Considerations in estimates of requirements and critical intake of zinc. Adaption, availability and interactions
In order to establish an adequate and safe dietary intake of zinc, several specific features of zinc metabolism have to be considered. Lacking sensitive indices of zinc status, estimates of zinc requirement have to be based on the factorial method, i.e., measurements of the daily losses of zinc via skin, intestine and kidney. The body maintains zinc homeostasis at different levels of zinc intake by changes in zinc absorption and excretion. After longer periods of a low zinc intake, daily inevitable zinc losses can be estimated to be approximately 1 mg, while at higher intakes losses are substantially higher. To translate physiological requirements to dietary requirement the availability of zinc has to be considered. From diets with a high content of phytate, less than 15% is typically absorbed while in refined animal protein based diets up to 40% is adsorbed. This means that the dietary requirement can vary from <5–>15 mg d–1. Zinc shows a low toxicity, but high doses of zinc interact with other trace elements, especially copper. (These interactions have already been observed at an intake of 50 mg.) Thus, the range between essential and potential deleterious intake of zinc is rather narrow.