Introduction / Background

This case study was created for an undergraduate introductory non-majors general biology class in conjunction with a 4- to 6-week unit on human anatomy and physiology. Homeostasis is the underlying theme as students examine various fad diet claims and their relationship to human physiology as well as social and psychological factors involved in the nation’s obsession with dieting. Students will recognize homeostasis in terms of negative feedback loops that apply to hormonal control of appetite, hormones and blood sugar, and energy balance. The case has been used in both small classes and large lecture sections.

Students enter an introductory biology class with a wide range of alternative conceptions. A cursory glance at dieting websites is all it takes to explain some of the prior conceptions students bring in about human health: that weight gain is caused by “toxins,” that “detoxification” is an essential part of a weight loss plan, that fat can be “flushed” from the body by various pills or herbs, that protein and complex carbs are always “good” while fat and sugar are “bad,” that sugar contains fat or vice versa. Students are often unused to thinking on the molecular scale, and may have only the most rudimentary ideas of the particulate nature of matter. If they have not learned the basics of biological molecules that make up cells, which make up body systems, they may express ideas about molecules that seem absurd to an instructor but make perfect sense in their own view of the world.

This case is designed to use written responses and classroom discussion to uncover prior conceptions about human physiology, and to apply new learning during a physiology unit to the case of a young man who is interested in dieting.

Objectives

• To properly define “energy” in a physiological sense.

• To understand the relationship between calories consumed, calories expended, and weight management.

• To critically analyze health claims using support from scientific sources.

• To understand how different body systems contribute to homeostasis, particularly endocrine control of homeostasis of appetite, body weight, and blood sugar.

• To develop empathy for people with weight management difficulties.

Classroom Management

I usually use this case in conjunction with a 4- to 6-week unit on human anatomy and physiology. I originally developed the case to go with a shorter unit on the biological molecules, and it can certainly be shortened and modified to accompany such a unit by emphasizing Part I and eliminating Parts III and IV. In expanding the case from molecules to anatomy, I used homeostasis as a unifying theme, exploring either in the case or in-class presentations how the body systems contribute to homeostasis.

The case is not the only activity occurring during the 4 to 6 weeks of the unit; rather, it is an activity to help unify the material the students are learning from lectures and labs during this unit. It is meant to supplement and complement—not replace—an existing unit. When I use this case, I generally show each part on the computer projector to the whole class, presenting each part in conjunction with readings and lectures that are appropriate to the case. Alternately, the separate parts of the case can be handed out as paper copies for students to read in class or as homework. Students then break into small groups to discuss their initial responses to the question sets, followed by a large-group discussion in which each group can contribute one or more of their ideas.

Each time I present a part of the case, I list student ideas on an overhead or on the board without comment or judgment. Often students come up with conflicting ideas that inspire discussion. Once the ideas are listed, and thereby “belong” to the class, the originators of the ideas often have less ownership over them and feel less personally threatened if the idea is challenged. Students often do challenge ideas that others come up with, particularly around the ideas of nutrition and health where there are so many non-scientific ideas in the popular media.

After the discussion, students are individually responsible for answering the questions and turning them in several days after the lecture. I instruct students to look up answers in their textbooks first (college-level non-majors textbooks will provide sufficient information), then direct them to some good online resources such as the National Institutes of Health (NIH) website. I warn them about popular dieting websites, which often carry misinformation. Students are often convinced that because the same information is available on many different diet websites, it must be true. I can sometimes show them examples of how the information from one site may be copied uncritically to another website, misspellings and all, and ask them to think about the reliability of such information. I stress that it is important for students to actually use resources rather than answering the questions from their own prior knowledge, because they might find out that something they know to be “true” is in fact unsupported by science. I do not want them to be intimidated by this, but rather to enjoy the process of learning. Depending on the class, I may only read and comment on the answers to the questions, and only evaluate the final essay for a grade. This allows students a safer place to explore their initial ideas, and gives them a chance to change their ideas before a final grade is applied.

In the same lecture where I present Part III of the case, or just before, I show students a short online video from the Nova Science Now website (see Resources below). This video presents new research on the hormone leptin, which is involved in appetite control. Depending on the time available, I may also have students read an article from Science News titled “Hungry Yet?” which presents research on the hormone ghrelin, also an appetite-control hormone (February 16th, 2002; Vol.161 #7). Students sometimes protest that the video makes it look as though all weight loss is a matter of genetics, but both the video and the article are good introductions to the complexities of digestive system hormones and help build empathy for people struggling with weight control.

Blocks of Analysis

Part I – Macronutrients and Energy

The first part uncovers students’ initial ideas about energy, calories, and the classes of macronutrients: proteins, carbohydrates, and fats.

