RD Resources for Consumers: Health Effects of Soy

Soyfoods are the only commonly consumed Soyfoods and Cancer foods that provide significant amounts of isoflavones. In Asia, women who eat the most soy have a lower risk of breast cancer Isoflavones, plant chemicals that are also called compared to women who eat little soy. phytochemicals, have biological activity but are not But, evidence suggests that this is true nutrients. These compounds are referred to as plant only if they also ate soy early in life. Eating soyfoods estrogens or phytoestrogens. Foods that are rich in during childhood and the teen years may protect isoflavones include Asian soyfoods like tofu, soymilk, breast tissue from cancer. Beginning soy consumption miso, and tempeh. later in life doesn’t appear to have any effect on risk of Most of these soyfoods contain about 3½ milligrams getting breast cancer. of isoflavones for every gram of protein. For example, However, women with breast cancer who eat soyfoods ½ cup of regular tofu has about 8 grams of protein and are less likely to see their cancer return and are less about 28 milligrams of isoflavones. Certain types of food likely to die from their cancer. The American Cancer processing reduce the amount of isoflavones in foods. Society states that women with breast cancer can Products such as soy-based meat analogs often have safely consume soyfoods. much lower amounts of isoflavones. Men may also benefit from eating soyfoods. In Asia, In Japan and urban areas of China, people consume men who eat the most soy have about one-half the about one to two servings of soyfoods per day. Older risk of getting prostate cancer compared to men who people whose diets are more traditional often have eat little soy. For men who have prostate cancer, soy much higher intakes than younger people. Since people may be helpful as well. One small study found that in the U.S. eat few soyfoods, their isoflavone intake is soy isoflavones reduced some of the side effects of very low. treatment for prostate cancer. Both isoflavones and the hormone estrogen bind to Soyfoods and Heart Health estrogen receptors in the breast and other tissues. There are two types of estrogen receptors in the body. Adding soy protein to diets can lower These are estrogen receptor-alpha and estrogen- blood cholesterol by as much as receptor-beta. Estrogen binds equally to both types, four percent. When soy is consumed but isoflavones prefer estrogen receptor-beta. In tissues in place of meat and other foods that have mostly estrogen-receptor alpha, estrogen has high in saturated fat, it reduces biological effects, but isoflavones may not. This means cholesterol even more. In older women, the isoflavones that isoflavones don’t always act like estrogen. The in soyfoods may improve the health of the arteries, effect of isoflavones likely depends in part on the type making them more flexible. of estrogen receptors in different tissues. RD Resources for Consumers: Health Effects of Soy Safety of Soyfoods Isoflavone Content of Selected Foods Human studies support Food Total Isoflavone the safety of both Content (mg) isoflavone supplements Soybeans, mature, ½ cup, cooked 47 and soyfoods. Isoflavones Tofu, silken, ½ cup 31.2* have no effect on Tofu, regular, ½ cup 29.7* estrogen levels in women Edamame (green soybeans), 28 or testosterone levels in ½ cup, cooked men. Clinical studies show Soymilk, 8 oz 23* they also have no effect Soy imitation chicken, 3 oz 13.2* on sperm or semen. Soy hotdog, 2 oz 9.7* Soyfoods have no effect on thyroid function in Soy cheese, 1 oz 8.8* people with normal thyroids. However, for those Soy veggie burger, 3 oz 8.4* who take thyroid pills, changes in soyfood intake Soy oil, 1 tbsp 0 may require changes in the amount of medicine needed. Your doctor can make these adjustments. * isoflavone content varies according to brand Soy infant formula has been used by more than 20 million Americans over the past five decades. Evidence clearly shows that it produces normal Soyfoods and Bone Health growth and development. In recent years the high In older Asian women, eating more soy isoflavone content of soy has made soy infant is linked to lower rates of bone fracture. formula controversial. However, the position of The same results have not been seen the American Academy of Pediatrics is that there is in studies among western women little concern about its safety. where researchers fed large amounts of isoflavones to postmenopausal women. This may be another case Optimal Soyfood Intake where lifelong soyfood intake is helpful but taking Average soyfood intake among Japanese adults supplements or eating soyfoods as an adult is not. ranges from about 1 to 2 servings per day. This can serve as a guide for a healthy amount of soy Menopause Symptoms to consume. But some studies show that disease The hot flashes that plague many rates are lower with greater soyfood intake, about women after menopause are rare 2-3 servings per day. Studies have not shown any in Japan. Isoflavones may be part of differences between the health effects of organic the reason for this. Results of more soyfoods versus those that are conventionally than 50 studies conducted mostly in grown. Western countries show that isoflavone supplements can reduce both the number and severity For More Information of hot flashes by about 50 percent. About two to three Soy: What’s the Harm by Jack Norris, RD servings of soyfoods per day may be helpful. http://www.veganhealth.org/articles/soy_wth

RD Resources are a project of the Vegetarian Nutrition Dietetic Practice Group. More topics available at www.VegetarianNutrition.net. Professional resources also available for members at www.VNDPG.org. © 2015 by VN DPG. Written by: Virginia Messina, MPH, RD Expires January 2020.