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NOVA scienceNOW: Obesity

Viewing Ideas


Before Watching

  1. Introduce the key components of a generalized hormone-receptor system. As a class, make a list outlining the key points about hormone-receptor systems. For example,

    • 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 products.

    • 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)

  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.
  3. 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 must:

    • 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)

After Watching

  1. 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 leptin.
    • 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.

  2. 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).


    Genetic Factors

    Environmental Factors

    Type 1 Diabetes

    Primarily a genetic condition related to the ability to produce insulin

    Influenced by environmental factors such as toxins taken into the body that can damage the pancreas

    Type 2 Diabetes

    Complicated interplay of genetics and the environment

    Lifestyle factors such as diet and obesity may trigger some of the genetic elements that cause Type 2 diabetes

    Skin Cancer

    Influenced by genetic factors, such as the skin's melanin levels and ability to repair damaged DNA

    Exposure to high levels of ultraviolet radiation can promote its development


    Mainly determined by genes determining bone length and bone mineral mass

    Adequacy of nutrition plays a role


    Genetic factors play a role in appetite, nutritional intake, and metabolism

    Influenced by nutrition and physical activity


    Genetics contribute to muscle fiber composition and size, and how skeletal muscle uses oxygen

    Physical training influences oxygen utilization and muscle fiber size and volume

  3. 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.

  4. 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 calories.)

  5. 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.

Links and Books

Web Sites

Hormones, Receptors, and Control Systems
Includes an overview of endocrinology and information on hormones, receptors, and target cells.

Leptin's Legacy
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 captions.

Teacher's Guide
NOVA scienceNOW: Obesity