An historical overview of zinc in poultry nutrition

Zinc is one of the most prevalent trace minerals in both the environment and in the body. It was used as a component for industrial coatings dating back to the 18th century and toxicity in smelter employees was a concern. Zinc was first reported in plants in the mid 1800's and later that century was the first report of zinc in the liver and muscle in mammals. At this point in nutrition history, there had been research interest in iron and copper with their visible, bright colors. The first series of experiments and publications on zinc in animal systems used the marine mollusc, Sycotypus canaliculatus, with the final paper reporting the storage of copper and zinc in the liver. One hurdle to recognizing zinc's importance to animal growth was the lack of easily observed deficiency symptoms as was the case with rickets (Vitamin D) and perosis (manganese). While zinc can accumulate in selected tissues (i.e., liver, bone), its role as a cofactor in the 2 key enzymes carbonic anhydrase and alkaline phosphatase will be the focus of this review as these enzymes are critically important to optimizing egg production, egg shell quality, skeletal development and embryonic development. These enzymes can occur as different isoforms and their biochemical importance was discovered prior to the recognition that zinc was an important co-factor. Over the last 20 yr, 3 factors have dominated the zinc literature: phytase, organic versus inorganic zinc supplements and the application of molecular techniques toward the discovery of previously ignored zinc containing proteins/enzymes. The objective of this review will be to discuss the history of zinc as an essential enzymatic co-factor regulating animal growth and development with an emphasis on the aforementioned poultry systems egg production, shell quality, and skeletal development.