Carbohydrate
The digestible carbohydrates are broken into simpler molecules
by enzymes in the saliva, in jui
ce produced by the pancreas, and in
the lining of the small intestine.
The three major sources of carbohydrate are starches, sucrose
(table sugar), and lactose (milk sugar). Minor sources include glycogen, alcohol, lactic acid, pyruvic acid, pectins, and dextrins. The diet also contains indigestible cellulose fibers. Starch
Starch is digested in two steps: First, salivary and pancreatic
amylase breaks the starch into the disaccharide (two-sugar) molecules called maltose and isoma
ltose. Second, enzymes in the
lining of the small intestine (maltase and isomaltase) split the maltose and isomaltose into glucose molecules that can be absorbed into the blood.
Starch digestion begins in the mouth with salivary amylase. By
the time food is swallowed, some
of the starch has already been
broken down into maltose. Even befo
re starch leaves the stomach to
be mixed with pancreatic amylase, more than a third has already been broken down to maltose. Sucrose
The sucrose of table sugar, another disaccharide, is digested in
the lining of the small intestine by
the enzyme sucrase into glucose
and fructose, each of which can be
absorbed from the intestinal
cavity into the blood. Lactose
The lactose of milk, a third type
of disaccharide, is changed into
absorbable molecules by the en
zyme lactase, also found in the
intestinal lining.
Into the Blood Stream
The final products of carbohydrate digestion that are absorbed
into the blood stream are all monosaccharide (one-sugar) molecules. In the ordinary diet, about 80% are glucose, about 10% fructose, and about 10% galactose. Fats (Lipids)
The major sources of lipids are triglycerides. Other fats include
phospholipids, cholesterol, and cholesterol esters.
A small, clinically unimportant quantity of fat is digested in the
stomach by gastric lipase.
The first step in overall digestion of a fat is emulsification—
dissolving it into the watery content of the intestinal cavity. The bile acids produced by the liver act as natu
ral detergents to dissolve fat in
water and allow pancreatic and enteric lipase enzymes to break the large fat molecules into smaller molecules, fatty acids.
The bile acids combine with the fatty acids and help molecules
to move into the cells of the mucosa. In these cells the small molecules are formed back into la
rge molecules, most of which pass
into vessels (called lymphatic) near the intestine.
These small vessels carry the reform
ed fat to the veins of the
chest, and the blood carries the fat to storage depots in different parts of the body. Protein
Giant molecules of protein must be digested by enzymes before
they can be used to build and repair body tissues.
Pepsin enzyme in the juice of the stomach starts the digestion of
swallowed protein. Further digestion of the protein is completed in the small intestine. Here, several enzymes including trypsin from the pancreatic juice and peptidases from the lining of the intestine carry out the breakdown of protein molecules into smaller molecules, amino acids.
Nutrition for Sports, Essentials of 40
