Final answer:
Calbindin-D requires activated vitamin D, specifically dihydroxyvitamin D or calcitriol, to bind calcium and facilitate its absorption in the small intestine. Vitamin D's activation occurs in the kidneys, and its presence is critical to maintain normal serum calcium levels and ensure proper bone health.
Step-by-step explanation:
Calbindin-D is a type of calcium-binding protein that is pivotal for the absorption of calcium in the small intestine. For Calbindin-D to bind to calcium effectively, it requires the presence of activated vitamin D. This is because vitamin D promotes the synthesis of calbindin and thereby facilitates the active transport of calcium through the intestinal mucosa. Specifically, it is the hydroxylated form of vitamin D, known as dihydroxyvitamin D or calcitriol, that is necessary for this process.
The activation of vitamin D involves its conversion to calcitriol in the kidneys. This activated form is then able to help maintain normal serum concentrations of calcium and phosphate, which are essential for various bodily functions including bone health, muscle contraction, hormone secretion, and neurotransmitter release. A deficiency in vitamin D can lead to decreased calcium absorption, resulting in skeletal issues such as rickets in children and osteomalacia and osteoporosis in adults.