Vitamin D refers to a group of fat-soluble secosteroids responsible for enhancing intestinal absorption of calcium, iron, magnesium, phosphate and zinc. The most important compounds in Vitamin D group is vitamin D3 (popularly know as cholecalciferol) and vitamin D2 (ergocalciferol).
Physiological Function of Vitamin D
There are two primary forms of vitamin D, vitamin D3 and vitamin D2. Vitamin D3 or cholecalcilerol, is formed in the skin upon UV exposure. The other, vitamin D2, or ergocalciferol, comes from foods or supplements. The cholecalciferol form of vitamin D comes from the conversion of 7-dehy-droxycholesterol to previtamin D3 via UV radiation from the sun. Previtamin D3 is then converted to vitamin D3. Then this Vitamin D binds with carrier proteins (such as vitamin D binding protein or DBP), vitamin D3 is then transported to the liver where it is hydroxylated to 25-hydroxyvitamin D3. The kidneys are stimulated by parathyroid hormone (PTH) to produce 1 ,25-dihydroxyvitamin D3 from circulating 25-hyroxyvitamin D levels. This conversion can also take place in other cells and tissues (prostate. colon, placenta, bone, macrophages. T-lymphocytes) via cytokine signals.
Vitamin D3 modulates its effects based on blood calcium levels or physiologic needs. Vitamin D is responsible for Increasing calcium and phosphorus in the blood via internal absorption, bone resorption and renal tubular absorption in the kidney.
Deficiency of Vitamin
There is growing concern over the occurrence of poor vitamin D status. Some prominent vitamin D researchers have identified vitamin D deficiencies as a pandemic. Vitamin D deficiencies are more common in those with limited sun exposure, a low intake of vitamin D, impaired absorption owing to kidney or digestive diseases, or older adults who have a decreased ability to synthesize this vitamin from the sun) UV exposure is the primary source of vitamin D for most people. Many populations are showing alarming rates of deficiencies. Most important Vitamin D forms, Cholecalciferol and ergocalciferol can be ingested from the diet and from supplements. The real challenge is that very few foods contain vitamin D. Therefore the most important form of Vitamin D is through supplementation.
Health concerns due to Vitamin D deficiency
Deficiency of Vitamin D results into the following Health concerns:
- Strong influence on the development of cancer
- Autoimmune diseases, such as insulin-dependent diabetes and multiple sclerosis’4’
Assessment of Vitamin D Status
The most frequently used assay for vitamin D assessment of status is serum 25-hydroxyvitamin D, a combination of both D3 (cholecalciferol) and D2 (ergocalciferol). 25-hydroxyvitamin D is the primary circulating form of vitamin D and the most reliable indicator of its status. It is the most abundant metabolite of the vitamin D and is the direct precursor of its active form.
Insufficient: <75nmol/L or <30ng/mL
Sufficient: 75-250nmol/L or 30-100ng/mL
Toxic: >250nmol/L or > 100ng/mL
Supplementation of Vitamin D
Adult Repletion: 700 – 10,000 IU/Day
Best or most Bio-Available or Active form of Vitamin D
Dietary Sources of Vitamin D
Dietary sources of Vitamin D are:
- Flesh of fatty fish (such as salmon, tuna, and mackerel)
- Fish liver oils
- Beef liver
- Egg yolks