March 31, 2009

Early soy consumption linked with reduced breast cancer risk

An article published in the American Association for Cancer Research journal Cancer Epidemiology, Biomarkers and Prevention reported that Asian-American women who consumed high amounts of soy during childhood had a lesser risk of developing breast cancer compared with women who consumed less soy. Although women living in Asia have a lower risk of breast cancer than those residing in the United States, breast cancer risk increases among the descendents of those who migrate to the U.S., leading researchers to believe that the Western diet may be involved.

Larissa Korde, MD, MPH of the National Cancer Institute and her colleagues analyzed data from 597 breast cancer patients and 966 healthy women of Chinese, Japanese or Filipino descent who resided in the San Francisco Bay Area, Los Angeles and Hawaii. For participants whose reported childhood intake of soy was among the highest one-third of subjects there was a 58 percent lower risk of breast cancer compared with those whose intake was in the lowest third. The reduction was similar for all three ethnicities and for those with and without a family history of breast cancer. High intake during adolescent or adult years was associated with a 20 to 25 percent lower risk of the disease.

"Since the effects of childhood soy intake could not be explained by measures other than Asian lifestyle during childhood or adult life, early soy intake might itself be protective," stated Dr Korde, who is a staff clinician at the National Cancer Institute's Clinical Genetics Branch. "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. Soy isoflavones have estrogenic properties that may cause changes in breast tissue. Animal models suggest that ingestion of soy may result in earlier maturation of breast tissue and increased resistance to carcinogens."

"Historically, breast cancer incidence rates have been four to seven times higher among white women in the U.S. than in women in China or Japan," noted coauthor Regina G. Ziegler, PhD, MPH, who is a senior investigator in the NCI Institute's Division of Cancer Epidemiology and Genetics. "However, when Asian women migrate to the U.S., their breast cancer risk rises over several generations and reaches that of U.S. white women, suggesting that modifiable factors, rather than genetics, are responsible for the international differences. These lifestyle or environmental factors remain elusive; our study was designed to identify them."

"This is the first study to evaluate childhood soy intake and subsequent breast cancer risk," she added. "The findings need to be replicated through additional research."

Breast cancer

Breast tumors often require hormones for growth, which poses a unique problem because the hormones involved in tumor growth are either estrogen, progesterone, or both. Estrogen and progesterone are naturally occurring and necessary hormones, produced mainly in the ovaries and adrenal glands in varying amounts throughout a woman's lifetime. These hormones are essential for many physiological functions, such as bone integrity. The stronger form of estrogen, estradiol, can be converted into the weaker form, estriol, in the body without using drugs. Estriol is considered to be a more desirable form of estrogen. It is less active than estradiol, so when it occupies the estrogen receptor, it blocks estradiol's strong "growth" signals. Using a natural substance, the conversion of estradiol to estriol increased by 50% in 12 healthy people (Michnovicz et al. 1991). Furthermore, in female mice prone to developing breast cancer the natural substance reduced the incidence of cancer and the number of tumors significantly. The natural substance was indole-3-carbinol (I3C).

Indole-3-carbinol (I3C) is a phytochemical isolated from cruciferous vegetables (broccoli, cauliflower, Brussels sprouts, turnips, kale, green cabbage, mustard seed, etc.). I3C given to 17 men and women for 2 months reduced the levels of strong estrogen, and increased the levels of weak estrogen. But more importantly, the level of an estrogen metabolite associated with breast and endometrial cancer, 16--a-hydroxyestrone, was reduced by I3C (Bradlow et al. 1991).

When I3C changes "strong" estrogen to "weak" estrogen, the growth of human cancer cells is inhibited by 54-61% (Telang et al. 1997). Moreover, I3C provoked cancer cells to self-destruct (kill themselves via apoptosis). Induction of cell death is an approach to suppress carcinogenesis and is the prime goal of cytotoxic chemotherapy. The increase in apoptosis induced by I3C before initiation of new tumor development may contribute to suppression of tumor progression. Nontoxic I3C can reliably facilitate apoptosis (12 week treatment in rats); thus, this phytonutrient may become a standard adjunct in the treatment of breast cancer (Zhang et al. 2003).

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Life Extension Update

• Greater soy consumption associated with lower breast cancer risk in women
• Meta-analyses find soy isoflavones prevent bone resorption, stimulate bone formation and increase bone mineral density in menopausal women

What's Hot

• Primate study shows soy does not increase breast cancer risk
• Soy isoflavones, miso, protect against breast cancer
• Soy lowers dangerous estrogen

Life Extension magazine

• New scientific options for preventing breast cancer
• Soy & cancer prevention

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Dayna Dye
Editor, Life Extension Update
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