Vitamin D- What Is It? What Are Its Main Functions?
Vitamin D is not really a vitamin at all but a hormone made from cholesterol. That’s right, the same stuff you have been told for years that you need to lower!
Vitamin D1 is a blend of D2 and lumisterol and is not worth taking or thinking about.
Vitamin D2 (calciferol, ergocalcifero, oleovitaminD2) is made by plants but not very active as it needs UV light to become the active form vitamin D3.
Vitamin D3 (cholecalciferol, oleovitaminD3, 7-dehydrocholesterol). This is the form our bodies make when sunlight hits our skin, and the form that gets converted to active 1,25-dihydroxyD3 (calcitriol).
Once Vitamin D3 (from sunlight on your skin, or a D3 supplement) gets into your blood it is transported to the liver by D-binding protein. In the liver, a hydroxy ( -OH ) group is added at carbon number 25 to make 25-hydroxyvitamin D3 (calcidiol), the storage form of vitamin D and the major type that is circulating in the blood and thus typically measured as an indicator of your vitamin D status.
In kidney, bone, and the placenta, the 25-hydroxyvitamin D3 is activated to 1,25-hydroxyD3 by the addition of another -OH group. This is the most potent form and active form of Vitamin D. Together with the parathyroid hormone, it controls calcium metabolism, and a host of other important functions. In the intestines and to some extent in kidney, bone, and placenta, some 24,25-OHD3 is made that influences the uptake of calcium.
Vitamin D has its main traditional role in the balance of calcium that must be kept in a narrow range for normal brain function, bone growth, and indeed cell function. If calcium levels are dropping too low the parathyroid gland (sits on top of the thyroid gland in your neck), will secrete more PTH (parathyroid hormone). PTH stimulates 1-hydroxylase enzyme in the kidney to make more active D3 (calcitriol). Increased active D3 does three things to raise blood calcium:
1. It increases intestinal absorption of calcium (this is a D3 dependent step).
2. It increases calcium movement from bone to the blood stream (also requires PTH).
3. It increases the kidney retention (reabsorbing) of calcium so it’s not lost in urine (also require PTH).
Notice that without enough active D3, you will not absorb the calcium from your diet, and all the milk or calcium supplements in the world will not help you! Lack of calcium in its most severe form is known as rickets (bowed weak bones in infants) and osteomalacia (soft painful bones) in adults.
Because Vitamin D Receptors (VDR) have been found in most cells and in our immune cells (T-lymphocytes), adequate Vitamin D seems necessary for good immunity and large doses may actually be beneficial in those with autoimmune disorders.