I vaguely understand that we possess (molecular?) receptors for various chemicals. I have been reading a lot lately about Vitamin D. Recent research suggests that current recommendations regarding Vitamin D are woefully inadequate. I find many of the arguments for higher intake compelling inasmuch as I understand them. However, one line of reasoning isn’t working for me.
Supposedly nearly every cell/every organ contains receptors for Vitamin D, and this is presented as one of the arguments for increasing our Vitamin D intake. That sounds simple enough - if we have receptors for it, our body must be evolved/designed to use it, right?
But wait, don’t we also have receptors for rhinovirus and THC?
Could someone explain to this layperson if/how the presence of a Vitamin D receptor is positive, neutral, or negative with regard to whether or not people need more Vitamin D?
First of all, we do not have receptors for rhinovirus and THC. There are many many types of receptors on brain cells that are there to respond to various chemicals, called neurotransmitters, that are produced naturally in the brain and are necessary to its proper operation. Some of those receptors will also respond to THC (probably because its chemical structure is similar in some way to one of the natural neurotransmitters) but not in quite the same way that they would respond to the substance that they are supposed to respond to. Thus your brain does not work quite the way it is meant to, and you get high. Most psychoactive drugs work in this sort of way.
If rhinovirus depends on receptors on your cells at all (I rather doubt it, but I am not sure) then something similar is happening. That is, the rhinovirus is making use of a receptor that is there to respond to some molecule that is naturally found in the body in order to fool the cell into letting the virus into it, like a gatecrasher sneaking into a party through a door left open for the convenience of invited guests.
Secondly, the fact that cells have receptors for Vitamin D really does not prove that we need more Vitamin D in our diets (although there does seem to be other evidence that many people do need more Vitamin D). We have receptors for all sorts of chemicals that we do not need in our diet because our body can make them for itself, and anyway, many of those chemicals are only needed in minute quantities. (Vitamins, roughly speaking, just are chemicals the body needs, in fairly small quantities, but cannot make for itself.) However, if there are Vitamin D receptors on every cell in the body, that does suggest (not prove) that we might need more of it than was previously thought. It used to be thought that Vitamin D was mainly important for the processes that build and maintain strong bones. However, if receptors for it are found on all cells, including those that have nothing to do with bone growth, that suggests that it is probably doing other important things too (probably regulating calcium metabolism in all our cells).