What Is Vitamin D?
Vitamin D is the principle regulator of calcium homeostasis (balance) in the body. This “vitamin” is really a prohormone, meaning it acts like a hormone but is not. Vitamin D does, however, contain cholesterol in its molecular structure like steroid hormones.
The physiological importance of vitamin D encompasses much more than the regulation of bone metabolism although this is a mighty function.
Q: How does vitamin D regulate bone metabolism?
A: In regulation of bone metabolism, vitamin D works in three ways: 1) enables active absorption of calcium from the small intestine, 2) enhances reabsortion of calcium by the kidneys that would otherwise be excreted in urine, and 3) plays an active role in skeletal development and bone mineralization. Mineralization gives strength to living bone tissue.
Vitamin D interacts with receptors within cells to effect transcriptional changes in many cell types including those in gut, bone, breast, prostate, brain, skeletal muscle, and the immune system.1
In regards to the essential role of vitamin D in muscle tissue, it has been recently shown that vitamin D regulates both muscle function and structure of primary myofibers.2
Vitamin D is converted in the body to a molecule that is biologically active. The active form is 1,25-dihydroxyvitamin D, usually referred to as vitamin D3. About 80% comes from sun exposure and the remaining from food.
Vitamin D3 is synthesized in the skin from 7-dehydrocholesterol via photochemical reactions requiring UV light (sunlight). That is, light that contains energy from the sun is incorporated into molecules of 7-dehydrocholesterol in the underlying dermis of skin to make this vitamin. This is why inadequate exposure to sunlight contributes to vitamin D deficiency.
Blood concentration of 25(OH)D is the best indicator of vitamin D status. It reflects vitamin D produced in the skin and that obtained from food and supplements and has a fairly long circulating half-life of 15 days.3
What Is Vitamin D Deficiency In Celiac Disease and/or Gluten Sensitivity?
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Girgis CM, Mokbel N, Cha KM, Houweling PJ, Abboud M, Fraser DR, Mason RS, Clifton-Bligh RJ, Gunton JE. The vitamin D receptor (VDR) is expressed in skeletal muscle of male mice and modulates 25-hydroxyvitamin D (25OHD) uptake in myofibers. Endocrinology. 2014 Sep;155(9):3227-37 ↩