Figure 511

Gastrointestinal absorption of dietary calcium (Ca). The normal recommended dietary intake of Ca for an adult is 800 to 1200 mg/d (20-30 mmol/d). Foods high in Ca content include milk, dairy products, meat, fish with bones, oysters, and many leafy green vegetables (eg, spinach and collard greens). Although serum Ca levels can be maintained in the normal range by bone resorption, dietary intake is the only source by which the body can replenish stores of Ca in bone. Ca is absorbed almost exclusively within the duodenum, jejunum, and ileum. Each of these intestinal segments has a high absorptive capacity for Ca, with their relative Ca absorption being dependent on the length of each respective intestinal segment and the transit time of the food bolus. Approximately 400 mg of the usual 1000 mg dietary Ca intake is absorbed by the intestine, and Ca loss by way of intestinal secretions is approximately 200 mg/d. Therefore, a net absorption of Ca is approximately 200 mg/d (20%). Biliary and pancreatic secretions are extremely rich in Ca. 1,25-dihydroxy-vitamin D3 is an extremely important regulatory hormone for intestinal absorption of Ca [1,2,17,18].

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Calmodulin

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