Soy and experimental cancer: animal studies.
Studies are reviewed that report consumption of soy protein diets inhibits the growth of various tumors in rats. The inhibitory effect has been attributed to the phytoestrogens (genistein and diadzein) or protein kinase inhibitor in soy protein products. Recent studies indicate that additional factors in soy protein products may also contribute to the inhibition of tumorigenesis, namely the deficiency of the essential amino acid methionine. Metastatic growth to the lungs of a primary rhabdomyosarcoma tumor was inhibited by feeding a soy protein diet. The effect was reversed by methionine fortification of the diet. Carcinogen-induced mammary tumor development was inhibited during the promotional phase in rats fed soy protein isolate diet and reversed with a methionine-supplemented diet. Additional studies demonstrated that after excision of the primary mammary tumor, growth of additional tumors was inhibited when the diet was changed from casein to soy protein isolate. Histopathologic evaluation of the mammary tumors revealed more benign fibroadenomas and lower-grade adenocarcinomas in the soy protein group. Before carcinogen administration (at 7 weeks of age), ornithine decarboxylase activity and polyamine concentrations in the rat mammary epithelium were significantly lower in the soy protein group. These data suggest an inhibitory effect on mammary epithelial growth in the soy-protein-fed group.