Soy Consumption in Childhood May Decrease Breast Cancer Risk, Epidemiologic Study Suggests

BOSTON—Consuming soy during childhood, adolescence, and adult life is associated with a decreased risk of breast cancer, but the strongest and most consistent effect is childhood consumption, according to a recent collaborative study led by researchers from the National Cancer Institute (NCI), and presented at the AACR Frontiers in Cancer Prevention Research meeting (abstract B163).

The scientists reported that women who ate the most soy-based foods such as tofu, miso, and natto (a traditional Japanese food made from soybeans) during ages 5 to 11 reduced their risk of breast cancer by 58%, compared with the women who ate the least amount. The reductions for adolescence and adulthood were about 25%. "In our study, soy intake during childhood is strongly protective against breast cancer later in life," said Larissa Korde, MD, MPH, of the NCI's Division of Cancer Epidemiology and Genetics.

The study was a case-control analysis of Asian-American women living in Los Angeles, San Francisco, and Oahu, Hawaii. Dr. Korde collaborated with epidemiologists at the University of Hawaii, Northern California Cancer Center, and University of Southern California.

The researchers interviewed approximately 1,500 women of Chinese, Japanese, and Filipino descent, ages 20 to 55, about their diet and lifestyle over the course of their lives—597 were breast cancer cases and 966 were controls. For study participants whose mothers were alive and living in the United States, the researchers also interviewed 255 mothers about their daughters' early exposures.

Soy intake was divided into thirds, based on frequency of consumption. Women in the highest category consumed, on average, 2 servings per week while those in the lowest category ate soy an average of once a month, resulting in a 10-fold difference in exposure to soy between the two extremes. By comparing the highest category to the lowest, the researchers found inverse associations between the risk of developing breast cancer and the amount of soy consumed. The strongest effect was for childhood consumption.

It is well known that women living in China and Japan have lower breast cancer rates than those living in western nations. Many studies have also shown that the breast cancer rates of women who migrate from Asian countries to the United States rise over several generations and approach that of US white women. This change in rates suggests that environmental or lifestyle factors play a role in the changing rates. One leading candidate is soy.

One of the advantages of working with migrant populations is that as they acculturate, they begin to acquire the habits of the host country over time and as a group have a wide range of exposures to environmental factors. This range of exposures would be difficult to observe outside a migrant community.

A number of studies have looked at adult soy intake and breast cancer risk in migrant populations and have found mixed results, Dr. Korde said. Two studies evaluating adolescent diet found protective effects, suggesting that adolescent diet may be more important than adult diet. "Our study is the first to look at childhood soy intake and breast cancer risk," Dr. Korde said, "and it appears that the timing of exposure may be critical."

She noted that the effect of childhood soy was "strikingly consistent." A protective effect was seen in women from all three ethnic groups, in all three centers, and in those with and without a family history of cancer. Furthermore, the protective effect of childhood soy was similar in Asian women born in Asia and in the West.

The analysis was adjusted for accepted breast cancer risk factors. In addition, to rule out a broader Asian lifestyle pattern that may have accounted for the reduced risk, the researchers adjusted for migration history, since recent migrants have a more Asian lifestyle, and for other measures of acculturation such as reading Asian newspapers, shopping in Asian stores, speaking Asian languages, and living in Asian neighborhoods. When these factors were considered, the soy effect was weakened in adolescence and adulthood, but not in childhood. These results suggest that soy intake in childhood may reflect real biological risk and not simply acculturation.

Regina Ziegler, PhD, MPH, senior investigator of the study, cautioned that it would be premature to recommend changes in childhood diet.

"This is the first study to evaluate childhood soy intake and subsequent breast cancer risk, and this one result is not enough for a public health recommendation," Dr. Ziegler said. "The findings need to be replicated through additional research."