In people with type 2 diabetes and kidney disease, taking 4000 IU of vitamin D daily for three months raises blood levels of vitamin D from 12.1 ng/mL to 34.5 ng/mL.
Mechanism
Synthesis from 1 study
Taking high-dose vitamin D pills raises blood levels of vitamin D even when kidneys are damaged because the liver can still process the vitamin without kidney help. The high dose overcomes a protein that normally traps vitamin D, allowing enough to circulate freely.
Most probable mechanism
When a person takes vitamin D pills, the body converts it into a form called 25(OH)D in the liver. Even when the kidneys are damaged, enough of this form still builds up in the blood because the liver can make it without help from the kidneys. Some people have too much of a protein that traps vitamin D, making it harder to absorb, but high doses of vitamin D overwhelm this trapping effect and still raise blood levels.
Oral cholecalciferol is absorbed from the intestine into the bloodstream.
Cholecalciferol is transported in the blood bound to vitamin D-binding protein, which sequesters a large fraction of the molecule and reduces its free concentration.
The liver hydroxylates cholecalciferol to form 25-hydroxyvitamin D (25(OH)D), a process independent of kidney function.
High-dose cholecalciferol supplementation saturates vitamin D-binding protein, increasing the proportion of unbound 25(OH)D available in circulation.
Elevated serum 25(OH)D levels accumulate despite impaired renal function because hepatic conversion remains intact and renal 1α-hydroxylase activity is not required for 25(OH)D production.
Evidence from Studies
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