Soy, billed as a healthy alternative to meat and dairy, its impact on a variety of health conditions, including cancer and heart disease, has been hotly debated.
The increasing emphasis on plant-based diets has made it more important than ever to understand whether soy should be celebrated for its health benefits or consumed with caution.
Culinary Medicine
by Lori A Smolin, PhD and
Contributor Stefanie Schwartz, MS, RD, CDN
Soybeans are a legume that have been part of the human diet for over 7,000 years.[1] Soybean products such as soymilk, tofu, and tempeh have their origins in Asia and these traditional foods, along with some modern soy products, are becoming more popular in the United States. You can sub soymilk for dairy in your morning coffee, enjoy a soy-based burger for lunch, toss tofu into your dinner stir fry, and snack on edamame. While soy is billed as a healthy alternative to meat and dairy, its impact on a variety of health conditions, including cancer and heart disease, has been hotly debated. The increasing emphasis on plant-based diets has made it more important than ever to understand whether soy should be celebrated for its health benefits or consumed with caution. [2,3]
Soy Nutritional Contributions and Concerns
Soy is high in protein; it is considered equivalent to animal protein in protein quality, making it a valuable addition to vegan diets. It is cholesterol free and low in saturated fat. Soy provides a good source of fiber, contains omega-3 polyunsaturated fatty acids, is rich in the vitamins thiamin, folate, and vitamin K and provides minerals including calcium, iron, magnesium, and potassium. In addition, it contains phytochemicals including isoflavones, which are a type of phytoestrogen (phyto=plant), tocopherols, carotenoids, and phytosterols.
As discussed in the sections below, soy has the potential to provide many health benefits but it is also one of the most common food allergens.[4] Foods containing soy must include it in the ingredient list and often highlight it at the end of the list with a statement such as: “Contains soy”. Soy may also interfere with the action of some medications. For example, soy can reduce the absorption of levothyroxine used to treat hypothyroidism. Those taking this medication need to avoid soy one hour before or within up to four hours after taking the medication or if they eat soy consistently, their dose of levothyroxine should be adjusted to account for the interference.[3] Soy can also interfere with some chemotherapy drugs and hormone-based cancer treatments; patients taking these medications should check with their oncologist before consuming soy.[3]
Breast Cancer
The relationship between soy and breast cancer has been highly debated. Much of the controversy has to do with how the isoflavones in soy affect estrogen activity in the body. Some types of breast cancer cells have proteins called estrogen receptors on their surface. When estrogen binds to these receptors it stimulates the cancer cells to grow. Isoflavones are structurally similar to estrogen so can bind to estrogen receptors in the body.[3] The physiologic effect of this binding is variable, in some situations having weak estrogenic effects and in others blocking estrogen activity.[3] In the case of breast cancer, epidemiological evidence supports an anti-estrogenic role for soy isoflavones; soy consumption is inversely associated with cancer incidence and deaths.[5]
However, over the years challenges have been made to the proposed breast cancer benefits of soy. In the late 1990s studies done on mice suggested that genistein, an isoflavone in soy, stimulated the growth of existing breast cancer cells.[6] This led clinicians to recommend that breast cancer patients avoid soy. However, since then it has been determined that the results of these studies cannot be applied to humans because mice metabolize genistein very differently. Human studies have found that consuming soybeans or isoflavones lowers the risk of breast cancer and in breast cancer survivors, lowers the risk of recurrence and death.[5,7] Thus, as our understanding has evolved, so have recommendations. It is currently the position of the American Cancer Society, the American Institute for Cancer Research, and the World Cancer Research Fund International that soy foods can be safely consumed by women with breast cancer.[3]
Prostate Cancer
Prostate cancer is another hormone-dependent cancer that may be affected by the isoflavones in soy. Epidemiology supports the benefits of soy; the incidence of prostate cancer is much lower in Asian countries where soy intake is high. Asian men who move to the United States and adopt a Western diet have an increased risk of prostate cancer, but those who continue to consume a traditional diet high in soy do not.[8] Although there is evidence to suggest that soy intake reduces the risk of prostate cancer, the data are currently too inconsistent to make conclusive recommendations.[3]
