   List three warning signs of dehydration resulting from diarrhea.


3.1     Why do We Want to Eat? – We eat to feed our cells.
3.1.1      Stimulates Senses
3.1.2      Psychological Factors
    • Comfort – Chocolate more than tastes good. Brain makes compounds that dull
        pain and increase level of well being – Americans spend more than $5 billion on
        chocolate each year. – G/S Nutrition every day choices, Wiley 2006
    • Love
    • Security
    • Reward and milestones
    • Punishment
    • Tradition
    • Available
    • Learned – Eat Insects? Waiter, there’s a fly in my soup and a worm in my apple.
    • Socially acceptable – My Pea Soup Story
    • Healthy
    • Genetics – Many of us crave sweet and salty food. We have evolved this way
        because we do need to eat to stay alive. And much earlier in our history, it was
        very hard to get salt. But today with abundant processed food, this craving is
        a problem.
3.1.3      Factors affecting Hunger and Satiation    Signals from Brain Hormones Amount and Type of Food
3.2     Are We Really What We Eat?
3.2.1      Atoms Join to Molecules
3.2.2      Food Composed of Molecules
3.2.3      Molecules Join to Form Cells Encased in a Functional Membrane Organelles Support Life
3.2.4      Cells Join to Form Tissues and Organs
3.2.5      Organs Make Up Functional Systems
3.3     What Happens to Food?
Gastrointestinal Tract

To better understand how weight loss surgery works, it is important to understand how your
gastrointestinal tract functions. As the food you consume moves through the tract, various digestive
juices and enzymes are introduced at specific stages that allow absorption of nutrients. Food
material that is not absorbed is then prepared for elimination. A simplified description of the
gastrointestinal tract appears below. Your doctor can provide a more detailed description to help
you better understand how weight loss surgery works.

   1. The esophagus is a long muscular tube, which moves food from the mouth to
      the stomach.
   2. The abdomen contains all of the digestive organs.
   3. The stomach, situated at the top of the abdomen, normally holds just over 3
      pints (about 1500 ml) of food from a single meal. Here the food is mixed with
      an acid that is produced to assist in digestion. In the stomach, acid and other
       digestive juices are added to the ingested food to facilitate breakdown of
       complex proteins, fats and carbohydrates into small, more absorbable units.
   4. A valve at the entrance to the stomach from the esophagus allows the food to
       enter while keeping the acid-laden food from "refluxing" back into the
       esophagus, causing damage and pain.
   5. The pylorus is a small round muscle located at the outlet of the stomach and
       the entrance to the duodenum (the first section of the small intestine). It
       closes the stomach outlet while food is being digested into a smaller, more
       easily absorbed form. When food is properly digested, the pylorus opens and
       allows the contents of the stomach into the duodenum.
   6. The small intestine is about 15 to 20 feet long (4.5 to 6 meters) and is where
       the majority of the absorption of the nutrients from food takes place. The
       small intestine is made up of three sections: the duodenum, the jejunum and
       the ileum.
   7. The duodenum is the first section of the small intestine and is where the food
       is mixed with bile produced by the liver and with other juices from the
       pancreas. This is where much of the iron and calcium is absorbed.
   8. The jejunum is the middle part of the small intestine extending from the
       duodenum to the ileum; it is responsible for digestion.
   9. The last segment of the intestine, the ileum, is where the absorption of fat-
       soluble vitamins A, D, E and K and other nutrients are absorbed.
   10. Another valve separates the small and large intestines to keep bacteria-laden
       colon contents from coming back into the small intestine.
   11. In the large intestines, excess fluids are absorbed and a firm stool is formed.
       The colon may absorb protein, when necessary.

How Your Digestion Works

Understanding the Gastrointestinal Tract

(Information reproduced from "Understanding Your Irritable Bowel" published by the IBS
Research Appeal)

In my experience one of the most helpful things we can offer IBS patients is a proper
understanding of why their symptoms occur. For example, patients will often be reassured if it
is explained to them that episodes of crampy pains in their lower abdomen are likely to be
occurring because the muscle surrounding the colon, which is responsible normally for
achieving movement, is going into spasm. There are at least 13 symptoms arising from the
intestines alone which can occur in IBS and a wide range of additional symptoms arising from
sites outside the gastrointestinal tract. Because gastrointestinal symptoms feature so
prominently in IBS, we present here a brief reminder of what the digestive system is and how
it works.
The Alimentary Canal

The digestive process takes place in tubes and sacs that extend from the mouth to the anus,
this system being known as the alimentary canal. Its organs are the mouth, the pharynx, the
oesophagus, the stomach, small and large intestines, the rectum and the anal canal. The
stomach and the intestines are referred to as the gastrointestinal tract. Other organs that
support the process are the teeth, tongue, salivary glands, pancreas, liver and gall bladder. In
short, the job of the digestive system is to ingest solids and liquids, to masticate (chew) them
with the teeth, to alter some foods chemically, to absorb useful products and to excrete the
useless residue.

