Isn't VitaminD used by immunity? Immunity or something else could be using up VitaminD in POIS or there's issue forming it so VitaminD deficiency causes/is related to POIS, not because of no sun/supplementation but as something prevents it from generating optimally.
It is low in auto-immune diseases, inflammatory bowel disease, MCAD, malabsorption and can lead to osteoporosis. Low Vit D is probably the most frequent abnormality I've encountered so far on this forum and yet only one POIS paper has stated that the role of vit D could be investigated.
"It is also important to establish any influence on males with POIS, of the 25-OH D vitamin deficiency, if we consider the epidemic character and the implications of this vitamin(7)."
RefThe enzymes that play a role in vit D metabolism are Cytochrome P450 enzymes (indicated with CYP):
https://www.wikipathways.org/index.php/Pathway:WP1531I haven't put time into this yet but noticed this today. The potential processes behind abnormalities seen in POISers, at least in this thread, might be driven by Cytochrome P450 enzymes (or their genes),
https://en.wikipedia.org/wiki/Cytochrome_P450:
"Cytochrome P450 enzymes are present in most tissues of the body, and play important roles in hormone synthesis and breakdown (including estrogen and testosterone synthesis and metabolism), cholesterol synthesis, and vitamin D metabolism."
Also these enzymes seem to need iron, low iron is also seen in POISers:
https://en.wikipedia.org/wiki/Cytochrome_P450#StructureThey also play a role in AA metabolism and could lead to inflammation by the molecules depicted in red, some POISers may actually suffer from an active inflammatory pathway via P450 instead of COX:
This family of enzymes can be affected by many things including diet. These enzymes are wide spread and I wonder if they can induce multi-system symptoms aside from conditions like MCAS.