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 used for high cholesterol, high blood pressure, and preventing diseases of the heart and blood vessels. It is also used for type 2 diabetes, asthma, lung cancer, endometrial cancer, prostate cancer, and thyroid cancer, as well as preventing weak bones (osteoporosis), and slowing the progression of kidney disease.
Other uses include treating constipation and diarrhea, as well as decreasing protein in the urine of people with kidney disease, improving memory, and treating muscle soreness caused by exercise.
Women use soy for breast pain, preventing breast cancer, preventing hot flashes after breast cancer, menopausal symptoms, and premenstrual syndrome (PMS).
In foods, soy is used as a milk substitute in infant feeding formulas, and as an alternative to cow's milk. 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.
How does it work?
Soy contains "isoflavones" which are changed in the body to "phytoestrogens," which are similar to the hormone estrogen.
Possibly Effective for:
- Breast cancer. Eating a high-soy diet is linked to a slightly reduced risk of developing breast cancer. Researchers think early exposure to soy may be key. Asian women who eat a traditional diet high in soy seem to be less likely to develop breast cancer. This benefit continues even when Asian women move to western cultures, where soy is less likely to be a regular part of the diet. This suggests that exposure to soy early in life (i.e., before menopause) provides the most protection against breast cancer.
- Diabetes. Most evidence suggests that taking soy products reduces blood sugar levels in people with diabetes. One study suggests that eating a whole soy diet, not just the protein from soy, reduces blood sugar in people with and without diabetes. However, there is also some evidence that suggests that soy and soy protein don’t affects blood sugar in people with diabetes.
- Kidney disease in people with diabetes. Soy isoflavones might help prevent or treat kidney disease in people with diabetes.
- Diarrhea. Feeding infants formula supplemented with soy fiber, alone or together with rehydration solution, seems to reduce the duration of diarrhea compared to cow’s milk formula or rehydration solution alone. However, in some studies formula supplemented with soy was no more beneficial than cow’s milk formula. In adults, early evidence suggests that taking soy fiber does not decrease the incidence of diarrhea.
- Trouble digesting the sugar galactose (galactosemia). Feeding a soy-based formula to infants who have galactosemia seems to be helpful.
- Trouble digesting the sugar lactose (hereditary lactase deficiency). Feeding a soy-based formula to infants who have hereditary lactase deficiency seems to be helpful.
- Trouble digesting the sugar lactose (lactose intolerance). Feeding a soy-based formula to infants who have lactose intolerance seems to be helpful.
- High cholesterol. Eating soy protein in place of other dietary protein or using soy fiber products seems to slightly reduce total cholesterol and “bad cholesterol” (low-density lipoprotein (LDL) cholesterol levels). However, not all evidence is positive. Some studies have shown no significant benefit of soy protein on cholesterol levels.
- Kidney disease. Taking soy protein by mouth seems to reduce protein in the urine in people with kidney disease.
- Menopausal symptoms. Eating soy protein seems to help hot flashes caused by menopause. However, it does not reduce other symptoms of menopause, such as vaginal dryness or itching. Also, soy does not seem to help hot flashes in women with breast cancer.
- Osteoporosis. Most evidence suggests that soy protein can increase bone mineral density (BMD) or slow BMD loss in women near or beyond menopause. Soy might also reduce the risk of fractures in some women. Soy does not seem to affect BMD in younger women.
Possibly Ineffective for:
- Colorectal cancer. Research suggests that taking soy protein does not reduce the progression of colorectal cancer.
- Muscle soreness caused by exercise. Taking soy isoflavone extract by mouth before exercising does not seem to prevent muscle soreness.
- Fibromyalgia. Drinking a soy protein shake containing soy isoflavones does not appear to improve physical function or symptoms of depression in people with fibromyalgia.
- Rheumatoid arthritis. Early evidence suggests that consuming a liquid diet containing soy protein (Top Up) does not improve pain, stiffness, or joint swelling in people with rheumatoid arthritis.
- Asthma. Early evidence suggests that people with asthma who eat soy foods have increased lung function.
- Heart disease. Some evidence suggests that postmenopausal women, but not men or pre-menopausal women, who eat a lot of soy in their diet might have a reduced risk of stroke, heart attack, or heart disease-related death.
- Mental function. There is conflicting evidence about the effect of soy on mental function. Some evidence suggests that eating more soy improves short-term and long-term memory. However, other research suggests that soy does not improve mental function, including memory.
- Crohn’s disease. Research suggests that taking soy by mouth, along with standard treatment, increases bowel movements and improves symptoms, such as fatigue and body weight, in people with Crohn’s disease.
- Endometrial cancer. Increasing soy intake might lower the risk of endometrial cancer. Endometrial cancer is less common in Japan, China, and other Asian countries where the usual diet is low in calories and high in soy and whole grain foods, vegetables, and fruits.
