Japan has one of the lowest rates of breast cancer among developed countries. That rate is climbing though, as more Japanese women have been diagnosed with this disease over the past several decades. While there are many possible reasons for this, the biggest suspect is a shift toward a more western diet. In particular, women who are trading in soyfoods for other sources of protein may be losing some of the protection of their traditional pattern of eating.
Researchers from Gifu University initiated the Takayama study to examine the role of diet among Japanese women in breast cancer risk. They enrolled 15,607 women from the city of Takayama who were at least 35 years old in 1992. Participants filled out an extensive lifestyle and health questionnaire at the beginning of the study. The questionnaire included questions about the intake of 169 different foods, including soyfoods such as miso soup, soymilk, boiled soybeans and various kinds of tofu.
Over 16 years of follow up, 172 women were diagnosed with breast cancer and most of the cases were invasive breast cancer. When the women were divided into four groups according to their soyfood intake, those who consumed three servings per day were only half as likely to have been diagnosed with breast cancer as those who consumed only about one-half serving of soy per day. A serving of soy is one cup of soymilk or about ½ cup of tofu.
This protective effect remained even after accounting for risk factors such as smoking, alcohol consumption, age at menarche and use of hormone therapy. The good news for women who are just beginning to add soyfoods to their diet is that, in this study, women who consumed just one or two servings of soyfoods per day derived almost as much protection as was seen with three servings. The researchers suggested that the isoflavones in soybeans, which are plant estrogens, might be responsible for the benefit.
Wada K, Nakamura K, Tamai Y, et al. Soy isoflavone intake and breast cancer risk in Japan: From the Takayama study. Int J Cancer 2013