MedlinePlus Herbs and Supplements: Soy (Glycine max [L.] Merr.)

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Soy (Glycine max [L.] Merr.)

Contents of this page:
Background Synonyms Evidence Dosing	Safety Interactions Methodology Selected references

Soy

BackgroundReturn to top

Soy is a subtropical plant native to southeastern Asia. This member of the pea family (Fabaceae) grows from 1-5 feet tall and forms clusters of 3-5 pods that each contain 2-4 beans. Soy has been a dietary staple in Asian countries for at least 5,000 years. During the Chou dynasty in China (1134-246 B.C.), fermentation techniques were discovered that allowed soy to be prepared in more easily digestible forms such as tempeh, miso, and tamari soy sauce. Tofu was invented in 2nd Century China.

Soy was introduced to Europe in the 1700s and to the United States in the 1800s. Large-scale soybean cultivation began in the United States during World War II. Currently, Midwestern U.S. farmers produce about half of the world's supply of soybeans.

Soy and components of soy called "isoflavones" have been studied for many health conditions. Isoflavones (such as genistein) are believed to have estrogen-like effects in the body, and as a result, they are sometimes called "phytoestrogens." In laboratory studies, it is not clear if isoflavones stimulate or block the effects of estrogen or both (acting as "mixed receptor agonists/antagonists").

SynonymsReturn to top

Coumestrol, daidzein, edamame, frijol de soya, genistein, greater bean, haba soya, hydrolyzed soy protein, isoflavone, isoflavonoid, legume, natto, phytoestrogen, plant estrogen, shoyu, soja, sojabohne, soya, soya protein, soybean, soy fiber, soy food, soy product, soy protein, Ta-tou, texturized vegetable protein.

EvidenceReturn to top

These uses have been tested in humans or animals. Safety and effectiveness have not always been proven. Some of these conditions are potentially serious, and should be evaluated by a qualified healthcare provider.

