Estrogen, soy, and breast cancer

Estrogen often plays a role in breast cancer development. Because of this, many of the therapies used to treat it work by blocking the action of estrogen in the body.

Soy foods can have both estrogen-like and anti-estrogen effects in the body, mainly due to compounds known as isoflavones. They also contain other substances that may prevent cancer, but not much is known about the use of soy foods by women who have had breast cancer. Some studies suggest that the estrogenic effects of soy isoflavones may promote cancer recurrence, while others have shown that soy may protect against it.

Soy has been eaten for thousands of years in Asia, but its popularity in Western nations has only caught on much more recently. Soy isoflavones are now frequently added to many processed foods and “exposure to isoflavones is becoming ubiquitous, which is heightening concern about soy food consumption among the rapidly increasing population of breast cancer survivors,” said the researchers.

The new study looked at more than 5,000 breast cancer survivors aged 20 to 75 years. During the average follow-up period of four years, 444 women died and 534 had a recurrence or died of breast cancer-related causes.

Soy seems safe

Eating soy foods was inversely related to mortality and to breast cancer recurrence. That is, compared with women who ate the least soy, those who ate the most (more than 15 grams of soy protein per day, or more than 63 mg of isoflavones per day) had a 29% reduced risk of death and 32% decreased risk of breast cancer recurrence.

The positive effect seemed to level off after about 11 grams of soy protein or 40 mg of soy isoflavone intake per day, suggesting, as the authors state, that “moderate soy food intake is safe and potentially beneficial for women with breast cancer.” Soy foods seemed to benefit women with both estrogen receptor (ER)-positive and ER-negative cancers.

The researchers also looked at how much soy women consumed who were also taking the anti-cancer drug, tamoxifen. Because soy and tamoxifen both bind to estrogen receptors, there has been some concern about using them together. They found that soy foods improved survival regardless of tamoxifen use, and that soy foods and tamoxifen may have comparable effects on breast cancer outcomes.

Since many soy preparations are newer to the human diet—such as concentrated soy isoflavone extracts and isolated soy protein—it might be prudent to focus on real food sources of soy, like tempeh, tofu, edamame, and miso. These foods have been eaten for centuries in parts of the world where breast cancer rates are lowest.

(JAMA 2009;302:2437–43)