Download Chapter 5 Summary

Chapter 5 Summary
Carbohydrates consisting of a single sugar are called monosaccharides, and those consisting of two
sugars are called disaccharides. Together, mono- and disaccharides are considered the simple
carbohydrates. Examples of monosaccharides include glucose, fructose, and galactose. Lactose, maltose,
and sucrose are disaccharides and consist of at least one glucose molecule. Lactose consists of glucose
bonded to galactose, sucrose consists of glucose bonded to fructose, and maltose consists of glucose
bonded to glucose.
Oligosaccharides and polysaccharides are classified as complex carbohydrates. Oligosaccharides consist
of 3 to 10 monosaccharides, and polysaccharides consist of hundreds of monosaccharides. Starch,
glycogen, and fiber are important polysaccharides. Amylose and amylopectin are two forms of plant
starch. Glucose molecules in amylose are arranged in a linear chain, whereas those in amylopectin are in
a highly branched arrangement. Glycogen, which also consists of glucose molecules bonded together in a
highly branched arrangement, is found in liver and skeletal muscle. Dietary fibers are a diverse group of
plant substances that are not digested or absorbed in the human small intestine. However, some are
broken down by bacteria in the large intestine. The physical properties associated with dietary fiber are
thought to determine their physiological effects. These include solubility in water, water-holding
capacity, viscosity, fermentability, and capacity to bind other substances such as bile and minerals.
Carbohydrate digestion involves a variety of enzymes produced in the salivary glands, pancreas, and the
small intestine. Although some starch digestion begins in the mouth, most occurs in the small intestine
via pancreatic -amylase. Digestion is completed by the enzyme maltase, resulting in glucose. Unlike
amylose, the digestion of amylopectin requires the enzyme -dextrinase to hydrolyze -1,6 glycosidic
bonds. Disaccharides are easily digested by the disaccharidases, which are produced in and found along
the brush border of the small intestine. Once carbohydrate digestion is complete, monosaccharides are
transported into the absorptive cells of the small intestine and carried to the liver in the blood. After
eating carbohydrate-rich foods, blood glucose levels increase, and the magnitude and duration of this rise
is referred to as the glycemic response. The glycemic index and glycemic load are rating systems used to
assess the glycemic response to specific foods.
Because glucose must be available to cells at all times, the body has a well-orchestrated homeostatic
system to maintain blood glucose concentrations. The pancreatic hormones insulin and glucagon play
major roles in this process. Insulin lowers blood glucose levels by (1) enabling glucose to cross cell
membranes, (2) stimulating the conversion of glucose to glycogen and fatty acids, and (3) decreasing the
use of amino acids for glucose production (gluconeogenesis). When blood glucose levels fall, glucagon
stimulates the breakdown of liver glycogen and the subsequent release of glucose into the blood. The
hormones epinephrine and cortisol also increase blood glucose levels during periods of stress. Once
glycogen is depleted, the body generates glucose via gluconeogenesis. To protect itself from excessive
muscle loss, the body decreases its need for glucose by using ketones as an energy source.
Carbohydrate consumption can influence human health in many ways. For example, high-fiber diets are
often associated with reduced risk of several chronic diseases, such as diabetes, cardiovascular disease,
and obesity. Similarly, some weight-loss “enthusiasts” tout low-carbohydrate diets as promoting weight
loss, although the evidence is inconsistent. However, the relationship between sugar consumption and
dental health is clear. Nutritionists recommend that children and adults limit foods high in sugars and
brush and floss teeth frequently and that infants not be allowed to fall asleep with a bottle of
carbohydrate-containing fluid.
In general, it is recommended that adults consume 45 to 65% of total energy as carbohydrates. In
addition, to decrease the risk for certain diseases, individuals are advised to consume 20 to 38 g of fiber
daily. As this amount is well above the average U.S. intake, consumption of high-fiber foods such as
whole-grain products and fruits and vegetables is encouraged. The term sugar is often used to refer to a
wide variety of sweet substances, including honey, corn syrup, and brown sugar. Current
recommendations suggest limiting added sugar intake to about 32 g/day.