What is it?
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. The boiled or roasted beans can also be eaten. Soy is sometimes used as a milk substitute. Soy is also taken by mouth and applied to the skin as a medicine.
Soy is used for high cholesterol, high blood pressure, heart disease, diabetes, symptoms of menopause, premenstrual syndrome (PMS), and many other conditions, but there is no good scientific evidence to support many of these uses.
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 the amount that is stated on the label. Paying more for a product doesn’t necessarily guarantee that the content shown on the label is accurate.
Possibly Effective for …
- Breast cancer. Eating large amounts of soy might help prevent breast cancer or breast cancer recurrence in some people. But there’s no reliable evidence that soy supplements work. For prevention, research has found that eating large amounts of soy is linked with a reduced risk of breast cancer. But the amount of soy eaten as part of a Western diet might not be enough to prevent breast cancer. Also, eating large amounts of soy may only be beneficial if women eat the large amounts as teens and young adults rather than later in life. For treatment, some research has found that women with breast cancer who eat large amounts of soy may have a reduced risk of breast cancer recurrence. Taking soy supplements doesn’t seem to have any benefit.
- Long-term kidney disease (chronic kidney disease or CKD). Taking soy protein by mouth seems to reduce protein in the urine in people with CKD. It also seems to reduce levels of certain nutrients and waste products, such as phosphorus and creatinine. These molecules can build up in the blood of people with CKD.
- Diabetes. Most research suggests that consuming soy products containing soy protein, soy fiber, or fermented soy reduces blood sugar levels 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.
- 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 “bad cholesterol” (low-density lipoprotein (LDL) cholesterol). Soy protein that contains higher amounts of an ingredient called isoflavones might work better than soy protein with little or no isoflavones. Also, soy might work better in people with high cholesterol that is more severe. Supplements containing purified soy isoflavones don’t seem to work. Soy doesn’t seem to lower triglycerides. Also, most research shows that soy doesn’t increase “good cholesterol” (high-density lipoprotein (HDL) cholesterol).
- High blood pressure. Most research shows that eating soy protein can reduce systolic blood pressure (the top number in a blood pressure reading) by about 4-8 mmHg and diastolic blood pressure (the bottom number) by about 3-5 mmHg in people with slightly high blood pressure.
- A long-term disorder of the large intestines that causes stomach pain (irritable bowel syndrome or IBS). Some research suggests that taking soy isoflavones can improve symptoms of IBS, such as stomach pain.
- 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 concentrated soy isoflavone extract seems to help hot flashes caused by menopause in some people. Taking soy products providing 100-200 mg of isoflavones in two or three divided doses per day might work better than taking lower or less frequent doses. In addition, using products that contain at least 15 mg of the specific isoflavone called genistein seem to work better than products that provide less genistein. Taking soy also seems to improve depression, blood sugar levels and body weight in women after menopause. It’s unclear if soy reduces vaginal dryness or itching that is associated with menopause. Soy does not seem to help hot flashes in women with breast cancer.
- A grouping of symptoms that increase the risk of diabetes, heart disease, and stroke (metabolic syndrome). Most research shows that consuming soy products containing soy protein helps lower blood sugar levels in people with metabolic syndrome. Also, eating a soy nut diet or a soy protein diet appears to reduce blood sugar and decrease “bad” cholesterol (low-density lipoprotein (LDL) cholesterol) in postmenopausal women with metabolic syndrome; however, the soy nut diet seems to work better than the soy protein diet.
- Weak and brittle bones (osteoporosis). Most evidence suggests that soy protein or soy extract can increase bone mineral density (BMD) or slow BMD loss in women near or beyond menopause. It appears that soy products need to contain at least 75 mg of an ingredient called isoflavones in order to work. Soy might also reduce the risk of fractures in some women. Soy does not seem to affect BMD in younger women.
Possibly Ineffective for …
- Age-related muscle loss (sarcopenia). Compared to milk protein, soy protein does not seem to slow the loss of muscle due to aging in women after menopause.
- Enlarged prostate (benign prostatic hyperplasia or BPH). Taking isoflavones isolated from soy doesn’t seem to improve urination or other symptoms in people with an enlarged prostate.
- Hot flashes in people treated for breast cancer. Research shows that drinking a beverage containing soy or taking tablets containing soy extract does not reduce hot flashes in breast cancer survivors.
- Colon cancer, rectal 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.
Insufficient Evidence to Make a Determination for …
- Alzheimer disease. Early research suggests that taking soy isoflavones does not improve brain function in patients with Alzheimer disease.
- Asthma. Early evidence suggests that people with asthma who eat soy foods have increased lung function, but taking supplements containing an ingredient in soy called isoflavones doesn’t appear to improve lung function or reduce asthma episodes in adults or children with poorly controlled asthma.
