What is the role of vitamin D in calcium homeostasis and bone health?

• A. Vitamin D inhibits intestinal calcium absorption.
• B. Vitamin D promotes hair growth.
• C. Vitamin D enhances intestinal calcium absorption and supports bone mineralization.
• D. Vitamin D has no role in bone health.

Answer: (C)

Vitamin D, in its active form calcitriol, is essential for calcium homeostasis because it increases the gut’s ability to absorb calcium (and phosphate) from the diet. It does this by upregulating calcium transport proteins in the intestinal lining, which boosts the amount of calcium that enters the bloodstream. This intestinal absorption works together with kidney handling and parathyroid hormone to keep blood calcium within a tight range. At the same time, sufficient vitamin D ensures minerals like calcium and phosphate are available for bone mineralization, helping form hydroxyapatite crystals in bone. When vitamin D is deficient, absorption drops, serum calcium falls, PTH rises, and bone mineralization becomes impaired, leading to conditions like rickets in kids and osteomalacia in adults. This role is why vitamin D is beneficial for bone health. It does not inhibit intestinal absorption, nor is it primarily linked to hair growth.