Uses based on scientific evidence	Grade^*
Dietary source of protein Soy products, such as tofu, are high in protein and are an acceptable source of dietary protein.	A
High cholesterol Numerous human studies report that adding soy protein to the diet can moderately decrease blood levels of total cholesterol and low-density lipoprotein ("bad" cholesterol). Small reductions in triglycerides may also occur, while high-density lipoprotein ("good" cholesterol) does not seem to be significantly altered. Some scientists have proposed that specific components of soybean, such as the isoflavones genistein and daidzein, may be responsible for the cholesterol-lowering properties of soy. However, this has not been clearly demonstrated in research and remains controversial. It is not known if products containing isolated soy isoflavones have the same effects as regular dietary intake of soy protein. Dietary soy protein has not been proven to affect long-term cardiovascular outcomes, such as heart attack or stroke.	A
Diarrhea (acute) in infants and young children Numerous studies report that infants and young children (2-36 months old) with diarrhea who are fed soy formulas experience fewer daily bowel movements and fewer days of diarrhea. This research suggests that soy has benefits over other types of formula, including cow milk-based solutions. The addition of soy fiber to soy formula may increase the effectiveness. Better quality research is needed before a strong recommendation can be made.Parents are advised to speak with qualified healthcare providers if their infants experience prolonged diarrhea, become dehydrated, develop signs of infections (such as fever), or have blood in the stool. A healthcare provider should be consulted for current breastfeeding recommendations and to suggest long-term formulas that provide enough nutrition.	B
Menopausal symptoms Overall, evidence suggests that soy products containing isoflavones may help reduce menopausal symptoms, such as hot flashes.	B
Breast cancer prevention Several large population studies have asked women about their eating habits and reported that higher soy intake (such as dietary tofu) is associated with a decreased risk of developing breast cancer. However, other research suggests that soy does not have this effect. Until better research is available, it remains unclear if dietary soy or soy isoflavone supplements increase or decrease the risk of breast cancer.	C
Cancer treatment Genistein, an isoflavone found in soy, has been found in laboratory and animal studies to have anti-cancer effects, such as blocking new blood vessel growth (anti-angiogenesis), acting as a tyrosine kinase inhibitor (a mechanism of many new cancer treatments), or causing cancer cell death (apoptosis). In contrast, genistein has also been reported to increase the growth of pancreas tumor cells in laboratory research. Until reliable human research is available, it remains unclear if dietary soy or soy isoflavone supplements are beneficial, harmful, or neutral in cancer patients.	C
Cardiovascular disease Dietary soy protein has not been shown to affect long-term cardiovascular outcomes, such as heart attack or stroke. Research does suggest cholesterol-lowering effects of dietary soy, which in theory, may reduce the risk of heart problems. Soy has also been studied for blood pressure-lowering and blood sugar-reducing properties in people with type 2 diabetes, although the evidence is not definitive in these areas. Further investigation is needed before a strong recommendation can be made.	C
Cardiac ischemia Cardiac ischemia occurs when blood flow to the heart is blocked. In women with suspected cardiac ischemia, high levels of the soy isoflavone genistein have been associated with blood vessel problems. Until more research is done, it is unknown if soy plays a role in cardiac ischemia.	C
Cognitive function It is unclear if soy isoflavone supplementation in postmenopausal women can improve cognitive function. Results from studies are mixed.	C
Colon cancer prevention There is not enough scientific evidence to determine if dietary intake of soy affects the risk of developing colon cancer. Study results are mixed, and more research is needed before a recommendation can be made.	C
Crohn's disease Due to limited human research, there is not enough evidence to recommend for or against the use of soy as a way to prevent Crohn's disease. Further research is needed before a recommendation can be made.	C
Cyclical breast pain It has been theorized that the "phytoestrogens" (plant-based compounds with weak estrogen-like properties) in soy may be beneficial to premenopausal women with cyclical breast pain. However, due to limited human research, there is not enough evidence to recommend for or against the use of dietary soy protein as a therapy for this condition.	C
Diarrhea in adults Due to limited human study, there is not enough evidence to recommend for or against the use of soy-polysaccharide/fiber in the treatment of diarrhea. Further research is needed before a recommendation can be made.	C
Endocrine disorders (metabolic syndrome) Soy nuts may help reduce inflammation, improve blood sugar control, and improve lipid profiles in postmenopausal women with metabolic syndrome. More research is needed in this area.	C
Endometrial cancer prevention There is not enough scientific evidence to determine if dietary intake of soy affects the risk of developing endometrial cancer.	C
Exercise capacity improvement (spinal cord injury patients) Soy appears to be less effective than whey protein at improving caloric expenditure and the distance and length of time that patients with spinal cord injuries are able to walk before feeling tired. More research is needed in this area.	C
Gallstones (cholelithiasis) Due to limited human research, there is not enough evidence to recommend for or against the use of soy as a therapy in cholelithiasis. Further research is needed before a strong recommendation can be made.	C
High blood pressure There is limited human research on the effects of dietary soy on blood pressure. Some research suggests that substituting soy nuts for non-soy protein may help improve blood pressure. Further research is needed before a firm recommendation can be made.	C
Kidney disease (chronic renal failure, nephrotic syndrome, proteinuria) Due to limited human study, there is not enough evidence to recommend for or against the use of soy in the treatment of kidney diseases, such as nephrotic syndrome. People with kidney disease should speak with their healthcare providers about the recommended amounts of dietary protein because soy is a high-protein food.	C
Menstrual migraine A phytoestrogen combination may help prevent menstrual migraine attacks. Further research is needed before a strong recommendation can be made.	C
Obesity/weight reduction Some research suggests that soy might be as effective as skim milk and more effective than a low-calorie diet alone in reducing weight. Other research has reported conflicting results. Further research is needed before a strong recommendation can be made.	C
Osteoporosis, post-menopausal bone loss It has been theorized that "phytoestrogens in soy" (such as isoflavones) may increase bone mineral density in post-menopausal women and reduce the risk of fractures. However, more research is needed before a conclusion can be made.	C
Prostate cancer prevention Early research has tested the effects of dietary soy intake on prostate cancer development in humans, but the results have not been conclusive. Better research is needed before a recommendation can be made.	C
Skin aging It is unclear if aglycones, a form of soy isoflavone, can improve aged skin in middle-aged women when it is taken by mouth. More research is needed.	C
Skin damage caused by the sun A soy moisturizing cream may help improve signs of sun damage, including discoloration, blotchiness, dullness, fine lines, and overall texture. Because the cream contained other ingredients besides soy, more research with soy alone is needed.	C
Stomach cancer Early research suggests that intake of soy products may be associated with a reduced risk of death from stomach cancer. Further investigation is needed before a conclusion can be drawn.	C
Thyroid disorders Early research suggests that soy supplements do not affect thyroid function. More research is needed.	C
Tuberculosis It has been suggested that soy may be beneficial for tuberculosis when taken with standard medications. According to early research, soy may improve the process of detoxification, have positive effects on the liver, reduce cell damage, and decrease inflammation. Therefore, soy supplements may allow patients to safely take higher doses of antimicrobial drugs that are used to treat tuberculosis.	C
Type 2 diabetes Several small studies have examined the effects of soy products on blood sugar levels in people with type 2 ("adult-onset") diabetes. Results are mixed, with some research reporting decreased blood glucose levels and other trials noting no effects. Overall, research in this area is not well designed or reported, and better information is needed.	C

