Effects of casein and fibrin on calcium absorption and calcium homeostasis in the rat.

To test the hypothesis that individual proteins have specific effects on calcium absorption, two proteins, casein and fibrin, were pair-fed in diets to rats for 3 weeks. Each protein was fed at normal (1.2%) and low (0.02%) calcium intakes. Diets were matched in other nutrients. Calcium absorption, measured in vitro as serosal-to-mucosal concentration ratio of 45Ca developed by duodenal sacs, was the same for both protein groups when calcium intake was normal and was increased by both low-calcium diets. Comparing low-calcium diets, absorption was greater in fibrin-fed than casein-fed groups. Balance studies showed that casein- and fibrin-fed rats taking normal calcium diets excreted the same amounts of calcium and phosphorus in urine and feces. Fibrin-fed rats taking low calcium excreted twice as much calcium as casein-fed rats and had decreased serum calcium. The balance data suggest that, compared to casein, fibrin prevents absorption or promotes excretion of calcium when calcium intake is low, and the response to calcium depletion is enhanced calcium absorption. It is concluded that individual proteins can alter calcium transport through effects on overall calcium homeostasis.