Download Where are we heading? Unit 4:

Unit 4:
Where are we heading?
Unit 1: What’s in your food?
Unit 2: How does your body use food?
Unit 4: Introduction
Unit 3: What is metabolic disease?
Unit 4: How do I identify ‘good’ and ‘bad’ food?
Unit 5: How does this knowledge apply to me?
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In Unit 3 we learned that the concept of ‘fast’ or ‘slow’ metabolism is
inaccurate, and that it is how our metabolic needs are balanced with
our caloric intake rather than our metabolic rate that dictates whether
or not we are at a healthy weight. We explored how the body regulates when and how much we eat, and learned that when the signals
regulating hunger and the feelings of pleasure and reward become
unbalanced, obesity can result.
In this unit we focus on the messages we receive about ‘good’ and
‘bad’ foods. We will prepare to critically evaluate some examples of
nutrition research in order to understand how the design limitations
of nutritional research contribute to confusion behind some nutritional messages.
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LESSON 4.1-2 WORKBOOK
Why are there contradictory
messages about 'good' and 'bad'
foods?
In these two lessons we begin to explore the processes and challenges of
nutritional research. Sometimes foods
or nutrients can be labeled as good one
day, and bad the next. How can this be
the case? In this lesson we will see that
understanding how research studies
are conducted impacts how we can
interpret study results and may clarify
apparent contradictions.
All nutrition research has similar key components
Wo r k b o o k
Lesson 4.1-2
Even though nutrition research is quite diverse, all studies have the same core components: They ask
a general BIG question, use scientific methods to answer that question, and result in data used to make
conclusions that can advance our understanding of nutrition and disease. Here we will begin to walk
through these main components. Later, in Lesson 4.3, we will learn about different methods used in nutrition research and we will explore how the methods limit the types of conclusions and claims we can make.
Finally, in Lesson 4.5 we will work with published nutrition data to formulate our own claims based on the
findings.
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150
LESSON READINGS
Results Observa(on Scientific methods commonly used in nutritional
science are iterative
Although there is no single scientific method, most nutritional studies use common activities that are sometimes
Background Experiment labeled the scientific method. But be careful, science
Research is not a linear process and the scientific method is not
intended to circumscribe an order for research, rather
Hypothesis it entails components used in an iterative process. This
means that the activities are modified and repeated as
Figure 1: Components of the scientific
we learn. When conducting research, scientists make
method are used in an iterative process.
observations that intrigue them or call what they know
into question. They will often then turn to background
research to see what others have found about the phenomenon. The observation and knowledge
gathered by others can then be synthesized into a hypothesis. Based on this hypothesis, experiments can
be planned and performed to test their theory. Finally, the data is used to make conclusions and update
biological models. As we will see, how the experiments are constructed limits the scope of our conclusions, a factor often missed by reports of scientific findings in the news.
Wo r k b o o k
Lesson 4.1-2
We can use the discovery of the cause of scurvy as an example of methods used to understand disease.
In the 15th and 16th centuries, European sailors often fell ill and died from scurvy. As mentioned in
Unit 1, symptoms of scurvy include general discomfort and lethargy, spots on the skin, and bleeding
spongy gums. Symptoms may also include paleness, depression and an inability to move. If the disease
progresses open wounds develop and teeth fall out. A Scottish naval surgeon named James Lind
observed how the sailors’ diet aboard a ship differed from their diets on
land. Namely, far fewer fruits and vegetables were available onboard a
ship. Hence, Lind found a correlation between high rates of scurvy and
a diet with few fruits and vegetables, and hypothesized that scurvy was
caused by a deficiency in some component of food. He then designed
an experiment to investigate this possibility. The experimental conditions Lind established were to give sailors salt water, vinegar, cider,
citrus juice or other liquids to drink. He found that the only the sailors
who drank citrus juice were resistant to developing scurvy. From this
he concluded that a component in the juice was able to replace the
missing fruits and vegetables in the sailors' diets, hence preventing/
Figure 2: James Lind
treating scurvy. We now know that the missing component replaced by
conducted the first
clinical trial to deterthe citrus juice is vitamin C. Today, investigators use similar processes
mine what foods prevent
to explore the relationship between food and health or disease.
scurvy.
