Calcium absorption is affected by which of the following?

ANSWER: (B) Vitamin DREF: Guyton 12th ed page 9621, 25-di hydroxycholecalciferol, the active form of vitamin D funtions as a type of hormone to promote intestinal absorption of calcium. It does this by increasing over a period of 2 days the formation of calbindin, a calcium binding protein in the intestinal epithelial cells. The protein functions in the brust border of these cells to transport calcium into the cell cytoplasm and calcium moves through the basolateral membrane of the cell by facilitated diffusion. The rate of calcium absorption is directly proportional to to the quantity of the calcium binding protein. And the protein remains in the cells for several weeks even after 1, 25 di hydroxy choie calciferol is removed causing prolonged absorption of calcium in the cells.