A randomized double-blind intervention was done in Huixian, People's Republic of China, a population with a high incidence of esophageal cancer. Once hyperplasia and dysplasia became evident in the mucous membrane of the esophagus and forestomach, zinc was given to the rats via their drinking water. The National Cancer Institute reports that the action of zinc is complex and that any possible benefit would depend on the dose, the route and the target site. The effects of zinc on the initiation and progression of cancer are not well established, although the negative effects of zinc deficiency on the immune system are clear. Some believe that a high zinc intake may contribute to colon cancer and accelerate the growth of prostate cancer.
Zinc ions seem to promote the emigration, implantation and outgrowth of circulating tumour cells.
Kroll also includes Alzheimer's disease on a list of toxic effects resulting from megadoses of zinc. Bespalov VG, et al . Blot JW, et al . Diamond WJ, et al . An alternative medicine definitive guide to cancer.