Early consumption of soy brings most benefits

Eating soy foods on a regular basis - especially during adolescence - might lower the risk of breast cancer, according to preventive medicine researchers at the Keck School of Medicine of USC and colleagues.

A study found that Asian-American women who consumed soy foods on a weekly basis during their teen years and adulthood had about half the risk of developing breast cancer compared to similar women who ate little soy during the same time periods.

Risk was also somewhat lowered for women who ate soy regularly during the teen years but consumed little during adulthood. However, preliminary data suggest 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 Dr 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, specifically looking at the importance of soy. From 1995 to 1998, they interviewed 501 Asian-American breast cancer patients and compared them to 594 healthy Asian-American women. The study in published in the September issue of Carcinogenesis​.

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 noted that larger studies must be conducted to confirm this result.

Scientists are still unsure about how soy works to protect health. Animal studies indicate that early life exposure to genistein - the main isoflavone in soybeans - seems to help protect against chemically induced breast tumours. It is thought that 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, suggest some scientists, and some studies show that genistein seems to inhibit some enzymes that are important in metabolising oestrogen.

Regardless of the protective mechanism, Wu noted that if the mounting soy research shows promise, many may ask, "How much soy is enough?" Wu said that 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.

Isoflavone levels in soy foods vary considerably, but a typical serving of store-bought tofu contains about 10 mg of isoflavones, Wu said.

She added that a much larger study is needed to determine the benefits of adult soy food intake and what 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.