Soy, its components, and cancer prevention: a review of the in vitro, animal, and human data.

Introduction The incidence of breast, prostate, and colon cancer, among others, is lower in Asian countries than in the West. People living in China, Japan, and Korea, for example, are 4 to 10 times less likely to be diagnosed with and die from breast and prostate cancer than are people in the United States (1). Traditionally, migrant populations have been examined to determine how much of the difference between ethnic groups is due to genetic tendencies and how much is due to environmental factors. In one such study, cancer incidence was observed among Japanese peoples migrating to the United States (2). Within two generations of migration, cancer mortality rates increased to levels similar to the native Caucasian population. This suggests that environmental and behavioral factors may be more important than genetic factors in determining cancer frequency between populations. Based on epidemiological calculations by Doll and Peto (3, 4), as many as 75-80% of all fatal cancers in the United States are due to extrinsic factors and thus may be preventable. Potentially avoidable environmental factors contributing to cancer frequency include diet, smoking, alcohol consumption, reproductive behavior, infection, geophysical factors ineluding sunlight, and prolonged exposure to extrinsic agents such as fossil fuel combustion products, radioactive waste, dust and fumes, pesticide residues, and food additives (4). Diet, in particular, is one such factor that can vary significantly from country to country and has been estimated to account for up to 35% of all cancer rate differences (4). Although Western and Eastern diets vary widely, one striking distinction between them is the source of protein. Those in the West generally rely on animal protein, whereas Eastern diets emphasize bean, particularly soybean, as a protein source. Soybeans have long been a major component of the Eastem diet. Asians consume an average of 20-80 g of soyfoods per day, whereas Americans eat only 1-3 g daily (5). Commonly consumed Asian soy products include soymilk, tofu, miso, yuba, and tempeh. Moreover, extensive epidemiological, in vitro, and animal data collected over the previous several years suggest that soybean consumption reduces the risk of developing several types of cancer including breast, prostate,