Vitamin D is a pre hormone obtained through diet (egg, fish oil) or via skin synthesis (sun exposure). Vitamin D is synthesized into its active form by kidney. When kidney is not functioning properly, its ability to convert vitamin D into its active form declines resulting in internal imbalance leading to progression of CKD.. Vitamin D plays a important role in renal, cardiovascular and immune system in the body apart from maintaining calcium and phosphorous homeostatis. Role of vitamin D also encompasses regulation of the renin-angiotensin system (RAAS).
Vitamin D is actually a hormone and not a vitamin. The body makes most of the vitamin D it needs; only about 10% comes from our food. The action of sunlight on our skin produces a substance called cholecalciferol, which is converted by the liver to calcidiol. This is further converted in the kidneys by the enzyme 1α-hydroxylase to calcitriol, the active form of vitamin D. It is advised that kidney patients should get adequate sun exposure in the early morning and evening (avoid noon times and hot summer) to replenish vitamin D requirement. Vitamin D is also available in eggs, fish oil, etc. In case of very low vitamin D, doctor may suggest oral doses of cholcalciferol tablets or powder sachet (to be taken with milk) or injections depending upon patient’s requirement. Doses should be strictly adhered to as some doses may be weekly or monthly.
The active form of vitamin D is produced primarily by the kidneys, but there are also a number of other tissues in the body that activate vitamin D. Excess cholecalciferol and calcidiol made during the summer are stored in our fat for use during the winter. Vitamin D modifies the activity of bone cells and is important for the formation of new bone in children and adults. It also regulates calcium levels in the blood by helping the body to absorb calcium from food and by preventing calcium loss from the kidneys. A fall in the concentration of calcium in the bloodstream is detected by the parathyroid glands, which then produce parathyroid hormone. Parathyroid hormone increases the activity of the enzyme 1α-hydroxylase which produces active vitamin D. This increase in the concentration of calcium together with vitamin D feeds back to the parathyroid glands to stop further parathyroid hormone release. The production of vitamin D is also directly regulated by calcium, phosphate and calcitriol.