MAMMALIAN DIGESTION
Introduction
The food you eat contains the nutrients that serve as building blocks, and provide energy
and nourishment throughout your body. In food, nutrients are contained in large molecules
that are chemically and physically bound together. Digestion is the process of breaking
down these tightly bound molecules into individual nutrients that can be taken into your
body and used to support its functions. Simply defined, digestion is cutting things down to
a size in which they can be absorbed into your body.
Digestion occurs in the gastrointestinal tract—the 20 to 30 foot long tube extending from
your mouth to your anus. Whatever you eat flows through this system, but until it is
absorbed through the intestinal tract, the nutrients in food are physically outside of your
body. This is because the gastrointestinal tract functions like an internal skin and provides
a barrier between whatever you ingest from the outside (external) world and your internal
bloodstream and cells. Part of the digestion process, then, is the selective transport of
nutrients through the cell wall that lines your intestinal tract. Once transported across the
intestinal barrier to the inside of your body, these nutrients can enter your bloodstream
and circulate to all of your tissues to maintain organ function, support your need for
energy, and provide for growth and repair of new cells and tissues.
While digestion can be simply defined, its mechanics are quite complex. This is because
your food contains so many different sizes, shapes, and types of individual molecules, all
tightly entwined, and because each of these types of molecules is chemically distinct.
Digestion uses both mechanical processes, such as chewing and grinding, which help
separate the different types of molecules, as well as chemical processes, in the form of
enzymes that can cut the bonds within the molecules, to release small nutrients into your
system. An analogy is two or more necklace chains of different types twisted, knotted, and
interlocked together. Digestion would be the process of untwisting and separating the
chains, usually requiring cutting them in a couple of places, and then pulling them apart
and further cutting each of them into many smaller pieces, so they can become building
blocks for other necklace chains.
Food is Complex and Contains Many Types of Molecules
Food is a very complex mixture of different types of very large molecules—the proteins
and some carbohydrates; mid-range sized molecules—such as fats; and a wide variety of
smaller molecules including vitamins, minerals, small carbohydrates like sugars, and other
phytonutrients, which are protective substances found in plants (phyto = plant). Most
foods you eat are a mixture of all of these different molecules, and since you need a
variety of types of nutrients, your body must be able to digest these varied types of
molecules in food.
The size, as well as the type of molecule, makes a difference in how a food is digested, the
nutrients that are derived from it, and where these nutrients are taken up by your body.
Each type of molecule has its own challenge with respect to digestion.
Proteins Provide Amino Acid Building Blocks For Growth and Repair
Proteins are extremely important because they constitute the majority of the structural
tissue in your body, such as bone and connective tissues that provide the shape and form
to which your cells attach. Proteins are involved in just about every function in the body as
well since enzymes are proteins, and enzymes are the molecules in the body that do much
of the work, like building new tissue or removing damaged tissue. Proteins are also
message carriers in your body, transporting hormones from one place to another, and
transporting signals across your cell membranes to your DNA.
Your body is constantly making new proteins to replenish what’s lost from tissue damage
or to provide for growth. Enzymes are continually being produced anew to replace older,
less functional enzymes. Therefore, to maintain optimal health, your body needs a
continuous supply of the nutrients to support protein production.
Proteins are made up of smaller molecules called amino acids that are strung together by
chemical bonds like beads on a chain. To become an active, functional protein, this string
of amino acids folds in on itself forming a twisted and entwined, three-dimensional
structure. An individual protein molecule can be as small as 200 to as large as 5,000
amino acids strung together.
How do I get the protein I need?
In order to make the protein your body needs, it must obtain the protein building blocks,
the amino acids, from the proteins in food. Although vegetables and grains do provide
some protein, you get most of your protein from nuts, legumes, eggs, fish, meats, and
dairy products. When you eat these protein-containing foods, your body must take the
large protein chains in them and cut them down to either individual amino acids or
dipeptides (two amino acids, di=two, peptide=amino acid) before you can absorb them.
Once absorbed, the amino acids are transported through your bloodstream to the tissues
that need them, such as muscles. Then, your body uses these amino acids to reconstruct
its own proteins in the forms you need to support your tissue’s growth and repair.
Your body produces enzymes called proteases to help break down the proteins in food to
the amino acids. Proteases cut proteins between specific amino acids to produce the
smaller peptide chains. Before the proteases can act on the protein, the protein must first
be untwisted, a process called denaturation, which results in a long single-chain protein.
Proteins are denatured in the stomach, with the help of the stomach acid (hydrochloric
acid), the mixing action of the stomach, and the protease pepsin.
