LESSON 3.3 WORKBOOK
How do we decide when and how
much to eat?
DEFINITIONS OF TERMS
Appetite — The psychological
desire to eat, driven by feelings of
pleasure from the brain.
Hunger — The biological or
physiological need to eat, caused
by a release of hormones from
the digestive tract.
For a complete list of defined
terms, see the Glossary.
In the last lesson we discussed what causes fast and
slow metabolism, and arrived at the idea that a person’s metabolic rate does not lead to obesity, rather,
restricting caloric intake to the metabolic needs of the
individual is the key to avoiding obesity. Why then is
it so hard to regulate weight? In this lesson we will
explore the signals that regulate sensations of hunger
and satiation, and will spend the next two lessons
relating these signals to appetite and obesity.
Hormones regulate our hunger
We know that glucose homeostasis is a highly regulated process, and if you’re healthy blood glucose
concentrations do not vary beyond a normal range. Yet, even when blood glucose concentrations are
normal we may have the urge to eat. So if low glucose isn’t the factor telling us to eat, how does the body
and mind sense hunger or satiety? There are two general driving forces that influence our desire to eat:
■■ Hunger – The primary physiological drive or need to find and eat food.
■■ Appetite – The primary psychological drive or desire to find and eat food that often occurs when
there is no obvious hunger.
Signals of hunger
Wo r k b o o k
Lesson 3.3
We have previously learned that the pancreas is tasked with the responsibility of sensing glucose concentrations in the blood and secreting hormones in response to too high, or too low glucose. Insulin is secreted
when glucose levels are high, and glucagon is secreted when glucose levels are low.
1. Wanting to eat food because nutrient
levels in the blood are low is an
example of:
aa. Hunger.
bb. Appetite.
cc. Addiction.
dd. Satiety
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
122
LESSON READINGS
As you know, these hormones can travel to
other cells and organs in the body, telling them to
either release stored energy or to store incoming
nutrients for later use. Likewise, other organs
will release hormones that will travel to the brain,
signaling that we either need to stop or start
eating. The important organs (other than the brain)
and the hormones in the process are:
DEFINITIONS OF TERMS
Circadian rhythm — A 24-hour
biological cycle of activity that
drives hormonal releases.
For a complete list of defined
terms, see the Glossary.
■■ The stomach releases ghrelin in response
to low nutrients in the blood – this stimulates
hunger.
■■ The pancreas releases insulin in response to
blood glucose levels increasing – this makes
you feel full.
Pancreas –
Insulin!
Small
Intestine –
CCK!
Stomach –
Ghrelin!
Large
Intestine –
PYY!
Adipose –
Leptin!
Figure 1: Hormones are released from
the digestive tract that send hunger or
satiety signals to the brain.
■■ The adipose tissue releases leptin when energy stores are growing – this makes you feel full.
■■ The small intestine releases cholecystokinin (CCK) when fatty acids and some amino acids enter
small intestine– this makes you feel full.
■■ The large intestine releases peptide YY (PYY) in response to feeding– this makes you feel full.
Hormones regulating hunger can follow your sleep cycle
Wo r k b o o k
Lesson 3.3
Throughout the day and night several hormones in our body are released in a cycle called the circadian
rhythm. Hormones that follow the circadian rhythm are responsible for making you sleepy at night time,
and alert during the day time.
Things like changing time zones
and being exposed to too much
blue light at nighttime can disrupt
our circadian rhythm. Blue light
is the wavelength of light we are
exposed to throughout the day.
Before electricity, the only way
we obtained blue light exposure
Midnight Noon Midnight was by the sun’s rays. Now we
are exposed to blue light coming
Figure 2: Hormones affected by the circadian rhythm are
from televisions, computer
released in a cyclical pattern throughout the day.
2. The circadian rhythm is:
aa. The natural 24-hour cycle of the
body.
bb. Disrupted by blue light.
cc. What makes you tired after
sunset.
dd. All of the above.
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
123
LESSON READINGS
screens, cell phone screens and fluorescent light bulbs, which can all mimic daytime light in our brain.
This in turn can have an effect on the release of some hormones. The release of ghrelin, insulin and leptin
are all cyclical and may be influenced by the circadian rhythm. For example, people that constantly lack
sleep, such as people that work the nightshift at their job, tend to have increased circulating ghrelin and
decreased circulating leptin levels in their blood. This in turn leads to increased feelings of hunger, and
may eventually lead to obesity. The next time you stay up all night see if you notice your hunger levels
changing the following day!
