NOVA scienceNOW: Obesity
Introduce the key components of a generalized hormone-receptor system. As
a class, make a list outlining the key points about hormone-receptor systems.
Glands and tissues produce and secrete hormones.
"Target" tissues and "target" cells have special receptors, for specific
hormones, that respond chemically to the hormones.
Hormones stimulate target tissues and cells to produce chemical
The chemical products produced by target tissues and cells affect the
body in particular ways.
Draw a diagram on the board that organizes these generalized components into a connected system. (To examine a specific hormone-receptor system, see Before Viewing question #2)
Use a concept map to illustrate a specific hormone-receptor system. Concept
maps are a way to visually show how the parts of a system relate to one
another. In a concept map, nouns are used to describe the components of the
system. The relationship between these components is shown by arrows, which
connect the related parts. Each noun is put in a box, and the arrows are
labeled with a verb describing the relationship between components. In the
paragraph below, the key nouns are underlined and the key verbs are italicized.
If students are familiar with a particular hormone-receptor system, have them
make a concept map for that system. Otherwise, provide student pairs the
paragraph below, and have them create a concept map to describe the production
and action of growth hormone in the body. When pairs have completed their
concept maps, create a class version on the board. Consider having students
make a concept map for leptin after they watch the segment.
Growth hormone is produced in the anterior lobe of the
pituitary gland in the brain. It circulates through the
bloodstream and helps regulate growth and
metabolism. One primary organ (i.e., a target tissue) that has
growth hormone receptors is the liver. There, the growth hormone
triggers the production of enzymes and other proteins that can
increase bone length and diameter.
Introduce the concepts of set point and homeostasis.In the segment,
scientists state that there is a natural stable weight for the body, called a
set point. One can think of it as a marble sitting in a bowl. Though the
marble can move from side to side, it returns to the middle, the position where
it is most stable. For a person's body to remain at its set point, the body
- monitor its fat content
- send a signal about the body's fat level to the glands and tissues that
help maintain body fat
- have an organ (i.e., the liver) make products that help the body store
fat if necessary
- signal the brain (i.e., the hypothalamus) to tell the body to continue
or to stop eating
Living things have the ability to maintain homeostasis, the property of
a system to regulate itself so as to maintain a stable condition. Make an
analogy by asking students how set point is similar to maintaining the correct
temperature in a room, refrigerator, or oven. Then, have them brainstorm body
systems that use feedback loops and that have a set point. (Body
temperature; the number of hours of sleep required for optimal performance;
heart rate; breathing rate; hormone levels; and levels of chemicals in the
blood, such as salt and water)
Create a concept map for leptin. As a class, make a list outlining the key
points about leptin. For example,
- Leptin is produced in fatty tissue.
- Cells in the hypothalamus and liver have specific receptors for
- Once stimulated by a hormone, target tissues and cells produce proteins
and other kinds of molecules.
- Leptin affects appetite, fat storage, and fat metabolism.
Have students develop a concept map that organizes these components into a connected system.
Examine nature (genetics) versus nurture (environment). Reproduce
the chart that follows, either on the board or on a handout. As a class, sort
the list into traits or conditions strongly influenced by genetics (i.e.,
height and Type 1 diabetes) and by environmental factors (i.e., Type 2
diabetes, skin cancer, weight, and muscularity). Discuss how, for each trait or
condition, both genetics and the environment play a role, though there is
typically a main factor. Then have students brainstorm personal behaviors that
may influence how a trait or condition expresses itself (e.g., obtain proper
nutrition, exercise regularly, get adequate sleep, avoid harmful chemicals,
practice good personal hygiene, maintain a healthy living environment).
Demonstrate an analogy for the role the environment plays in the expression of
a genetic factor. Fill three glasses with water—one-third full,
two-thirds full, and almost full. Tell students that these represent different
degrees of genetic predisposition—slightly predisposed, moderately
predisposed, and greatly predisposed. To each glass, add five-milliliter
increments of water (i.e., the environmental factor). Make the point that for
people with strong (and even moderate) genetic predispositions, environmental
factors can cause a genetically-based trait or condition to express itself
(i.e., the water spilling over).