After denaturation in the stomach, the long single-chain protein is transported to the
proximal small intestine, the duodenum, which contains several types of proteases. These
proteases act on the protein chain, cutting it further until only dipeptides and single amino
acids are present. The amino acids and dipeptides are absorbed in the small intestine,
primarily in the middle section, the jejunum.
How much protein do I need?
A healthy adult is estimated to need around 40 to 65 grams of protein per day. If this is
not provided in the food you eat, your body will begin to break down muscle and other
tissues to obtain the amino acids it needs. Inadequate intake and digestion of amino acids
from protein can lead to stunting, poor muscle formation, thin and fragile hair, skin
lesions, a poorly functioning immune system, and many other symptoms.
In plant and animal foods, the amino acids you need are mainly provided in the form of
large protein molecules that require all aspects of protein digestion—denaturation in the
stomach and protease action in the intestines—before absorption. Free amino acids, which
require no processing by the body before absorption, may also be present but are
generally not found in large amounts.
In processed foods, protein is sometimes provided as hydrolyzed protein, which means it
has been chemically cut into smaller chains from two to 200 amino acids called peptides.
These peptide fragments may be easier for your body to digest; that is, they may not need
to be denatured in the stomach, but are still too large for direct absorption and must be
digested in the intestine. Some specially produced foods for hospital or healthcare use are
made of elemental amino acids; these products provide the amino acids themselves and
require no digestion before absorption.
Fats Insulate Your Body's Cells From the Outside World
Fats, also called lipids, are required for many important functions in your body. Fats are a
main component of the membranes of all the cells in your body: without fats, your cells
would have no covering or boundary. By providing the membrane around all your cells,
fats are vital for insulating your body from the outside world. Fats also can be used to
provide energy and are involved in supporting the immune system, brain health, and
cardiovascular function.
There are many different types of fats, but only a few are essential, which means your
body cannot create them internally, so you must take them in through your diet. These
essential fats include an omega-6 fatty acid (linoleic acid), and an omega-3 fatty acid
(linolenic acid), and are found in the highest amount in nuts, seeds, and fish. Meat
contains high levels of fats that are not considered essential, called the saturated fatty
acids, and it also contains cholesterol, which is also not essential and is digested in the
same way as fats. High amounts of the non-essential saturated fats, and too little of the
essential fats can result in problems with the immune system, hardened arteries, and
scaling skin, among other symptoms.
As well as being a necessary part of your diet, during digestion, fats also act as carriers of
the fat-soluble vitamins (A, D, E, and K) and the carotenoids, thus enabling their
absorption. (Carotenoids, such as beta-carotene, are a group of highly colored fat-soluble
compounds in plants with a wide range of health protective effects.) Without fats in your
diet, you would also not be able to absorb these important vitamins, and would show
deficiency symptoms such as problems with blood clotting (vitamin K), weak bones
(vitamin D), or vision disturbances (vitamin A).
What happens when I eat a food containing fat?
Fats are present in food primarily as three fat molecules attached to a backbone molecule
called glycerol, but your body can’t absorb this molecule directly. Like protein, your body
must first break down this larger molecule into smaller ones. For example, after you eat a
piece of salmon, which contains essential fats, your body must first remove, or strip-off
the fat molecules from the glycerol backbone to which they are attached. This process is
called hydrolysis, and the types of enzymes that hydrolyze fats from glycerol are called
lipases. Lipases are secreted under the tongue, in the stomach, and from the pancreas;
therefore, fat hydrolysis begins the minute fats enter your mouth and continues in your
stomach, where the majority of fat hydrolysis occurs.
After hydrolysis, the absorption of fats is complicated by the fact that, like any oil, they are
insoluble in water, and therefore the body has a system in place to provide a solubilized fat
aggregate. The body uses bile acids, which act as detergents, to make fat globules, or
aggregates. After aggregation with bile, the fat aggregates, also called miscelles are
transported to the small intestine, where they can be taken up directly by the intestinal
cells and absorbed into the body.
Absorption of the fat from the miscelles begins in the first part of the small intestine, the
duodenum, with the majority of absorption occurring in the mid-section of the intestine,
the jejunum. The bile acids generally stay behind in the intestinal tract, acting more as a
shuttle.
Carbohydrates Support Your Need for Energy and Provide Fiber for Intestinal
Health
Carbohydrates are a varied combination of both very small and very large molecules and
comprise about 40 to 45 percent of the energy supply for your body. You get most of your
carbohydrates from cereals, fruits and vegetables. Small carbohydrates, like table sugar
(sucrose) or glucose, provide a sweet taste to foods. Larger carbohydrates, like starches or
fiber, provide substance to foods. Examples of these larger carbohydrates include gums,
gels, or pastes, like you get with bread or cookie dough. When cooked, these foods have a
structure, like a slice of bread or a cracker, but are mainly composed of different types of
carbohydrates.
