Vitamin D is a prohormone that must undergo two hydroxylations in the liver and kidneys to become its active form, calcitriol. Calcitriol regulates calcium and phosphate absorption in the intestines and their levels in the blood through interactions with parathyroid hormone. Vitamin D deficiency can lead to rickets in children, causing soft and deformed bones, and osteomalacia in adults, making bones brittle. The main sources of vitamin D are skin exposure to sunlight and dietary intake from oily fish and fortified foods.