Expect many students with alternative conceptions and no conception at all, even if they have learned about the macromolecules in prior courses. I have learned to make no assumptions about their prior knowledge. Students with a poor grasp of the particulate nature of matter often struggle to understand the nature of the macromolecules, and are more likely to cling to their prior conceptions about nutrients. Depending on the ideas the students come up with, this may be a good time to review the major nutrients. Physical models shown next to samples of protein, sugar, starch, etc., can help students who are struggling to link the idea of “protein” as an amorphous substance with “proteins” as distinct molecular entities. A physical model or an illustration of an amino acid can also be used to show where the amine group is located to give students clues to one of the questions. Be sure to point out that the “balls” in a “ball and stick” model represent atoms—students with a poor understanding of the particulate nature of matter are often unclear on that point.

Students often have some awareness of dietary sources of the major macronutrients, and this is important to add to the discussion. In fact, starting with the students’ own knowledge is a good way to kick off the discussion. However, it’s likely that students will reveal common misunderstandings during the discussion.

Students may equate the source with the substance; that is, when asked to name a protein, students may say, “Meat.” Students need to recognize the difference between a pure protein, carbohydrate, or fat and its dietary source. Some discussion of the roles the macromolecules play in building cells may help, such as showing how cell membranes are made up of lipids and proteins.

I’ve found that students with a poor understanding of molecules tend to lump “bad” and “good” nutrients together and expect them to have similar properties. For example, when I’ve asked students to point out fats on a nutrition label, some students list sodium, sugar, and calories. All these things, to these students, are “bad for you,” so they must be fats, which are also “bad for you.” Students also have a tendency to believe that starchy foods are fatty because “they make you fat.” Here again, physical models placed next to samples of substances can help demonstrate the structural differences.

Students are also asked to take a critical view of dietary information they discover on the Internet. The female character in the story, Janine, is highly skeptical and disparaging of some of the information that the male character, Mitchell, has gleaned from typical diet books and websites. Students who hold these beliefs themselves may feel threatened by the activity, which is why it is important to list ideas uncritically on the board, and then once the ideas are separated from the student, to engage in critical analysis. While it’s tempting to hold the NIH website as the gold standard, students often resist being told what is scientifically acceptable until they understand why the information on the NIH website is more plausible than the information on a site whose main intention is to sell diet products. Like the NIH website, diet websites will cite studies. A critical discussion should encourage students to ask questions such as: Who conducted the study? How many people were involved? What sort of people? What was their stake in the research? How was the study conducted? Are the results specific to the study group, or can they be generalized to the population at large? What does this study mean to my health?

Students may also notice that scientific information about nutrition seems to change rapidly and may ask “Why can’t scientists make up their minds?” Again, encourage students to look past sensational headlines (“Oat bran lowers cholesterol!”) and take a critical view of any dietary information seen in the popular media (“A new study reports that oat bran reduced blood cholesterol in overweight people with high cholesterol levels.”). This is also a good opportunity to discuss the nature of science and scientific inquiry: that all scientific information is tentative and subject to change. (Note: The term scientific inquiry is preferable to “the scientific method.” Scientific methodology is broader than the single-variable controlled experiments that “the scientific method” implies. It is, however, important for students to understand the nature of hypothesis formation and testing, the importance of double-blind controlled experiments in health research, and why testimonials and historical use are of little value in judging the efficacy of a health remedy.)

The unifying theme of homeostasis is introduced in this first section. Students should see several examples of negative feedback loops throughout the unit—better yet, students should draw negative feedback loops for themselves to understand how the body regulates itself. In my class, each time we encounter a body system involving an important negative feedback loop (such as temperature regulation, water balance, blood sugar, etc.), I present the key players in the system and ask students to attempt to draw the negative feedback loop before I present diagrams from the textbook or other resources.

Part II – Metabolism and Detox

This section of the case raises the subject of “detoxification.” This is the point at which I find I get the most resistance from certain students. Beliefs come with high emotional stakes, and students who already have a strong belief in diet products and diet claims are uncomfortable with having their beliefs challenged. They may already know from experience in using “detoxifying” products or drinking large amounts of water that they must make more trips to the toilet than usual, and may believe that they are ridding their body of “toxins” in the process. The scale may tell them that they have lost weight after using a “detoxifying” product, so their prior experience strongly reinforces their beliefs. Or they may report “feeling better” on a detoxifying diet, which is not an unusual feeling if they are consuming more fruits and vegetables than usual as part of the diet. Other students believe just as strongly that “detoxifying” diuretics only result in the loss of water weight and are useless for long-term weight control. It is important, therefore, to continue to use boardwork to separate ideas from the students who hold them. This keeps the conversation from devolving into personal attacks.

