Zinc transport in Caco-2 cells and zinc balance in rats: influence of the heat treatment of a casein-glucose-fructose mixture.

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.