Overview

Soy (Glycine max) comes from soybeans. The beans are a legume that come from China. They can be processed into soy protein, soy milk, or soy fiber.

Soy contains isoflavones which are changed in the body to phytoestrogens. Phytoestrogen molecules are similar in chemical structure to the hormone estrogen. In some cases, these phytoestrogens can mimic the effects of estrogen. In other cases, these phytoestrogens can block the effects of estrogen.

Soy is used for high cholesterol, high blood pressure, heart disease, diabetes, symptoms of menopause, and premenstrual syndrome (PMS). It is also used for many other conditions, but there is no good scientific evidence to support many of these uses.

Uses & Effectiveness

Possibly Effective for

  • Breast cancer. Eating large amounts of soy might help prevent breast cancer or breast cancer recurrence in some people. But taking soy supplements doesn't seem to help.
  • Long-term kidney disease (chronic kidney disease or CKD). Taking soy protein by mouth seems to reduce protein in the urine and other measures of kidney function in people with CKD.
  • Diabetes. Eating a diet high in soy seems to reduce the risk of developing diabetes. It is unclear if eating a diet high in soy reduces the risk of developing gestational diabetes. It is also unclear if taking soy protein improves blood sugar or cholesterol levels in people who already have diabetes.
  • Diarrhea. Feeding infants with formula supplemented with soy fiber seems to reduce diarrhea. But taking soy by IV doesn't seem to reduce diarrhea in adults. IV products can only be given by a healthcare provider.
  • An inherited disorder in which the body is unable to properly digest the sugar galactose (galactosemia). Feeding a soy-based formula to infants who have galactosemia might help to reduce symptoms.
  • High levels of cholesterol or other fats (lipids) in the blood (hyperlipidemia). Eating soy protein in place of other dietary protein or using soy fiber products seems to slightly reduce total cholesterol and low-density lipoprotein (LDL or "bad") cholesterol. But taking supplements containing purified soy isoflavones by mouth don't seem to help.
  • High blood pressure. Eating soy protein seems to reduce blood pressure by a small amount in people with slightly high blood pressure.
  • Inability to properly digest the sugar lactose (lactose intolerance). Feeding a soy-based formula to infants who have lactose intolerance might help to reduce symptoms.
  • Symptoms of menopause. Eating soy protein or taking soy isoflavone extracts by mouth seems to help reduce hot flashes caused by menopause in some people.
  • A grouping of symptoms that increase the risk of diabetes, heart disease, and stroke (metabolic syndrome). Eating soy protein products helps lower blood sugar levels and improve other measures of the disease in people with metabolic syndrome.
  • Muscle strength. Taking soy protein by mouth seems to increase muscle strength. It seems to work as well as whey protein, dairy protein, and beef protein.
  • Weak and brittle bones (osteoporosis). Taking soy protein or soy extract by mouth can increase bone mineral density (BMD) or slow BMD loss in females near or beyond menopause.

Possibly Ineffective for

  • Enlarged prostate (benign prostatic hyperplasia or BPH). Taking soy by mouth doesn't seem to improve urination or other symptoms in people with an enlarged prostate.
  • Hot flashes in people treated for breast cancer. Taking soy by mouth doesn't reduce hot flashes related to breast cancer.
  • Colon cancer, rectal cancer. Taking soy by mouth does not reduce the risk for colorectal cancer.
  • Muscle soreness caused by exercise. Taking soy isoflavone extract by mouth before exercising doesn't seem to prevent muscle soreness.

There is interest in using soy for a number of other purposes, but there isn't enough reliable information to say whether it might be helpful.

Side Effects

When taken by mouth: Soy is commonly consumed in foods. Dietary supplements containing soy extracts are possibly safe when used for up to 6 months. Soy can cause some mild stomach and intestinal side effects such as constipation, bloating, and nausea. It can also cause allergic reactions involving rash, itching, and breathing problems in some people.

Special Precautions and Warnings

When taken by mouth: Soy is commonly consumed in foods. Dietary supplements containing soy extracts are possibly safe when used for up to 6 months. Soy can cause some mild stomach and intestinal side effects such as constipation, bloating, and nausea. It can also cause allergic reactions involving rash, itching, and breathing problems in some people.

Pregnancy: Soy is commonly consumed in foods. However, soy is possibly unsafe when used in the larger amounts found in medicine when pregnant. Higher doses during pregnancy might harm development of the baby.

Breast-feeding: There isn't enough reliable information to know if soy is safe to use in the larger amounts found in medicine when breast-feeding. Stay on the safe side and stick to food amounts.

Children: Soy is commonly consumed in foods. Giving infants soy formula doesn't seem to cause health or reproductive problems later in life. But soy milk that is not designed for infants should not be used as a substitute for infant formula. Regular soy milk 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.

Breast cancer: The effects of soy in people with breast cancer are unclear. Because there isn't enough reliable information about the effects of soy in females 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.

Allergy to peanuts and related plants: Soy might cause serious allergic reactions in people who are allergic to peanuts and other members of the Fabaceae plant family.

