What is the main reason for calcium loss in immobilized individuals?

The primary reason for calcium loss in immobilized individuals is increased osteoclast activity. When a person is immobilized, such as due to illness or injury, the mechanical strain on bones is significantly reduced. This lack of mechanical loading leads to a decrease in signals that normally stimulate bone formation by osteoblasts, the cells responsible for building bone. In response, osteoclasts, which are the cells that break down bone tissue and release calcium into the bloodstream, become more active. This imbalance results in greater bone resorption than formation, contributing to calcium loss and making bones more susceptible to fractures.

In conditions of immobilization, the physiological response focuses on maintaining serum calcium levels, leading to more calcium being released into the bloodstream from the bones. As a result, the overall bone density decreases, further emphasizing the impact of elevated osteoclast activity in this scenario.