- [Voiceover] Let me ask you a question.
Does this gentleman here
look hungry or full?
Well, we can't really tell
just by looking at his face.
We'd have to look at the level of hormones
in his bloodstream, because depending on
whether this person just ate or not,
you'll have a change in
the level of hormones
that'll then talk to
your brain to tell you
whether you should eat more
food or if you're full.
The part of your brain that determines
whether you're hungry or not is called
the hypothalamus, the hypothalamus.
Your hypothalamus helps
make the distinction
if your body is rich in energy
or if your body is poor in energy.
Or, in other words, are
you hungry or are you full?
Well, imagine we just ate,
and the amount of glucose in our blood,
or the serum or blood concentration
of glucose is high.
That means we're going to
release a hormone called insulin.
Insulin is released to store the glucose
you have in your blood from
whatever meal you just ate,
and it'll go and bind
receptors in the hypothalamus
and block these receptors
to indicate to the brain
and the hypothalamus that
you're full, you're not hungry.
Now on the flip side, if you have a low
blood concentration of glucose,
you're not going to be releasing insulin,
and so, you're not going to be
inhibiting the hypothalamus.
Now what if we ate a really fatty meal,
and we have high levels of lipids
or fat in our bloodstream?
Something really greasy.
Well, that'll cause the release
of a hormone called leptin.
Leptin is similar to insulin
in that it represents
the presence of energy-rich
nutrients in the bloodstream.
So it'll go to the hypothalamus
and bind receptors there
to inhibit the feeling of hunger,
which means that when
lipid concentration is low,
you're not going to be releasing leptin,
and you're not going to be
inhibiting the hypothalamus.
Now the final thing that talks
to our brain is our stomachs.
After we've eaten a bunch,
our stomach will be pretty full with food,
however, if we haven't eaten in a while,
our stomachs can be pretty empty.
And I'm sure you've
had this happen to you.
If you're stomach is empty,
it starts talking to you.
It starts making noises.
It actually starts to growl,
and if you listen closely,
the stomach is actually saying,
ghrelin, ghrelin,
which just happens to be
the name of the hormone
that's released into the bloodstream
to tell the hypothalamus
that we are pretty hungry.
We've got a pretty empty stomach.
And instead of a bar,
I'll put a plus sign here
to show that the presence of ghrelin
will tell the hypothalamus
that we're hungry
and motivate us to find some food.
These three hormones,
insulin, leptin, and ghrelin
are the main players that
determine whether we're hungry.
One of the interesting things
that I should mention, though,
about leptin, which I
can probably write here,
leptin levels rarely change,
and the reason why is
because leptin levels are based more on
the amount of adipose in your body
than the amount of lipid
you have in your blood,
which makes sense because
even though we'll eat
some fatty, greasy meal
that'll release lipids
into our bloodstream, we'll
definitely have a lot more
fat tissue in our body beforehand.
So any change in lipid
concentration in the blood
will be very small
relative to how much fat
or adipose tissue we
have stored in our body.
