Dietary protein increases intestinal calcium absorption in part by increasing tight junction expression of Claudin‐2

Abstract

High protein diets increase calcium (Ca) absorption but the mechanism is not fully understood. Gene profiling was undertaken to identify metabolic pathways involved in dietary protein's effect on Ca absorption. Female Sprague-Dawley rats were fed a low (5%, n=9) or high (40%, n=9) protein diet for 7 days. On day 7, duodenal mucosa was harvested for isolation of total RNA. Microarray analyses identified 208 genes whose expression was at least 1.5 fold different between groups. None of the known transcellular Ca transport proteins were identified, but the Ca permissive tight junction protein, claudin-2, was found to be upregulated by the high protein diet. Exposing Caco-2 Bbe cells to a mixture of amino acids increased claudin-2 mRNA expression and redistributed claudin-2 to tight junctions. Using Caco-2 Bbe cells we evaluated the acute effect of amino acids on Ca transport. Including amino acids in the transport buffer increased paracellular Ca flux. We conclude that in rats, a high protein diet increases Ca absorption not only by increasing transcellular Ca transport as previously reported, but also by increasing claudin-2 expression and paracellular Ca absorption. The effect of dietary protein on claudin-2 expression appears to be directly mediated by amino acids. Funded by NRI Grant #2005-00806 and AFRI Grant #2009-02903 from the USDA NIFA, by a HATCH grant from UConn, and by the Beef Checkoff.