Download LESSON 3.5 WORKBOOK Homeostasis gone awry: How

LESSON 3.5 WORKBOOK
Homeostasis gone awry: How
does the satiety pathway relate to
obesity?
In the last lesson we explored how the body
regulates the sensations of hunger and satisfaction. In this lesson we will work our way
through a primary research paper that shows
how changing diet and lifestyle impacts homeostatic signals. This data has important implications for people attempting to lose weight.
Our goal will be to use research and data to
evaluate health claims and advertisements,
allowing us to make more informed choices.
Weight loss relapse: why is losing weight so difficult?
With great effort and constant attention, someone can successfully lose weight and maintain weight loss.
Unfortunately, people who lose weight often eventually regain that weight. It is very hard to lose weight and
keep that weight off, but why?
Mixed communications from adipose to the brain
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Lesson 3.5
When a person gains weight, extra energy is being stored as fat in the adipose tissue. As the size of the
adipose cells grow, the amount of leptin that is produced increases in tandem. Remember that leptin is the
hormone that is produced in the adipose and sent to the brain to signal feelings of satiety.
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LESSON READINGS
Normally, increased leptin signals to the brain that
energy stores are full and no more food is needed,
thereby maintaining homeostasis and preventing
excessive weight gain. However, there seems to be a
point in weight gain when these signals fail, and the
brain is no longer able to 'see' the messages from
leptin. This is called leptin resistance, and is very
common in obese individuals. Leptin resistance is also
correlated with another type of resistance that we will
learn about in the following lesson: insulin resistance.
With leptin resistance there is increased leptin in the
blood, but the normal response in the brain (release of
α-MSH and stimulation of the satiety center) malfunctions. The result is a loss of feeling full, and increased
hunger despite consuming excess nutrients.
Decreased Food Intake Lep7n Increased Body Weight Figure 1: Increased adipose stores
increase circulating leptin, which
should result in decreased hunger. This
can be disrupted in obese individuals.
Resetting homeostasis
In general, our bodies are reluctant to change and want to maintain homeostasis. For example, if you
eat a huge meal at lunchtime, you probably won’t have much appetite for dinner. Over the long term
however, weight gained slowly over months and years will eventually reset homeostasis so that the body’s
new 'normal' is a higher weight. It is this resetting that makes losing weight, and maintaining weight loss
difficult. From an evolutionary standpoint, it was desirable to keep extra energy in our adipose stores
because we were likely to live through a time of food
shortage. Therefore, our bodies are programmed to hold
extra calories, and are reluctant to shed fat.
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Lesson 3.5
Figure 2: Our body wants to
maintain the balancing act of
homeostasis, and is reluctant to
change.
The increased circulating leptin associated with obesity
is reduced back to normal levels after weight loss. While
at first glance this seems like it is beneficial, the drop in
leptin triggers metabolic changes in the thyroid gland that
may prevent further weight loss. Recall that the thyroid
is responsible for determining your metabolic rate. So,
when restricting calories, the thyroid responds to the
drop in leptin levels by balancing the person's current
state of starvation! The thyroid then goes into starvation
mode, and lowers the basal metabolic rate in an effort to
save energy in the body and promote weight re-gain.
1. What is the result of leptin
resistance?
aa. Decreased hunger.
bb. Increased satiety.
cc. Increased hunger.
dd. All of the above.
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LESSON READINGS
As discussed in the last lesson, the other central hormone that regulates food intake is ghrelin, which
makes us feel hungry. If you were to compare the levels of ghrelin circulating in a lean person to those
in someone that has lose substantial weight, what would you expect to find? Similar to the case of the
thyroid gland reducing basal metabolic rate, ghrelin concentrations increase after weight loss because
the body 'thinks' it is starving, and tries to hold on to energy and re-gain energy stores. The body sends
signals to the brain that energy storage reserves are emptying and need to be refilled, causing increased
hunger and food consumption. How long would you expect it to take before the body reaches a new
homeostatic set point: days, weeks, months or even years?
DEFINITIONS OF TERMS
Ghrelin — A hormone produced
in the stomach that stimulates
hunger.
