Soy comes from soybeans. The beans can be processed into soy protein, which is a powder; soymilk, which is a beverage that may or may not be fortified with extra calcium from the soybeans; or soy fiber, which contains some of the fibrous parts of the bean.
Soy is taken by mouth for high cholesterol, high blood pressure, and preventing diseases of the heart and blood vessels. It is also used for type 2 diabetes and kidney disease associated with diabetes, diabetes during pregnancy, Alzheimer's disease, asthma, as well as preventing weak bones (osteoporosis), preventing joint pain and stiffness in people with arthritis, and slowing the progression of kidney disease. Soy is also taken by mouth to prevent different kinds of cancer.
Soy is also taken by mouth for treating constipation, diarrhea, Crohn's disease, hepatitis C, irritable bowel syndrome, metabolic syndrome, fibromyalgia, weight loss, enlarged prostate, as well as decreasing protein in the urine of people with kidney disease, improving memory and mental function, improving muscle strength, and treating muscle soreness caused by exercise.
Women take soy by mouth for breast pain, preventing hot flashes after breast cancer, menopausal symptoms, premenstrual syndrome (PMS), disorders of the ovary such as polycystic ovary syndrome, and migraine headaches during menstruation.
Soy is used as a milk substitute in infant feeding formulas, and as an alternative to cow's milk. It is fed to infants who are unable to process the sugar galactose, who are lactose intolerance, who have a condition called hereditary lactase deficiency, or who have infant colic.
Soy is applied to the skin to improve photo-aged skin or wrinkled skin.
Soy is applied inside the vagina to treat vaginal swelling due to decreased lubrication and thinning vaginal tissue (vaginal atrophy).
In foods, soybeans are eaten boiled or roasted. Soy flour is used as an ingredient in foods, beverages, and condiments.
The active ingredients in soy are called isoflavones. A study of the quality of commercially available soy supplements suggests that less than 25% of products contain within 90% of labeled isoflavone content. Paying more for a product doesn't necessarily guarantee that the content shown on the label is accurate.
Natural Medicines rates effectiveness based on scientific evidence according to the following scale: Effective, Likely Effective, Possibly Effective, Possibly Ineffective, Likely Ineffective, Ineffective, and Insufficient Evidence to Rate.
The Effectiveness ratings for Soy are as follows:
Possibly Effective for...
Possibly Ineffective for...
Insufficient Evidence to Rate Effectiveness for...
More evidence is needed to rate soy for these uses.
Consuming foods containing soy protein or taking soy protein products is LIKELY SAFE. Taking dietary supplements with soy extracts is POSSIBLY SAFE when used short-term (up to 6 months). Soy can cause some mild side effects such as constipation, bloating, and nausea. It can also cause allergic reactions involving rash, itching, and anaphylaxis in some people.
Long-term use of high doses of soy dietary supplements is POSSIBLY UNSAFE. There is concern that taking high doses might cause abnormal tissue growth in the uterus.
Pregnancy and breast-feeding: Soy protein is LIKELY SAFE to be used during pregnancy and breast-feeding when consumed in amounts normally found in food. However, soy may be POSSIBLY UNSAFE when used during pregnancy in medicinal amounts. Higher doses during pregnancy might harm development of the baby. Not enough is known about the safety of higher doses during breast-feeding. Stay on the safe side and avoid larger doses.
Children: Soy is LIKELY SAFE for children when used in amounts commonly found in food or infant formula. Using soy formula does not seem to cause health or reproductive problems later in life. However, soymilk that is not designed for infants should not be used as a substitute for infant formula. Regular soymilk could lead to nutrient deficiencies.
Soy is POSSIBLY UNSAFE when used as an alternative to cow’s milk in children who are allergic to cow’s milk. Although soy protein-based infant formulas are often promoted for children with milk allergy, these children are often allergic to soy as well.
Don’t give children soy in amounts larger than what is found in food or formula. Researchers don’t know whether soy is safe for children at higher doses.
Cystic fibrosis: Soymilk can interfere with the way children with cystic fibrosis process protein. Don’t give these children soy products.
