How does your body know you're full? - Hilary Coller
Hunger claws at your grumbling belly.
It tugs at your intestines,
which begin to writhe, aching to be fed.
Being hungry generates a powerful,
often unpleasant physical sensation
that's almost impossible to ignore.
After you've reacted by gorging
on your morning pancakes,
you start to experience
an opposing force, fullness,
but how does your body actually know
when you're full?
The sensation of fullness is set in motion
as food moves from your mouth
down your esophagus.
Once it hits your stomach,
it gradually fills the space.
That causes the surrounding
muscular wall to stretch,
expanding slowly like a balloon.
A multitude of nerves wrapped
intricately around the stomach wall
sense the stretching.
They communicate with the vagus nerve
up to the brainstem and hypothalamus,
the main parts of the brain
that control food intake.
But that's just one input your brain uses
to sense fullness.
After all, if you fill
your stomach with water,
you won't feel full for long.
Your brain also takes into account
chemical messengers
in the form of hormones produced
by endocrine cells
throughout your digestive system.
These respond to the presence of specific
nutrients in your gut and bloodstream,
which gradually increase
as you digest your food.
As the hormones seep out,
they're swept up by the blood
and eventually reach the hypothalamus
in the brain.
Over 20 gastrointestinal hormones
are involved in moderating our appetites.
One example is cholecystokinin,
which is produced in response to food
by cells in the upper small bowel.
When it reached the hypothalamus,
it causes a reduction in the feeling
of reward you get when you eat food.
When that occurs, the sense
of being satiated starts to sink in
and you stop eating.
Cholecystokinin also slows down
the movement of food
from the stomach into the intestines.
That makes your stomach stretch
more over a period of time,
allowing your body to register
that you're filling up.
This seems to be why when you eat slowly,
you actually feel fuller
compared to when you consume your food
at lightning speed.
When you eat quickly, your body doesn't
have time to recognize the state it's in.
Once nutrients and gastrointestinal
hormones are present in the blood,
they trigger the pancreas
to release insulin.
Insulin stimulates the body's fat cells
to make another hormone called leptin.
Leptin reacts with receptors
on neuron populations in the hypothalamus.
The hypothalamus has two sets of neurons
important for our feeling of hunger.
One set produces the sensation of hunger
by making and releasing certain proteins.
The other set inhibits hunger through
its own set of compounds.
Leptin inhibits the hypothalamus neurons
that drive food intake
and stimulates the neurons
that suppress it.
By this point, your body has reached
peak fullness.
Through the constant exchange
of information between hormones,
the vagus nerve,
the brainstem,
and the different portions
of hypothalamus,
your brain gets the signal
that you've eaten enough.
Researchers have discovered
that some foods produce
more long-lasting fullness than others.
For instance, boiled potatoes are ranked
as some of the most
hunger-satisfying foods,
while croissants
are particularly unsatisfying.
In general, foods with more protein,
fiber, and water
tend to keep hunger at bay for longer.
But feeling full won't last forever.
After a few hours, your gut and brain
begin their conversation again.
Your empty stomach produces
other hormones, such as ghrelin,
that increase the activity of
the hunger-causing nerve cells
in the hypothalamus.
Eventually, the growling beast of hunger
is reawakened.
Luckily, there's a dependable
antidote for that.