For a complete list of defined
terms, see the Glossary.
Understanding research data
A quick glance through a newspaper or health magazine will
reveal many headlines about nutrition and exercise. Journalists
use results from scientific studies to make claims about what
diets and foods are good or bad. These articles often confuse
readers by misinterpreting or overstating claims. For instance,
one week running may be good for you; the next week running
may be the cause of an early death! So was the research wrong,
and how can you make informed choices when messages flipflop? The only way to effectively evaluate these claims is
to know how the researchers did the experiments.
Figure 3: Nutrition research
often makes news headlines.
Knowing how the research
is conducted helps us sort
through the misleading reports.
Using the QMDC method
Reading a scientific paper is at first daunting, but it is a skill that grows with
practice and is critical for understanding health claims. All primary science
papers have a similar structure that we can use to navigate through the
complex web of ideas. We can simplify the structure of a scientific paper into
four parts, its QMDC:
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Lesson 3.5
Figure 4: Asking
what the BIG
question is before
reading through
a scientific paper
will make the
other parts easier
to understand.
Q uestion: What is the main question of the paper?
M ethod: How do the authors investigate this question?
D ata: The data is represented in figures, and each figure has its
own QMDC.
C onclusion: What conclusions can you make based on the data?
2. Losing weight results in:
aa. A slowing of metabolism.
bb. Reduced concentration of
circulating leptin.
cc. Increased ghrelin.
dd. All of the above.
3. Leptin is associated with ____, and
ghrelin is associated with ____.
aa. Hunger; satiety.
bb. Satiety; hunger.
cc. Fullness; satiety.
dd. Pleasure; hunger.
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LESSON READINGS
Each paper has a primary, BIG Question that is usually reflected in the overall title of the paper. Then
each figure in the paper asks a more fine-tuned Question that relates to an aspect of the big question.
Each figure also uses a specific Method to address the big question. The Data from the figures allow
us to draw Conclusions about whether the more limited questions have been addressed, which again
relates to the BIG Question.
Understanding methods is key
We will learn more about the different types of scientific studies in Unit 4. For now it is important to know
that the type of study the researchers conducted can greatly impact the conclusions we can make. When
determining the methods used in a study, ask yourself three questions: (1) What were the methods used?
(2) How many measurements were made, and was the length of the study appropriate? (3) What did the
researchers measure? Essentially, the more alternative explanations you can come up with to
explain the results, the farther you are from making a solid health claim!
Consider a study that asks the question of whether
weight loss affects the risk of cardiovascular
disease. Does the method of weight loss matter?
Did the participants cut their calories, exercise
more, or did they undergo a weight-loss surgery?
Each method may give us a different answer
to our BIG question. We would also need to
consider how often measurements were made.
Can a weight loss study be completed in a month?
Probably not, because cardiovascular disease
takes years to develop! To really understand the
impact weight loss has on risk for cardiovascular
disease, measurements would need to be taken
for a long period of time.
Figure 5: Determining what methods
scientists used in a study allows us to
make proper conclusions.
Finally, how was the weight loss measured? Did the researchers simply measure the number of pounds
lost, or did they measure changes in body composition? For example, if the participants in the study were
exercising they may even gain weight but reduce their body fat. As you read through a scientific paper try
to focus on what results the methods can give you. You may find that researchers do not always use ideal
methods, but rather those that are possible.
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Lesson 3.5
4. Understanding the methods used
in scientific research helps you
interpret:
aa. The BIG question.
bb. How the study was conducted.
cc. What conclusions are
reasonable.
dd. Both B & C.
ee. All of the above.
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STUDENT RESPONSES
We have discussed the evolutionary benefits of our bodies' drive to store extra energy, and how this makes it difficult to lose
weight. Even so, relative to human history obesity is a recent health problem. Given this information, would you argue that
obesity is caused by genetics, by our environment, or a mixture of the two? Explain your reasoning. _____________________________________________________________________________________________________
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Remember to identify your
sources
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TERMS
TERM
Ghrelin
DEFINITION
A hormone produced in the stomach that stimulates hunger.
For a complete list of defined
terms, see the Glossary.
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Lesson 3.5
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