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LESSON 3.4 WORKBOOK
Can you become addicted to food?
DEFINITIONS OF TERMS
Dopamine — A compound that sends
signals from one neuron to another, and
is made from the amino acid tyrosine.
Dopamine reward pathway — A
circuit in the brain that when activated
leads to feelings of pleasure. Rewarding experiences such as food and
sex stimulate the dopamine reward
pathway.
Nucleus accumbens (NAc) — A
region of the brain that is involved in
reward, pleasure, addiction, fear and
laughter.
Prefrontal cortex (PFC) — The
anterior part of the brain (located
below the forehead) that plays a role in
personality, decision-making and social
behavior.
Ventral tegmental area (VTA) — A
group of neurons that is the start of the
dopamine reward pathway. Dopamine
is released from the VTA to the NAc
and PFC.
For a complete list of defined terms,
see the Glossary.
Wo r k b o o k
Lesson 3.4
In this lesson we will examine yet another complicating factor that can thwart our intentions to maintain a healthy weight – the similarities between
how our brain behaves when confronted with food
and how it behaves when confronted with drugs of
abuse. The realization that there are many commonalities between the addicted brain and the
obese brain is a recent one, and it has significant
implications for future treatments of obesity.
The dopamine reward pathway
The various feeding and satiety messages we learned about in the previous lesson do not single-handedly
determine what and when we eat. Almost everyone has inhaled a mouth-watering dessert even on a full
stomach; we often eat because food is in front of us. It smells good, tastes good, and looks good! We
might eat because it is the right time of day, we are celebrating, or we are trying to overcome sadness.
After a meal, pleasant memories reinforce our appetite, giving us the desire to eat.
The dopamine reward pathway is responsible for our feelings of pleasure
Our desire to eat is controlled by the dopamine reward pathway, which originates in an area of the brain
called the ventral tegmental area (VTA). The dopamine neurons in the VTA send connections to the
nucleus accumbens (NAc) and the prefrontal cortex (PFC). Don’t worry if you don’t get these
terms right away, we’ll be discussing the reward pathway for the rest of this lesson.
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LESSON READINGS
Dopamine Reward Pathway
Pleasurable s1muli PFC NAc VTA The connections between the VTA, NAc and
PFC are collectively called the reward pathway
because they are activated during pleasurable
experiences, such as eating, during sex, consuming drugs of abuse or when given praise. Because
the same reward center in the brain is responsible
for positive feelings after using drugs and after
eating, overeating may in fact impact the brain like
abusing a drug.
Eating is associated with dopamine release in the
reward pathway, and the amount of dopamine
Figure 1: The dopamine reward pathway
released during a meal can be used to predict
is made up of the prefrontal cortex (PFC),
how pleasurable the experience of eating was. As
nucleus accumbens (NAc) and the ventral
tegmental area (VTA).
expected, different foods produce different levels
of dopamine release, leading to different levels of
pleasure from a meal. Typically, food that is high in
sugars and fats are deemed more pleasurable, though this varies between people. Also as expected, the
amount of dopamine released in the nucleus accumbens is reduced as a meal continues, meaning that
the first bite of a food will be the most pleasurable, and all following bites will get more and more boring!
Food cravings are related to the dopamine reward pathway
Dopamine regulates food consumption not only because it acts on the
reward pathway while eating, but also because we can be conditioned
to stimuli that then drive our motivation to consume food. For example,
whenever you sit down to watch a movie you may feel the urge to eat
popcorn. One of the first descriptions of a conditioned response was
by a scientist named Ivan Pavlov, who showed that after dogs were
exposed to repeated pairings of a tone with a piece of meat, the tone
itself would cause the dogs to salivate even when no food was present.
It has since been discovered that this conditioning increased dopamine
release in the reward pathway upon associations with food.
Wo r k b o o k
Lesson 3.4
Figure 2: Pavlov conditioned dogs to expect
food every time a bell
was rang.
This increase in dopamine in the reward pathway is the cause of the intense food cravings we all experience. Like Pavlov's dogs, humans can be conditioned to associate eating with stimuli that have previously
been tied to food. For some people, it is the “need” to drink coffee every morning, for others it is the “need”
to finish the day with a sweet dessert. Sometimes we can pinpoint the root cause of the craving, but other
times we don’t know what sets these cravings off.
1. Which best describes the dopamine
reward cycle?
aa. A circuit between organs of the
body and regions of the brain.
bb. A series of chemical reactions
that release energy.
cc. Regions of the brain that are
connected by neurons.
dd. Neurons in the hypothalamus
that release signaling proteins.
2. Pleasurable stimuli:
aa. Activate the dopamine reward
pathway.
bb. Can become addictive.
cc. Lead to dopamine release.
dd. All of the above.
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130
LESSON READINGS
Stress triggers overeating
DEFINITIONS OF TERMS
Adrenal glands — Glands that
sit on top of the kidneys that produce hormones that help control
heart rate, blood pressure, and
stress reactions.
Cortisol — A steroid hormone
that is released in response
to stress, and increases blood
glucose, suppresses the immune
system and speeds up metabolism.
For a complete list of defined
terms, see the Glossary.
Wo r k b o o k
Lesson 3.4
Stress can play a particularly critical role in overeating. Moderate stress will stimulate appetite and lead
to overeating in a variety of mammals, from rodents
to humans. This suggests that there is some stressevoked biochemical signal that modifies the reward
pathway to trigger overeating. In the short-term,
stress hormones released from the brain actually
suppress the appetite. If stress persists, glands
sitting on top of the kidneys, called adrenal glands,
will release a stress hormone called cortisol.
