The Truth About Soy
According to Jack Norris, Registered Dietitian and the President and Executive Director of Vegan Outreach, there is a great deal of controversy surrounding soy foods, mostly due to their isoflavones which can bind to estrogen receptors and affect thyroid hormone.
There is significant evidence that eating moderate amounts (one to two servings per day) of traditional soy foods, whether fermented or not, can reduce the risk of prostate cancer and can lower LDL cholesterol.
People have been concerned that moderate amounts of soy could increase the risk of breast cancer or be harmful to women with breast cancer, especially if their cancer is estrogen receptor positive. However, the research to date has been quite reassuring, showing mostly benefits for breast cancer prevention.
Everyone who eats soy should make sure they are getting enough iodine. People with hypothyroidism might need their synthetic thyroid hormone dosage adjusted if they start eating more soy due to the possibility that soy might interfere with it. There is some concern that eating soy could push some people with subclinical hypothyroidism into overt hypothyroidism, so limiting soy for such people might be a good idea.
Soy infant formula has been shown to be safe except possibly for infants with congenital hypothyroidism, whose thyroid function should be monitored. Soy formula is not intended for pre-term infants.
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