Vitamin D Metabolism
There are two forms of vitamin D – vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Vitamin D3 is produced in the skin by the action of ultravioiet B (UVB) light on skin cells. Vitamin D2 is plant derived. Both Vitamin D2 and D3 can be obtained from certain foods in the diet or as supplements.
On average, only about 20% of our daily vitamin D intake is obtained from the diet, due to the limited range of food that contains vitamin D. Thus sunlight is usually our major source of vitamin D.
Once absorbed, vitamin D is converted in the liver to 25-hydroxy vitamin D [25(OH)D]. This is further converted in the kidneys to 1.25-dihydroxy vitamin D. This is the active torm of vitamin D, which is responsible for adjusting calcium absorption in the gut to maintain calcium homeostasis. Adequate levels of calcium in the blood is required for mineralisation of bones, the process by which calcium is deposited onto bones, thus making them strong.
A lack of vitamin D would lead to reduced amounts of calcium being absorbed through the gut, and leading to lower levels in the blood. As a result, production of a hormone called parathyroid hormone (PTH) is stimulated. PTH compensates for the reduced calcium absorption from the gut by mobilising calcium from the bones. Through this process, the amount of calcium in the bones can get less and the bones can get weaker.
Severe vitamin D deficiency leads to a condition called rickets in children, where their bones are weak and are more liable to fracture or deform; in adults, vitamin D deficiency can give rise to a condition called osteomalacia, where the bones are not mineralised properly. leading to aching and weakness.