How does calcitriol affect calcium homeostasis?

How does calcitriol affect calcium homeostasis?

• A. Calcitriol decreases intestinal calcium absorption.
• B. Calcitriol decreases bone resorption.
• C. Calcitriol increases renal calcium excretion.
• D. Active vitamin D increases intestinal calcium absorption, promotes bone resorption, and increases renal calcium reabsorption.

Answer: (D)

Calcitriol acts to raise the availability of calcium from all the main sources in the body. In the gut, it increases the absorption of dietary calcium by boosting the expression of calcium-transport proteins, so more calcium enters the bloodstream after meals. In bone, it promotes turnover by enhancing signals that recruit osteoclasts, releasing calcium from bone when blood levels are low. In the kidneys, it increases calcium reabsorption in the distal tubules, reducing urinary calcium loss. Putting these effects together, active vitamin D elevates intestinal absorption, can promote bone resorption to mobilize calcium, and prevents calcium loss in urine, which is why this option best describes calcitriol’s role in calcium homeostasis. The other statements either claim the opposite effect on the gut, limit to one aspect, or state increased excretion, which does not align with calcitriol’s actions.

Calcitriol acts to raise the availability of calcium from all the main sources in the body. In the gut, it increases the absorption of dietary calcium by boosting the expression of calcium-transport proteins, so more calcium enters the bloodstream after meals. In bone, it promotes turnover by enhancing signals that recruit osteoclasts, releasing calcium from bone when blood levels are low. In the kidneys, it increases calcium reabsorption in the distal tubules, reducing urinary calcium loss. Putting these effects together, active vitamin D elevates intestinal absorption, can promote bone resorption to mobilize calcium, and prevents calcium loss in urine, which is why this option best describes calcitriol’s role in calcium homeostasis. The other statements either claim the opposite effect on the gut, limit to one aspect, or state increased excretion, which does not align with calcitriol’s actions.