 |  Perspectives in Nutrition, 5/e Gordon M. Wardlaw,
Ohio State University
Margaret W. Kessel,
Ohio State University
Carbohydrates
Chapter 5 Summary- The common monosaccharides are glucose, fructose, and galactose.
Once these are absorbed from the small intestine and delivered to the liver,
much of the fructose and galactose is converted to glucose.
- The major disaccharides are sucrose (glucose plus fructose), maltose
(glucose plus glucose), and lactose (glucose plus galactose). When digested,
these yield their component monosaccharides.
- One major group of polysaccharides consists of storage forms of
glucose: starches in plants and glycogen in humans. In these polymers, the multiple
glucose units are linked by alpha bonds, which can be broken by human digestive
enzymes, releasing the glucose units. The main plant starches--straight-chain
amylose and branched-chain amylopectin—are digested by enzymes in the mouth
and small intestine. In humans, glycogen is synthesized in the liver and muscle
tissue from glucose. Under the influence of hormones, liver glycogen is readily
broken down to glucose, which can enter the bloodstream.
- Dietary fiber is composed primarily of the polysaccharides cellulose,
hemicellulose, pectin, gum, and mucilage, as well as the noncarbohydrate lignins.
These substances are not hydrolyzed by human digestive enzymes. However, soluble
dietary fibers are metabolized by bacteria in the large intestine.
- Some starch digestion occurs in the mouth. Carbohydrate digestion
is completed in the small intestine. Some plant fibers are digested by the bacteria
present in the large intestine; undigested plant fibers become part of the feces.
Single sugars mostly follow an active absorption process in the small intestine.
They are then transported via the portal vein to the liver.
- Carbohydrates provide energy (on average, 4 kcal/gram), protect
against wasteful breakdown of food and body protein, prevent ketosis, and impart
flavor and sweetness to foods. Although no RDA for carbohydrate has been set,
a daily intake of at least 50 to 100 g is required to prevent ketosis. If carbohydrate
intake is inadequate to supply the body's needs, protein is metabolized to provide
glucose (gluconeogenesis) for energy needs. However, the price is loss of body
protein, ketosis, and eventually a general body weakening. For this reason,
low-carbohydrate diets are not recommended for extended periods (greater than
4 to 6 weeks).
- Dietary fiber provides mass to the feces, thus easing elimination.
In high doses, soluble fibers can help control blood glucose in diabetic people
and lower blood cholesterol.
- Diets high in complex carbohydrates are encouraged as a replacement
for high-fat diets. A goal of about half of energy as complex carbohydrates
is a good one, with about 60% of total energy coming from carbohydrates in general.
Foods to consume are whole-grain cereal products, pasta, legumes, fruits, and
vegetables. Many of these foods are rich in dietary fiber.
- Moderating sugar intake, especially between meals, in turn reduces
the risk of dental caries. Other health benefits also occur, such as a reduced
glycemic index (GI) for a meal or snack. Alternative sweeteners, such as aspartame,
aid in reducing intake of simple sugars.
- The ability to digest large amounts of lactose often diminishes
with age. People in some ethnic groups are especially affected. This condition
develops early in childhood and is referred to as lactose intolerance.
Undigested lactose travels to the large intestine, resulting in such symptoms
as abdominal gas, pain, and diarrhea. Most people with lactose intolerance can
tolerate cheese and yogurt; tolerance of other dairy products varies among affected
individuals.
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2002 McGraw-Hill Higher Education