What happens when I eat a bowl of cereal?
Only the individual small sugar molecules, called monosaccharides (mono=one;
saccharide=sugar), can be absorbed directly. Glucose and fructose are examples of
monosaccharides. Since carbohydrates exist in food not only as monosaccharides, but also
as many combinations of these monosaccharides linked together, your body has to cut
these carbohydrates down to their individual monosaccharide units.
Many of the simple sugars that give food its sweet taste are found as two small sugars
bonded together. For example, when you eat a bowl of cereal, your body must digest the
sucrose (table sugar), which is made of two small sugars, to its monosaccharides. To do
this, it uses an enzyme called sucrase, which cuts sucrose to produce glucose and fructose,
a process called hydrolysis. The milk on the cereal gets its sweet taste from the
carbohydrate called lactose, which is cut (hydrolyzed) into monosaccharides by lactase, to
produce galactose and glucose. The majority of carbohydrate hydrolysis occurs in the small
intestine; that is, these carbohydrates are mainly transported to the small intestine before
they are cut into the monosaccharides glucose, galactose, and fructose. After hydrolysis,
these individual monosaccharides are then absorbed directly in the duodenum and
jejunum.
Cereals are also high in fiber and provide your body with this important nutrient. Fiber is
made of very large carbohydrates containing types of chemical structures that aren’t
broken down, or digested, by your body. Fiber travels through your gastrointestinal tract
intact and ends up in the large intestine, where it provides nutrition for the intestinal
bacteria that ferment it. Fiber is called soluble or insoluble, depending on its ability to take
up water and to be fermented in the large intestine.
What is starch?
Plants store their energy by stringing together many glucose molecules into a long
complex of several hundred to several thousand glucose molecules. Plant foods that have
stored energy, for example seeds that must provide energy for the young plant when it
starts growing, are high in starch. When the young plant starts growing, the starch is
broken down to form glucose for energy. Starch is found in food as amylose starch, which
is a straight chain starch, and amylopectin starch, which is a branched chain starch.
When you eat foods with starch, like corn or potatoes, your body digests this very large
carbohydrate in much the same way as it digests protein. Your body uses a number of
enzymes to cut down a large, linear starch chain into the small individual units that are
linked together, the glucose molecules, which can then be absorbed in the intestines. The
enzymes that breakdown starches are called amylases. Amylases are very important
because starch is prevalent in our diet and a main source from which we derive glucose,
the primary sugar molecule the body uses for energy. Amylases actually cut starch down
to two-sugar units, maltose and isomaltose, and then other enzymes, called maltase and
isomaltase, hydrolyze these two sugars into the individual monosaccharide glucose.
Amylases are produced in the mouth and, therefore, when you eat starch it is immediately
acted upon, beginning the process of starch breakdown. This is one of the reasons why
thoroughly chewing rather than gulping your food is so important. Since the smaller sugars
that come from amylase action on starch are sweeter tasting, if you hold a cracker in your
mouth and swish saliva around it, you may notice the appearance of a sweeter taste.
One special kind of starch is found in some foods, such as raw, green bananas. It is called
resistant starch, and gets its name because it is resistant to digestion. Therefore, resistant
starch is more like a fiber, traveling through the intestinal tract undigested until it reaches
the large intestine where, like fiber, it may be fermented by the bacteria in the colon.
Vitamins and Minerals are Absorbed Selectively
Vitamins and minerals are quite varied in structure and amount in the foods you eat. They
can be found in food in a free form, chemically bound to a larger molecule, or tightly
encased inside a food aggregate. In most cases, they are liberated during eating by the
mechanical process of grinding. They may also be liberated during the breakdown of the
large molecules like proteins and starch, in which they may be encased.
Since your body requires specific amounts of these key nutrients, most vitamins and some
minerals have active transports in place for absorption and are taken into the body in very
specific ways. These active transports act as shuttles, picking up the vitamin or mineral
and taking it through the intestinal cell wall into the body, where it may be directly
released or transferred to another transport molecule. Since vitamins and minerals are
small and are usually found in much lower levels than amino acids, carbohydrate, and fats,
these active transports must select and pull these important molecules out of the food and
take them into your body. Active transports require energy to function properly.
Calcium and iron are examples of minerals that are taken into the body by active
transport. Most of the water-soluble vitamins have an active transport in place as well, and
these active transports are primarily found in the middle section of the small intestine, the
jejunum. Some minerals, like iron and calcium, are absorbed in the first part of the small
intestine as well as the jejunum. The fat-soluble vitamins (vitamins A, D, K, and E), as
discussed above, are absorbed with fat miscelles, and therefore require fat to be present
for their full absorption.
