Why does vitamin D need to be exogenous?

Question

Vitamin D is a either a hormone or a precursor to hormones. It is very unlike any other vitamins, which are either cofactors or antioxidants, or may be other chemicals necessarily performing catalytic function, just like transitional metal.

But in all sources I found so far it says that Vitamin D doesn't really do anything chemistry-bound, but rather just works as a hormone matching some arbitrary protein receptors or something. E.g. it signals something to the body. But what? And if nothing, then why does not evolution not select out Vitamin D in favor of endogenous chemical or nothing at all?

Answer 1

Vitamin D is vitally important for the uptake of calcium, iron, magnesium, phosphate, and zinc, as well as for bone maintenance:

Vitamin D promotes calcium absorption in the gut and maintains adequate serum calcium and phosphate concentrations to enable normal mineralization of bone and to prevent hypocalcemic tetany. It is also needed for bone growth and bone remodeling by osteoblasts and osteoclasts. Without sufficient vitamin D, bones can become thin, brittle, or misshapen. Vitamin D sufficiency prevents rickets in children and osteomalacia in adults. Together with calcium, vitamin D also helps protect older adults from osteoporosis.

It also acts as a hormone, binding to the Vitamin D receptor in the nucleus of target cells which then acts as a transcription factor for genes related to multiple processes:

Vitamin D has other roles in the body, including modulation of cell growth, neuromuscular and immune function, and reduction of inflammation [1,3,4]. Many genes encoding proteins that regulate cell proliferation, differentiation, and apoptosis are modulated in part by vitamin D. Many cells have vitamin D receptors, and some convert 25(OH)D [an inactive form] to 1,25(OH)₂D [an active form].

So, the truth is far from your statement that "Vitamin D doesn't really do anything chemistry-bound, but rather just works as a hormone matching some arbitrary protein receptors or something." In fact, a cursory reading of the first couple of Google results for "Vitamin D" shows that it is broadly involved across the body in many different processes ranging from inflammation and cancer to bone maintenance and mineral absorption.

Sources:

• Vitamin D - Fact Sheet for Health Professionals. NIH Office of Dietary Supplements. November 2014.
• Vitamin D Wikipedia
• Effectiveness and safety of vitamin D in relation to bone health. A Cranney, T Horsley, S O'Donnell, et al. Evid Rep Technol Assess (Full Rep). 2007 Aug;(158):1-235.
• Vitamin D WebMD

Answer 2

I'm going to answer your question as asked.

It doesn't need to be exogenous. In fact, it's often not exogenous at all. The truth is, vitamin D can be (and is) be synthesized de novo in adequate amounts by most mammals exposed to sufficient sunlight.^[1]

Now, obviously, the presence of the correct wavelength of UVB simply allows the reaction to proceed at a reasonable rate. But the first misunderstanding is that calling "cholecalciferol" a vitamin is simply a misnomer. In reality it is not an essential dietary factor. As you allude to, it's better to consider it a hormone.

You next question is "why does not evolution not select out Vitamin D in favor of endogenous chemical or nothing at all?" Again, answering this question:

It doesn't.

Vitamin D is an endogenous chemical!

It has been in many organisms for millions of years! And very useful too. Ergosterol, previtamin D2, and its photoproducts could have acted as an ideal UVB sunscreen since they could absorb UVB radiation and dissipate its energy by the rearrangement of the double bonds.

Of course, the photochemistry of vitamin D is important and the synthesis in the body is very dependent on a variety of factors. Firstly, environment and the incidence of solar radiation with wavelengths of 290–315 nm on the skin. Other factors include the zenith angle; nitrous oxide, ozone sulphur dioxide pollution, altitude, sunscreen, skin pigment, aging, latitude, season, etc.^[2]

It's vital for maintaining the skeletal calcium balance by promoting calcium absorption in the intestines, promoting bone resorption by increasing osteoclast number, maintaining calcium and phosphate levels for bone formation, and allowing proper functioning of parathyroid hormone to maintain serum calcium levels. 1,25-dihydroxyvitamin D influences a large number of biologic pathways. Hence rickets, etc.

One thing to be aware of is that vitamin D either from endogenous synthesis or the diet must be biologically activated through enzymatic hydroxylation in the liver and kidney.