Essentials of Nutrition for Sports

(Nandana) #1
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
Free download pdf