How about that. You need to be iron replete for your vitamin D to work:
The Effect Of Iron Dextran On Vitamin D3 Metabolism In SD Rats, Nutrition and Metabolism, 16 July 2022
Our study found that iron deficiency may affect the metabolic level of vitamin D3. It is suggested that iron may play a role in 25 hydroxylation by regulating CYP2R1 and CYP27A1 enzymes, and 1α-hydroxylation by regulating CYP27B1 and CYP 24A1 enzymes, thus affecting the levels of 25-(OH)D3 and 1,25-(OH)2D3.
Conclusion: Iron may be involved in the metabolism of VD3 by regulating the expression of VD3 hydroxylase, suggesting that appropriate iron supplementation might promote the activation of VD3.