Zinc absorption may be modified by concomitant protein ingestion.
Increasingly, the Western diet is being shown to cause zinc deficiencies. This is concerning because zinc has an important role to play as a cofactor to many human proteins. The factors that that affect zinc absorption are therefore of interest to nutritional scientists because decreasing zinc absorption even a small amount can have a significant impact on health if the zinc status of the individual is poor. A number of factors are known to affect zinc absorption including other minerals such as iron or copper, ascorbic acid, chelating agents and cirrhosis of the liver. Also diseases such as celiac’s disease and crohn’s disease may negatively affect zinc absorption. Interestingly the amount of zinc required to cause a rise in plasma levels of zinc is above that normally present in the diet. Therefore plasma levels of zinc are a poor indicator of absorption. In addition because zinc is absorbed over the entire length of the small intestine, the initial rise in plasma levels may not accurately reflect zinc absorption as a whole.
Therefore zinc absorption is highly complex. Studies have tried to elucidate the questions surrounding zinc absorption and many have been highly beneficial in this regard. For example, in one study>sup>1, researchers administered oral doses of 25 mg radiolabeled zinc as zinc chloride to 20 healthy volunteers. Half of the subjects received the zinc along with water, while the other half received the zinc along with turkey. The zinc absorption from the two test meals was 42 %. However, the data showed that the absorption characteristics of the two zinc treatments was quite different. The zinc in water showed a significantly higher peak absorbance and a significantly greater area under the curve for plasma zinc concentrations compared to the zinc in turkey. Therefore although the total zinc absorbed from the test meals were identical, the zinc administered with the turkey was absorbed considerably more slowly that the zinc administered in water. Therefore a discrepancy exists between the total zinc absorption and its rate in the two meals.
This differing rates of zinc absorption but the similar total zinc absorption was suggested to be related to changes in the gastric transit time caused by the turkey protein. Protein in able to delay gastric emptying and therefore increases the time taken for food to reach the small intestine. Because zinc is absorbed from the small intestine, this delays the absorption of zinc. Taken as a whole, these results suggest that the absorption of zinc can be significantly delayed when accompanied by protein. In addition, the zinc may have bound temporarily with the proteins within the turkey meat, being released to solution upon complete digestion of the protein or through pH changes from the stomach to the small intestine. Meat can therefore delay zinc absorption, but does not inhibit zinc absorption. This has significance to any study that has not taken these physiological interactions into account when measuring peak plasma levels following an oral zinc load.
