Final answer:
Vitamin D is crucial for calcium absorption, bone health, and muscle function, and its systemic importance is so broad that it's considered by some to be more like a hormone than a vitamin.
Step-by-step explanation:
An important characteristic about vitamin D is its role in the absorption and regulation of calcium (Ca++) and phosphate in the body. This vitamin undergoes activation via a hydroxylation reaction in the kidney, converting calcidiol into calcitriol (1,25-dihydroxycholecalciferol).
Active vitamin D is crucial for the absorption of calcium in the digestive tract, reabsorption in the kidneys, and maintenance of normal serum concentrations of calcium and phosphate. Calcium is essential for bone health, muscle contraction, hormone secretion, and neurotransmitter release.
Vitamin D is so systemic in its effects that some scientists argue that it should be classified as a hormone rather than a vitamin. Additionally, certain vitamins like B7 and K are nonessential as they are synthesized by bacteria in the large intestine, and vitamin D can also be synthesized in the skin upon UV light exposure.