As we know, the process begins in the mouth where the teeth tear and crush the food. The
tongue tastes it and the salivary glands lubricate the food and begin to digest some of the
starches. The act of swallowing pushes the food through the throat and passes it to the
oesophagus where the action of the oesophageal muscle propels it to the stomach. The
stomach is a digestive bag that holds the food during the initial digestive action. Muscles in the
wall of the stomach churn the food as gastric juices, secreted by glands in the gastric wall of
the stomach, act on the food and digest it. These glands secrete about a quart (1 litre) of
gastric juices every 24 hours. Certain cells secrete a mucus that forms a layer in the stomach,
preventing the enzyme pepsin from digesting the stomach itself since some of the stomach's
secretions are extremely powerful - hydrochloric acid being one of them. The gastric juices
begin to digest or break down the food and liquid that we eat and drink.

The average meal remains in the stomach for about 1 - 3 hours while this process takes place.
This food, by now partially digested and at this stage known as chyme, is then discharged into
the upper intestine. Here the food mixes with bile and the digestive enzymes produced in the
pancreas. The small intestine, which is about 4 - 6 metres long, is the power house of food
handling. We eat about 80 grammes of protein a day, 80 grammes of fat a day and 250
grammes of carbohydrate a day. These are all complex food stuffs which are broken down to
their basic constituents before absorption can take place. The digestive juices produced by the
pancreas and important chemical enzymes in the lining of the intestine are responsible for the
complex break-down process. Thus the fat we eat is absorbed in the form of the simple
compounds fatty acids and glycerol. All the different proteins we eat are absorbed in the form
of single molecules called amino acids and small peptides composed of two or three of these.
The complex sugars that we eat are all absorbed in the form of single molecules called
monosaccharides. The processes of digestion and absorption are enormously efficient. Most of
the food we eat is absorbed in the first 80 cm or so of the small intestine.


On average we drink about 1.5 litres, or 2.5 pints, of fluid a day. This is not the end, however,
as far as the gastrointestinal tract is concerned, as the digestive juices add another 7.5 litres
to the volume of fluid the gut has to deal with every day. Normally we pass around 200 ml or
200 grammes of stool a day, so it follows that the gastrointestinal tract has to absorb a lot of
fluid every day.

Figure 2.2

Approximate daily volumes of fluid (ml)
handled by the human gastrointestinal tract.

                                 Fluid volume (ml)
Input         Diet                           1500
              Digestive juices               7500
              Total                          9000
Absorption    Small intestine                7500
              Colon                          1300
Output        Stool                           200

As shown in figure 2.2, most of this fluid is absorbed in the small intestine. This is achieved in
conjunction with the absorption of digested food. Some 1.5 - 2 litres of fluid is absorbed during
the passage of contents through the colon or large intestine

Part of the food we eat is derived from plants, ie. cereals, pulses, vegetables and fruit. The
cells wall of all plants contains a carbohydrate different in structure from the starch in our diet,
called non-starch polysaccharide. This is generally known as fibre. Unlike the starch in our diet,
fibre is not broken down and absorbed in the small intestine. Thus, following the digestion and
absorption of food, fibre, together with the digestive juices and a few other chemicals, travels
down the small intestine in liquid form, and a volume of 1.5 - 2 litres a day enters the colon.
Here the fluid and electrolytes are absorbed, the stool is manufactured and proportions of the
fibre are broken down to the chemicals (short chain fatty acids) which are responsible for
maintaining the integrity of the lining of the colon. Some 150 - 200 grammes of solidified
contents pass out of the colon everyday in the form of stools.


It follows from this discussion of the events that take place to achieve the digestion and
absorption of food that a very important function of the gastrointestinal tract is a propulsive
one - that is, the moving of ingested food and digestive juices from the stomach to the colon
and anus. It is the contractile or motile function of the gut that is responsible for inducing this
movement. In IBS it is the abnormalities of this motility (contractility) of the intestines that
lead to many of the symptoms from which patients suffer. Gastrointestinal physiologists have
over the years developed many different techniques for assessing the rates of movement of
contents through the gastrointestinal tract and have usually expressed their results in terms of
transit time. There are broadly three components to gastrointestinal transit - emptying of the
stomach (gastric emptying), transit through the small intestine and transit through the colon.

Gastric Emptying
Many different factors, including importantly the nutrient content of the meal, influence
emptying times, but in general the greater part of a solid meal has emptied from the stomach
within three hours. Small intestinal tract Bearing in mind that the small intestine is 4 - 6
metres long, its motility or contractility is a very efficient process, as all its contents are moved
right the way through in about 4 hours. In the immediate response to eating, the
characteristics of motility (contractility) are geared up to preventing movement of contents
from the upper regions, so as to allow digestion and absorption to take place here. The rate of
movement of the residual contents after these events is then very rapid. Movement is achieved
by the powerful contractile forces that move down the intestine in an orderly way, called the
migrating motor complex, or MMC.

