Calcium and Vitamin D Supplementation

Calcium is needed for neuromuscular transmission, skeletal mineralization, cardiac conduction, vascular tone, and blood coagulation. Serum calcium must remain within a narrow range because even modest changes can alter muscle, nerve, and cardiac function. Vitamin D is a prohormone rather than the final active hormone. Vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol) are converted in the liver to 25-hydroxyvitamin D, written as 25(OH)D. This is the storage form measured in blood testing. The kidneys then convert 25(OH)D to calcitriol, the active form of vitamin D. Calcitriol increases intestinal absorption of calcium and phosphate. When serum calcium falls, parathyroid hormone (PTH) works with calcitriol to preserve serum calcium, partly by increasing renal calcium conservation and mobilizing calcium from bone. This is why vitamin D deficiency, kidney disease, hypoparathyroidism, and inadequate calcium intake can all disturb calcium balance through different mechanisms. A supplement is not automatically needed because a nutrient supports bone health. The clinical question is whether intake is inadequate, absorption is impaired, or a specific condition requires prescribed calcium or active vitamin D therapy.