“The Shocking Effect of Soy on BOTH Sexes!”

 “10 Reasons to Never Drink Soy Milk”

We’ve all seen news headlines like these and we’re all thoroughly confused. Soy is one of the most widely consumed foods, but even today it remains shrouded in mystery. It makes up everything from animal feed to biofuel to the Silk soymilk in our dining halls, yet we still aren’t sure whether soy is healthy or harmful.

Soybeans are touted as the “king of beans,” containing 38% protein — twice as much as pork, three times more than eggs, and twelve times more than milk. Soy is known to lower cholesterol and blood pressure, help treat type II diabetes, and prevent osteoporosis and cancer. Yet, there are concerns that soy consumption may have health hazards, particularly when it comes to male fertility and potential feminizing effects on men. So, is soy safe? The answer requires a closer look at the molecular interactions between soy and the human body.

The Benefits

Soy contains isoflavones, a class of phytoestrogens. These compounds are similar in structure to estrogen. As a result, soy isoflavones are also known as weak estrogens. During female reproductive years, estrogen secreted by the ovaries regulates the menstrual cycle and is crucial for bone formation. Estrogen also plays a role in blood clotting and even affects mood — chronically low estrogen levels are linked to depression. Estrogen is also important for sperm maturation in males.

As women enter menopause, estrogen levels decline, which explains why postmenopausal women are susceptible to mood swings and osteoporosis. For years, hormone replacement therapy (HRT), a treatment involving prescribed female hormones like estrogen, was commonly used to combat these symptoms.

However, in a 2002 study conducted by the U.S. National Institutes of Health for the Women’s Health Initiative, it was found that menopausal women taking HRT experienced higher rates of breast cancer and cardiovascular disease from blood clots in the heart, brain, and lungs. A call was made for safer alternatives as a result. Drug companies scrambled to produce a selective estrogen receptor modulator — a compound that has pro-estrogenic effects in some tissues, like bone, which could enhance bone density, but antiestrogenic effects in other tissues, like the breast, to prevent breast cancer.

This is where phytoestrogens enter the picture. Phytoestrogens are plant-derived compounds that mimic estrogen because they have similar structures. Soy products are rich in isoflavones — a class of phytoestrogens. These appear to function as natural selective estrogen receptor modulators. The original theory for how soy phytoestrogens control breast cancer growth was that they compete with our own estrogens for binding with the estrogen receptor. This accounts for soy’s antiestrogenic effects, but how can its pro-estrogenic effects on other tissues, like bone, be explained?

This mystery was solved in a Swedish study conducted by the Karolinska Institute Department of Biosciences and Nutrition, which suggests there are two types of estrogen receptors in the body — the classic estrogen receptor alpha and the newly discovered estrogen receptor beta. Soy phytoestrogens preferentially bind to the beta receptors. Essentially, the effects of estrogen and phytoestrogens depend on which type of receptor certain tissues in the body contain.

Estrogen binds to alpha receptors. In HRT, the synthetic estrogen increases the risk of fatal blood clots because it prompt the liver to release clotting factors. But the liver only contains alpha receptors. Similarly, the uterus only contains alpha receptors, explaining why HRT increases the risk of endometrial cancer, while consuming soy phytoestrogens actually reduces the risk of endometrial cancer, and offers protection from gynecological cancers in general. In fact, in a meta-analysis published in An International Journal of Obstetrics and Gynaecology, women with the highest intake of soy who averaged 136.4 grams of soy foods per day had 30% lower rates of endometrial cancer and cut their ovarian cancer in risk in half compared to the women who ate the least amount of soy (at about 7.1 grams of soy foods per day).

By contrast, bone tissue only contains beta estrogen receptors, explaining why soy phytoestrogens appear to significantly increase bone mineral density, without the negative effects of estrogen drugs. This is consistent with data published in the Journal of Bone and Mineral Research suggesting that just a single serving of soy per day — about a cup of soymilk — not only prevents bone loss, but actually enhances bone formation even more than the synthetic hormones in HRT.

The Myths

Concerns about safe soy consumption still linger, though. Studies conducted on rodents have found that high doses of phytoestrogens impair male rats’ ability to produce offspring. However, in 2010, a meta-analysis conducted on men and published in the journal Fertility and Sterility suggested otherwise. Its findings showed that soy isoflavones have no effect on free testosterone levels, male estrogen levels, sperm count and motility, or semen quality. Researchers concluded that the contrasting results in rodent studies were not applicable to men because of differences in isoflavone metabolism between rodents and humans as well as the excessively high amounts of isoflavones the rodents were exposed to.

An upper limit on soy consumption does appear to exist.

The Catch

An upper limit on soy consumption does appear to exist. A 2007 study published in the Nutrition and Cancer journal found that soy isoflavones increase production of insulin-like growth factor-1, or IGF-1. Though this well known growth factor is important for development in children, according to joint studies between Harvard Medical School, Harvard School of Public Health, and Brigham and Women’s Hospital, the growth factor increases tumor formation in adults.

Other Harvard studies published in the Journal of the National Cancer Institute found that high levels of IGF-1 significantly increases risk of colorectal, breast, and prostate cancer. In fact, one study found that men with the highest levels of IGF-1 had more than four times the risk of prostate cancer than those with the lowest levels.

What Now?

It’s clear that consuming too much soy may neutralize its anti-cancer and bone building benefits, so how do we determine a safe amount to eat? In the 2007 study on soy and IGF-1, subjects consumed 7-18 servings of soy per day for a year. That’s the equivalent of four quarts of soy milk each day — unsurprisingly, having that much soy is harmful.

Findings published in The Journal of Clinical Endocrinology and Metabolism showed that 5-10 servings of soy per day also increased IGF-1 levels. However, a Japanese study found that three servings still offered soy’s benefits but had no effect on IGF-1.

Where do we go from here? To reap the health benefits of soy without overstepping safe levels, around 2-3 servings a day seems optimal. This could be as simple as a cup of soymilk and a tennis ball-size serving of edamame.

Like many cases in nutrition, it appears that moderation is key.

About The Author

Elisabeth Slighton