Contributed by Dr. Steven T. Devor – Director of Performance Physiology for MIT and OhioHealth, and Associate Professor of Exercise Physiology, Department of Human Sciences, and Department of Physiology and Cell Biology, The Ohio State University
Soybeans contain all of the essential amino acids necessary for human nutrition and have been grown and harvested for thousands of years. In the last ten years nutritional soy products have become increasingly popular, and are consumed daily in various forms by millions of Americans. Dietary soy products include soybeans (also called edamame), and other food products made from soybeans, including soymilk, tofu, tempeh, miso, and vegetarian meat and dairy substitutes like soy meats and soy cheeses.
Like nearly all foods, the most healthful soy choices are those that have been minimally processed so they retain all of their original nutrients. Additionally, since soybeans are often one of the most frequently genetically modified food products by the food industry, I recommend you always choose non-genetically modified organism (non-GMO) soy products.
One way to be certain you are getting non-GMO soy products is to buy organic, as the use of GMOs is prohibited in organic products. This means, for example, an organic farmer is not permitted to plant GMO seeds, an organic cow cannot be fed GMO alfalfa or corn, and an organic soup producer may not utilize any GMO ingredients. To meet the USDA organic regulations, farmers and food processors must demonstrate and document they are not using GMOs. And that they are protecting their products from contact with prohibited substances, such as GMOs, from the farm all the way to the table.
However, because soy products are so widely consumed, the issue has been raised as to whether they are safe. So what I would like to do is look at the latest medical and nutritional research findings in several important health related areas and summarize the primary results.
Cancer Prevention and Survival
Large scale epidemiological studies have consistently found that dietary soy protein intake can reduce the risk of developing certain cancers including breast, colon, and prostate. Studies consistently show that women who include soy products in their dietary intake are less likely to develop breast cancer, compared with other women. In January 2008, scientists at the University of Southern California found that when compared with women who consumed little or no soy products in their diets, women averaging one cup of soymilk or about one-half cup of tofu daily had approximately 30 percent less risk of developing breast cancer.
Women who have been previously diagnosed with breast cancer were studied in The Women’s Healthy Eating and Living Study. The results showed that women
received a significant advantage by incorporating nutritional soy products into their diets. Those who consumed the most soy products cut their risk of cancer recurrence or mortality in half. Similarly, a study in the Journal of the American Medical Association reported results based on 5,042 women previously diagnosed with breast cancer who were participating in the Shanghai Breast Cancer Survival Study (SBCSS) over a four-year period. The SBCSS demonstrated that women who regularly consumed soy products, such as soymilk, tofu, or edamame, had a 32 percent lower risk of recurrence and a 29 percent decreased risk of death, compared with women who consumed little or no soy.
Women who avoid soy products appear to have no advantage at all, while those who include soy products in their diets appear to cut their risk of cancer recurrence. A 2012 analysis that combined the results of prior studies, including a total of 9,514 women from the United States and China, showed that those women who consumed the most soy products were 25 percent less likely to have their cancer return, compared with those who tended to avoid soy products.
Other concerns include whether soy products have a negative effect on reproductive health. However, studies in both men and women have shown that soy protein intake did not hinder reproduction. Also, adults who had been fed soy infant formula as infants were found to have no difference in their reproductive health when compared with adults who had been fed cow milk formula.
Soy products have no adverse effects on men and their natural hormone profiles and may help prevent certain cancers in men. A meta-analysis published in the journal Fertility and Sterility, based on more than 50 treatment groups, showed that neither soy products nor isoflavone supplements from soy influenced testosterone levels in men. Additionally, an analysis of 14 studies published in the American Journal of Clinical Nutrition showed that increased intake of soy resulted in a 26 percent reduction in prostate cancer risk. Finally scientists noted a 30 percent risk reduction for prostate cancer with unfermented soy products such as soymilk and tofu.
Thyroid Health Clinical studies show that soy products do not cause hypothyroidism. However, soy isoflavones may take up some of the iodine that the body would normally use to make thyroid hormones. The same is true of fiber supplements and some other medications. In theory, then, people who consume soy might need slightly more iodine in their diets. (Iodine is found in many plant foods,
and especially in seaweed and iodized salt.) Soy products can also reduce the absorption of medicines used to treat hypothyroidism. Individuals that use these sorts of medications should check with their health care providers to see if their doses may need to be adjusted.
Soy products are typically high in protein, and some food manufacturers have exploited this fact, packing isolated soy protein into shakes and turning it into vegetarian meat substitutes. However, it may be prudent to avoid highly concentrated proteins from any source, including soy. It has long been known that milk from cows increases the amount of insulin-like growth factor in the bloodstream, and this compound is linked to higher cancer risk. Some evidence suggests that highly concentrated soy proteins (known as “soy protein isolate” on food labels) can do the same. Simple unprocessed soy products, such as tofu, tempeh, edamame, or miso, are likely the best choices. And this is almost certainly due to them being minimally processed.
Overall, dietary soy protein is well tolerated in the diets of most people, and contains all of the essential amino acids found in animal proteins. Because soy is a complete source of protein shown to lower cholesterol, I recommended it as a dietary substitution for higher saturated fat animal products. Evidence to date indicates that soy products may reduce the risk of breast cancer and breast cancer recurrence. They do not appear to have adverse effects on the thyroid gland, but may reduce the absorption of thyroid medications. Further, they do not appear to have any negative influence on the natural hormone profile in men. The benefits of soy products appear to relate to traditional soy products, not to concentrated soy proteins, or highly processed soy products.