Soy is an Asian plant found in a variety of foods, including tofu, miso and protein powder. It is often used in supplement form to treat hormonal imbalances, such as menopause, in women. Ongoing research is attempting to determine its exact effects on estrogen. While soy is generally considered safe, it may cause negative effects in certain people, especially if used long-term. This is especially true for soy supplements, which is why you should check with your doctor before starting them.
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Soy products may interfere with the way your thyroid works. Soy contains isoflavones. If you take in a large amount of isoflavones, they may stop certain thyroid hormones from working, according to a study review published in the June 2002 issue of "Environmental Health Perspectives." This is especially true if you are also deficient in iodine. This combination can cause symptomatic hypothyroidism and goiters. A goiter is a swollen thyroid gland, and develops when there isn't enough thyroid hormone in the body.
Soy can be a highly reactive allergen for some people. If you are allergic to tree nuts, eggs, fish, milk, peanuts or wheat you may have an allergic reaction to soy. Reactions range from mild to severe. Occasionally, people develop anaphylactic reactions to soy. Anaphylaxis is a severe, full-body allergic reaction characterized by hives, difficulty breathing, swelling of the throat and circulatory collapse. It can cause death if it is not promptly treated.
People who consistently take in large amounts of soy products, especially tofu, may be up to four times more likely to develop problems with cognitive thinking later in life, according to a review published in the April 2000 issue of "Journal of the American College of Nutrition." A high intake of soy is also associated with low brain weight and enlarged ventricles in the brain. Memory and learning aptitude are most affected.
Other Negative Effects
Eating a large amount of soy products can have immediate adverse effects in some people. Soy can cause gastrointestinal problems, such as nausea and constipation, cause you to have trouble falling or staying asleep and migraine headaches. This is especially true if you are not use to eating it. You can avoid these effects by adding soy to your diet slowly so your body has time to adjust.