Calcium absorption is affected by-

Ans. is 'd' i.e., All of the above * With an average intake of 1000 mg of calcium its net intestinal absorption is only 150-250 mg/day. Calcium is absorbed mainly in the duodenum and jejunum (proximal intestine) by an active transport mechanism regulated by 1, 25 (OH)2 D3 (calcitriol). Parathormone indirectly promotes absorption of calcium by increasing the renal synthesis of 1, 25 (OH)2 D3.* Dietary lactose, proteins and an acidic environment promote calcium absorption. On the other hand, phytates, phosphates, oxalates, tetracycline and and alkaline environment impair calcium absorption.* Calcium is transported across the brush border of intestinal epithelial cells via channels known as transient receptor potential vaniloid type 6 (TRPV6) and binds to an intracellular protein known as calbindin. Calbindin sequesters the absorbed calcium so that it does not disturb the epithelial signaling process that involve calcium. The absorbed Ca+2 is thereby delivered to the basolateral membrane of the epithelial cells, from where it can be transported into the blood stream by either a Sodium/Calcium exchanger (NCX1) or a calcium-dependent ATPase.