Carbohydrates are sources of energy for
vital metabolic processes and also are constituents of
cellular substances such as nucleic acids, and are
enzyme cofactors and structural components of cell walls
and cell membranes.
The digestive system breaks down the carbohydrates into
simple sugars and ultimately absorbs glucose into the
bloodstream. Any excess glucose is converted into
glycogen which is then stored around the body. Thus an
excess of carbohydrates can lead to an increase in body
fat and a gain in weight.
Carbohydrates are conveniently classified into three
major groups polysaccharides (complex carbohydrates),
monosaccharides and disaccharides (simple
carbohydrates). Complex carbohydrates contain many more
nutrients than simple carbohydrates. These complex
carbohydrates, such as whole wheat bread, potatoes and
rice have a high nutrient density and are a good source
of dietary fiber, and should be a major ingredient in
any persons diet. Between 55% and 75% of calories should
come from carbohydrates.
They have a low fat content, but are usually consumed
with meat, cheese, oil or butter, all of which are very
fattening. It is a good idea to try and limit the amount
of fatty foods and protein rich foods combined with
carbohydrates. For instance a pasta dish with a garlic,
chili or tomato sauce is far healthier than a similar
pasta covered in a cream, cheese or meat topping.
Simple carbohydrates are usually refined foods such as
white sugar, white bread or cola. These foods contain
little else besides energy (ie. empty calories). Any
food that does not have a large variety of nutrients is
considered to have a low nutrient density. Foods that
have low nutrient densities should be limited to small
portions and only eaten as a special treat if the bulk
of your diet consists of nutrient dense, natural,
wholefoods (eg. bananas, fruit juice, nuts, granola,
beans and green vegetables).
Monosaccharides:
Monosaccharides are simple sugars (glucose, fructose,
and galactose) that do not need to be further digested
to be absorbed. The most important dietary
monosaccharide is glucose, also called dextrose. It
constitutes about 0.1% of the blood of mammals and is
essential to life. Glucose, either free or combined with
other molecules, is probably the most abundent organic
compound. It is the ultimate hydrolysis product of
starch and cellulose.
Fructose, also known as fruit sugar, occurs free, along
with glucose and sucrose, in many fruits, vegetables,
and honey.
Disaccharides:
Are sugars formed from two monosaccharides. Ordinary
cane sugar (sucrose) is a disaccharide composed of
glucose and fructose monosaccharide units. Sucrose is
the most important disaccharide, it occurs in all
photosynthetic plants where it appears to serve as an
easily transported energy source. Its two main
commercial sources are sugar beets and sugar cane. The
juices, which contain about 20% sucrose, are put through
a rather extensive purification process to remove
impurities.
Maltose is a disaccharide of two glucose molecules and
is found in beer and cereals. It is formed by the action
of an enzyme from malt on starch, further hydrolysis of
maltose, catalyzed by the enzyme maltase (from yeast)
gives only glucose. Lactose is the sugar present in
milk, human milk contains five to eight percent and
cow's milk, four to six percent. It is composed of one
molecule of glucose and one of galactose.
Polysaccharides:
Starch, glycogen, cellulose and most types of fiber are
polysaccharides. Starch is the reserve carbohydrate in
many plants and comprises large percentages of cereals,
potatoes, corn, and rice. Under the microscope, the
appearance of the granules of starch from these
different sources varies both in shape and size.
Chemically, however, they are similar.
Complete hydrolysis of starch yields glucose but partial
hydrolysis gives maltose as well. Partial hydrolysis of
starch transforms it into dextrins, polysaccharides of
smaller molecular weight than starch. They are more
readily digested than starch and are used, mixed with
maltose, in infant foods. A dried mixture of dextrins,
maltose, and milk is the preparation used for making
malted milk. Dextrins are sticky when wet and are used
in manufacturing mucilage (gum) for postage stamps and
envelopes. In laundries, starched materials become stiff
and shiny due to the transformation of the starch to
dextrins by the heat of the iron.
When starch is ingested, it is hydrolyzed enzymatically
in a stepwise fashion. Initiated in the mouth by the
enzyme amylase, present in saliva, hydrolysis is
continued by additional amylase in the pancreatic
juices. The maltose produced in this way is, in turn,
hydrolyzed to glucose with the aid of an enzyme present
in the intestines. The glucose is absorbed from the
intestines into the blood and transported to the liver,
muscles, and other sites where it is converted to
another glucose polymer, glycogen, and stored.
Glycogen, the reserve carbohydrate of animals, is found
mainly in the liver and muscles. Glycogen helps maintain
the proper amount of glucose in the blood by removing
and storing excess glucose derived from ingested food or
by supplying it to the blood when it is needed by the
body cells for energy.
Cellulose is the main structural material of plant life,
being the chief ingredient of cell walls of cotton, wood
pulp, straw, corn cobs, and many other materials. Fiber
includes a variety of carbohydrates and other
components.
The chemical linkages in starch and glycogen can be
split by the human intestinal enzymes, but those of
polysaccharides found in fiber are indigestible,
although some fiber components can be broken down by
enzymes released by intestinal bacteria to short-chain
fatty acids that can be reabsorbed.
