Blacklab,
I had the same question as Brew... Man flu?
The following slide illustrates the two vitamin D3 metabolism paths and major consumers of 25(OH)D and the final metabolite 1,25(OH)2D3, a.k.a. calcitriol, the hormonal form of vitamin D3.
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Assuming normal liver, kidney and parathyroid functions, roughly 20% of the vitamin D3 metabolized into 25(OH)D by the liver is further metabolized by the kidneys to support calcium homeostasis. This is called the endocrine path.
The rate at which 25(OH)D is metabolized to 1,25(OH)2D3 in the kidneys is tightly controlled by PTH (Parathyroid Hormone) in order to keep total calcium serum concentrations in a very narrow range and to maintain bone mineral density (BMD).
The remaining 80% of the 25(OH)D produced in the liver is consumed at the cellular level in what is called the autocrine/paracrine path. This is the path that many vitamin D3 experts claim results in genetic expression of over 2000 genes in the human genome.
From our standpoint as cluster headache sufferers, it is clearly possible that 1,25(OH)2D3 down regulates/suppresses the production of CGRP (Calcitonin Gene-related Peptide) in brain cells that is elevated during both cluster headache and migraines. This is just a SWAG (sophisticated wild-ass guess) at this point. However, if we can ever get researchers interested and funded to study the causal relationship between cluster headache and a vitamin D3 deficiency... we might just get confirmation.
The active form of vitamin D, 1,25-dihydroxvitamin D3 (1,25(OH)2D3), is a pleiotropic hormone whose actions include the regulation of calcium homeostasis, control of bone cell differentiation and modification of immune responses. Synthesis of 1,25(OH)2D3 from the major circulating metabolite, 25-hydroxyvitamin D3(25(OH)D3), is catalyzed by a mitochondrial cytochrome P450 enzyme, 25-hydroxyvitamin D-1-a hydroxylase (1-a OHase).
A few years ago it was though this enzyme was only found in the kidneys. It was later found in the gut, lungs, and in all the body's tissues making the extrarenal metabolism of 25(OH)D to 1,25(OH)2D3 possible throughout the body.
If you haven't broken the code, researchers name enzymes for the product they produce followed by "ase", i.e., 1-a OHase which adds the (OH) radical at the first position in the 25(OH)D3 molecular chain.
As you could can see in the slide, 1,25(OH)2D3 activates our immune system's T-Cells. Newly formed T-Cells are naive, (they don't know what to do... until activated). However, once they're supplied with sufficient concentrations of 25(OH)D and 1,25(OH)2D3 they start becoming aware of genetic fragments of virus floating around them.
Once sensitized to these viral fragments, the T-Cells start multiplying rapidly into little pack man cells that gobble up that particular virus. Some of the T-Cells just remember the specific virus in case it ever becomes present again. - In a word, immunity... to that virus.
The highest concentrations of 1-a OHase and naive T-cells are found in the gut and lungs... Not all that surprising given the gut and lungs are exposed to more pathogens than any other part of the body.
Accordingly, viral infections like colds and flu trigger our immune system's T-Cells and that consumes available 25(OH)D and 1,25(OH)2D3 at much higher rates leaving little to trigger the genetic expression (down-regulation/suppressed production of CGRP) that prevents our cluster headaches.
There's another consumer not shown on this slide. It's 24-OHase, an enzyme that adds an (OH) radical to the 24th position on the 25(OH)D3 molecule to yield another metabolite of vitamin D3, 24,25,(OH)2D3. This metabolite is inactive with respect to the endocrine and autocrine functions. It's also easily broken down in the kidneys and pumped over the side in urine. This is one of the body's self-regulating mechanisms that prevents 1,25(OH)2D3 concentrations from reaching toxic levels...
In short, we've got a lot of yin-yang functions going on with respect to vitamin D3 metabolism. These functions can and will result in fluctuations in cluster headache frequency, severity and duration until the 25(OH)D serum concentration stabilizes around 85 ng/mL.
Hope this helps.
Take care,
V/R, Batch