Vitamin D is a fat-soluble vitamin essential for humans. It is a secosteroidal prohormone that can be obtained by humans from skin synthesis and dietary intake, of which skin synthesis is regarded as the main approach for humans. Exposed to enough UV B radiation of sun, Vitamin D is synthesized in the skin from cholesterol. Vitamin D is identified as the prevention and treatment of rickets.
Vitamin D is converted into calcifediol (25-hydroxycholecalciferol) in the liver, and then calcifediol carried to the kidneys is converted into calcitriol (1,25-dihydroxycholecalciferol), an active form of vitamin D. Calcitriol is then released into the blood circulation and binds to vitamin D-binding protein that transports calcitriol to target organs. Vitamin D receptor (VDR) is activated by calcitriol, which modulates the calcium absorption in small intestine and kidneys, phosphate loss, immune function, and leads to autophagy and an autophagic transcriptional signature in breast cancer (BC) cells.
Figure 1. Vitamin D metabolism
Vitamin D deficiency results in rickets, osteomalacia, osteoporosis, and is associated with multiple diseases including cancer, Alzheimer's disease, rheumatoid arthritis, diabetes, hypertension, glucose intolerance, and multiple sclerosis. In general, humans get vitamin D via sunlight exposure, yet people in areas lacking sunlight would take vitamin D supplement. Vitamin D mostly refers to vitamin D2 and vitamin D3, and following is the catagories of vitamin D as well as some impurities we provide:
Vitamin D3: Vitamin D3, also known as cholecalciferol, is the most significant of all the vitamin D and their derivatives. It is naturally synthesized in human skin from 7-dehydrocholesterol via UV B radiation exposure. Vitamin D3 itself is inactive, and undergoing two hydroxylations, it is converted into the active form calcitriol with high affinity for vitamin D-binding protein. Vitamin D3 is essential for calcium balance and bone growth that can be used for the treatment of low blood calcium, rickets, osteomalacia, etc.
Vitamin D2: Vitamin D2, also known as ergocalciferol, is similar to vitamin D3 in structure and function. Vitamin D2 is produced by plant such as mushrooms in response to UV radiation. Study indicates that calcifediol and calcitriol metabolized from vitamin D2 is less active to vitamin D-binding protein in comparison with that of vitamin D3.
Calcitriol: Calcitriol is 1,25-dihydroxycholecalciferol metabolized from vitamin D via two hydroxylations respectively in the liver and the kidneys. Calcitriol increases the uptake of calcium from the gut into the blood thus leads to an increase of the calcium level in the blood. Calcitriol is used for the prevention and treatment of low calcium level and bone disease, which is commonly caused by kidney damage or parathyroid gland disorders.
Calcipotriol: Calcipotriol is a derivative of calcitriol with 1% activity of calcitriol. Under the trade name Dovonex, it is used as an ointment approved for the treatment of psoriasis, which is caused by excessive multiplication of human skin outer layer cells. Calcipotriol inhibits the cell proliferation of skin with psoriasis. Calcipotriol is also used in combination with betamethasone, a medication in the class of corticosteroids, to reduce inflammation at the same time.
Maxacalcitol: Maxacalcitol is an analogue of calcitrol used in the treatment of psoriasis and secondary hyperparathyroidism.
Paricalcitol: Paricalcitol is an analogue of calcitriol used as a medication for secondary hyperparathyroidism with the trade name Zemplar. It increases the level of calcium then resulting in a decrease of the level of parathyroid hormone (PTH) in patients with chronic renal failure.