*Key to grades
A: Strong scientific evidence for this use;
B: Good scientific evidence for this use;
C: Unclear scientific evidence for this use;
D: Fair scientific evidence against this use;
F: Strong scientific evidence against this use.

Grading rationale

Uses based on tradition or theory

The below uses are based on tradition or scientific theories. They often have not been thoroughly tested in humans, and safety and effectiveness have not always been proven. Some of these conditions are potentially serious, and should be evaluated by a qualified healthcare provider.

Anemia, anorexia, antifungal, antioxidant, antithrombotic (preventing clots), atherosclerosis (hardening of the arteries), attention deficit hyperactivity disorder (ADHD), autoimmune diseases, breast enlargement, cystic fibrosis, diabetic neuropathy, fever, gastrointestinal motility, headache, hepatitis (chronic), inflammation, insect repellant, lymphoma, memory enhancement, nosebleed (chronic), osteosarcoma, pancreatic cancer, ovarian disorders (premature ovarian failure), respiratory problems (cough, phlegm), rheumatoid arthritis, urinary tract cancer, vaginitis, vasoregulator.

DosingReturn to top

The below doses are based on scientific research, publications, traditional use, or expert opinion. Many herbs and supplements have not been thoroughly tested, and safety and effectiveness may not be proven. Brands may be made differently, with variable ingredients, even within the same brand. The below doses may not apply to all products. You should read product labels, and discuss doses with a qualified healthcare provider before starting therapy.

Adults (over 18 years old)

Soy is typically consumed as a protein drink, soy flour, isolated soy protein (e.g. Supro®), extract, fiber/cereal, or milk beverage. Studies have examined the effects of 10-80 grams of soy with an isoflavone content of about 40-120 milligrams taken daily for up to six months by mouth.

A dose of 25-50 grams of soy protein taken daily by mouth has been studied in people with high cholesterol. Isoflavone content has ranged from 60-90 milligrams daily. Cholesterol and low-density lipoprotein levels have been reduced in people using 28 grams daily of soy protein with a high isoflavone content, or with Abacor®, a brand that contains 26 grams of soy protein. There is limited study of soymilk (400 milliliters daily) in pre-menopausal women, with reported benefits on cholesterol levels. Additional doses have been studied but are not recommended due to a lack of available scientific evidence.

Children (under 18 years old)

Due to potential safety concerns, a qualified healthcare provider should be consulted regarding the choice of infant formula.

SafetyReturn to top

The U.S. Food and Drug Administration does not strictly regulate herbs and supplements. There is no guarantee of strength, purity or safety of products, and effects may vary. You should always read product labels. If you have a medical condition, or are taking other drugs, herbs, or supplements, you should speak with a qualified healthcare provider before starting a new therapy. Consult a healthcare provider immediately if you experience side effects.

Allergies

Soy may act as a food allergen similar to milk, eggs, peanuts, fish, and wheat. Symptoms of an allergic reaction range from a runny nose to a sudden drop in blood pressure.

Side Effects and Warnings

Soy has been a dietary staple in many countries for more than 5,000 years and it does not appear to cause long-term toxicity. Aside from allergic reactions, limited side effects have been reported in infants, children, and adults.

Soy protein taken by mouth has been associated with stomach and intestinal difficulties, such as bloating, nausea, and constipation. More serious intestinal side effects have been uncommonly reported in infants fed soy protein formula, including vomiting, diarrhea, growth failure, and damage/bleeding of the intestine walls. People who experience intestinal irritation (colitis) from cow's milk may also react to soy formula.

Based on human case reports and animal research, soy may affect thyroid hormone levels in infants. There have been rare reports of goiters (enlarged neck due to increased thyroid size). Hormone levels became normal again after stopping soy. Infants fed soy or cow's milk formula may also have higher rates of atopic eczema than infants who are breastfed.

