Soy is a near-perfect food, with rich sources of high-quality protein, fiber, and 12 essential nutrients. As a result, it’s one of the most widely studied foods on the planet, and also one of the most controversial: A number of health scares about soy have circulated, from its feminizing effects on males to promoting breast cancer. Some have a thread of credibility, while most are pure myth.
Soy safety in question. Anti-soy messages are rampant on the internet, for a number of reasons, including the high amount of genetically modified soy grown in the U.S. (primarily for animal feed) and poor interpretation of results of studies on soy. However, it’s true that in the late 1990s researchers raised questions about the safety of soy foods because of soy’s rich isoflavone (phytoestrogen) content, which was thought to promote cancer in rats. Scientists now know that rodents metabolize phytoestrogens differently than humans, and that they bind to a different estrogen receptor site that may actually suppress tumors. “There has been a mistaken equating of soy phytoestrogens with the hormone estrogen. However, extensive human research has shown isoflavones and estrogen often act very differently,” says Mark Messina, Ph.D., professor at Loma Linda University, and executive director of the Soy Nutrition Institute.
• Breast cancer. While earlier studies in mice suggested that soy isoflavones might stimulate the growth of estrogen-sensitive breast tumors, recent research quells these fears. “In the past few years, five population studies and one major analysis of several studies all found that women who’d had estrogen receptor-positive breast cancer showed either decreased recurrence or decreased deaths with moderate consumption of soy foods, or no effect,” says Karen Collins, M.S., R.D.N., nutrition advisor for the American Institute for Cancer Research (AICR). Both the AICR and the American Cancer Society conclude that moderate intake of soy is safe, even for breast cancer patients.
• Adverse hormonal effects in men. There is no proof to support this popular notion. “Clinical research shows that neither soy nor isoflavones lower testosterone or adversely affect sperm or semen. In fact, Italian researchers suggested that isoflavones could be a treatment for low sperm count,” says Messina.
Soy’s health potential. First cultivated in China in 1100 BC, soy is a staple in many Asian countries where chronic disease rates are lower than in the U.S. While studies have linked soy with enhanced bone health, reduced wrinkling and fewer hot flashes, some of the most intriguing associations include heart health and cancer.
• Heart health. There are many ways that soy may protect the heart. “Soy foods provide heart-healthy fat, lower blood cholesterol and possibly blood pressure; the isoflavones appear to improve arterial health in postmenopausal women,” says Messina. In the Women’s Isoflavone Soy Health study, which involved 350 healthy postmenopausal women ages 45-92, soy was found to inhibit atherosclerosis (Stroke, 2011).
• Cancer. Some studies suggest that soy may help protect against certain types of cancer, such as breast, prostate, and lung. It may be that soy intake during childhood or adolescence is crucial for gaining protection against breast cancer, in particular. “It’s nearly impossible to reach definitive conclusions about diet and cancer relationships because such data require large, long-term clinical trials with cancer as an endpoint,” says Messina.
Soy is safe. For now, science indicates moderate consumption of soy is safe—and healthy. According to the AICR, a moderate level is 1 – 2 daily servings of whole soy foods, such as tofu, soy milk, and edamame—though they concede that some studies indicate even up to three daily servings is safe. One serving (see chart) provides 7 g of protein and 25 mg isoflavones.
—Sharon Palmer, R.D.