Here are some studies about vitamin D cofactors:
Vitamins D and K as Pleiotropic Nutrients: Clinical Importance to the Skeletal and Cardiovascular Systems and Preliminary Evidence for Synergy (2016)
Note: Overview of the research demonstrating vitamin D and K’s synergistic relationship.
Up-Regulatory Impact of Boron on Vitamin D Function – Does it Reflect Inhibition of 24-Hydroxylase? (2004)
Conclusion: There is evidence that the mineral boron improves vitamin D levels by slowing down catabolization of vitamin D, improving it’s metabolism, and making it last longer in your body. Multiple studies have found that increasing boron intake increases vitamin D levels.
Conclusion: This study is of 30 men and women suffering from vitamin D-deficiency diseases (osteoporosis, osteomalacia, and hyperparathyroidism) and two normal healthy women who were given various types and sizes of vitamin D supplements. The researchers found that following supplementation the subjects’ calcium absorption greatly increased and their magnesium absorption increased slightly, but significantly. Several other studies referenced also have found that vitamin D supplementation slightly improves magnesium absorption.