- Heart disease. Some research has found that postmenopausal women, but not premenopausal women or men, from Japan who eat a lot of soy might have a reduced risk of stroke, heart attack, or heart disease-related death. There is also evidence that Chinese or Japanese people who eat more fermented soy have a lower risk of death due to heart disease. But other forms of soy don’t seem to be linked with a lower risk of death due to heart disease. Higher soy intake does not seem to be linked with a lower risk of heart-related events in women from Western countries. This might be because the amount of soy eaten in Western diets is low, even among women with highest intake.
- Cancer of the cervix. Early research shows that consuming a diet that contains large amounts of soy might help to prevent cervical cancer, but only in women who also drink green tea.
- Child growth. Early research shows that giving a soy protein supplement to children 2-7 years old for 1 year does not increase height or weight for most children.
- Memory and thinking skills (cognitive 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. Some soy formulations might work better than others.
- Excessive crying in infants (colic). Early research shows that feeding infants a soy-based formula might reduce the duration of colic symptoms in infants who have trouble digesting cow’s milk. But higher-quality research suggests that feeding infants a soy-based formula does not improve the duration of crying in infants with colic. Also, soy-based formula doesn’t seem to improve crying compared to the drug dicyclomine.
- A type of inflammatory bowel disease (Crohn 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 disease.
- Kidney damage in people with diabetes (diabetic nephropathy). Research shows that eating soy protein in place of animal protein as part of the diet might help prevent or treat kidney disease in people with diabetes. But some early research shows that drinking soy milk doesn’t help.
- Cancer of the lining of the uterus (endometrial cancer). Eating more soy seems to 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. It’s too early to know if soy supplements affect endometrial cancer risk.
- Stomach cancer. Eating more foods that contain soy might reduce the risk of getting stomach cancer. However, it is not known if soy supplements affect stomach cancer risk. Also, eating one type of fermented soy, called miso, might increase the risk of stomach cancer in men.
- Swelling (inflammation) of the liver caused by the hepatitis C virus (hepatitis C). Early research suggests that taking soy protein reduces the buildup of fat in the liver and lowers markers of liver injury in people with hepatitis C.
- Lung cancer. Men and women who consume a higher amount of dietary phytoestrogens, such as isoflavones from soy, seem to be less likely to develop lung cancer than people who consume smaller amounts. Soy seems to prevent lung cancer more in men than women. However, some research suggests that only non-smokers, not smokers, who eat soy have a lower risk of developing lung cancer.
- Breast pain (mastalgia). Drinking soymilk might reduce monthly breast pain in some women.
- Migraine. 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 shows that taking soy protein can increase lean tissue mass or strength in untrained and experienced athletes participating in resistance training. Soy protein added to resistance training also seems to improve strength in women after menopause. Early research also shows that soy protein seems to be as effective as whey protein, dairy protein, or beef protein for improving muscle strength.
- Yellowing of the skin in infants (neonatal jaundice). Research shows that when women who have diabetes during pregnancy include soy protein in their diets, it decreases the risk of neonatal jaundice in the baby after birth.
- Build up of fat in the liver in people who drink little or no alcohol (nonalcoholic fatty liver disease or NAFLD). Early research shows that drinking soy milk while on a low-calorie diet might help control blood sugar and blood pressure in people with NAFLD.
- Obesity. Some research shows that eating a soy-based, low-calorie diet can reduce weight in obese and overweight people more than a low-calorie diet alone. Eating foods containing soy fiber might also improve weight loss. Replacing meat protein with soy protein in the diet might improve weight loss in women, but not all research agrees. Supplementing with soy-based meal replacements doesn’t seem to improve weight loss or prevent weight gain. Soy protein also doesn’t seem to improve weight loss when eaten as part of a free-choice diet. Finally, soy protein also doesn’t seem to improve weight loss in postmenopausal women.
- Osteoarthritis. Early research 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.
- Death from any cause. Eating more soy does not seem to help people live longer.
- A hormonal disorder that causes enlarged ovaries with cysts (polycystic ovary syndrome or PCOS). Women with PCOS have higher levels of cholesterol in the blood, higher levels of male hormones, and problems regulating blood sugar. Some research suggests that taking soy isoflavones improves the regulation of blood sugar and hormone levels in women with this condition. But taking soy does not seem to improve cholesterol levels.
- Preterm birth. Research shows that when women who have diabetes during pregnancy include soy protein in their diets, it does not reduce the risk of early labor or caesarean section.
- 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 is conflicting. Men who eat more soy seem to have a lower risk of prostate cancer. However, since Asian men eat the most soy, it is unclear whether it is the soy in the diet of these 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. Soy protein doesn’t seem to reduce hot flashes in prostate cancer patients.
