How does calcitriol regulate calcium and phosphate balance?

How does calcitriol regulate calcium and phosphate balance?

• A. Increases intestinal absorption of calcium and phosphate and suppresses PTH production.
• B. Decreases intestinal absorption of calcium and phosphate and stimulates PTH production.
• C. Increases renal phosphate reabsorption and decreases bone mineralization.
• D. Suppresses intestinal calcium absorption and enhances PTH synthesis.

Answer: (B)

Calcitriol, the active form of vitamin D, raises calcium and phosphate levels mainly by increasing their absorption from the intestine. It does this by upregulating calcium-binding proteins and transport systems in enterocytes, so more dietary calcium and phosphate enter the bloodstream. At the same time, calcitriol feeds back to the parathyroid glands to reduce PTH synthesis, helping to keep bone turnover and renal handling of minerals in balance. So, the statement that calcitriol increases intestinal absorption of calcium and phosphate and suppresses PTH production best reflects its actions. Concepts that suggest decreased gut absorption or stimulation of PTH, or that emphasize only renal reabsorption or bone mineralization in a way that contradicts the usual effect, don’t fit with calcitriol’s normal role.

Calcitriol, the active form of vitamin D, raises calcium and phosphate levels mainly by increasing their absorption from the intestine. It does this by upregulating calcium-binding proteins and transport systems in enterocytes, so more dietary calcium and phosphate enter the bloodstream. At the same time, calcitriol feeds back to the parathyroid glands to reduce PTH synthesis, helping to keep bone turnover and renal handling of minerals in balance.

So, the statement that calcitriol increases intestinal absorption of calcium and phosphate and suppresses PTH production best reflects its actions. Concepts that suggest decreased gut absorption or stimulation of PTH, or that emphasize only renal reabsorption or bone mineralization in a way that contradicts the usual effect, don’t fit with calcitriol’s normal role.