Magnesium is a mineral of tremendous importance for bone health, energy production, and
overall healthy functioning throughout the body since it activates more than 300 cellular
enzymes. Like calcium, magnesium must be constantly supplied to maintain optimal
function. Magnesium doesn’t have an active transport, but depends entirely on dietary
intake and a healthy intestinal lining for its absorption, and can be absorbed throughout
the entire small intestine and even in the colon. Low intakes of magnesium, or loss of
ability of the intestinal tract to absorb magnesium due to intestinal inflammation or
disease, can result in a variety of problems such as muscle twitching or tremors,
weakness, irritability and restlessness, depression, and weak bones. Magnesium is found
at highest levels in whole foods such as grains but is often removed during processing.
Whole grain bread and cereals will have a much higher amount of magnesium than white
bread, which is made from refined flour.
Vitamin B12 is also absorbed differently from the other vitamins and minerals. First, it is
most commonly found attached to proteins, and therefore requires protein breakdown to
be liberated. Then, it requires a protein made in the stomach, called intrinsic factor, for its
absorption, but is not absorbed until the vitamin B12-intrinsic factor complex reaches the
final part of the small intestine, the ileum. Optimal digestion of vitamin B12 is dependent
on your ability to make a healthy amount of stomach acid, since protein breakdown
requires stomach acid and research has shown that intrinsic factor is also not secreted in
adequate levels when stomach acid is low.
DIGESTION
Click on eat to start the animation. Run your mouse over the parts of the digestive track
to see what they do.
Where does digestion occur?
The whole process of digestion involves many different organs, which are called the
digestive system, and include the mouth, esophagus, stomach, small intestines, large
intestines, rectum and anus. Other organs are involved in supporting the digestive process
as well, but are not technically considered part of the digestive system. These organs are
the tongue, the glands in the mouth that produce saliva, the pancreas, liver and
gallbladder.
What happens in the mouth?
Digestion begins in the mouth with the chewing of food (mastication). Mastication not only
breaks down very large aggregates of food molecules into smaller particles and allows
saliva and enzymes to enter inside the larger food complexes, but also sets off a signaling
message to the body to start the entire digestive process. Research has shown that the
activation of taste receptors in your mouth and the physical process of mastication signal
the neural (nervous) system. For example, the taste of food can trigger the stomach lining
to produce acid, a process called the cephalic phase of digestion; therefore, your stomach
begins to respond to food even before any food leaves your mouth.
Saliva is secreted by the salivary glands in your mouth and moistens the food to improve
the chewing and grinding. Saliva also contains some enzymes that begin the breakdown of
starches and fats. For example, carbohydrate digestion begins with the salivary enzyme
alpha-amylase, and fat digestion begins with the secretion of the enzyme lingual lipase by
glands under your tongue.
What happens in the esophagus?
The esophagus, sometimes called the gullet, connects the mouth to the stomach. It
delivers the saliva-mixed food from the mouth to the stomach and serves as an air lock
between the outside world and the digestive tract. The importance of the esophagus’
ability to separate the mouth and stomach can be seen in the condition known as GERD
(gastroesophageal reflux disease), in which the esophageal barrier is not effective, so the
acid contents of the stomach can escape into the esophagus. Everyone experiences some
gastroesophageal reflux, and the esophagus, with the help of another helpful component
of saliva, salivary bicarbonate, has the ability to clear any stomach acid that escapes. In
many people, however, this reflux occurs more frequently than it should, causing pain and
affecting healthy digestion. This situation is called GERD and is one of the most commonly
seen conditions in medicine today.
What happens in the stomach?
The esophagus opens into the stomach, which is a large chamber consisting of the fundus,
the body and then the antrum. The entire involvement of the stomach in digestion is called
the gastric phase of digestion. The stomach is the primary place where proteins are
disassembled and broken down into small peptides. Due to its acidic environment, the
stomach is also a decontamination chamber for bacteria and other potentially toxic
microorganisms that may have entered your gastrointestinal system through your mouth.
The fundus and body of the stomach, which are usually referred to together and constitute
the majority of the stomach in size, are where the stomach stores food before it is
delivered to the intestine. When the food enters the fundus and body of the stomach, the
lining of the fundus (called the gastric fundal mucosa) produces hydrocholoric acid (HCl).
This acidic environment is critical for destroying toxins in foods, such as bacteria, as well
as for untwisting the complex three-dimensional protein chains, a process called
denaturation of the proteins.