1. Which of the following is NOT true
about the scientific method?
aa. It is a set of steps that determine
the order of scientific processes.
bb. It is a steadfast rule that dictates
how research is conducted.
cc. It will produce results that will
always be interpreted the same
way.
dd. It is an iterative process used to
update biological models.
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LESSON READINGS
The human factor
DEFINITIONS OF TERMS
Cell culture — A process in
which cells are grown under controlled conditions in a laboratory.
Research question — The primary question a scientific study is
aimed to answer.
For a complete list of defined
terms, see the Glossary.
In Unit 3 we briefly discussed how different methods used in
research yield different conclusions. The same holds true for
the overall type of study that was conducted. For example, did
the study use human participants, or was the study carried out
on animals, or on mice cells grown on a plate (cell cultures)?
Each of these types of research has strengths and weaknesses. Using humans as research participants may seem like
Figure 4: Using animal
the most direct way to test the impacts of something in people.
models has benefits like
However, humans differ greatly from one another in both genetcontrolled environments and
ics and lifestyle. So, depending on your research question
genetics. However, a mouse is
humans may or may not be the ideal participant! There are
a mouse not a human!
also several ethical issues that are raised when using humans
as research participants. For
example, it is unethical to purposefully cause a nutrient deficiency in people.
Therefore, a study analyzing the effects of a nutrient deficiency would need
to be conducted with laboratory animals, or in a human population that
has a naturally occurring deficiency. It is also easy to determine the exact
nutrient intake in mice, but very difficult to track the diet of free-living humans
throughout the months or years of a study period. Given these limitations in
human research, causal links between risk factors and a disease are often
shown through experiments with animals and cell cultures. Unfortunately,
conclusions that are made from animal or cell culture studies may not
directly apply to humans simply because each of these organisms have
Figure 5: Cell
cultures grown on
different biology. Hence, our limited ability to study human participants and
a plate are useful in
the significant biological differences between humans and other species
biological research.
has contributed to our evolving understanding of 'good' and 'bad' foods.
The constant evolution of 'good' and 'bad' foods
Are there really 'bad' nutrients?
Wo r k b o o k
Lesson 4.1-2
To further understand how the classification of 'good' and 'bad' foods is continually evolving we will take a
look at the nutrients and foods that are commonly associated with disease (so-called 'bad' nutrients), and
then explore how current research either supports or contradicts this classification. It is a common belief
that diets high in fats, sodium, cholesterol and red meats increase the risk for developing heart disease.
2. Animal models are often used for
nutrition studies because:
aa. Using mice can avoid ethical
limitations.
bb. Diets of mice can be controlled.
cc. Genetics of mice can be
controlled.
dd. All of the above.
3. Human-based studies are
challenging because:
aa. People have similar living habits.
bb. People eat the same types of
foods.
cc. People misreport what they eat.
dd. People have identical genetics.
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LESSON READINGS
However there are many examples of nutritional studies that do not support these conclusions. To understand these discrepancies we will look to the experimental methods used in two studies to analyze the link
between dietary fat and heart disease.
Does dietary cholesterol lead to heart
disease?
Levels of low-density lipoprotein (LDL) in
the blood correlate positively with increased
rates of heart disease, and elevated blood
LDL levels also correlate with diets rich in fats
and cholesterol. From these data one may
hypothesize that modifying diets to limit LDL
levels will decrease the risk for heart disease.
Figure 6: Dietary cholesterol is found in animal
products, such as meat, dairy and eggs.
Data published in 1972 in the American Journal of Clinical nutrition entitled Effect of dietary cholesterol
on serum cholesterol in man supported this idea. In this study 70 male participants were fed diets that
were identical in all aspects except for varying amounts of cholesterol. Men who consumed more dietary
cholesterol had increased blood levels of LDL cholesterol, confirming a positive correlation between
dietary cholesterol intake and blood LDL concentrations. But does this change in LDL correlate with a
change in heart disease? The expectation was that lowering dietary cholesterol intake would thereby
decrease blood levels of LDL (which the study confirmed), and as a consequence the risk of heart
disease would also decrease. However, lowering dietary cholesterol can have the opposite effect, leading
to increased incidence of heart diseases! As recent data suggests, LDL is not the only character in the
heart disease story. It turns out that low levels of HDL positively correlate with risk of heart disease,
suggesting that perhaps the ratio of HDL to LDL may be important.