Breast cancer: The effects of soy in people with breast cancer are unclear. Some research finds that soy might “feed” certain breast cancers because it can act like estrogen. Other studies have found that soy seems to protect against breast cancer. The difference in effects might have something to do with the amount taken. Because there isn’t enough reliable information about the effects of soy in women with breast cancer, a history of breast cancer, or a family history of breast cancer, it’s best to avoid using soy supplements until more is known.
Endometrial cancer: Long-term use of concentrated soy isoflavone tablets might increase the occurrence of precancerous changes in the tissue lining the uterus. Don’t take concentrated soy isoflavone supplements if you have endometrial cancer.
Kidney failure: Soy contains a chemical called phytoestrogens. Very high levels of phytoestrogens can be toxic. People with kidney failure who use soy products might be at risk for blood levels of phytoestrogens becoming too high. If you have kidney failure, avoid taking large amounts of soy.
Kidney stones: There is some concern that soy products might increase the risk of kidney stones because they contain large amounts of a group of chemicals called oxalates. Oxalates are the main ingredient in kidney stones. Another concern is that people with serious kidney disease aren’t able to process some of the chemicals in soy. This could lead to dangerously high levels of these chemicals. If you have a history of kidney stones, avoid taking large amounts of soy.
Milk allergy: Children who are very allergic to cow’s milk might also be sensitive to soy products. Use soy products with caution.
Urinary bladder cancer: Soy products might increase the chance of getting bladder cancer. Avoid soy foods if you have bladder cancer or a high risk of getting it (family history of bladder cancer).
Under-active thyroid (hypothyroidism): There is a concern that taking soy might make this condition worse.
Asthma: People with asthma are more likely to be allergic to soy hulls. Avoid using soy products.
Hay fever (allergic rhinitis): People with hay fever are more likely to be allergic to soy hulls.
Diabetes: Soy might increase the risk of blood sugar levels becoming too low in people with diabetes who are taking medication to control blood sugar.
Interaction Rating = Moderate Be cautious with this combination.
Talk with your health provider.
Antibiotics are used to reduce harmful bacteria in the body. Antibiotics can also reduce friendly bacteria in the intestines. Friendly bacteria in the intestines seem to help increase the effectiveness of soy. By reducing the number of bacteria in intestines, antibiotics might decrease the effectiveness of soy. But it is too soon to know if this interaction is a big concern.
Large amounts of soy might have some of the same effects as estrogen. But soy isn't as strong as estrogen pills. Taking soy along with estrogen pills might decrease the effects of estrogen pills.
Some estrogen pills include conjugated equine estrogens (Premarin), ethinyl estradiol, estradiol, and others.
Levothyroxine is used for low thyroid function. Soy seems to decrease how much levothyroxine is absorbed by the body in infants, but not adults. If levothyroxine absorption is decreased, it might not work as well. The dose of levothyroxine may need to be adjusted if soy is being used regularly, such as in soy-based formulas. If soy is used occasionally, levothyroxine and soy should be taken at least 4 hours apart.
Some brands that contain levothyroxine include Armour Thyroid, Eltroxin, Estre, Euthyrox, Levo-T, Levothroid, Levoxyl, Synthroid, Unithroid, and others.
Interaction Rating = Minor Be watchful with this combination.
Talk with your health provider.
Some medications are changed and broken down by the liver. Soy might increase how quickly the liver breaks down some medications. But it is too soon to know if this interaction occurs in all people or if it affects how well the medication works.
Some medications changed by the liver include carvedilol (Coreg), fluvastatin (Lescol), losartan (Cozaar), phenytoin (Dilantin), and many others.
Interaction Rating = Major Do not take this combination.
Fermented soy products such as tofu and soy sauce contain tyramine. Tyramine is a naturally occurring chemical that is involved in blood pressure regulation. Tyramine is broken down by monoamine oxidase. Some medications for depression (MAOIs) can decrease the breakdown of tyramine. Consuming more than 6 mg of tyramine while taking one of these medications can increase the risk of serious side effects such as blood pressure getting too high. The amount of tyramine in fermented soy products is usually small, often less than 0.6 mg per serving; however, there can be variation depending on the specific product used, storage conditions, and length of storage. Storing one brand of tofu for a week can increase tyramine content from 0.23 mg to 4.8 mg per serving. If you take one of these medications, avoid fermented soy products that contain high amounts of tyramine.