The gastric fundus mucosa also secretes the enzyme pepsinogen, which is present in the
stomach much of the time but is inactive until the acid is present, when it becomes
activated as pepsin. Pepsin acts on the denatured proteins by hydrolyzing, or cutting, the
bonds between amino acids in the protein chain, resulting in several smaller chains, or
peptides.
Fat hydrolysis is very active in the stomach. The fats have already been exposed to lipase
in the saliva, which begins the hydrolysis, but it is the gastric lipase, secreted by the
stomach, that is primarily responsible for fat hydrolysis in humans.
The antrum, or lower part of the stomach, is the site for the stomach’s grinding action and
contains a sensor mechanism, called gastrin, for regulating the level of acid produced in
the body of the stomach. The antrum also controls the emptying of food into the intestine
through the pyloric sphincter. This way the food can be delivered into the intestine in a
controlled manner. Once the food-acid-enzyme mixture leaves the stomach, it is called
chyme. The movement of chyme through the pyloric sphincter stimulates the intestine to
release the hormones secretin and cholecystokinin, which signal the pancreas to release its
contents, the pancreatic juice, inside the lumen (the lining) of the duodenum (the first
segment of the small intestine).
What happens in the small intestine?
The small intestine, which is specifically designed to maximize the digestion and absorption
process, has an expanded surface area with inner folds, called plicae, villi and microvilli, to
increase its surface area and enhance its ability to absorb nutrients. All together, this
surface is called the brush border of the small intestine. Some enzymes are present on the
surface of the brush border, such as disaccharidases like sucrase, maltase, and lactose,
which hydrolyze disugars (sugars composed of two monosaccharides) to their two
individual sugar molecules.
The duodenum, the part of the small intestine that is closest to the stomach, is a
neutralization chamber in which the chyme from the stomach is mixed with bicarbonate,
which appears again, this time in the pancreatic juice. Bicarbonate lessens the chyme’s
acidity, thus allowing more enzymes to function and furthering the breakdown of
macromolecules still present. The pancreatic juice also contains many of the enzymes
necessary for digestion of proteins, such as trypsin and chymotrypsin, enzymes that cut
proteins and peptides down into one-, two-, and three-amino acid chains; and amylase, an
enzyme that continues the hydrolysis of starch.
A few nutrients, like iron and calcium, are taken up most efficiently in the duodenum;
however, the jejunum, the middle section of the small intestine, is the place where most
nutrients are actively absorbed. The amino acids as well as most vitamins and minerals are
absorbed in the jejunum. The process of absorption used by the jejunum is called active
absorption since your body uses energy to select the exact nutrients it needs. Protein
carriers or channels hook-up to these nutrients and take them through the cell wall of the
jejunum and into the portal vein, which carries them to the liver.
Active fat absorption also occurs in the duodenum and the jejunum, and requires that the
fat be put into small aggregates that can be transported into your body directly. The body
uses bile as a detergent to solubilize the fat. Bile is produced by the liver, stored in the gall
bladder, and released into the duodenum and jejunum after a meal. It then can form
miscelles, small fat droplets, for fat absorption. This process is particularly important for
the absorption of the fat-soluble vitamins (vitamins A, D, E, and K), and for cholesterol
absorption.
The majority of starch is also digested in the duodenum and jejunum, the first and second
segments of the small intestine. The monosaccharide products of carbohydrate digestion,
glucose and galactose, are actively absorbed through the intestine by a process that
requires energy. Fructose, another common monosaccharide product of carbohydrate
digestion, and also a common sweetener for many processed foods, is absorbed more
slowly by a process called facilitated transport. Facilitated transport does not require
energy.
The ileum is the final part of the small intestine. The ileum is responsible for completing
the digestion of nutrients and for reabsorbing the bile salts that have helped to solubilize
(keep in solution), the fats. Although most nutrients are absorbed in the duodenum and
jejunum, the first two segments of the small intestine, the ileum is the place where
vitamin B12 is selectively absorbed into your body.
At the end of transport through the small intestine, the chyme has been depleted of
around 90 percent of its vitamins and minerals and the majority of its other nutrients. In
addition, around eight to 10 liters of fluid is also absorbed in the small intestine each day.
Complex carbohydrates that resist the enzyme degradation, such as fiber and resistant
starch, remain, as do a small amount of other food molecules and nutrients that have
escaped the digestion process. For example, about 3-5% of ingested protein normally
escapes digestion and continues to the large intestine.
What happens in the large intestine?
The large intestine is not designed for enhancing absorption but is particularly specialized
to conserve the sodium and water that escape absorption in the small intestine, although it
only transports about one liter of fluid per day. The large intestine is about five feet long,
including its final segments, the colon and the rectum.
