Soy: Myth vs. Fact
Soy foods include a wide variety of items including edamame (whole soybeans), soy flour, soymilk, tofu, soy protein isolates, textured vegetable protein, soy oil, and fermented soy products like tempeh and miso.
Then there are all of those processed items that are made from soy products including soy "meats," soy cheeses, soy "ice creams," to name a few. With all of these products in the marketplace, soy has become a popular target for many false claims. Let’s take a look at some of the more often repeated myths.
Myth #1: The plant estrogens contained in soy promote the growth of certain cancers.
Fact: Christa Novelli, M.P.H., writes, “While a few studies have found that animals who are implanted with cancer cells and then fed soy protein isolates show increased growth of the cancer cells, the majority of the research shows soy to have an inhibitory effect on cancer growth.” A recent study published in the journal Cancer Epidemiology, Biomarkers, and Prevention looked at lifetime soy intake and breast cancer risk among American women of Asian descent. The researchers found that soy intake during childhood was associated with decreased breast cancer risk, suggesting early exposure may have the most positive preventive effect. Still, women who have a history of breast cancer in their family may want to be cautious with their soy consumption and be careful not to consume too much of the overly-processed forms of soy.
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Myth #2: Soy leads to less masculine males, infertility in women, and hormone imbalances.
Fact: A few studies of the offspring of rodents that were fed soy isolates during pregnancy and lactation, and rodents who were injected with soy isoflavones in infancy, have found evidence of negative reproductive outcomes. However, Novelli writes that there is a significant difference in the amount of estrogen to which fetal rats and fetal humans are normally exposed. Further, these results have not been replicated in humans, and research on human infants exposed to soy in utero and during infancy suggests no statistically significant differences in sexual development or reproductive health--other than one study that found slightly longer menstrual periods in women who were fed soy formula as infants.
Myth #3: Soy causes thyroid cancer and/or reduces thyroid function.
Fact: Some older studies of infants fed soy-based formula not fortified with iodine showed reduced thyroid functioning. Further research has shown that, in the absence of an iodine deficiency, soy does not reduce thyroid function. As for the claim of thyroid cancer, large population-based studies show that soy not only does not increase the risk of thyroid cancer, but it actually has a protective effect.
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Myth #4: Soy causes Alzheimer's disease, dementia, and reduced brain function.
Fact: The truth actually is the complete opposite. The majority of the research relating to soy consumption and brain health shows soy to have a protective effect on the brain, not a damaging effect. One article that has frequently been cited as "proof" that soy damages the brain found some possible benefits and some possible harm for male rats fed a diet high in phytoestrogens. The researchers who conducted this study have also conducted a number of other studies which concluded that soy phytoestrogens are protective to the brain.
Myth #5: Soy has no beneficial effect on heart health.
Fact: In April 2009, a Journal of Nutrition report summarized findings from the 8th International Soy Symposium held the previous year in Tokyo. The most comprehensive systematic review presented at the symposium, covering the years 1978 through the present, found that about two-thirds of the studies judged to be of high or moderate quality showed a statistically significant reduction in either total cholesterol or LDL (bad) cholesterol. In addition, the meta-analysis presented showed a net reduction in LDL (bad) cholesterol of approximately 5 percent, which is in line with other data.
The Journal of Nutrition. (2010). “Soy Summit: Exploration of the Nutrition and Health Effects of Whole Soy.” Retrieved from http://jn.nutrition.org/content/140/12/2288S.full.pdf+html
PR Newswire. (2008 March 26). “New Report in the Journal of Nutrition Shows Soy is Beneficial for Heart and Bone Health.” Retrieved from http://www.prnewswire.com/news-releases/new-report-in-the-journal-of-nutrition-shows-soy-is-beneficial-for-heart-and-bone-health-61954012.html
VegFamily. “Being Vegan and Eating Soy: Myths, Truths, and Everything in Between.” Retrieved from http://www.vegfamily.com/health/vegan-soy-information.htm