In the first video of what I hope to be a comprehensive series, today we're going to
be taking our first look at the enigmatic nutrient vitamin D, with a focus on how we don't overdose
on vitamin D from the Sun, some interesting similarities with vitamin C, why deficiency
is by no means limited to vegans and why the heck we need to supplement a hormone, anyway!
How are ye doing guys, and welcome back to the channel. I've wanted to cover
this mammoth topic for a long time and especially now since we're heading into
Winter here in the Northern hemisphere with our vitamin D levels dropping,
and it has been probably my most-requested video topic. I've been working away at it in my free
time for the last few months and I am delighted to finally be able to get to the bottom of this
complicated and controversial topic and share my findings and recommendations with you guys.
Today we're going to begin this series by taking a first look at what vitamin D is and how that
makes it technically a hormone even though unlike all other hormones it is one that most of us,
vegan or not have to take in through diet and supplements. You are probably well aware
as well of the attention vitamin D has received recently in relation to COVID-19, so I will also
be discussing that later on in the series as well. Vitamins are organic micronutrients that cannot be
synthesised by the body, but that must be taken in in the diet for the body to function normally.
I have covered some other important vitamins and minerals for vegans previously in my Nutrients of
Concern series, which I will be adding to in the future beyond vitamin D, so check that one out and
subscribe to the channel if you haven't already - I'll link to the playlist in the usual places.
But vitamin D is not just of concern for vegans. They do appear to have lower intakes of the
nutrient on average, but as I will propose by the end of the series I think this actually gives them
an advantage when it comes to optimising vitamin D function in our modern World. But everybody in
the EPIC-Oxford study missed the minimum target by quite a margin anyway, regardless of diet - so
this is far more than just a vegan problem. Vegans were the best at supplementing vitamin D,
but obviously not enough to get into the minimum adequate intake range. Vitamin D is required for
normal functioning, which makes it like a vitamin, but as we shall see it can actually be synthesised
by the body and all our needs can be met without taking in a single molecule of the nutrient
through our diet - which makes it a hormone and not actually a vitamin. We do not need to
take in any from our diet to maintain completely normal vitamin D function. Indeed, there are very
few foods which naturally contain vitamin D - a very short list populated mostly with fatty fish,
especially their livers, and mushrooms. The other foods we commonly associate with vitamin D
and from which we get most of our dietary supply like dairy products and breakfast cereals
do not contain any appreciable amount of vitamin D naturally, but are instead fortified with it. The
problem with our own production process is that we require exposure to ultraviolet radiation to allow
one of the steps in the formation of vitamin D to proceed - ultraviolet radiation that we received
huge doses of during most of our evolution in close geographical proximity to the equator and
without any clothes, but conditions that obviously very few humans fulfill today. All vitamin D
actually is is a signalling molecule, in this case a cholesterol precursor found in the skin
that becomes chemically altered by sunlight and heat and that travels through the blood to affect
tissues far away from where it was produced. Of course the one place where plenty of sunlight has
access to is our skin, so we can expect plenty of the cholesterol precursor to be converted to
vitamin D - or at least we could until relatively recently in our development as a species.
In fact vitamin C and vitamin D share similar stories as they pertain to the
evolution of humans and animals. Unlike humans, most animals can synthesise their own vitamin C.
This process is quite resource-demanding, and any mutations in an organism that save precious
energy give it a selective survival advantage - though normally one that results in the inability
to produce a nutrient essential for life of course will not be passed on to the next generation since
the organism will not survive. But we humans, some other primates, guinea pigs and Indian fruit bats
did lose this ability during our evolution. It is thought that it was possible for us to
survive this because of the plant-rich diets our ancestors ate which provided plentiful vitamin C,
among some other very interesting hypotheses, and aided by a unique ability to recycle vitamin C
through red blood cells found only in us and these other animals, and this meant that we did not need
to maintain the metabolically costly ability to synthesise it ourselves, and that ability
was lost forever from our genome as these more energy-efficient ancestors quickly took over. Then
as we moved away from our traditional plant-rich diet, as was classically observed in migrations,
the lack of vitamin C intake on which we are now dependent, combined with the inability to
synthesise it ourselves resulted in the deficiency disease scurvy. Like with humans and vitamin C,
some animals, notably cats and dogs, have largely lost their ability to make vitamin D since they
could source their needs sufficiently through their meat-rich diets, unlike most herbivores.
