Today in the second part of the series, we're going to be taking
a closer look at what vitamin D actually is, what it does and why it's important.
How are ye doing guys, welcome back to the channel. Last time out in part 1 of this series
on this huge topic of vitamin D we discussed why it is the only hormone we have to supplement,
how we make vitamin D ourselves, how we manage to make just the right amount and will never
overdose if getting it from the Sun and some of the potential pitfalls with supplementation and
a little bit on the minutiae of the downsides of getting this nutrient orally instead, all of
which we will get into in more detail in a later video along with some practical recommendations.
But to inform and justify those recommendations, I want to turn our attention now to what vitamin D
actually does for us and why it's important to get enough and not neglect this vital micronutrient.
If you haven't seen part 1 yet there'll be a link to it in all the usual places. And make sure you
subscribe to the channel so you don't miss the rest of this series! The effects of vitamin D can
be divided into two groups - the classical effects and non-classical or extra-skeletal effects.
The classical effects all relate to what everyone thinks of when we think of vitamin D - calcium and
bone health. But how does vitamin D allow us to acquire and maintain healthy bones?
To cut a millions of years long story short, when the earliest animals left the water
their ample supply of calcium in the aquatic world was greatly reduced and became much more labile
on land. But animals need to keep blood calcium levels in a very tight range because a precise
level is required for normal nerve conduction, muscle contraction and clotting, amongst other
physiological functions. So how did animals manage to keep their calcium levels up during periods of
time when there was a paucity of calcium in the diet and they were no longer surrounded by an
abundant supply in the water? The answer is that they evolved to be able to easily absorb it from
their own skeletons if required, and to do that a complex calcium and phosphate homeostasis emerged
over time involving several interdependent hormones: parathyroid hormone or PTH, FGF-23
and calcitonin in addition to the hormone vitamin D. When our blood calcium level gets too low, the
parathyroid glands in the neck sense this abnormal state and release parathyroid hormone, or PTH,
in response. The main function of this hormone is to stimulate resorption of calcium from the
skeleton and inhibit bone deposition, along with stimulating maximal re-absorption of calcium in
the kidneys, with the overall effect of bringing our blood level of calcium back up to normal.
But if this were to happen continuously, our bones would be resorbed more and more over time
and would become very weak and brittle. This is where vitamin D comes into play - parathyroid
hormone also acts on the kidney to increase the rate at which it performs the final chemical
modification step on vitamin D, converting it to it's active form, which can then act on the gut
to increase absorption of calcium from the diet. Likewise, when calcium gets too high,
parathyroid hormone is suppressed and calcitonin is released from the thyroid gland instead,
which overall stimulates bone deposition, promotes loss of calcium via the kidneys and
inhibits conversion of vitamin D to it's active form, preventing us from absorbing calcium from the
diet. This way we can have the best of both worlds - maximal absorption of this potentially scarce
mineral from the diet and our own bones, but also the ability to quickly down-regulate our blood
calcium in times of abundance and lay that calcium down in the bone for the next rainy day,
and never get out of that sweet spot for normal nerve and muscle function. Pretty awesome! But
of course, the correct functioning of this whole system is predicated on the body being able to
release each of these hormones in the correct quantities. The kidney cannot perform the final
chemical modification on vitamin D to convert it to it's active form if there is not enough
circulating vitamin D to begin with. Active vitamin D exerts it's effects by acting on the
vitamin D receptor, or VDR, which is present in these tissues we have just discussed - the bones,
kidneys, intestine and so on, and this is how it participates in calcium and phosphate homeostasis.
But it turns out that the vitamin D receptor and other enzymes associated with vitamin D function
are present in many other tissues throughout the body such as the cardiovascular system,
immune cells, pancreas, muscle, lung, central nervous system, reproductive system and indeed
most nucleated cells of the body. Since these tissues are not a part of our calcium regulation
system, it would be reasonable to assume that vitamin D plays a big role in the body beyond
calcium homeostasis - and indeed, this seems to be the case, as is being more and more appreciated in
recent years. These so-called "non-classical" or "extra-skeletal" or "non-calcaemic" effects of
vitamin D seem to affect every organ system in the body, with better outcomes observed in cancer, diabetes,
cardiovascular disease, aberrant immune function, metabolic syndrome, muscle weakness and many other
specific health problems all associated with a better vitamin D status, and these observations
all have corresponding mechanisms identified in mostly animal and in-vitro pre-clinical studies
to lend biological plausibility. These effects are realised in the form of reduced all-cause
mortality seen in human interventional studies, though notably not all studies have identified
this effect as we will explore in a later video. Vitamin D has been found to affect anything up to
thousands of gene variants, with many of these having nothing to do with calcium homeostasis,
but instead playing a role in things like cancer suppression and immunity, which makes
sense given the better outcomes in these aspects of health observed in people with a better vitamin
D status. It is thought that these non-skeletal effects may have persisted throughout evolution
due to the functions the vitamin D receptor had before we left the water and developed our calcium
homeostasis - so these other functions of the protein may be even older and in fact the original
purpose of it, and this may be why we see so many of these non-skeletal health benefits associated
with good vitamin D status. Rickets is the manifestation of severe vitamin D deficiency in
children that results in the bowing of their legs as they grow, amongst other nasty effects. Severe
vitamin D deficiency manifests as the disease osteomalacia in adults whose fully-grown bones
do not deform like we see in growing children but instead become painful and very susceptible to
fractures due to inadequate bone mineralisation. Thankfully overt vitamin D deficiency leading to
rickets and osteomalacia is now rare thanks to public health measures like food fortification,
though it may once again be on the rise owing to further social changes and depending on certain
demographic factors as I will get into in more detail in a later video. But the tricky part is
that it is much easier to attain the levels required to prevent these most severe bone
diseases than the higher levels that have been associated with the extra-skeletal health benefits
we have discussed, like reducing the risk of cancer and infectious diseases. Once we do attain
these higher levels, the benefits observed would appear to be highly impressive for such a simple,
inexpensive intervention - so why is there not more talk about vitamin D supplementation, massive
public health efforts to promote awareness and get the message out there about how important it is to
have enough? And furthermore, why does there seem to be such a huge disparity in recommended level
and dose between various institutions, all of them highly-respected? How can these different groups
look at the same body of evidence and come up with what amounts to completely different
recommendations for intake? Stay tuned for the rest of this series on vitamin D during which I
will be exploring the complex answers to these questions, talking about what I recommend and
what I take myself to maintain a healthy level, including real-life results from blood tests.
In summary, the effects of vitamin D can be divided into two groups - the classical effects
on forming strong bones, and the non-classical effects like promoting good immune function and
preventing cancer. The non-classical effects have been increasingly studied in recent years, and may
exist because of how old vitamin D functioning is on an evolutionary scale. Rickets and osteomalacia
are the manifestations of vitamin D deficiency as it pertains to the classical effects, but we may
need much more vitamin D to get the benefits on the non-classical effects. Stay tuned for part 3
when I will be explaining why defining the amount we need to get the full benefits from vitamin D
is no easy task, and how that will inform my own recommendations. Catch ye then, guys! Sláinte.