Vitamin D is the other fat-soluble
vitamin I want to talk about here.
Now, vitamin D is important in a
variety of functions in the body.
One of the things that we know
about for it best is it regulates
the intestinal absorption of calcium,
iron, magnesium, phosphate, and zinc.
We’re going to focus
on calcium here.
Vitamin D is not
exactly a vitamin.
Technically, vitamins are
something that we can’t make,
but we actually make most of
the vitamin D that we need.
Vitamin D therefore acts
more like a hormone
then it does like a vitamin, but
nonetheless the name persists.
Vitamin D is ultimately derived from
cholesterol or by supplements that we take.
And people who don’t get
enough sunlight for example
or live in areas where
there’s limited sunlight
may need supplements
to get the full amount
of vitamin D that they
need for their bodies.
Cholecalciferol is a form
of vitamin D known as D3,
that’s created by
exposure to sunlight.
And that form of vitamin D3
is also the form that we take
when we take oral forms of
vitamin D as supplements.
The formation of cholecalciferol or vitamin
D can happen as a result of reaction
that you see in the
slide on the right.
In this reaction, 7-dehydrocholesterol
is converted to cholecalciferol
simply by the
presence of UV light.
So if you get enough UV light and
you have a light enough skin,
getting sufficient vitamin
D isn’t a problem.
However, if you don’t get
enough exposure to UV light
or you’re in an area where you have
dark skin and the light levels are low,
you may be deficient
in the vitamin D.
Now, cholecalciferol or vitamin D3
is not the active form of vitamin D.
It can be converted into the active
form and that is done within the body.
You can’t take active vitamin D because
if the body gets active vitamin D,
it will simply convert
it into something else
because it’s very important to regulate
the proper amount of vitamin D.
The body doesn’t want to
have too much vitamin D,
but too little vitamin
D is also a problem.
If you have too
little vitamin D,
you can develop a very serious
condition known as rickets.
So it’s important to monitor
your body’s level of vitamin D.
The next time you talk to your doctor,
that’s something that you
should have checked.
Well, vitamin D, as I said is
very important for calcium.
The body levels of calcium are very
delicately balanced in a variety of ways.
Vitamin D is involved in the dietary
absorption of calcium as you can see here.
There’s other factors in the
calcium levels in our body
that we have to take
Calcium, of course, is a
component of our bones.
Calcium is carried within our blood.
And calcium is also used within the cells.
Calcium is a very important ion
for the process known as
signalling as we shall see.
In this slide, I want to show the
chemical reactions that are involved
in converting vitamin D3, cholecalciferol,
into the active form of vitamin D.
This occurs in the liver.
The reactions necessary to make
this happen are hydroxylations.
There’s two different hydroxylations
that happens to cholecalciferol.
The first hydroxylation creates
a compound called calcifediol.
And calcifediol has a hydroxyl group placed
way out there in that
red box as you can see.
Calcifediol is also hydroxylated
to form calcitriol.
And calcitriol puts an extra hydroxyl
in the red box that you can see here.
This calcitriol that is the
active from of vitamin D.
And as I said, this is very carefully
regulated inside the body.
And we’ll see how that regulation
happens in just a minute.
There are also chemically-modified
forms of vitamin D
that are sometimes used in
supplementation that are also
converted into very active forms
of vitamin D as we can see.
The first of these is
known as ergocalciferol
or vitamin D2 as it
is sometimes called.
This is not a natural vitamin.
It’s one that is actually manmade
but the body recognizes and treats it as
if it were in fact a natural vitamin.
We can see here that the only
difference in structure between
ergocalciferol and cholecalciferol
are a double bond
in the ergocalciferol that you
can see in the green box.
hydroxylated in the liver
just like cholecalciferol did to
create 25-hydroxy ergocalciferol.
A second hydroxylation occurs
on top of that molecule
to give 1,25-ergocalciferol
as you can see here.
And again, a structure very, very similar
to the active natural form of vitamin D.
Both 1,25-ergocalciferol and calcitriol act
completely in the same way in the body.