But in contrast to our furry friends, our mostly hairless skin was exposed to so much ultraviolet
radiation during the vast majority of our evolution that we became dependent on this
large dose of sunlight. As we migrated away from our tropical home and started to wear clothes,
suddenly we weren't synthesising enough vitamin D, which resulted in an explosion of the vitamin
D deficiency diseases rickets and osteomalacia in the industrialised world despite a partial
adaptation to these lower-radiance conditions in the form of lighter skin. And it is not just we
humans that have had to overcome problems with vitamin D after being removed from our natural
environment. Llamas and alpacas are very sensitive to vitamin D deficiency since like us they evolved
in a high-radiance environment. Horses have evolved a different calcium homeostasis altogether
which means they do not require as much vitamin D from their hair-covered skin.
And some animals may even absorb some vitamin D that is produced on their fur
orally during grooming, bypassing the need for it to be absorbed through their covered skin,
although this hypothesis has been contested for cows following the result of mad experiments like
this involving covering up different parts of their bodies. Anyway, I digress, I think we'll
stick with human biology for the rest of this discussion! As a result of our migration away
from our tropical home, spending most of our time indoors and wearing clothes, vitamin D deficiency
and insufficiency is now widespread depending on how you define it as we will discuss in a later
video. But do not confuse this with dietary deficiency. There is no biological need to
consume any foods that naturally contain vitamin D if we are actually exposed to enough sun. Like
all hormones, under optimal conditions the body will produce exactly the amount of vitamin D it
needs and no more for normal physiology - but unfortunately our conditions are just no longer
optimal. And like all hormones, if exogenously supplementing vitamin D it is possible to under-
or overdo it, since we are bypassing some of the body's own autoregulation mechanisms.
This scenario of replacing under-produced hormones and getting the balance right when we have to
bypass the body's own homeostasis is actually very common in clinical practice. Some broadly
simplified examples - too much thyroxine causes thyrotoxicosis, too little insulin results in
diabetes mellitus, too much glucocorticoid causes Cushing's syndrome and so on. Just like too little
vitamin D results in the deficiency diseases rickets and osteomalacia - and there is something
to be said for viewing it the same way as these other hormones. But how does the body autoregulate
it's own natural production of vitamin D? How does it have the ability to stop producing it once we
have the right amount, despite ongoing exposure to the sun? During the normal physiological
production of vitamin D in the skin, before we are initially exposed to light, say before
getting up in the morning, we start off with a vitamin D skin concentration of zero obviously,
since we have not yet been exposed to the light. As soon as the light hits us, a cholesterol
precursor present in the skin starts to break down into previtamin D3 as it absorbs UV radiation,
which then goes on to form vitamin D3 by the heat from our body. But previtamin D3 and vitamin D3
also absorb UV light and break down themselves. So as their concentration increases, more and more of
the UV is absorbed by these molecules, thus more and more is broken down into inactive metabolites,
while less is absorbed by the cholesterol precursor. Thus we eventually reach a state of
equilibrium where further exposure to UV radiation will not result in any more vitamin D production,
with any further cholesterol precursor broken down ending up becoming one of these inactive
metabolites, making it almost impossible to overdose on vitamin D from the sun.
What's more, vitamin D produced in the skin is all released into the blood bound to it's specific
binding protein, from which it is released slowly to the tissue that is to perform the penultimate
step in activating the hormone - meaning that we get a more efficient and sustained supply
of vitamin D when it is produced on exposure to sunlight, as opposed to the non-physiological
spike in activity in the body when it is taken orally and is metabolised all at once following
release from a non-specific carrier. In addition to the control of production of vitamin D in the
skin, we have evolved many other mechanisms to regulate the production and activity of this
hormone. Remember darker skin produces less vitamin D for the same amount of UV exposure,
so our skin gets darker the more we are exposed to the sun, preventing over-production. There is
also built-in negative feedback in the vitamin D signalling cascade, tolerance to stimulation
of the target organ and control over the rate at which the two enzymes that convert the hormone
to it's active form work. So there are multiple in-built steps all the way along from synthesis
to end organ effects that overall result in a very tight control over not only the level of vitamin D
transported in the blood but also the effects of the active hormone - important when one is
bathed in the sun all day long! Unfortunately, we bypass some of these control mechanisms when
we supplement vitamin D instead of obtaining it the natural way, which means it is possible to
overdose from supplements and we require a bit of care and attention when attempting to mimic these
natural processes and obtain a healthy vitamin D status with supplements and diet - something
hopefully you will have a better idea of how to do by the end of this series! In summary,
vitamin D is a hormone that has become a vitamin because of the circumstances of our modern World.
Our body can make the exact amount it needs and no more if it is producing it naturally
following sun exposure. But since this natural process is no longer available to most people
we must use a certain amount of finesse to replicate our body's own natural production,
just like we would if replacing any other hormone. Stay tuned for the rest of this series in which I
will be outlining my recommendations on how to do just that! Talk to ye soon guys, sláinte!