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:
- High cholesterol. Eating soy protein in place of other dietary protein 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.
- Hot flashes caused by menopause. Eating soy protein seems to help hot flashes caused by menopause, but it doesn't seem to help hot flashes in women with breast cancer.
- Reducing the risk of osteoporosis (weak bones), especially in women who have passed menopause. Most evidence suggests that soy protein can increase bone mineral density (BMD), or slow BMD loss in women near or beyond menopause. Soy does not seem to help BMD in younger women.
- Reducing the 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.
- Reducing the duration of diarrhea in infants.
- Preventing and treating diabetic nerve problems.
- Providing nutrition to infants who can't digest milk sugars.
- Reducing protein in the urine of people with kidney disease.
- Treating diabetes type 2.
- Treating infants who have trouble digesting lactose (galactosemia, hereditary lactose deficiency, lactose intolerance).
Possibly Ineffective for:
- Reducing muscle soreness caused by exercise.
- Preventing thyroid cancer. Getting a lot of soy from the diet might help to prevent thyroid cancer.
- Endometrial cancer. There is some evidence that 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.
- 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.
- Prostate cancer. Research on the effect of soy on prostate cancer risk has been mixed. 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. But, it's unclear whether it’s 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.
- Improving memory. Some research suggests that a high soy diet might slightly improve performance on memory tests.
- Reducing breast pain. There is some limited evidence that soymilk (34 grams soy protein/day) might reduce monthly breast pain in some women.
- Weight loss. Limited evidence suggests that eating soy protein along with a low-calorie diet for six months seems to reduce weight in obese and overweight people more than a low-calorie diet alone.
- 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. This is a relatively small reduction.
- Premenstrual syndrome (PMS).
- Heart disease.
- Other conditions.
Side Effects & Safety
Consuming soy foods such as soy protein is LIKELY SAFE. Taking soy dietary supplements also seems to be safe for most people when used short-term (up to six 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 in pregnancy and during breast-feeding when consumed in amounts normally found in food. 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 disease: 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 kidney disease or a history of kidney stones, avoid taking large amounts of soy.
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.
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.