Vitamin D and the absorption of calcium

Vitamin D increases calcium absorption by the intestines. Calcitriol, the active form of vitamin D, increases the production of calcium-binding proteins in the lining of the small intestine. The resulting increase in plasma calcium suppresses the secretion of preformed PTH by chief cells of the parathyroid gland.

The rate of calcium absorption seems to be directly proportional to the quantity of calcium binding proteins. Calcitriol’s primary regulatory role involves keeping blood calcium and phosphorus levels within a normal range. Calcitriol (1,25(OH)2D3) is the dihyroxylated form of vitamin D3.

Calcitriol

Calcitriol acts in concert with two other hormones: parathyroid hormone (PTH) from the parathyroid glands and calcitonin from the thyroid gland.

Together, levels of these three harmonies continually rise and fall, adjusting blood calcium levels by changing urinary calcium excretion, intestinal calcium absorption and the flow of calcium in and out of bone.
Vitamin D and the absorption of calcium