Some diseases reduce vitamin D getting to blood or cells

Some diseases restrict vitamin D by changing gene activation,
   by one or more of the following

• Restrict conversion of light into vitamin D in the skin
• Restrict oral absorption in the gut
• Restrict semi-activation in the liver
• Restrict free semi-activated vitamin D from getting to the kidneys
• Restrict full activation in the kidneys
• Restrict semi and fully activation in the cells
• Restrict activated Vitamin D from entering cell mitochondria (VDR)
• Destroy the vitamin D before it gets to the cells

Some diseases restrict vitamin D without changing genes

• The disease just uses/consumes the vitamin D
• The disease upsets the gut, which reduces bioavailability of oral form
  • unless a gut-friendly form is used

References

• COVID changes 100 vitamin D-related genes in the lungs
• Reasons for low response to vitamin D
• Genetics category has 332 items along with related searches
• The risk of 44 diseases at least double with poor VDR as of Oct 2019