It is interesting, given that most digestion and absorption occurs prior to the large
intestine, that food, which at this point is primarily fiber, will spend more time in your
large intestine than anywhere else during digestion. On average, food travels through the
stomach in 1/2 to two hours, continues through the small intestine over the next two to six
hours, and spends six to 72 hours in your large intestine before final removal by
defecation.
One reason food stays longer in the large intestine may be that the large intestine is
capable of generating nutrients from food. The food that makes it into the large intestine is
primarily fiber, and the large intestine contains an ecosystem of bacteria that can ferment
much of this fiber, producing many nutrients necessary for the health of the colon cells.
Colonic fermentation also produces a series of short-chain fatty acids, including
proprionate, acetate, and butyrate, which are required for healthy colonic cell growth and
have many other health promoting functions in your body.
The friendly bacteria that are responsible for the primary amount of healthy colonic
fermentation are called the probiotics (pro-life) and include the Bifidobacteria and
Lactobaccillus genuses. Along with providing beneficial fermentation products, probiotic
bacteria keep pathogenic, or disease-promoting bacteria, from colonizing your colon.
Certain fibers in food, called prebiotics, specifically support these probiotic bacteria.
Prebiotics include such molecules as inulin and fructooligosaccharides, which are found in
chicory and Jerusalem artichoke, and may include some other carbohydrates such as
galactooligosaccharides, arabinogalactans, and arabinoxylans, which are found in soy and
rice fibers, and in larch tree extracts.
Some fiber isn’t fermented, but it is also important because it provides bulk for stool
excretion, and can bind toxins and waste products for their removal through the stool.
Finally, the rectum and the anus allow for controlled elimination of stool.
What happens in the pancreas?
The pancreas can be thought of as a protein factory. It produces and secretes many of the
enzymes necessary for digestion, which include the enzymes that digest protein (trypsin,
chymotryosin, carboxypeptidase, and elastase), enzymes that digest fat (lipase and
phospholipase), and the enzyme that digests carbohydrate (alpha-amylase). The pancreas
releases these enzymes in a pancreatic juice, which is enriched with bicarbonate. The
bicarbonate is used to neutralize the acid in chyme. More than a liter of pancreatic juice is
released per day in response to signals from eating a meal.
Since your body’s tissues are made of protein, the pancreatic enzymes that digest protein
have the ability to digest your own tissues. Your body has an intricate protection from selfdigestion by these enzymes. The stomach and intestinal tract lining have a mucous layer
protecting the tissue from direct digestion by these enzymes. The pancreas uses other
mechanisms for protection. Primarily, it produces the enzymes in an inactive form, called
zymogens or proenzymes. For example, trypsin is produced as the inactive proenzyme
trypsinogen. Trypsinogen is transported to the intestine where it is activated to trypsin by
a protease enzyme on the brush border of the intestinal cells. All pancreatic enzymes
except lipase and alpha-amylase are secreted as proenzymes, and are therefore inactive
within the pancreas.
What happens in the liver?
The liver is one of the most active organs in your body. The liver is the clearinghouse for
all nutrient absorption through the gastrointestinal system. The liver reviews the
compounds that have been taken in and has the ability to distinguish toxins and other
molecules. It has a detoxification system, in which drugs and toxins are chemically
converted to molecules that can be eliminated through the kidneys (urine) or the intestine
(stool). The liver is also responsible for synthesizing most of the proteins that circulate in
your blood, and it produces bile, which is important for the digestion of fats and is used for
the excretion of cholesterol and other fat-soluble molecules.
The liver is the major organ involved in maintaining healthy blood sugar (glucose) levels.
It monitors your body’s glucose needs and provides glucose from digestion, or obtains
glucose by breaking down glycogen, the form in which glucose is stored in your liver. The
liver has only about a 24-hour supply of glycogen. In prolonged fasting, when glucose is
not provided in the diet and glycogen stores have been used, your liver will synthesize
glucose from amino acids and other molecules.
The liver is also the primary organ in which fats are metabolized. The liver can make
cholesterol and is the primary place where cholesterol is removed from the blood. The liver
eliminates cholesterol in the form of bile acids. Every day, your liver secretes about 500
milliliters of bile acids, which are used during digestion to solubilize fats.
What happens in the gallbladder?
The gallbladder is the storage site for the bile acids produced by the liver. After a meal is
consumed, the gallbladder is signaled to release its contents into the duodenum and
jejunum, where they are available for fat digestion.