DEFINITIONS OF TERMS
Hypothalamus — A region in
the brain that coordinates homeostatic activity, controlling body
temperature, thirst and hunger.
Satiety — The feeling of being
full, or sated.
For a complete list of defined
terms, see the Glossary.
Wo r k b o o k
Lesson 3.3
Our brain interprets signals sent from the body
The organs of the body are important sensors and signalers of eating because they are the “feet on the
ground” that know when nutrient and energy levels are getting too high or too low. But how does the brain
interpret these signals into behaviors like eating or not eating?
The hypothalamus is the control center of hunger
The homeostatic regulation of food intake is under the
control of the hypothalamus: a tiny structure in the base of
the brain that is the master regulator of most of the body’s
homeostatic mechanisms. The hypothalamus receives,
coordinates and responds to metabolic cues and signals
from the digestive system. By integrating these signals, the
hypothalamus tells us when we need to eat to maintain our
body weight. It is clear however, that higher brain centers
above the hypothalamus have a huge influence on what and
how much food we eat. The reward pathway, for example,
controls our desire to eat, and may be to blame for food
cravings. As we will see, our inability to forego these rewarding aspects of food can override the long-term homeostatic
control of food intake, and can contribute to obesity.
Hypothalamus Figure 3: The hypothalamus
in the brain is the homeostatic
regulator of food intake.
We have known for many years that the hypothalamus plays a central role in driving our need to eat. In
animal studies, placing a tiny lesion in the hypothalamus can cause the animal to become obese or lean,
depending on where the lesion is put. These experiments have allowed us to determine which areas of
the hypothalamus are 'hunger' centers (telling us to eat), or 'satiety' centers (telling us to stop eating). As
we will see below, the hunger and satiety centers of the brain are like a toggle, switching back and forth as
the combination of signals received from the body fluctuate.
3. How does the hypothalamus
regulate energy homeostasis?
aa. It makes us feel hungry or sated.
bb. It regulates metabolic rate.
cc. It leads to glucose uptake in the
muscle.
dd. It causes stored energy to be
broken down.
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
124
LESSON READINGS
The part of the hypothalamus that plays a key role in interpreting hunger and satiety signals is called the
arcuate nucleus (ARC), as shown in the figure below. The ARC has two sets of neurons that control
hunger in opposing ways:
■■ One set of neurons produces two molecules: neuropeptide Y (NPY) and agouti-related peptide
(AgRP) that stimulate feeding and promote weight gain.
DEFINITIONS OF TERMS
Agouti-related Peptide (AgRP)
— A peptide that is synthesized
in the arcuate nucleus. Its release
leads to feelings of hunger.
Alpha-melanocyte-stimulating
hormone (α-MSH) — A peptide
that sends messages between
neurons in the hypothalamus.
Its release leads to feelings of
fullness.
Arcuate Nucleus (ARC) — A
group of specialized neurons
(nerve cells) in the hypothalamus.
Neuropeptide Y (NPY) — A
peptide that sends messages
between neurons in the hypothalamus. Its release leads to
feelings of hunger.
For a complete list of defined
terms, see the Glossary.
Wo r k b o o k
Lesson 3.3
■■ The other set of neurons produces a hormone called alpha-melanocyte-stimulating hormone
(α-MSH) that reduces appetite and promotes weight loss.
What makes us eat?
When nutrients in the blood get low,
ghrelin is released from the stomach,
which travels to the ARC in the hypothalamus and activates the release of
AgRP and NPY. These molecules then
go to the hunger center in the hypothalamus and tell us that we need to
eat. We then get the feeling of hunger
and eat!
What makes us stop eating?
Figure 4: Regulation of hunger and satiety in the
hypothalamus involves hormones from the body
communicating with the arcuate nucleus, which leads
us to eat or to stop eating.
A number of factors can be sensed
and trigger us to stop eating. Food
entering our digestive tract, glucose concentrations rising in our blood and triglycerides being sent off
to storage in the adipose tissue are all cues that it is time to stop eating. When these things happen,
leptin, insulin, CCK and PYY are all released from their respective tissues, and all travel to the ARC in the
hypothalamus. This is where things start to get tricky! The presence of leptin, insulin, CCK and PYY in
the ARC stimulates the production of α-MSH, which signals the satiety center and tells us to stop eating.
