Vitamin D’s influence on the body is complex and multifaceted. It functions more like a hormone than a typical vitamin. It increases calcium absorption in the intestines, reduces calcium excretion in the kidneys, and promotes calcium release from bones when needed. This tightly controlled calcium balance is essential for bone health, muscle function, and nerve transmission. Beyond calcium regulation, calcitriol interacts with vitamin D receptors (VDRs) found in various tissues throughout the body, including immune cells, muscle cells, and cells in other organs. When calcitriol binds to VDRs, it influences gene expression, affecting a wide range of cellular processes. This explains why vitamin D plays a role in immune function, cell growth and differentiation, and potentially even mood regulation. The precise mechanisms by which vitamin D exerts its non-calcium-related effects are still being actively researched, but its influence on gene expression through VDRs is a key area of study.