The effects of the heat treatment of casein in the presence of glucose-fructose on Zn bioavailability were studied. Changes in Zn speciation were compared after in vitro digestion of heated (HC) and unheated mixture (C) alone and as part of the diet. The uptake and transport of digested soluble Zn was investigated in Caco-2 cells grown in bicameral chambers; balance studies were done in rats fed diets containing the different samples. After in vitro digestion, the precipitated Zn was significantly higher in HC than in C. In assays with Caco-2 cells, the amount of Zn transferred from the apical to the basolateral chamber was significantly greater when the culture medium contained raw or heated casein. However, because a larger proportion of Zn was precipitated by in vitro digestion, Zn utilization was less efficient in the presence of casein. In biological experiments, food efficiency of the heated casein-glucose-fructose diet was lower, and feeding this diet increased the urinary Zn excretion and lowered Zn absorption and retention. The effects of browning products generated during food processing should be taken into account, especially in diets containing marginally adequate levels of Zn, to prevent possible deficiency.
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.
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