Additionally, leptin, insulin, CCK and PYY in the ARC will inhibit the functions of ghrelin, thereby making us
feel full in two ways: one, by stimulating the satiety center, and two, by inhibiting the hunger center.
Some foods may make us feel fuller than others. Because CCK is only released in response to fat and
some amino acids entering the small intestine, a high carbohydrate meal may not sufficiently trigger CCK
secretion, and may not give us feelings of satiation. Also, recall that foods with a high glycemic index - like
highly processed, sweet foods - cause blood glucose to peak and fall quickly. This produces a fast storage of energy followed by a drop in blood glucose that will lead to a fall in insulin and leptin, which allows
ghrelin to stimulate hunger.
4. Eating a meal would stimulate the
release of all of the following except:
aa. Leptin.
bb. Insulin.
cc. Ghrelin.
dd. CCK.
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
125
LESSON READINGS
Stimulants of appetite
When do you feel the urge to eat? Is it only when your body truly
needs nutrients and energy? Probably not! We all get the urge to eat
for a multitude of reasons, like boredom, or passing by our favorite
ice cream shop. So if ghrelin isn’t stimulating this need to eat, what
is?
How are our senses linked to appetite?
Figure 5: Simply
thinking of food may
make us feel hungry!
We can be driven to eat simply by seeing or smelling food. This is
because food activates the brain's reward center, which we will learn
about in greater detail in the next lesson. By activating the reward
center, we feel good after eating. Simply seeing or smelling a favorite
food can activate the reward center and make us want to eat. There are some foods that give us more
pleasure than others, and some people are more sensitive to getting pleasure from food than others. In
general, people who experience more pleasure from eating certain foods tend to eat more, leading some
researchers to believe that the brain’s reward center may play a central role in the development of obesity.
On the other side of the equation, being hungry can actually increase our sense of smell. The release of
ghrelin from the stomach can travel to cells that are responsible for smell and make them more sensitive.
This is why the smell of food cooking is especially mouthwatering when you are hungry!
Marketing food to our senses
We live in a food-rich environment, and are
constantly exposed to advertisements for food
products. Food marketers can use our senses
to sell us food even when we are not hungry.
Some studies have shown that seeing a picture
of a tantalizing food (like the pizza to the right)
can activate the brain's reward pathways and
make us want to eat. Using this knowledge,
we can be driven to purchase a food simply by
seeing a picture of it.
Wo r k b o o k
Lesson 3.3
Figure 6: Seeing images of our favorite
foods can make us feel hungry even when
we don't need nutrients.
5. Wanting to eat food because you
smell it (not because your body
needs it) is an example of:
aa. Hunger.
bb. Appetite.
cc. Addiction.
dd. Satiety.
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
________________________________
126
STUDENT RESPONSES
How can we use the knowledge of hypothalamus and hunger and satiety signals to formulate a drug for weight loss? What
process or processes would you activate or inhibit and what would you predict the outcome and side effects to be?
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________
____________________________________________________________________________________________________
Remember to identify your
sources
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________
Wo r k b o o k
Lesson 3.3
_____________________________________________________________________________________________________
___________________________________________________________________________________________
127
TERMS
TERM
For a complete list of defined
terms, see the Glossary.
Wo r k b o o k
Lesson 3.3
DEFINITION
Agouti-related Peptide
(AgRP)
A peptide that is synthesized in the arcuate nucleus. Its release leads to feelings of hunger.
Alpha-melanocyte-stimu­
lating hormone (α-MSH)
A peptide that sends messages between neurons in the hypothalamus. Its release leads to feelings of
fullness.
Appetite
The psychological desire to eat, driven by feelings of pleasure from the brain.
Arcuate Nucleus (ARC)
A group of specialized neurons (nerve cells) in the hypothalamus.
Circadian Rhythm
A 24-hour biological cycle of activity that drives hormonal releases.
Hunger
The biological or physiological need to eat, caused by a release of hormones from the digestive tract.
Hypothalamus
A region in the brain that coordinates homeostatic activity, controlling body temperature, thirst and hunger.
Neuropeptide Y (NPY)
A peptide that sends messages between neurons in the hypothalamus. Its release leads to feelings of
hunger.
Satiety
The feeling of being full, or sated.
128