You Are What You Eat?
Fats. Fat molecules are a rich source of
energy for the body. The first step in digestion of a fat such
as butter is to dissolve it into the watery content of the
intestine. The bile acids produced by the liver dissolve fat
into tiny droplets and allow pancreatic and intestinal enzymes
to break the large fat molecules into smaller ones. Some of
these small molecules are fatty acids and cholesterol. The bile
acids combine with the fatty acids and cholesterol and help
these molecules move into the cells of the mucosa. In these
cells the small molecules are formed back into large ones, most
of which pass into vessels called lymphatics near the
intestine. These small vessels carry the reformed fat to the
veins of the chest, and the blood carries the fat to storage
depots in different parts of the body.
Vitamins. Another vital part of food that
is absorbed through the small intestine are vitamins. The two
types of vitamins are classified by the fluid in which they can
be dissolved: water-soluble vitamins (all the B vitamins and
vitamin C) and fat-soluble vitamins (vitamins A, D, E, and K).
Fat-soluble vitamins are stored in the liver and fatty tissue
of the body, whereas water-soluble vitamins are not easily
stored and excess amounts are flushed out in the urine.
Water and salt. Most of the material
absorbed through the small intestine is water in which salt is
dissolved. The salt and water come from the food and liquid you
swallow and the juices secreted by the many digestive
How is the digestive
The major hormones that control the functions of the
digestive system are produced and released by cells in the
mucosa of the stomach and small intestine. These hormones are
released into the blood of the digestive tract, travel back to
the heart and through the arteries, and return to the digestive
system where they stimulate digestive juices and cause organ
The main hormones that control digestion are gastrin,
secretin, and cholecystokinin (CCK):
- Gastrin causes the stomach to produce
an acid for dissolving and digesting some foods. Gastrin is
also necessary for normal cell growth in the lining of the
stomach, small intestine, and colon.
- Secretin causes the pancreas to send
out a digestive juice that is rich in bicarbonate. The
bicarbonate helps neutralize the acidic stomach contents as
they enter the small intestine. Secretin also stimulates
the stomach to produce pepsin, an enzyme that digests
protein, and stimulates the liver to produce bile.
- CCK causes the pancreas to produce the
enzymes of pancreatic juice, and causes the gallbladder to
empty. It also promotes normal cell growth of the
Additional hormones in
the digestive system regulate appetite:
- Ghrelin is produced in the stomach and
upper intestine in the absence of food in the digestive
system and stimulates appetite.
- Peptide YY is produced in the
digestive tract in response to a meal in the system and
Both of these hormones work on the brain to help regulate
the intake of food for energy. Researchers are studying other
hormones that may play a part in inhibiting appetite, including
glucagon-like peptide-1 (GPL-1), oxyntomodulin (+ ), and
Two types of nerves help control the action of the digestive
Extrinsic, or outside, nerves come to the
digestive organs from the brain or the spinal cord. They
release two chemicals, acetylcholine and adrenaline.
Acetylcholine causes the muscle layer of the digestive organs
to squeeze with more force and increase the “push” of food and
juice through the digestive tract. It also causes the stomach
and pancreas to produce more digestive juice. Adrenaline has
the opposite effect. It relaxes the muscle of the stomach and
intestine and decreases the flow of blood to these organs,
slowing or stopping digestion.
The intrinsic, or inside, nerves make up a
very dense network embedded in the walls of the esophagus,
stomach, small intestine, and colon. The intrinsic nerves are
triggered to act when the walls of the hollow organs are
stretched by food. They release many different substances that
speed up or delay the movement of food and the production of
juices by the digestive organs.
Together, nerves, hormones, the blood, and the
organs of the digestive system conduct the complex
tasks of digesting and absorbing nutrients from the foods and
liquids you consume each day.
Additional Resources for the Digestive System:
For More Information
American Dietetic Association
120 South Riverside Plaza, Suite 2000
Chicago, IL 60606–6995