Acute migraine headache has been reported with the use of a soy isoflavone product. Based on animal research, damage to the pancreas may theoretically occur from regularly eating raw soybeans or soy flour/protein powder made from raw, unroasted, or unfermented beans.

The use of soy is often discouraged in patients with hormone-sensitive cancers, such as breast, ovarian, or uterine cancer, due to concerns about possible estrogen-like effects (which theoretically may stimulate tumor growth). Other hormone-sensitive conditions, such as endometriosis, may also theoretically be worsened. In laboratory studies, it is not clear if isoflavones stimulate or block the effects of estrogen or both (acting as a "receptor agonist/antagonist"). Until additional research is available, patients with these conditions should be cautious and speak with a qualified healthcare practitioner before starting use.

It is not known if soy or soy isoflavones share the same side effects as estrogens, such as increased risk of blood clots. Early studies suggest that soy isoflavones, unlike estrogens, do not cause the lining of the uterus (endometrium) to build up.

There has been a case report of vitamin D deficiency rickets in an infant nursed with soybean milk (not specifically designed for infants). Patients should consult their qualified healthcare practitioners for current breastfeeding recommendations and use formulas with adequate nutritional value.

Pregnancy and Breastfeeding

Soy as a part of the regular diet is traditionally considered to be safe during pregnancy and breastfeeding, although scientific research is limited in these areas. The effects of high doses of soy or soy isoflavones in humans are not clear, and therefore are not recommended.

Recent study demonstrates that isoflavones, which may have estrogen-like properties, are transferred through breast milk from mothers to infants. High doses of isoflavones given to pregnant rats have resulted in tumors in female offspring, although this has not been tested in humans.

In one human study, male infants born to women who ingested soymilk or soy products during pregnancy experienced more frequent hypospadias (a birth defect in which the urethral meatus, the opening from which urine passes, is abnormally positioned on the underside of the penis). However, other human and animal studies have examined males or females fed soy formula as infants, and have not found abnormalities in infant growth, head circumference, height, weight, occurrence of puberty, menstruation, or reproductive ability.

Research in children during the first year of life has found that the substitution of soy formula for cow's milk may be associated with significantly lower bone mineral density. Parents considering the use of soy formula should speak with qualified healthcare practitioners to make sure the appropriate vitamins and minerals are provided in the formula.

InteractionsReturn to top

Most herbs and supplements have not been thoroughly tested for interactions with other herbs, supplements, drugs, or foods. The interactions listed below are based on reports in scientific publications, laboratory experiments, or traditional use. You should always read product labels. If you have a medical condition, or are taking other drugs, herbs, or supplements, you should speak with a qualified healthcare provider before starting a new therapy.

Interactions with Drugs

Soy contains "phytoestrogens" (plant-based compounds with weak estrogen-like properties), such as isoflavones. It is not clear if isoflavones stimulate or block the effects of estrogen or both (acting as a "receptor agonist/antagonist"). It is not known if taking soy or soy isoflavone supplements increases or decreases the effects of estrogen on the body, such as the risk of blood clots. It is unclear if taking soy alters the effectiveness of birth control pills containing estrogen.

It is not known what the effects of soy phytoestrogens are on the anti-tumor effects of selective estrogen receptor modulators (SERMs) such as tamoxifen. The effects of aromatase inhibitors such as anastrozole (Arimidex®), exemestane (Aromasin®), or letrozole (Femara®) may be reduced. Because of the potential estrogen-like properties of soy, people receiving these drugs should speak with their oncologists before taking soy in amounts greater than normally found in the diet.

Soy protein may interact with warfarin (Coumadin®), although this potential interaction is not well characterized. Patients taking warfarin should check with a doctor and pharmacist before taking soy supplementation.

Interactions with Herbs and Dietary Supplements

The effects of soy protein or flour on iron absorption are not clear. Studies in the 1980s reported decreases in iron absorption, although more recent research has noted no effects or increased iron absorption in people taking soy. People using iron supplements as well as soy products should consult their qualified healthcare practitioners to follow blood iron levels. Calcium and phosphate levels may be altered.

Some experts believe that there may be a potential interaction between soy extract and Panax ginseng , although this possible interaction is not well understood.

Prebiotics (complex sugars) do not appear to affect how the body absorbs soy. It is unclear if probiotics (commonly found in cultured milk products like yogurt) affect the absorption of soy.