- Rheumatoid arthritis (RA). Early research suggests that consuming a liquid diet containing soy protein does not improve pain, stiffness, or joint swelling in people with rheumatoid arthritis.
- Stroke. Early research shows that drinking soymilk every day might help older adults who have had a stroke to recover faster during rehabilitation therapy.
- Thyroid cancer. Diets that include plenty of soy seem to be linked to a reduced risk of thyroid cancer.
- Thinning of vaginal tissue (vaginal atrophy). Applying a vaginal gel containing soy extract seems to reduce vaginal dryness and pain and improve vaginal tissue in postmenopausal women with symptoms of vaginal atrophy.
- Skin wrinkles from sun damage. Consuming soy isoflavones or applying a moisturizer containing soy seems to improve the elasticity of skin and the appearance of fine wrinkles.
- Decline in memory and thinking skills in older people that is more than what is normal for their age.
- Other conditions.
More evidence is needed to rate soy for these uses.
Long-term use of high doses of supplements containing soy extract is POSSIBLY UNSAFE. There is some concern that this might cause abnormal tissue growth in the uterus. However, eating large amounts of soy does not seem to have this effect.
Special Precautions & Warnings:
Pregnancy and breast-feeding: Consuming soy protein in amounts normally found in food is LIKELY SAFE when pregnant or breast-feeding. 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. 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 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.
Hay fever (allergic rhinitis): People with hay fever are more likely to be allergic to soy hulls.
Asthma: People with asthma are more likely to be allergic to soy hulls. Avoid using 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.
Cystic fibrosis: Soymilk can interfere with the way children with cystic fibrosis process protein. Don’t give these children soy products.
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.
Endometrial cancer: Long-term use of concentrated soy isoflavone tablets might increase the occurrence of precancerous changes in the tissue lining the uterus. However, conflicting evidence exists. Use supplements containing soy isoflavones cautiously if you are at risk for endometrial cancer. Soy foods are likely safe.
Under-active thyroid (hypothyroidism): There is a concern that taking soy might make this condition worse.
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.
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.
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).
Interaction Rating=Moderate Be cautious with this combination.
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.
Interaction Rating=Moderate Be cautious with this combination.
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.
Medications for depression (MAOIs)
Interaction Rating=Major Do not take this combination.
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.
Interaction Rating=Moderate Be cautious with this combination.
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).
Interaction Rating=Moderate Be cautious with this combination.
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.
Herbs and supplements that might lower blood pressure: 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.
Herbs and supplements that might lower blood sugar: 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.
Iron: 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.
Manganese: 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.
Zinc: 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.
- For type 2 diabetes:
- Touchi extract 300 mg three times daily for 3-6 months. Touchi is a traditional Chinese food prepared from soybeans.
- Fibrous soy hulls 26 grams daily for 4 weeks. Single doses of soy fiber 7-10 grams have also been used.
- 30 grams of soy protein, containing 132 mg of phytoestrogens, daily for up to 12 weeks.
- For diabetes during pregnancy: Protein 0.8 grams/kg daily for 6 weeks, consisting of 35% soy protein as textured soy protein (Sobhan), 35% animal protein, and 30% other plant proteins.
- For high cholesterol: 20-50 grams per day of soy protein.
- For high blood pressure: 18-66 grams of soy protein, containing 34-143 mg of isoflavones, daily for up to 12 weeks has been used. Also, 4.5 grams of black soy peptide daily for 8 weeks has also been used.
- For irritable bowel syndrome (IBS): Soy containing 40 mg isoflavones daily for 6 weeks has been used.
- For long-term kidney disease (chronic kidney disease or CKD): a diet limited to 700-800 mg/kg soy protein daily has been used.
- For menopausal symptoms such as hot flashes:
- 15-60 grams per day of soy protein providing 34-80 mg isoflavones.
- concentrated soy isoflavone extracts providing 35-200 mg/day of isoflavones.
- Genistein, a soy isoflavone: 54 mg per day.
- For menopausal symptoms such as depression: 100 mg of soy containing 50 mg of isoflavones has been used along with sertraline 50 mg daily.
- To reduce body weight during menopause: Soy isoflavones <100 mg daily for up to 6 months have been used.
- For metabolic syndrome. 30 grams of soy nuts per day as part of a diet high in fruit and vegetables for 8 weeks has been used.
- For weak bones (osteoporosis): 40 grams per day soy protein containing 2-2.25 mg isoflavones per gram has been used for 3-6 months to prevent osteoporosis. Soy extract 1 gram containing 80 mg of isoflavones has been used for one year.
- For diarrhea: Soy fiber-fortified formula containing 18-20 grams of soy protein per liter has been used in infants.
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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