Type 1 Diabetes
a genetic condition related to the ability to produce insulin
by environmental factors such as toxins taken into the body that can damage the
interplay of genetics and the environment
factors such as diet and obesity may trigger some of the genetic elements that
cause Type 2 diabetes
by genetic factors, such as the skin's melanin levels and ability to repair
to high levels of ultraviolet radiation can promote its development
determined by genes determining bone length and bone mineral mass
of nutrition plays a role
factors play a role in appetite, nutritional intake, and metabolism
by nutrition and physical activity
contribute to muscle fiber composition and size, and how skeletal muscle uses
training influences oxygen utilization and muscle fiber size and volume
Contrast the health effects of different types of fats. At the end
of the segment, Dr. Jeffrey Freidman makes three suggestions to help maintain
health, including eating a heart-healthy foods. For many years, this meant
eating foods low in fat and low in cholesterol. Studies show that this
information is overly simplistic—not all fats are "bad." Saturated and
trans fats can increase the risk of certain diseases, including heart disease.
However, monounsaturated and polyunsaturated fats can lower the risk of certain
diseases, including heart disease. Divide the class into four groups and have
them research a type of dietary fat (i.e., trans, saturated, polyunsaturated,
or monounsaturated). Have them describe what effect the fat might have on
health. Have teams choose a food that is a source of the fat they selected and
write a public-service announcement for it. When possible, have them include
information such as:
- Trans fats should be minimized.
- Saturated fats are okay only in limited amounts.
- Monounsaturated and polyunsaturated fats are "good fats" and, for the
average person, heart healthy when eaten in the recommended amounts.
For homework, have students visit the following Web sites and find examples of
a daily healthful amount of "good fat" items.
Determine a healthy daily intake of fat. Emphasize to students that
fat is a necessary part of their diets—bodies need fat in order to
function properly. Fat is an important source of energy, it is used in making
certain tissues, and it helps bodies absorb nutrients, such as vitamins A, D,
E, and K. Fat intake becomes unhealthy if it is regularly above 25%-35% of a
person's daily caloric intake or if it is primarily composed of trans or
saturated fats. Americans typically obtain about 40% of their calories from
fat. Have students calculate how many of their daily calories should come from
fat. A heart-healthy diet for typical middle and high school students has
25%-35% of the daily calories coming from fat, mainly "good fats." And the
recommended number of daily calories for middle and high school students who
exercise moderately is 1,800 for girls and 2,200 for boys. Thus, fats should
contribute, on a daily basis, 450-630 calories for girls and 550-770 calories
for boys. Have student pairs research two or three foods and the number of
servings it would take to meet the daily healthful amount of "good" (i.e.,
unsaturated) fat. (Saturated fats should equal less that 10% of total fat
Identify the components of a "heart-healthy" diet. Divide the class
into four teams and assign each one a meal—breakfast, lunch, or dinner.
Have teams research an inexpensive, simple-to-prepare, heart-healthy menu for
their meal. (The USDA's "My Pyramid" Web site is a good source of information.)
Have them consider what makes a food "heart healthy," and discuss the typical
serving size for the foods they chose, the total number of calories in the
meal, and the balance among protein, carbohydrate, and fat. Consider having
teams gather materials and ingredients and prepare and present their meal to
the class, as though they were on a television cooking show.
Hormones, Receptors, and Control Systems
Includes an overview of endocrinology and information on hormones, receptors,
and target cells.
Reviews research on leptin's role in the body.
Molecular Studies of Food Intake and Body Weight
Discusses studies that focus on leptin's action and how leptin is regulated.
Chipping Away at Leptin's Effects
Describes how researchers have identified genes that are specifically regulated
by the hormone, leptin.
Biology Coloring Book
by Robert D. Griffin. Collins, 1986.
Provides detailed diagrams of human body structures and systems.
The Concise Encyclopedia of the Human Body
by David Burnie. Dorling Kindersley, 1995.
A reference book with clear, detailed illustrations and informative text and