A good biology textbook should have some explanation of how kidneys remove wastes from the bloodstream, and how the liver deals with environmental toxins. Review the role of the skin and sweat, also, in controlling body temperature; many students believe that the main role of sweat is to rid the body of “toxins.” A discussion of adipose tissue can lead students to an understanding of how the body stores energy, while lessons within a unit on the endocrine system can reveal how and when the body uses fat stores for energy. Students may bring up the point that some toxic materials, such as certain illegal drugs or pesticides, can accumulate in body fat. It is true that the body may resist metabolizing fat if fat metabolism causes a release of these stored toxins. This is a good time to ask students, “Would a diuretic remove these toxins from fat cells? Would it help to drink extra water?”

Body image is raised in this section, and may have already come up in discussions during Part I. Anorexia and bulimia are often associated with teenage girls, but it is important for students to understand that anyone can have body image problems. Mitchell may have a body image issue, or he may be the victim of peer pressure. Students may find value in a discussion about body image and the popular media.

The question on metabolism can tie together knowledge about muscles, the nervous system, and the endocrine system. A discussion about what exactly metabolism is would be useful here. Students may have encountered many products claiming to suppress appetite or raise metabolism. The NIH website reviews several such products and is a good resource for discussing the safety and efficacy of these products.

Part III – Hormones and Homeostasis

I present this section after students have studied the endocrine system. I find that students are often confused by the endocrine system because of the many new names they encounter as they learn about different hormones. I limit the hormones that I expect them to know, and have students repeatedly draw negative feedback loops involving a small set of hormones that I want them to know about in detail. Hormones are a confusing topic indeed. Science is continuing to discover new hormones as well as new information about known hormones, adding to the volume of information in the textbooks.

My purpose in using the NOVA video and articles from Science News about hormones such as leptin and ghrelin is not to add to the confusion, but to show genetic aspects of weight management that are presently beyond our own control. I have had occasional students who insist that the only reason people are overweight is that they are “lazy.” The readings and video make it clear that some people do have a genetic issue that affects their weight. A discussion about gene-environment interactions is valuable here. It can also be useful for students to hunt for articles in Science News, Scientific American, or similar reliable sources for the latest discoveries regarding appetite and hormones. Some suggested questions for the NOVA video:

• What do scientists mean by weight “set point”?

• What do genes have to do with hormones?

• Are they saying your “set point” is permanent, or can it be changed?

• What else besides your genes affects your weight?

• Even though Americans are more exercise-conscious than ever, we’re also heavier than ever. What are some contributing factors?

I also bring in (or show pictures of) a dinner plate from a set of vintage china and a dinner plate from a modern set and compare the sizes, and talk about how when I was in high school there were no snack or soda machines in the school. This sparks a discussion on changing portion size and the psychological effects of simply putting more food in front of people.

Part IV – Summary Essay

The summary essay that students are asked to write should pull in all the information that they have learned in the unit that relates to the case. Students should demonstrate some critical thinking toward diet claims and present Mitchell with sensible advice about his diet and weight. They may also notice that Janine is thinking about her weight and her diet and they may have some unasked-for advice for her as well.

While the essay alone gives students an opportunity to draw the case to a conclusion, it is often helpful to have a small-group discussion followed by a class discussion to share ideas about what they have learned before students write their essays.

If desired, instead of individual summaries, groups may be responsible for preparing and delivering a presentation or writing and preparing a short skit in which they take the roles of Janine, Mitchell, and the “you” in the case and deliver their advice orally.

Answer Key

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Resources

USDA My Pyramid:

http://mypyramid.gov/index.html

NOVA Science Now: Obesity

http://www.pbs.org/wgbh/nova/sciencenow/3313/03.html

Science News:

http://www.sciencenews.org/

“The Hunger Hormone?” (February 16, 2002; Vol.161 #7)

“Still Hungry?” (April 2, 2005; Vol 167 #14)

National Institutes of Health Weight Loss:

http://health.nih.gov/topic/WeightLossDieting

http://health.nih.gov/topic/Nutrition/WellnessLifestyle

Set Point Theory (MIT Medical):

http://medweb.mit.edu/pdf/set_point_theory.pdf

WebMD article on detox diets:

http://www.webmd.com/food-recipes/features/detox-diets-purging-myths

BBC news article on detox diets:

http://news.bbc.co.uk/2/hi/health/7808348.stm

Acknowledgements: This case was published with support from the National Science Foundation under CCLI Award #0341279. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Date Posted: June 26, 2009.

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