In animal models, caffeine intake can result in negative calcium balance through increased calcium excretion. Homeostasis in bone is achieved through a balance of bone formation and bone resorption. Accelerated bone resorption is seen in the pathogenesis of osteoporosis and other bone diseases. Research has suggested that caffeine may block A2A and A2B receptors, and its effect on bone mineral density may arise from blockade of other adenosine receptors. Adenosine A2A and A2B knockout mice have diminished bone density20.
Human studies have suggested that caffeine may impair the efficiency of calcium absorption, increase calcium excretion in urine, and decrease vitamin D receptor protein expression and 1,25(OH)2 vitamin D3-stimulated alkaline phosphatase activity of osteoblasts. Together these effects could reduce bone mineral density and in turn increase the risk of fractures and osteoporosis8.