caffeine caused a significant decrease of PTH secretion and PTH gene expression. This decrease occurred in parallel with a decrease of the intracellular cAMP level, protein kinase A activity, and ADORA1 gene expression

chronic administration of caffeine could lead to negative calcium balance when there is an impaired ability to increase the efficiency of calcium absorption. Such a situation exists in elderly human subjects

Caffeine decreases vitamin D receptor protein expression and 1,25(OH)2D3 stimulated alkaline phosphatase activity in human osteoblast cells

Estimates with interaction of caffeine and duration of treatment

Vitamin D concentration among the groups was 16.2±5.3 mL and 80.3% of them were considered to be vitamin D-deficient. All the groups appeared to have deficiencies compared to males who did not consume caffeine