Some of these medications include phenelzine (Nardil), tranylcypromine (Parnate), and others.
Soy can decrease blood sugar levels. Diabetes medications are also used to lower blood sugar. Taking soy along with diabetes medications might cause your blood sugar to be too low. Monitor your blood sugar closely. The dose of your diabetes medication might need to be changed.
Some medications used for diabetes include glimepiride (Amaryl), glyburide (DiaBeta, Glynase PresTab, Micronase), insulin, metformin (Glucophage), pioglitazone (Actos), rosiglitazone (Avandia), and others.
Soy might reduce blood pressure. Using soy with drugs that lower blood pressure can increase the effects of these drugs and may lower blood pressure too much.
Some medications for high blood pressure include captopril (Capoten), enalapril (Vasotec), losartan (Cozaar), valsartan (Diovan), diltiazem (Cardizem), amlodipine (Norvasc), hydrochlorothiazide (HydroDiuril), furosemide (Lasix), and many others.
Bone loss might occur in women with osteoporosis who combine progesterone with soymilk containing isoflavones.
Some types of cancer are affected by hormones in the body. Estrogen-sensitive cancers are cancers that are affected by estrogen levels in the body. Tamoxifen (Nolvadex) is used to help treat and prevent these types of cancer. Soy seems to also affect estrogen levels in the body. By affecting estrogen in the body, soy might decrease the effectiveness of tamoxifen (Nolvadex). Do not take soy if you are taking tamoxifen (Nolvadex).
Warfarin (Coumadin) is used to slow blood clotting. Soy has been reported to decrease the effectiveness of warfarin (Coumadin). Decreasing the effectiveness of warfarin (Coumadin) might increase the risk of clotting. It is unclear why this interaction might occur. Be sure to have your blood checked regularly. The dose of your warfarin (Coumadin) might need to be changed.
Early research suggests that soy can increase urine production. The effects seem to be similar to those of "water pills." Taking soy along with "water pills" might increase the risk for side effects.
Soy might lower blood pressure. It has the potential to add to blood pressure lowering effects of other herbs and supplements that also lower blood pressure. Other herbs and supplements that can lower blood pressure include andrographis, casein peptides, cat's claw, coenzyme Q-10, fish oil, L-arginine, lycium, stinging nettle, theanine, and others.
Soy can lower blood glucose levels. Using it with other herbs or supplements that have the same effect might cause blood sugar levels to drop too low. Some herbs and supplements that can lower blood sugar include devil's claw, fenugreek, garlic, guar gum, horse chestnut, Panax ginseng, psyllium, and Siberian ginseng.
Soy might increase or decrease the absorption of iron. The potential differences might be related to the presence of a chemical called phytic acid, which is found in some soy products. Phytic acid reduces iron absorption. However, the amount of phytic acid in soy is reduced if soy is fermented.
Soy can reduce the availability of manganese in the body. This is due to the presence of a chemical called phytic acid, which is found in some soy products. Phytic acid reduces manganese absorption. However, the amount of phytic acid in soy is reduced if soy is fermented.
Soy can reduce zinc levels in the blood. This is due to the presence of a chemical called phytic acid, which is found in some soy products. Phytic acid reduces zinc absorption. However, the amount of phytic acid in soy is reduced if soy is fermented.
Soy protein reduces the absorption of iron from plant sources.
The following doses have been studied in scientific research:
Soy foods contain variable amounts of isoflavones. Soy flour contains 2.6 mg isoflavones per gram of soy flour, fermented soybeans contain 1.3 mg per gram, boiled soybeans contain 0.6 mg per gram, soymilk contains 0.4 mg per gram, soybean curd contains 0.5 mg per gram, fried soybean curd contains 0.7 mg per gram, soybean paste contains 0.4 mg per gram, and soy sauce contains 0.016 mg per gram.
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