New paper below [1].
Vitamin D circulates in the blood as inactive 25-D. The kidneys will convert 25-D to the active hormone 1,25-D, as needed, to maintain circulating calcium levels.
Elsewhere, cells that need 1,25-D first produce the enzyme that performs the conversion of 25-D.
It is normal for hormones created in a cell to be given a short window in which to operate. As soon as 1,25-D is created, the enzyme CYP24A1 will be produced to destroy it [2].
The situation is altered in PCa. The enzyme to create 1,25-D is downregulated and the enzyme to destroy it is upregulated. Cells produce less 1,25-D and clear it faster.
The Japanese authors:
"... propose the potential feasibility of vitamin D metabolism-blocking therapy in various types of human malignancies that express constitutive CYP24A1."
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I have the impression that many men with PCa are aware of the importance of 25-D, but ignore 1,25-D. We can do little to make PCa cells produce more 1,25-D, or to keep it around longer. But we can do something to improve access to 1,25-D in the blood.
Briefly, high intake of calcium will reduce access; fructose will increase access. In my case, I do not use calcium supplements & I sweeten my coffee with fructose.
-Patrick