
Digestion is the process by which the body breaks down food
into absorbable nutrients. The body absorbs and assimilates everything that we
ingest. Digestion includes physical actions such as chewing and peristalsis
(involuntary contraction and dilation of muscles to force forward movement), as
well as the chemical actions of
enzymes, bile and acids.
An
enzyme is a protein that acts as a catalyst in a biological
reaction. It binds itself to a substance and converts it into another substance.
Enzymes are very specific in their functions, which is why there are
different
enzymes for different biological reactions. In the case of digestion,
distinct forms of food require specific
enzymes. Food is digested in three areas of the body: the mouth, the stomach
and the intestines.
The Mouth
Beginning in the mouth with chewing, food is physically broken up into small
pieces to facilitate the beginning of complete digestion. Chewing thoroughly is
an important step because smaller pieces offer more surface area for contact
with the digestive
enzymes and acids in the stomach and are therefore more easily digested.
Chewing also stimulates the release of hormones that control appetite. The
longer you chew, the less likely you are to overeat.
Only one digestive
enzyme, amylase (for starch), is present in the mouth. However, food is
normally not in the mouth long enough to permit complete digestion and little
nutrient absorption takes place in the mouth.
[Karl Note: This is a very deceptive
description of the action of chewing and the absorption of certain components of
food INTO and THROUGH the mouth tissues. Western Science has not studied
this -- Dr. Ayyangar has.]
The Stomach
The primary function of the stomach is to break large proteins into smaller
peptides and peptones. The acidic environment in the stomach (1.5 to 7.0 pH) is
home to the
enzyme pepsin, used to digest proteins. Pepsin breaks up proteins into
smaller peptides of varying lengths. Other
enzymes such as gelatinase (for gelatin) digest specific proteins.
Amylase (for starch) is inactivated, or destroyed, when stomach pH falls below
6.5.1 Before this happens however, up to 50% of starches may be partially broken
down. Some fat is emulsified (broken up into smaller pieces) in the stomach by
bile acids, and the
enzyme lipase, to aid digestion in the small intestine. At the average
stomach pH of 2.0, however, most fat is formed into large globules that pass
unchanged into the small intestine. Small amounts of sucrose (table sugar) may
be broken apart into glucose and fructose by acid hydrolysis from the bile acids
in the stomach.
The Intestines
Virtually all absorption of nutrients (macronutrients such as carbohydrates and
fats and micronutrients such as vitamins and minerals) occurs in the intestines.
Absorption of nutrients is, in fact, the primary function of the small
intestine. Most carbohydrates, for instance, are absorbed in the small
intestine. The
pancreatic enzymes secreted into the small intestine also contain amylase
which breaks down starches into a disaccharide (a complex carbohydrate) called
maltose.2 In the intestines,
enzymes such as maltase and lactase break disaccharides into single sugars,
such as glucose. When adults and older children do not have enough lactase to
digest lactose (the sugar in milk) lactose intolerance results and milk cannot
be completely digested.
When food leaves the stomach, digestion is completed in the small intestine with
the help of the
enzymes secreted by the pancreas (lipase, amylase, protease, maltase,
trypsin and chymotrypsin). The pH of food mass increases from about 2.0 to 6.5
(still slightly acidic), as it passes from the stomach through the small
intestine to the colon. Very few nutrients, except water, is absorbed by the
large intestine (colon).
WATER is the most important macro nutrient we consume. Over 50% of our body
weight is water. Beverages provide just under half of the water we require each
day. The rest of the water is from food. Water also aids digestion as part of
the food mass in the intestines. Motility, the movement of food in the digestive
tract, is more effective if more water is retained in the food. When food
reaches the colon, most of the water is reabsorbed.
CARBOHYDRATE digestion occurs primarily in the small intestine. Most
carbohydrates are easily digested and 90% to 98% are absorbed.1 Bile acids in
the stomach aid in acid hydrolysis to break sucrose down into glucose and
fructose. Before the stomach pH falls below 6.5, destroying amylase, up to 50%
of starches may be partially broken down, but digestion of most carbohydrates is
done in the small intestine.1
FIBER, a complex carbohydrate, makes up the remaining 2% to 10% of unabsorbed
carbohydrates. Dietary
fiber can be further subdivided into soluble and insoluble fibers.1
Insoluble fibers (such as lignins and cellulose from vegetables, grains and
nuts) increase fecal bulk and decrease transit time through the intestines.3
They are partially digested by intestinal bacteria but are not absorbed well.
