Spontaneous cancers, mostly of the thyroid, have developed in hamsters kept on an iodine-deficient diet, whereas animals receiving adequate iodine remained cancer-free. When rats have been given thyroid or radioactive iodine, the number of spontaneous cancers markedly increased. Cancer of the thyroid is unusually high in countries where the iodine intake is low. In America it decreased after the introduction of iodized salt.

Although over a billion dollars has now been spent on cancer research, the effect of various mineral deficiencies on cancer susceptibility appears to have been studied little. Increasing the copper intake "significantly retarded development" of cancers in animals, and decreased the liver damage and cirrhosis caused by cancer-inducing materials.