Under-active thyroid (hypothyroidism): Some people with this condition also have low iodine levels. Taking soy might make this condition worse in people who have low iodine levels.

Kidney failure: Soy contains a chemical called phytoestrogens. People with kidney failure who use soy products might have blood levels of phytoestrogens become too high. If you have kidney failure, avoid taking large amounts of soy.

Kidney stones: Soy products might increase the risk of kidney stones. Soy products contain large amounts of chemicals called oxalates. Oxalates are the main ingredient in kidney stones. 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.

Bladder cancer: Soy products might increase the chance of getting bladder cancer. Avoid soy foods if you have bladder cancer or are at high risk of getting it.

Interactions

    Moderate Interaction

    Be cautious with this combination

  • Medications for depression (MAOIs) interacts with Soy

    Fermented soy products such as tofu and soy sauce contain tyramine. Tyramine is a naturally occurring chemical that is involved in blood pressure regulation. MAOIs can decrease how quickly the body breaks down tyramine. Consuming more than 6 mg of tyramine while taking one of these medications can increase the risk of serious side effects, such as very high blood pressure. If you take one of these medications, avoid fermented soy products that contain high amounts of tyramine.

    Some common MAOIs include phenelzine (Nardil), selegiline (Zelapar), and tranylcypromine (Parnate).

  • Estrogens interacts with Soy

    Large amounts of soy might have some of the same effects as estrogen. Taking soy along with estrogen pills might decrease the effects of estrogen pills.

  • Tamoxifen (Nolvadex) interacts with Soy

    Tamoxifen affects the activity of estrogen in the body. Soy seems to also affect estrogen activity in the body. Taking soy with tamoxifen might change the effects of tamoxifen. Speak with a healthcare provider if you are taking tamoxifen.

  • Warfarin (Coumadin) interacts with Soy

    Warfarin is used to slow blood clotting. Soy has been reported to decrease the effects of warfarin. This might increase the risk of clotting. Be sure to have your blood checked regularly. The dose of your warfarin might need to be changed.

  • Medications changed by the liver (Cytochrome P450 2C9 (CYP 2C9) substrates) interacts with Soy

    Some medications are changed and broken down by the liver. Soy might change how quickly the liver breaks down these medications. This could change the effects and side effects of these medications.

  • Medications for diabetes (Antidiabetes drugs) interacts with Soy

    Soy might lower blood sugar levels. Taking soy along with diabetes medications might cause blood sugar to drop too low. Monitor your blood sugar closely.

  • Medications for high blood pressure (Antihypertensive drugs) interacts with Soy

    Soy might lower blood pressure. Taking soy along with medications that lower blood pressure might cause blood pressure to go too low. Monitor your blood pressure closely.

  • Levothyroxine (Synthroid, others) interacts with Soy

    Levothyroxine is used for low thyroid function. Soy seems to decrease how much levothyroxine is absorbed by the body in infants, but not adults. This might decrease the effects of levothyroxine in infants. The dose of levothyroxine may need to be adjusted if soy is being used regularly, such as in soy-based formulas. Otherwise, take levothyroxine and soy at least 4 hours apart.

  • Progesterone interacts with Soy

    Some research shows that taking soy milk along with progesterone might increase bone loss in females with osteoporosis.

  • Water pills (Diuretic drugs) interacts with Soy

    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.

  • Caffeine interacts with Soy

    Soy contains the chemical genistein. Genistein might slow down how quickly the body gets rid of caffeine. This might increase the effects of caffeine.

    Minor Interaction

    Be watchful with this combination

  • Antibiotic drugs interacts with Soy

    Antibiotics are used to reduce harmful bacteria in the body. Antibiotics can also reduce friendly bacteria in the intestines. Friendly bacteria in the intestines help to convert soy to its active form. By reducing the number of bacteria, antibiotics might decrease the effects of soy. But it is too soon to know if this interaction is a big concern.

Dosing

Soy foods are available in many different forms, including tofu, miso, and soy milk. Soy protein products have most often been used by adults in doses of 40 grams by mouth daily for up to 5 months. The active ingredients in soy, called soy isoflavones, have been used in supplements in doses of 120 mg by mouth daily for up to 6 months.

Soy is also used in topical products such as gels and moisturizers. Speak with a healthcare provider to find out what type of product and dose might be best for a specific condition.
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CONDITIONS OF USE AND IMPORTANT INFORMATION: This information is meant to supplement, not replace advice from your doctor or healthcare provider and is not meant to cover all possible uses, precautions, interactions or adverse effects. This information may not fit your specific health circumstances. Never delay or disregard seeking professional medical advice from your doctor or other qualified health care provider because of something you have read on WebMD. You should always speak with your doctor or health care professional before you start, stop, or change any prescribed part of your health care plan or treatment and to determine what course of therapy is right for you.

This copyrighted material is licensed from Therapeutic Research Center, LLC. Information from this source is evidence-based and objective, and without commercial influence. For professional medical information on natural medicines, see Natural Medicines Comprehensive Database Professional Version.
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