As a vegan (for ethical not health reasons), I need good quality plant protein. Soy delivers the most plant protein and in a variety of different forms than any other single source I’m aware of. Bottom line is, I eat a lot of soy products: soy milk, tofu, soy hot dogs (I know–not much better for you than “real” hot dogs), tempeh, edamame.

I first heard rumblings about the risks of eating soy from another vegan friend who tries to avoid it. Whenever I asked why, she said it was because of “all she’d read.” What she’d read were reports that linked soy to breast cancer.

As explained in this article (and many others), soy contains natural plant compounds (phytochemicals) called isoflavones. These are chemically similar to estrogen. This article talks about the link between estrogen and some cancers in women:

It is well established that estrogen is linked to hormonally-sensitive cancers in women, such as breast and endometrial cancer. Breast cells contain estrogen receptors, and when the “key” (estrogen) joins with the “lock” (the estrogen receptor), a series of signals are sent which can spur on estrogen-receptor (ER) positive breast tumor growth. Common risk factors for breast cancer include conditions that involve longer exposure of breast tissue to estrogen.

These risk factors include having children late, not having children, or post-menopausal obesity. I qualify under the second one of these since I have never given birth.

If soy acts like estrogen and estrogen is linked to these forms of cancer, then soy is also linked to these forms of cancer, right? Not so fast.

Studies on rats and mice seemed to suggest risk, but the consequences for humans are not easily read from these studies because they apparently metabolize isoflavones differently than humans.

And also here is more about isoflavones:

Isoflavones behave like very, very weak forms of the body’s own estrogen. Isoflavones compete for the same place on cells that estrogen does. That means isoflavones can affect the action of estrogen in your body, but not increase the level of estrogen. For example, since isoflavones can bind to estrogen receptors on breast cells, they prevent a woman’s own, more potent, estrogen from taking that spot. Some of the risks of excess estrogen, including breast and uterine cancer, are thought to be lowered in this way.

This makes it sound like soy is a good thing that can lower the risk of cancer in women. It appears also to lower the risk of prostate cancer in men:

…a study conducted in more than 12,000 Californian men found that those who drank a soy beverage daily, compared to those who never drank it, were 70 per cent less likely to develop prostate cancer. It’s thought that isoflavones can help keep testosterone levels in check (prostate cancer cells feed off testosterone). Soy beans also contain other phytochemicals that have cancer-fighting actions.

Large studies in Asia seem to suggest that women who consume soy have a lower incidence of breast cancer. But since soy consumption patterns differ between Asian women and North American women, it’s not clear how well these results translate over for non-Asian women. In other words, lifelong soy consumption might yield different results.

Most of what I read suggests that, at the very least “soy foods, such as tofu, tempeh, edamame and soymilk, are safe and do not pose cancer risk.” The question then becomes: “is including soy foods as part of your healthy eating habits ‘neutral’ in effect, or will it actually help reduce breast cancer risk? The answer, it seems, may be ‘it depends’…”

Variables include quantity of consumption, at what time in a woman’s life the soy is consumed (pre- or post-puberty, pre- or post-menopause), and genetic differences between people that have an impact on how they metabolize soy compounds.

I’m less concerned with whether soy reduces risk of breast cancer than I am with whether it poses a health risk. My primary reason for consuming in it the quantities that I do is because I am seeking a sound source of protein, not because I am seeking cancer prevention.

Based on what I’ve read so far, no study shows that soy poses a risk for humans and no link between soy and an increased risk of breast cancer has been established. This comes as a great relief to me. I will continue to enjoy the soy products that I’m consuming, and will also continue to monitor the latest research for any significant new findings.

Source materials:

Really? The Claim: Eating Soy Increases Risk of Breast Cancer (by Anahad O’Connor)

Soy and Breast Cancer: Where Are We Now? (by Karen Collins)

The Bottom Line on Soy and Breast Cancer Risk (by Marji McCullough)

Is Soy Milk Suitable for Men? (by Leslie Beck)