Carbohydrates are an important source of fuel for body cells. Carbohydrates are
organized in molecules that are in single (monosaccharide), double (disaccharide), or
multiple (polysaccharide) units. Glucose, fructose, and galactose are examples of
monosaccharides; sucrose, lactose, and maltose of disaccharides; and starch and fiber
There are three primary carbohydrate groups: sugars, oligosaccharides, and
polysaccharides. Examples of sugars include the monosaccharides glucose and fructose;
the disaccharides sucrose and lactose; and the polyols sorbitol and mannitol. Examples
of oligosaccharides include maltodextrin and raffinose. Examples of polysaccharides
include starch and fiber.
Digestion of carbohydrate begins in the mouth with salivary amylase, and continues in
the small intestine with pancreatic amylase. For absorption to take place in the
intestine, disaccharides and polysaccharides must be further reduced. This is
accomplished by enzymes called disaccharidases. Sometimes, necessary enzymes are
missing and an intolerance results. Lactose intolerance is caused by an inability to
produce lactase, an enzyme which is necessary for digestion of milk sugar.
When body supplies of glucose, a carbohydrate, are adequate, ingested protein is used
for body maintenance, repair, and growth. When it is not, protein is used for fuel.
Recommendations for daily carbohydrate intake are based on the minimal amount
needed to prevent ketosis a condition characterized by incomplete combustion of fats
resulting in elevated blood ketone levels. Recommended daily intake of carbohydrate is
less than most people eat.
First described in the 1970s, the term dietary fiber describes carbohydrate derived from
plant cell walls including cellulose, hemicellulose, and nonstarch polysaccharides. There
is a lack of agreement concerning which types of carbohydrate qualify as dietary fiber.
Although widely used, the terms soluble and insoluble are inexact. Fiber has three
forms: fermentable, nonfermentable, and viscous.