Ways to Support Healthy Digestion
Healthy digestion requires support for all the different components of digestion:

Chew thoroughly. Chewing is the physical process of breaking the food down into
smaller fragments. Thorough chewing mixes food well with saliva, which moistens
the food particles and provides a means for enzymes, like amylase and lipase, to
get to the pieces of food and begin the process of starch and fat digestion.
Chewing also signals the body to begin the digestion process, alerting the stomach
to prepare to make stomach acid, and signaling the pancreas to prepare to secrete
its contents into the lumen of the small intestinal tract.
When a meal is not well chewed, the food fragments are too big. Since the
digestive enzymes can only work on the surface of the food fragments, inadequate
chewing results in incomplete digestion. This means not only nutrients being left in
the food and unabsorbed, but also extra food for bacteria in the colon. This extra
bacterial food results in bacterial overgrowth, gas and symptoms of indigestion.
Eating should always begin with thorough chewing of food to allow for complete
digestion to occur.

Ensure adequate amounts of digestive factors. After chewing, the food’s next stop
is the stomach, where an adequate amount of stomach acid (hydrochloric acid) is
the next necessity. Stomach acid is required for adequate breakdown of proteins.
Without adequate stomach acid, not only is protein digestion ineffective, but also
digestion of vitamin B12 is seriously affected. Vitamin B12 digestion and
absorption requires that it be liberated from protein. In addition, intrinsic factor,
the protein that is necessary for vitamin B12 absorption, is low when stomach acid
is low.
Low stomach acid (hypochlorhydria) is common, especially in older people since as
we age, we make less stomach acid. Research suggests that as many as half of the
people over 60 years old have hypochlorhydria. A variety of factors can inhibit
sufficient stomach acid production including the pathogenic bacteria, Helicobacter
pylori, and frequent use of antacids. Hypochlorhydria is also associated with many
diseases, such as asthma, celiac sprue, hepatitis, rheumatoid arthritis,
osteoporosis, and diabetes mellitus. Signs of hypochlorhydria include a sense of
fullness after eating, bloating, excessive belching, indigestion, multiple food
allergies, undigested food in the stool, and peeling and cracked fingernails.
In addition to hydrochloric acid, the production of pancreatic enzymes and
bicarbonate is also compromised in some people. If necessary, these digestive
factors can be replaced with appropriate supplementation. Digestive enzyme
support can also be obtained from fresh pineapple or papaya, which contain the
enzyme bromelain, and other fresh vegetables and herbs. Processed foods, like
canned pineapple, contain little enzyme activity since digestive enzymes are
proteins, which are destroyed by heating, such as in the sterilization process. So
beginning a meal with fresh fruits or salad can provide support for healthy
digestion.
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Identify and eliminate food allergens. The intestinal brush border (the absorptive
surface of the small intestine) can be negatively affected by food allergies, which
cause inflammation along the intestinal tract wall. When a food allergic reaction
occurs, the immune system perceives specific food molecules as hostile invaders,
and forms antibodies, which latch on to these allergens to assist in their removal.
As part of the immune system’s defensive action against food allergens,
inflammation can occur along the intestinal tract lining, interrupting the absorption
process and causing damage to the lining. Gastrointestinal inflammatory
diseases—such as diverticulosis or inflammatory bowel disease—and celiac sprue
(intolerance of gluten found in wheat products) also result in damage to the
intestinal wall. Most common food allergens include milk proteins, wheat, soy,
some shellfish, and peanuts.
Support the gastrointestinal barrier. The gastrointestinal cell wall is the barrier
between what you ingest and the inside of your body; therefore, the integrity of
this barrier is vital to your health. Support for the mucus that covers the cells in
the gastrointestinal tract is very important, especially in the stomach. The mucus
layer is one way the stomach and upper small intestine protect themselves against
the damaging effects of stomach acid. Alcohol, over-the-counter anti-inflammatory
drugs, called NSAIDS (e.g. aspirin), and the pathogenic bacteria, Helicobacter
pylori can reduce the mucous layer, leading to lesions in the stomach and small
intestinal tract walls.
Choline provides nutritional support for a healthy mucous layer and is found in
vegetables such as cauliflower and lettuce. Choline can be obtained from lecithin
(phosphatidylcholine) as well, which is high in eggs and soybeans. Some foods also
help combat or protect against the damage of Helicobacter pylori, and these
include catechins found in green tea, some spices such as cinnamon, carotenoids
found in vegetables, and vitamin C, found in citrus foods.
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Provide a healing environment for the small intestine. Research studies have
shown that the small intestinal tract barrier can become leaky under some
conditions. That is, the cells loose their attachments to each other, resulting in a
wall with holes between the cells instead of the cells forming a strong, connected
and continuous surface. When this “leaky gut” happens, molecules can get inside
the body that normally wouldn’t be transported through the intestinal cell wall.
