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In a typical adult, if 1,000 mg of Ca are ingested in the diet per day, approximately 200 mg will be absorbed in the intestine, and about 10 g of Ca will be filtered daily through the kidney. The majority of the Ca will be reabsorbed, with about 5 mmol (200) mg being excreted in the urine. The normal 24-h excretion of Ca may, however, vary between 2.5 and 7.5 mmol (100 and 300 mg per day). If bone turnover is “in balance,” approximately 500 mg of Ca is resorbed from bone per day and the equivalent amount is deposited.

The portion of Ca absorbed from the diet may range from 20 to 60%, varying with age and the amount of Ca ingested. Thus, rates of net Ca absorption are high in growing children, during growth spurts in adolescence, and during pregnancy and lactation, and decline with age in men and women. The efficiency of Ca absorption increases during prolonged dietary Ca restriction. Fecal Ca losses vary between 2.5 and 5.0 mmol (100 and 200 mg) per day. Fecal Ca is composed of unabsorbed dietary Ca and Ca contained in intestinal, pancreatic, and biliary secretions. Secreted Ca is not regulated by hormones or serum Ca.

About 90% of Ca absorption generally occurs in the large surface area of the duodenum and jejunum. Intestinal epithelial Ca transport includes both an energy-dependent, cell-mediated saturable active process that is largely regulated by 1,25(OH)₂D [24], and a passive, diffusional paracellular path of absorption that is largely driven by transepithelial electrochemical gradients, but may also be modulated by 1,25(OH)₂D by regulating claudin 2 and claudin 12, which form paracellular calcium channels. Active transcellular intestinal Ca absorption generally accounts for absorption of 10–15% of a dietary load. Active absorption involves 3 sequential cellular steps: first, a rate-limiting step involving transfer of luminal Ca into the intestinal cell via the epithelial apical Ca channel of the transient receptor potential vanilloid (TRPV) family, TRPV6; second, transport of Ca across the cell via a channel-associated protein, calbindin-D9K; and finally extrusion of Ca across the basolateral membrane into ECF by an energy-requiring process via the basolateral Ca ATPase system, PMCA1b [25]. Reductions in dietary Ca intake can increase 1,25(OH)₂D production, which can then increase expression of the proteins involved in transport resulting in enhanced fractional Ca absorption and compensation for the dietary reduction. Fractional Ca absorption may thus increase from 55 to 70% in children and young adults and from 20 to 45% in older men and women.