- High blood pressure. Some evidence suggests that eating soy protein might reduce systolic blood pressure (the top number in a blood pressure reading) by about 4 mmHg and diastolic blood pressure (the bottom number) by about 3 mmHg in people with pre-high blood pressure or mild high blood pressure. However, this is a relatively small reduction. Also, some research suggests that soy protein does not affect blood pressure.
- Colic in infants. Early research suggests that feeding infants a soy-based formula might reduce the duration of colic symptoms in infants who have trouble digesting cow’s milk. However, other research suggests that feeding infants a soy-based formula does not improve crying in infants with colic.
- Lung cancer. Research suggests that men and women who consume a higher amount of dietary phytoestrogens, such as isoflavones from soy, are less likely to develop lung cancer than people who consume smaller amounts. Soy seems to prevent lung cancer more in men than women.
- Breast pain (mastalgia). Drinking soymilk might reduce monthly breast pain in some women.
- Memory. Some research suggests that a high-soy diet might slightly improve performance on memory tests.
- Metabolic syndrome (a condition that increases risk for diabetes and heart disease). Eating a soy nut diet appears to reduce blood sugar and decrease low-density lipoprotein (LDL or “bad”) cholesterol in postmenopausal women with metabolic syndrome better than a soy-protein diet or a DASH diet.
- Migraines. Research suggests that taking a combination of soy isoflavones, dong quai, and black cohosh reduces the frequency of migraines associated with menstruation.
- Muscle strength. Research suggests that taking soy protein can increase lean tissue mass in people participating in resistance training. Other research suggests that consuming a specific soy protein product (Supro) can increase body mass and strength and reduce fatigue in athletes. However, other conflicting research shows that soy protein might not improve strength.
- Osteoarthritis. Early evidence shows that taking soy protein can improve motion, pain, and quality of life in people with osteoarthritis. However, taking milk-based protein also seems to have these effects.
- Sun-damaged skin. Evidence suggests that applying a soy moisturizer to the skin can improve skin color, fine lines, and texture.
- Premenstrual syndrome (PMS). Research suggests that taking soy protein for two menstrual cycles can reduce cramps and swelling associated with PMS.
- Prostate cancer. Research regarding the effect of soy on prostate cancer risk has been conflicting. Men who eat an Asian diet, which contains 10 times more soy than the average American diet, seem to have a lower risk of prostate cancer. However, it is unclear whether it is the soy in the diet of Asian men or other factors (such as genetic differences or differences in dietary fat) that protect against prostate cancer. Some research shows that taking soy protein can reduce the risk of prostate cancer in at-risk men. However, there is conflicting evidence about whether soy can affect the progression of prostate cancer.
- Thyroid cancer. Diets that include plenty of soy seem to be linked to a reduced risk of thyroid cancer.
- Weight loss. Some evidence suggests that eating soy protein along with a low-calorie diet can reduce weight in obese and overweight people more than a low-calorie diet alone. However, other evidence suggests that consuming soy-meal replacements while following a calorie-restricted diet does not improve weight loss in overweight people.
- Wrinkled skin. Evidence suggests that consuming soy isoflavones can improve the elasticity of skin and the appearance of fine wrinkles. >
- Other conditions.
Side Effects & Safety
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 and itching 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.
Special Precautions & Warnings: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 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.
Major Interaction Do not take this combination
- Medications for depression (MAOIs) interacts with SOY
Fermented soy products such as tofu and soy sauce contain tyramine. Tyramine is an amino acid 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.
Moderate Interaction Be cautious 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 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.
- Estrogens interacts with SOY
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.
- Tamoxifen (Nolvadex) interacts with SOY
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) interacts with SOY
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.
Minor Interaction Be watchful with this combination
- 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 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.
The following doses have been studied in scientific research:
- For high cholesterol: 20-50 grams per day of soy protein.
- For preventing weak bones (osteoporosis): 40 grams per day soy protein containing 2-2.25 mg isoflavones per gram.
- For menopausal symptoms such as hot flashes:
- 20-60 grams per day of soy protein providing 34-76 mg isoflavones.
- concentrated soy isoflavone extracts providing 50-120 mg/day of isoflavones.
- Genistein, a soy isoflavone: 54 mg per day.
- For protein in the urine of people with kidney disease: a diet limited to 700-800 mg/kg soy protein daily has been used.
- For diarrhea in infants, soy fiber fortified formula containing 18-20 grams of soy protein per liter.
- For type 2 diabetes, touchi extract 300 mg three times daily. Touchi is a traditional Chinese food prepared from soybeans.
- For type 2 diabetes in postmenopausal women, 30 grams of soy protein daily, containing 132 mg of phytoestrogens daily for up to 12 weeks.