Furthermore, studies have shown that this leaky gut can also cause problems in
the normal transport of nutrients. This is probably because most nutrients are
taken into the body through the cells in the intestinal wall by the selective process
of active transport, and they may need to go through the cells and not around
them to get to the right transport systems in your body. Therefore, with leaky gut,
the things that shouldn’t get in do, and those that should can’t get where they
need to be for adequate transport through the body. The result is the body doesn’t
get the nutrition it needs.
Anything that irritates the lining of the gastrointestinal tract can cause leaky gut,
but a major contributor is inflammation (e.g., food allergies). Leaky gut occurs
under stress (see below), and is found after radiation treatments for cancer, after
some chemotherapy, with diseases such as inflammatory bowel disease, and with
bacterial infections, which can result in bacterial overgrowth in the small intestine.
Eliminating foods to which you are intolerant or allergic can help provide a healing
environment in the small intestine. Carotenoids, (a precursor to vitamin A), may
be particularly important since vitamin A supports the maturation of epithelial
cells, which are the type of cell that line the intestinal tract, and it is the mature
epithelial cells that form the strongest barrier in the intestinal tract. Carotenoids
are found at high levels in vegetables, especially the orange- and red-colored
vegetables.
Glutathione, a small peptide found in the highest concentrations in fresh
vegetables, fruits, and lean meats is also beneficial to the small intestine, since it
can directly act as an antioxidant in the intestinal tract and help decrease
damaging molecules that may be produced during inflammation. Vitamin C, from
citrus fruits, and vitamin E, found in whole grain cereals and nut oils, are important
antioxidants for the small intestine and work with glutathione to support intestinal
healing.
The cells that line the intestinal tract need fuel to continue their process of nutrient
uptake. The preferred fuel for these cells is the amino acid glutamine, which can be
obtained from proteins. Some studies have shown that short-chain fatty acids may
also support the small intestinal tract barrier because they can serve as an
alternate fuel for the cells that make up the intestinal lining. The small intestinal
tract cells also require energy to maintain integrity of the cell wall, and production
of energy requires healthy levels of vitamin B5. Mushrooms, cauliflower, sunflower
seeds, corn, broccoli, and yogurt are concentrated sources of vitamin B5. The
intestinal tract cells also require a number of vitamins, so adequate overall
nutrition is necessary.
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Support the growth of probiotic bacteria. When a good balance of probiotic
bacteria have colonized the colon, they crowd out pathogenic bacteria and other
microorganisms that compromise your health, preventing them from growing. By
fermenting the fiber your body couldn’t directly digest, these healthy colonic
bacteria also produce short-chain fatty acids that the cells of the colon use for their
own nourishment. In addition, these short-chain fatty acids are absorbed into the
body and have beneficial effects on the small intestine and the system in general.
For example, they may help maintain healthy blood sugar and lipid (fat) levels,
and may also increase the amount of calcium taken in by the small intestine, and
promote the movement of food through the intestinal tract. Foods that will supply
probiotic bacteria include some yogurts, kefir, and other foods that have been
fermented with Lactobacillus or contain Bifidobacteria, the beneficial types of
bacteria. Foods that will nourish probiotic bacteria include foods that contain soy
fiber, inulin (from chicory or Jerusalem artichoke), and rice fiber.
Provide for healthy intestinal transit. The movement of the food, or chyme,
through the digestive tract is very important. Healthy intestinal transit is
supported, in part, by the short-chain fatty acids produced by fermentation of
prebiotic fibers in the colon. Fiber, in general, supports overall transit of the chyme
and healthy elimination. Some fibers, like those found in rye, wheat and flax, also
can bind to environmental toxins, such as pesticides, and carry them through the
digestive tract for direct elimination, decreasing the amount that is absorbed into
your body.
Learn how to deal with stress effectively. Research has shown that the intestine
responds negatively to stress, during which the intestinal lining becomes leaky,
absorption is less effective, and your body is unable to selectively take up the
nutrients it needs. The reasons for these effects of stress on the intestinal tract are
not entirely known, however many neurotransmitters (brain-produced signaling
molecules) are found surrounding the intestinal tract. Furthermore,
neurotransmitter receptors, which can bind and respond to these signaling
molecules, are located along the intestinal tract. Therefore, it is known that brain
signaling molecules can affect the intestinal tract. Foods with a calming effect
include herb teas, like chamomile. Alcohol, caffeine, and refined carbohydrates,
like table sugar, should be avoided. Eating meals at regular times and in a relaxed
environment can also help decrease stress.