which is made of glucose and galactose; maltose, made
of two glucose units; and sucrose, made of glucose and
fructose. Monosaccharides can be absorbed directly
into the bloodstream, but disaccharides need to be
broken down into their monosaccharide components
before they can be absorbed.
When food is eaten, the digestion of carbohy-
drates begins in the mouth, where an enzyme in saliva
breaks down starch molecules into the disaccharide
maltose. The food then moves into the stomach
where it mixes with the stomach’s acid and other jui-
ces. In the small intestine, starch is further broken
down into disaccharides and small polysaccharides
by an enzyme released from the pancreas. Cells lining
the small intestine then secrete an enzyme that further
splits these disaccharides and polysaccharides into
monosaccharides. The cells lining the small intestine
can absorb these monosaccharides, which are then
taken to the liver. The liver converts fructose and
galactose to glucose. If there is an excess of fructose
or galactose, it may also be converted to fat. Lastly,
the glucose is transported to the body’s cells by the
circulatory system, where it can be used for energy.
When there is an excess of glucose, the muscle and
liver cells often convert it to glycogen, which is the
storage form of glucose. The muscles store two thirds
of the body’s glycogen solely for themselves, and the
liver stores the other one third, which can be used by
the brain or other organs. When blood glucose levels
decline, the body breaks down some of its glycogen
stores, and uses the glucose for energy. If blood glu-
cose (sugar) levels are too high, the excess glucose is
taken to the liver where it is converted to glycogen and
stored for future use.
Fiber
One of the complex carbohydrates, fiber, is a pol-
ysaccharide in which the bonds holding it together
cannot be digested by humans. Fiber can be either
water-soluble or water-insoluble. Even though these
compounds cannot be digested by humans, they serve
several important functions. The main function of
insoluble fiber is to bind bile acids, which reduces fat
and cholesterol absorption. Sources of insoluble fiber
include wheat bran, whole grains, and brown rice.
Soluble fiber, which helps decrease low-density lipo-
protein (LDL) cholesterol, also called the ‘‘bad’’ cho-
lesterol, can be found in barley, fruit, legumes, and
oats.
Fiber is an extremely important part of the diet. It
aids in weight control by displacing calorie-dense fats
in the diet. Fiber also absorbswaterand slows the
movement of food through the digestive tract, pro-
moting a feeling of fullness. Recommended intakes
of fiber should be about 27 to 40 grams per day. The
United States Department of Agriculture (USDA)
Dietary Guidelines were designed by health professio-
nals to help consumers make nutritious food choices.
The guidelines, released in 2005, replace the food pyr-
amid that the USDA used for many years. Instead of
recommending a certain number of servings per food
group, as the food pyramid did, the new guidelines
advise consumers to eat a diet that emphasizes fruits,
vegetables, whole grains, and fat-free or low-fat milk
and milk products; includes lean meats, poultry, fish,
beans, eggs, and nuts; and is low in saturated fats,
trans fats, cholesterol, salt, and added sugars. The
guidelines recommend that 45–65% of total calories
come from carbohydrates and that foods containing
complex carbohydrates (such as whole-grains) are pre-
ferred over simple carbohydrates (such as table sugar
and white flour.) As an example, one cup of whole-
grain brown rice has more nutritional value and fiber
that processed white rice.Precautions
A common concern among consumers is that a
high intake of carbohydrate-rich foods will cause
weight gain. Consuming too much of any particular
food can cause an increase in weight, but eating a
balanced diet with plenty of fruits, vegetables, and
grains will help promote weight management. GeneralKEY TERMS
Diabetes—A condition characterized by inad-
equate use of insulin preventing a person from
controlling blood sugar levels.
Fructose—A monosaccharide known as fruit sugar.
Galactose—A monosaccharide known as milk sugar.
Glucose—A monosaccharide used for energy; also
known as blood sugar.
Lactose—A disaccharide known as milk sugar.
Low-density lipoprotein (LDL)—The so-called bad
cholesterol that contains a large amount of choles-
terol and transports lipids (fats) to other tissues in
the body.
Maltose—A disaccharide known as malt sugar.
Sucrose—A disaccharide known as table sugar.
Polysaccharides—Long chains of glucose units
linked together.Carbohydrates