Soluble fibers (such as pectins and gums from fruits, beans and grains) delay
stomach emptying, slow the absorption rate of glucose and help to lower the
levels of fats and cholesterol in the blood.4 Pectins are neither digested nor
absorbed, but gums may be partially digested by intestinal bacteria and
absorbed1 providing food for the cells lining the colon.
FATS, (or lipids) such as those found in egg yolks, can be broken into glycerol
and fatty acids by stomach acids, but most fat digestion occurs in the small
intestine. Bile, secreted by the gall bladder, emulsifies large fat particles
and breaks them into a smaller size so that pancreatic and intestinal lipases
can digest the fats. The lipase
enzymes are responsible for the majority of fat digestion. Fats are 95% to
100% digestible in healthy people.1 Bile salts also facilitate absorption of
fatty acids.
PROTEIN digestion starts with stomach
enzymes (proteases) such as pepsin that break apart specific links in the
amino acid chains that comprise proteins. These chains are reduced to much
shorter lengths, called peptides and peptones. The partially digested proteins
move onto the small intestine where pancreatic protease continues digestion,
breaking the peptides and peptones into individual amino acids which are
absorbed directly or attached (chelated) to minerals, thus allowing mineral
absorption.
Protein digestibility varies widely 1:
from vegetables and fruits: 65% to 85%
from legumes: 78%
From cereals and grains: 76% to 84%
from milk, meat and eggs: 97%
If protein digestion is incomplete, longer peptide chains of undigested proteins
can be absorbed, possibly causing allergic reactions.5 Symptoms of food
allergies vary widely and can affect any part of the body.
BACTERIA and
PROBIOTICS such as
Lactobacillus acidophilus also are important digestive aids and sources of
nutrition. Intestinal bacteria synthesize biotin and vitamin K, both essential
nutrients.5
Lactobacillus acidophilus and other friendly intestinal bacteria can help
the intestines maintain a healthy colon against an onslaught of unhealthy
pathogenic bacteria such as E. coli or salmonella, especially in infants.6 These
friendly
probiotic bacteria also help the digestion of proteins, carbohydrates and
fats.6, 7
Probiotics (or friendly bacteria) can be found naturally in the body, but
the majority are ingested in foods or as
supplements.
The digestion of foods and the absorption of nutrients is the first, and most
important, step to good health. Our bodies are designed to get the highest
possible benefit from food, but
digestive aids are sometimes needed. Many of us also supplement our diet to
ensure we obtain adequate nutrition. The proper absorption of dietary
supplements is just as important as the proper
digestion of food, and the two work closely together.
Remember, eat a healthy balanced diet, chew your food thoroughly, drink plenty
of water and take
digestive aids for nutritional insurance.
References:
1. Guthrie, J.A., Ph.D. Introductory Nutrition, The C.V. Mosby Company, St
Louis; 1975.
2. Pike, R.L., Ph.D. & Brown, M.L., Ph.D., Nutrition: An Integrated Approach,
John Wiley & Sons, Inc. New York;1975
3. Kundsen, K.E.B., et al. Physiological Implications of Wheat and Oat Dietary
Fibers in New Developments in Dietary Fiber, 1990; Advances in Experimental
Medicine and Biology, 1990;270:135-147.
4. Anderson, J.W. and Ward, K. High carbohydrate, high fiber diets for
Insulin-treated men with diabetes mellitus. American Journal of Clinical
Nutrition, 1979;32-2312.
5. Alpers, D.A. M.D., Clouse, R.E., M.D. & Stenson, W.F., M.D. Manual of
Nutritional Therapeutics, Little Brown & Company, 1984.
6. Friend, B.A. et al. Nutritional and Therapeutic Aspects of Lactobacilli.
Journal of Applied Nutrition, 1984;36(2):125-153.
7. Fernandes, C.F., et al. Therapeutic role of dietary Lactobacilli and
Lactobacillus fermented dairy products. Fed of Eur Microbiol Rev,
1987;46:343-356 .
*These statements have not been evaluated by the Food and Drug Administration.
These products are not intended to diagnose, treat, cure or prevent any disease.
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