Colonic Transit
Remember that 1 - 2 litres of fluid enter the colon each day. Unlike the emptying of the
stomach and the movement through the small intestine, transit through the colon is slow and
in normal subjects it takes up to 48 hours for the contents to move through the ascending,
transverse and descending segments of the colon to the bottom portion or rectum. Again, it is
the disturbances of motility or contractility of the colon that are responsible for abnormal
propulsion, giving rise to symptoms in IBS sufferers. It follows that in IBS, movement of
contents is abnormally slow in those with constipation and abnormally rapid in those with
diarrhoea. Absorption and evacuation After the various foods have been digested to their
soluble and easily usable products, they are absorbed through the intestinal lining cells, mostly
in the small intestine. The amino acids, glucose, vitamins and other substances diffuse into the
blood system to drain to the liver before passing into general circulation. The products of fat
digestion (glycerol and fatty acids) are absorbed and reformed into neutral fats and carried
away from the intestine by the lymphatic system. Alcohol and some drugs are absorbed in the
stomach. Water is absorbed in the small and large intestines. At the junction of the stomach
and the small intestine, and at the junction of the small and large intestine, are circular
muscles called sphincters. When they relax, food passes from one structure to another. After
passing through the sphincter between the small and large intestines, the intestinal contents
pass successively through the ascending colon, then into the transverse colon and then the
descending colon, the sigmoid colon, the rectum and the anus. Water continues to be absorbed
as contents pass along the large intestine and solidified material is formed. This solidified
material is stored as faeces in the lower end of the colon, or rectum, and finally passed
through the anal canal by means of a coordination of muscles in this area of the body, a
process that is called defaecation.

In the revised edition of the acclaimed book, 'Understanding Your Irritable Bowel', Dr David
Silk gives a detailed and easy to follow explanation of this complex condition.

Our online catalogue shows extracts from the book. Click here to visit the catalogue and read
about the book, or access the catalogue from our 'Publications' page linked in the sidebar at
the left.

All proceeds from donations and sales go towards furthering our research into the
causes of IBS and into helping develop cures for it.
Before the mouth: When you see some food you want, your saliva starts to flow even
though the food has yet entered your mouth.
3.3.1     Mouth
3.3.2     Esophagus
3.3.3     Stomach
3.3.4     Small Intestine Gallbladder and Pancreas Specialized Lining in Small Intestine Small Intestine Cells Absorb Vitamins, Minerals, and Water Blood and Lymph Transport Nutrients and Fluids Liver Regulates Blood Nutrients
3.3.5     Large Intestine
3.4    Body Coordination and Regulation of Digestion
3.4.1     Muscles in GI Mix and Move Food
3.4.2     Enteric Nerves Coordinate and Regulate Digestive Activities
3.5    Disorders related to Eating
3.5.1     Heartburn and GERD
3.5.2     Ulcers
NUTRITION Everyday Choices G/C Wiley 2006 – Over half the world’s population is
infected with the bacterium Helocobacter Pylori. This can cause ulcers and long-term
infection is associated with development of gastric cancer.
3.5.3     Food Allergies and Intolerances Celiac Disease Food Allergies
3.5.4     Irritable Bowel Syndrome
3.5.5     Diarrhea and Constipation

OTHER RESEARCH – G/S NUTRITION Everyday Choices Wiley 2006

   • Food takes an average of 24 hours to pass from the mouth to the anus, but it
     depends on what you ate. High fat/high protein meal will empty slower and
     keep you full longer than high carbohydrate meal.
  • Food spends 3 to 5 hours in the small intestine.
  • Figure out your transit time by eating 3 or 4 whole beets. It will turn your
     feces a deep red color.
  • Large intestine alone may hold food for 24 hours. This resident time allows
     many important bacteria to flourish.
  • One byproduct of bacteria is “gas”. Normal adults produce between 200 and
     2000 ml of “gas” each 24 hours. 2000 ml = a 2 liter bottle of soda.
  • Brown color in feces is not the color of food. It’s the bilirubin found in bile.
     Bacterial action turns the bilirubin from yellow to brown.

   1. You are taking medication that gives you “dry mouth” meaning that your saliva
      content is significantly low. How will this affect your food intake?
   2. Your teeth are sore because of some recent dental work. You have an apple,
      but do not chew it very well before swallowing. What has happened to the
      nutritional content of that apple.
   3. You have a nutritious meal that is followed by a dessert with lots of fat. How
      has the time of food transit through your body changed?
   4. Your pancreatic enzymes are not being produced in sufficient quantities. How
      does this affect the adsorption of your meal?
   5. You eat a good lunch high in fiber, but do not drink anything. How does this
      affect your feces?

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