Effects of heat treatment of casein in the presence of reducing sugars on calcium bioavailability: in vitro and in vivo assays.

Casein-glucose-fructose mixtures unheated (C) or after heating (HC) were added to a solution of ionic calcium to study calcium speciation and included in diets (C-D, HC-D) for rats. Samples and diets were digested in vitro. Supernatants of digested samples were used for transport experiments with Caco-2 cells. Total soluble and ionic calcium levels were lower and precipitated calcium levels higher with HC compared to C. Dialyzed calcium from the diets was highly ionic and lower in HC-D compared to C-D. Nondialyzed soluble calcium was also lower, whereas precipitated calcium was higher, in HC-D. HC increased calcium transport in Caco-2 cells compared to C, but transport efficiency decreased due to lower calcium solubililty after digestion. Urinary calcium increased with HC-D consumption without changes in calcium absorbed and retained. Maillard reaction products in HC decrease calcium solubility, but enterocyte metabolism and calcium absorption and retention seem to be unaffected. Nevertheless, urinary calcium losses increase.