This may explain why low cholesterol diets can have negative effects
on heart diseases; a low cholesterol diet may lower both LDL and HDL.
Wo r k b o o k
Lesson 4.1-2
Figure 7: The HDL to
LDL ratio may be more
effective at determining
heart disease risk than
HDL or LDL alone.
What happens to the HDL/LDL ratio when people eat low fat and low
cholesterol diets? In a study entitled Randomized Clinical Trials on the
Effects of Dietary Fat and Carbohydrate on Plasma Lipoproteins and
Cardiovascular Disease, reducing the levels of fat intake overall seems
to lead to a reduction in both LDL and HDL cholesterol levels, and an
increase in blood triglycerides. So a diet with less total fat can reduce
heart disease risk by lowering LDL, but at the same time it increases
heart disease risk by lowering HDL and increasing triglycerides. Based
on these studies, would you classify dietary fat and cholesterol as 'bad' or 'good'?
4. High LDL cholesterol is correlated
with heart diseases. Therefore,
reducing dietary LDL:
aa. May cause heart diseases.
bb. May prevent heart disease.
cc. May have no effect.
dd. All of the above.
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153
LESSON READINGS
The parts don't equal the sum of the
whole!
DEFINITIONS OF TERMS
Incidence — The occurrence,
rate, or frequency of a disease.
Osteoporosis — 'Porous bones'.
A condition in which the bones
become brittle and fragile from
loss of tissue.
Placebo — A substance with no
therapeutic effect that is used as
a control in a study.
For a complete list of defined
terms, see the Glossary.
Wo r k b o o k
Lesson 4.1-2
In many cases it is difficult to determine what
component of food is causing or preventing
a disease or ailment. Even if the causal nutrient is identified, it often has different effects
when eaten in the absence of the other
components of the whole food. For example,
imagine that you make the observation that
women who drink at least three cups of milk
a day have a lower incidence of osteoporosis than those who don’t drink milk
Figure 8: Supplements may not have the
at all. Milk is high in calcium, an important
same biological activity as whole foods rich
building block of bones, so you design a
in the nutrient.
study to test the hypothesis that calcium
intake will prevent osteoporosis. In this study,
women are randomized to take either a calcium supplement or a placebo, after which bone density is
measured. Unfortunately you find no effect, so you conclude that calcium intake is not related to bone
density. However, calcium in milk is in a complex mixture of lipids and other vitamins including vitamin D,
which investigators later found are required for the calcium to be absorbed effectively. Hence, the results
from the study of calcium alone may be reported in the news as 'increasing calcium intake does not
prevent osteoporosis'. But years later, after the discovery that calcium intake is vitamin D-dependent, the
study may be reported as 'increasing calcium intake does prevent osteoporosis'. Which statement is
correct? Or are they both correct or both incorrect?
5. The conclusions of a nutrition
study are definitive and cannot be
changed.
aa. True.
bb. False.
6. The results of a study analyzing the
effect of a nutrient can be different
than analyzing the effect of a whole
food.
aa. True.
bb. False.
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STUDENT RESPONSES
What are the pros and cons of conducting nutritional studies in humans vs. mice?
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Remember to identify your
sources
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Wo r k b o o k
Lesson 4.1-2
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TERMS
TERM
For a complete list of defined
terms, see the Glossary.
Wo r k b o o k
Lesson 4.1-2
DEFINITION
Cell Culture
A process in which cells are grown under controlled conditions in a laboratory.
Incidence
The occurrence, rate, or frequency of a disease.
Osteoporosis
'Porous bones'. A condition in which the bones become brittle and fragile from loss of tissue.
Placebo
A substance with no therapeutic effect that is used as a control in a study.
Research Question
The primary question a scientific study is aimed to answer.
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