Cortisol is the stress hormone that is responsible
for increasing appetite, though the reason why is
not yet completely understood.
Figure 3: Eating high calorie, sweet
and fatty foods is a common response to
chronic stress.
Stress can also affect food preferences, making you want to eat food that is high in fat and sugar. These
foods in turn will activate the dopamine reward pathway, giving us a sense of pleasure, and perhaps
combatting our stress for a short period of time. Some research has suggested that men and women deal
with stress differently. For example, women are more likely to turn to food when they are stressed, and
men are more likely to consume alcohol or smoke.
Can you inherit an addiction?
People suffering from addiction or from obesity are
often stigmatized as having little will power. Yet addiction and obesity are complicated diseases that result
in structural changes in the brain that are difficult to
remedy. A portion of the vulnerability to addiction may
be attributed to genetic differences. For example, addictive behaviors tend to run in families, yet the contribution
Figure 4: Nature vs. nurture: are
of genetics and environmental factors is complex and
some people more likely to be
addicted to drugs or food.
an area of active research. Just as the genetic and
environmental influences in addiction vary between
cultures and people, so does the interaction between
genetics and environmental conditions leading to obesity. Understanding the relationship between
innate (nature) drive for food and learned (nurture) associations that lead to obesity is an area of intense
research. But one thing is clear, being addicted to substances or foods leads to dramatic changes in the
structures of the brain that perpetuate the cycle of addiction.
3. Long term stress results in:
aa. Suppression of the appetite.
bb. Release of dopamine.
cc. Feelings of pleasure.
dd. Release of cortisol.
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131
LESSON READINGS
This is your brain on obesity
Why does the brain 'promote' obesity?
Our reward pathways are particularly sensitive to high-calorie foods containing sugar and fat. This makes
sense evolutionarily, because as hunters and gatherers we often failed to find food, and sometimes had to
wait several days between meals. So craving calorie rich foods offered a survival advantage; it was in our
best interest to seek high calorie foods, so our bodies adapted a mechanism that would find high calorie
foods rewarding, motivating us to eat them.
DEFINITIONS OF TERMS
Inverse — Opposite in direction
or effect.
For a complete list of defined
terms, see the Glossary.
For a greater part of human evolution, sweet
taste was associated with only fruits. Now
that we live in a time when sugars and fats
are abundant, the dopamine reward pathway
can backfire. Instead of being there to protect
us from starvation, our preference for high
calorie foods can harm us by leading to
weight gain and obesity.
Figure 5: In addition to protecting us from starvation our food drive can promote obesity when
an abundance of calories are available.
Changes in the dopamine reward pathway
Eating, smelling and seeing images of foods can lead to increased dopamine release in the reward
pathway in the brain. Some research suggests that the magnitude of the dopamine response is associated with an individual’s body mass index (BMI). In general,
lean individuals have an increased dopamine response
when stimulated with food relative to overweight or obese
individuals. This indicates that obese people may develop a
tolerance to the activation of the reward pathway in response
to consuming food.
Wo r k b o o k
Lesson 3.4
Figure 6: Dopamine is a
simple molecule, that when
released gives us feelings of
pleasure.
This blunting of the reward pathway in obese individuals may
be due to a reduction in dopamine receptors present on the
neurons that make up the reward pathway. In one study,
the number of dopamine receptors in the reward pathway
was inversely related to BMI in an obese population. The
idea being that over time overconsumption of food causes a
decrease in sensitivity of the reward pathway, which sets up
a vicious cycle of needing excess food to feel satisfied.
4. We have evolved to prefer lowcalorie, micronutrient-light foods.
aa. True.
bb. False.
5. A reduction in dopamine receptors
in the dopamine reward pathway
would:
aa. Numb the feelings of pleasure
from food.
bb. Heighten the sense of pleasure
from food.
cc. Increase sensitivity of the reward
pathway.
dd. Lead to severe weight loss.
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STUDENT RESPONSES
Some scientists have attempted to make an anti-obesity prescription drug by targeting the reward pathway. What sort of side
effects would you expect the drugs to produce if they are successful in inhibiting the reward pathway? Would these drugs
work for reducing obesity? Why or why not?
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Remember to identify your
sources
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Wo r k b o o k
Lesson 3.4
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TERMS
TERM
For a complete list of defined
terms, see the Glossary.
Wo r k b o o k
Lesson 3.4
DEFINITION
Adrenal Glands
Glands that sit on top of the kidneys that produce hormones that help control heart rate, blood pressure, and
stress reactions.
Cortisol
A steroid hormone that is released in response to stress, and increases blood glucose, suppresses the
immune system and speeds up metabolism.
Dopamine
A compound that sends signals from one neuron to another, and is made from the amino acid tyrosine.
Dopamine Reward
Pathway
A circuit in the brain that when activated leads to feelings of pleasure. Rewarding experiences such as food
and sex stimulate the dopamine reward pathway.
Inverse
Opposite in direction or effect.
Nucleus Accumbens
(NAc)
A region of the brain that is involved in reward, pleasure, addiction, fear and laughter.
Prefrontal Cortex (PFC)
The anterior part of the brain (located below the forehead) that plays a role in personality, decision-making
and social behavior.
Ventral Tegmental Area
(VTA)
A group of neurons that is the start of the dopamine reward pathway. Dopamine is released from the VTA to
the NAc and PFC.
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