A combination of eating soy foods on a regular basis during adolescence together with a regular intake during adulthood could signficantly reduce the risk of breast cancer, according to a new study of Asian-American women released this week.
Scientists at the Keck School of Medicine, University of Southern California carried out a study on Asian-American women who consumed soy foods on a weekly basis during their teen years and adulthood. They found that these women had about half the risk of developing breast cancer compared to similar women who ate little soy during the same time periods. Results of the study are published in the September issue of Carcinogenesis.
The scientists added that the risk was also lowered for women who ate soy regularly during the teen years but consumed little during adulthood. However, preliminary data suggests that there is little added benefit for women who ate little soy during adolescence but a high amount of soy during adulthood.
"There has been a lot of talk and controversy about the Asian diet and connections between soy food intake and breast cancer. We wanted to look at soy very carefully, to better understand if soy by itself is protective or if the level of soy consumption is just a marker for acculturation," said Anna H. Wu, professor of preventive medicine at the Keck School of Medicine.
Wu and colleagues conducted a case-control study of breast cancer among Chinese, Japanese and Filipino women in Los Angeles County, and specifically looked at the role of soy. From 1995 to 1998 they interviewed 501 Asian-American breast cancer patients and compared them to 594 healthy Asian-American women.
The researchers asked about eating habits, including how many times each week during adolescence they ate tofu. They also asked about the frequency and amounts of whole soy foods, such as tofu, soymilk, miso and fresh soybeans, usually eaten during adulthood.
Intake was highest among Chinese (26.8 milligrams of isoflavones a day), intermediate among Japanese (18.4 mg of isoflavones a day) and lowest among Filipinas (9.3 mg of isoflavones a day). Migrants ate a little more soy than American-born women did. Most of the Chinese and Filipino women in the study, more than 90 per cent, were born in Asia, compared to less than 30 per cent of the Japanese women.
When women were grouped by how often they ate soy during adolescence and adult life, researchers found that women who were high consumers during both time periods had a 47 per cent reduction in risk. Those who ate little soy during adult life but were regular soy consumers during adolescence showed a 23 per cent reduction in risk. Women who were low consumers during adolescence and high consumers during adulthood showed little reduction in risk. However, the number of such women was small, and researchers note that larger studies must be conducted to confirm this result.
Animal studies indicate that early life exposure to genistein, the main isoflavone in soybeans, seems to help protect against chemically induced breast tumours. Getting genistein early in life may help the mammary glands develop in a favourable way.
Eating soy might also lead to lower levels of oestrogen circulating within the body and some studies indicate that genistein appears to inhibit some enzymes that are important in metabolising oestrogen.
Wu notes that if the mounting soy research shows promise, many may ask questions about the right quantity to eat. According to Wu there may be a point at which eating more soy does not further reduce risk. Researchers are still seeking to understand the relationship between soy food dose and risk reduction, though in this study the greatest risk reduction was seen among women with the highest level of consumption: 12.68 mg or more of isoflavones per 1000 kilocalories during adulthood and eating soy foods four or more times a week during adolescence.
Wu stressed that a much larger study is needed to clarify the benefits of adult soy food intake and which levels of soy intake are most helpful. Her study looked only at traditional soy foods, not at genistein or other isoflavones that may be contained in pills and supplements.
Full findings are published in Carcinogenesis, September 2002, Vol. 23, No. 9, pp. 1491-1496.