Methodology Return to top

This information is based on a professional level monograph edited and peer-reviewed by contributors to the Natural Standard Research Collaboration (www.naturalstandard.com): Ethan Basch, MD, MSc, MPhil (Memorial Sloan-Kettering Cancer Center); Dawn Costa, BA, BS (Natural Standard Research Collaboration); Cynthia Dacey, PharmD (Northeastern University); Paul Hammerness, MD (Massachusetts General Hospital); Shaina Tanguay-Colucci, BS (Natural Standard Research Collaboration); Catherine Ulbricht, PharmD (Massachusetts General Hospital); Christine Ulbricht, BS (University of Massachusetts); Mamta Vora, PharmD (Northeastern University); Wendy Weissner, BA (Natural Standard Research Collaboration); Jen Woods, BS (Natural Standard Research Collaboration).

Methodology details

Selected references Return to top

Allison DB, Gadbury G, Schwartz LG, et al. A novel soy-based meal replacement formula for weight loss among obese individuals: a randomized controlled clinical trial. Eur J Clin Nutr 2003;57(4):514-522.
Anderson JW, Luan J, Hoie LH. Structured weight-loss programs: meta-analysis of weight loss at 24 weeks and assessment of effects of intervention intensity. Adv Ther 2004;21(2):61-75.
Campbell CG, Brown BD, Dufner D, et al. Effects of soy or milk protein during a high-fat feeding challenge on oxidative stress, inflammation, and lipids in healthy men. Lipids 2006 Mar;41(3):257-65.
Cohen LA, Crespin JS, Wolper C, et al. Soy isoflavone intake and estrogen excretion patterns in young women: effect of probiotic administration. In Vivo 2007 May-Jun;21(3):507-12.
Fournier LR, Ryan Borchers TA, Robison LM, et al. The effects of soy milk and isoflavone supplements on cognitive performance in healthy, postmenopausal women. J Nutr Health Aging. 2007 Mar-Apr;11(2):155-64.
Giampietro PG, Bruno G, Furcolo G, et al. Soy protein formulas in children: no hormonal effects in long-term feeding. J Pediatr Endocrinol Metab 2004;17(2):191-196.
Izumi T, Saito M, Obata A, et al. Oral intake of soy isoflavone aglycone improves the aged skin of adult women. J Nutr Sci Vitaminol (Tokyo) 2007 Feb;53(1):57-62.
Kerstetter JE, Wall DE, O'Brien KO, et al. Meat and soy protein affect calcium homeostasis in healthy women. J Nutr 2006 Jul;136(7):1890-5.
Koo WW, Hammami M, Margeson DP, et al. Reduced bone mineralization in infants fed palm olein-containing formula: a randomized, double-blinded, prospective trial. Pediatrics 2003;111(5 Pt 1):1017-1023.
Kreijkamp-Kaspers S, Kok L, Grobbee DE, et al. Effect of soy protein containing isoflavones on cognitive function, bone mineral density, and plasma lipids in postmenopausal women: a randomized controlled trial. JAMA 2004;292(1):65-74.
MacGregor CA, Canney PA, Patterson G, et al. A randomised double-blind controlled trial of oral soy supplements versus placebo for treatment of menopausal symptoms in patients with early breast cancer. Eur J Cancer 2005;41(5):708-714.
Nelson HD, Vesco KK, Haney E, et al. Nonhormonal therapies for menopausal hot flashes: systematic review and meta-analysis. JAMA 2006 May 3;295(17):2057-71.
Qin LQ, Xu JY, Wang PY, et al. Soyfood intake in the prevention of breast cancer risk in women: a meta-analysis of observational epidemiological studies. J Nutr Sci Vitaminol (Tokyo) 2006 Dec;52(6):428-36.
Teas J, Braverman LE, Kurzer MS, et al. Seaweed and soy: companion foods in Asian cuisine and their effects on thyroid function in American women. J Med Food 2007 Mar;10(1):90-100.
Tice JA, Ettinger B, Ensrud K, et al. Phytoestrogen supplements for the treatment of hot flashes: the Isoflavone Clover Extract (ICE) Study: a randomized controlled trial. JAMA 2003;290(2):207-214.

March 01, 2008.

Natural Standard Logo This evidence-based monograph was prepared by the Natural Standard Research Collaboration. The information provided should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. Talk to your health care provider before taking any prescription or over the counter drugs (including any herbal medicines or supplements) or following any treatment or regimen. Copyright© 2008 Natural Standard (www.naturalstandard.com). All Rights Reserved.