EFFECT OF CITRATE ON ZINC BIOAVAILABILITY FROM MILK, MILK FRACTIONS AND INFANT FORMULAS

It is recognized that zinc is well absorbed from milk. While part of this high bioavailability may be due to milk proteins, it is also possible that low molecular weight anions complex zinc facilitating its absorption. We investigated the bioavailability of zinc from fresh cow's milk, skimmed milk, commercial casein, commercial whey protein concentrates, ultrafiltered milk, milk ultrafiltrate (LMW) and citrate-free ultrafiltrate (LMW -C). The effect of adding citrate to casein and whey protein concentrates was also investigated. In a second experiment, milk based infant formulas were separated by ultracentrifugation in three fractions; fat, whey and casein. Zinc-65-labeled diets were fed by gastric intubation to fasted 20 day old suckling rats. Animals were killed after 4 h and tissues removed and counted. Zinc absorption was high from whole milk (53%), skim milk (58%) and casein (52%) while it was lower for the whey proteins (28-37%). When citrate was added to casein or whey protein concentrate, zinc absorption increased significantly. Ultrafiltration of skim milk caused a reduction in zinc absorption, although this was not statistically significant. An enhancing effect on zinc bioavailability was observed when citrate was added to fat or pellet from formula. This study shows that citrate enhances zinc absorption from milk fractions and that supplementation of formulas with citrate may be advantageous.