Abstract
Efficient intestinal calcium (Ca) and phosphate (Pi) absorption is necessary for optimal bone mineralization during growth, the protection of bone in adults, and the prevention of osteoporosis. Ca and Pi absorption each occurs through regulated, saturable pathways that predominate when dietary intake is low and through diffusion pathways that predominate when dietary intake is high. Habitual low Ca or Pi intake upregulates their saturable transport pathway through increased renal 1,25-dihydroxyvitamin D production, while low vitamin D status limits Ca and Pi absorption by reducing the saturable pathways. VDR or CYP27B1 gene deletion severely limits basal Ca and Pi absorption and the adaptation of mice to low Ca, but not low Pi, diets. Nonsaturable Ca absorption in the ileum may also be regulated by vitamin D status, but the mechanism is unclear. The vitamin D-regulated saturable pathways are transcellular, but significant gaps still exist in our understanding of these processes.
Published Version
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