You Are What You Eat?
Finally, the digested nutrients are absorbed through the
intestinal walls and transported throughout the body. The waste
products of this process include undigested parts of the food,
known as fiber, and older cells that have been shed from the
mucosa. These materials are pushed into the colon, where they
remain until the feces are expelled by a bowel movement.
The digestive glands that act first are in the mouth—the
salivary glands. Saliva produced by these glands contains an
enzyme that begins to digest the starch from food into smaller
molecules. An enzyme is a substance that speeds up chemical
reactions in the body.
The next set of digestive glands is in the stomach lining.
They produce stomach acid and an enzyme that digests protein. A
thick mucus layer coats the mucosa and helps keep the acidic
digestive juice from dissolving the tissue of the stomach
itself. In most people, the stomach mucosa is able to resist
the juice, although food and other tissues of the body
After the stomach empties the food and juice mixture into
the small intestine, the juices of two other digestive organs
mix with the food. One of these organs, the pancreas, produces
a juice that contains a wide array of enzymes to break down the
carbohydrate, fat, and protein in food. Other enzymes that are
active in the process come from glands in the wall of the
The second organ, the liver, produces yet another digestive
juice—bile. Bile is stored between meals in the gallbladder. At
mealtime, it is squeezed out of the gallbladder, through the
bile ducts, and into the intestine to mix with the fat in food.
The bile acids dissolve fat into the watery contents of the
intestine, much like detergents that dissolve grease from a
frying pan. After fat is dissolved, it is digested by enzymes
from the pancreas and the lining of the intestine.
Transport of Nutrients
Most digested molecules of food, as well as water and
minerals, are absorbed through the small intestine. The mucosa
of the small intestine contains many folds that are covered
with tiny fingerlike projections called villi. In turn, the
villi are covered with microscopic projections called
microvilli. These structures create a vast surface area through
which nutrients can be absorbed. Specialized cells allow
absorbed materials to cross the mucosa into the blood, where
they are carried off in the bloodstream to other parts of the
body for storage or further chemical change. This part of the
process varies with different types of nutrients.
Carbohydrates. The Dietary Guidelines for
Americans 2005 recommend that 45 to 65 percent of total daily
calories be from carbohydrates. Foods rich in carbohydrates
include bread, potatoes, dried peas and beans, rice, pasta,
fruits, and vegetables. Many of these foods contain both starch
The digestible carbohydrates—starch and sugar—are broken
into simpler molecules by enzymes in the saliva, in juice
produced by the pancreas, and in the lining of the small
intestine. Starch is digested in two steps. First, an enzyme in
the saliva and pancreatic juice breaks the starch into
molecules called maltose. Then an enzyme in the lining of the
small intestine splits the maltose into glucose molecules that
can be absorbed into the blood. Glucose is carried through the
bloodstream to the liver, where it is stored or used to provide
energy for the work of the body.
Sugars are digested in one step. An enzyme in the lining of
the small intestine digests sucrose, also known as table sugar,
into glucose and fructose, which are absorbed through the
intestine into the blood. Milk contains another type of sugar,
lactose, which is changed into absorbable molecules by another
enzyme in the intestinal lining.
Fiber is undigestible and moves through the digestive tract
without being broken down by enzymes. Many foods contain both
soluble and insoluble fiber. Soluble fiber dissolves easily in
water and takes on a soft, gel-like texture in the intestines.
Insoluble fiber, on the other hand, passes essentially
unchanged through the intestines.
Protein. Foods such as meat, eggs, and
beans consist of giant molecules of protein that must be
digested by enzymes before they can be used to build and repair
body tissues. An enzyme in the juice of the stomach starts the
digestion of swallowed protein. Then in the small intestine,
several enzymes from the pancreatic juice and the lining of the
intestine complete the breakdown of huge protein molecules into
small molecules called amino acids. These small molecules can
be absorbed through the small intestine into the blood and then
be carried to all parts of the body to build the walls and
other parts of cells.