Vitamin D (25-hydroxyvitamin D) is a fat-soluble prohormone produced from sun exposure or dietary intake. It regulates calcium absorption, bone mineralisation, immune function, and gene expression throughout the body.

Deficiency is extremely common in Australia despite abundant sunshine, due to sun avoidance, indoor work, and sunscreen use. Deficiency causes rickets in children, osteomalacia in adults, bone pain, muscle weakness, and secondary hyperparathyroidism with accelerated bone loss.