Breast Cancer Risk and Childhood Soy Intake in Asian-American Women

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Breast Cancer and Soy

In a novel study of Asian-American women, a team of researchers led by National Cancer Institute (NCI) investigators has found that a decreased risk of breast cancer is associated with consuming soy during childhood, adolescence and adult life, but that the strongest and most consistent effect was seen for childhood intake.

They found that women who ate the most soy-based foods (such as tofu, miso, natto) during ages 5-11 reduced their risk of developing breast cancer by 58 percent, compared to women who ate the least amount. The corresponding reductions for adolescent and adult intake were about 25 percent.

"Childhood soy intake was significantly associated with reduced breast cancer risk in our study, suggesting that the timing of soy intake may be especially critical," said the study's lead investigator, Larissa Korde, M.D., MPH, a staff clinician at the NCI's Clinical Genetics Branch, in the Division of Cancer Epidemiology and Prevention. Korde worked in collaboration with epidemiologists at the University of Hawaii, the Northern California Cancer Center, and the University of Southern California.

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The underlying mechanism is not known. However, Korde said that one hypothesis for the decreased risk associated with childhood intake is that soy isoflavones have estrogenic effects that cause changes in breast tissue, leading to decreased sensitivity to carcinogens. A similar protective effect has been found in studies of overweight girls, perhaps because fat tissue also secretes estrogens, she added.

"Hormonal exposures in adulthood, such as use of estrogen and progesterone replacement therapy, are established breast cancer risk factors. However, a growing body of evidence suggests that hormonally related exposures early in life may also modify susceptibility to breast cancer," Korde said.

Studies investigating adult soy intake and breast cancer risk have had mixed results, but the two studies that looked at adolescent consumption found that the risks of developing breast cancer later in life were cut in half. This study is the first to address the relationship between soy consumption during childhood and future risk of breast cancer.

As provocative as the findings are, the senior investigator on the study, Regina Ziegler, Ph.D, MPH, 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," she said. "The findings need to be replicated through additional research."

The researchers conducted a case-control study of women of Chinese, Japanese and Filipino descent who were living in the San Francisco Bay area, Los Angeles, or Oahu, Hawaii. Included were 597 Asian-American women with breast cancer and 966 women without the disease, who answered questions about their adult and adolescent diet and lifestyle. In addition, for a subset of 255 participants whose mothers were alive and living in the US, the mothers were asked about their daughter's early childhood exposures.

Soy intake was then divided into thirds, based on frequency of consumption, and by comparing the highest category to the lowest, the researchers found an inverse association between the risk of developing breast cancer and the amount of soy consumed. The childhood relationship held in all three races and all three study sites, and in women with and without a family history of breast cancer. Since the effects of childhood soy could not be explained by other measures of Asian lifestyle during childhood or adult life, researchers concluded that early soy intake might itself be protective.

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