Heart Health
Lowering blood levels of LDL cholesterol is an important strategy for reducing the risk of cardiovascular disease (CVD). The cholesterol-lowering properties of soy protein have been studied for decades. In the 1990s, data supporting a cholesterol-lowering effect of soy prompted the FDA to allow a health claim on food labels highlighting the benefit of soy protein in reducing the risk of heart disease. But the hypocholesterolemic effects of soy protein became controversial when subsequent studies did not find soy protein to have a significant cholesterol-lowering effect.[9] Today, meta-analyses support a modest, yet clinically relevant, cholesterol-lowering effect for soy protein.[3]
Most of the studies on the cholesterol-lowering properties of soy have investigated the effects of the protein in soy. But whole soy has other nutritional properties, such as being low in saturated fat and high in polyunsaturated fat and fiber, that could contribute to its cholesterol-lowering effects.[9] A long-term epidemiological study found that replacing red meat with soy foods, beans, and nuts was associated with a 14% lower risk of heart disease.[10]
Soy may also have other physiological effects that reduce CVD risk. Soy intake may help to reduce blood pressure.[9] The mechanism is not known and in most studies the reduction is modest, but even modest blood pressure lowering can have a significant impact on CVD risk. Soy intake is also negatively associated with the risk of type 2 diabetes, an important risk factor for CVD.[11]
Hot Flashes
The fluctuating levels of estrogen and progesterone that occur with menopause cause hot flashes and other menopausal symptoms. Increasing intake of soy isoflavones, which bind to estrogen receptors, has been hypothesized to help reduce hot flashes. Women in Asian countries have less frequent hot flashes and higher blood levels of isoflavones than women in the US, but it is unclear whether the less frequent hot flashes are due to soy intake.[12] Reviews of studies on the effect of supplemental isoflavones on hot flashes found conflicting results. This may be due to the use of different types of isoflavones in different trials.[3,13,14] For example, clinical trials using supplements derived from whole soybeans generally support its effectiveness at reducing hot flashes, while those using supplements made from the germ portion of the bean do not.[3] The fact that isoflavones are safe and have other health benefits makes them an appealing alternative to hormone therapy for hot flashes.[13]
Osteoporosis
Osteoporosis risk increases dramatically in postmenopausal women when estrogen levels drop. A meta-analysis found supplements of soy isoflavones or consumption of soy protein or soy foods resulted in significant improvement in bone mineral density of the spine, hip, and femoral neck in menopausal and perimenopausal women.[15] However, this is clinically significant only if it results in a reduction in bone fractures. A meta-analysis of studies investigating the association of soy food intake with bone fractures found that intake correlated with a reduced fracture risk in women but the association was not significant in men.[16] More information is needed before recommending isoflavones to improve bone health, but including soy foods, which are a source of isoflavones and protein, and are often fortified with calcium, may have benefits for those with concerns about bone loss.
Cognitive Function
Some cognitive decline occurs in adults as they age but decline that interferes with daily life is not normal.[17] In postmenopausal women evidence suggests that low levels of estrogen may contribute to impaired cognitive function.[2] Soy isoflavones are thought to exert beneficial effects on cognition function through their estrogen-like activity. [18] Although the results of both observational studies and randomized controlled trials evaluating the effects of isoflavones on cognitive decline have been inconsistent, recent meta-analysis support the potential of isoflavones in soy foods to reduce cognitive decline and dementia.[3,18]
Bottom Line
Soy has been extensively studied for its impact on various health conditions, often with conflicting results. Some of the inconsistences may be due to differences in the type of study, the study population, and the amount and type of soy used. Despite continuing controversy, there is significant evidence that soy intake has protective effects against breast and prostate cancers and heart disease. In addition, there is data supporting a beneficial impact of soy on the symptoms of menopause, osteoporosis, and cognitive function. Although there is still insufficient evidence to make specific recommendations for intake of soy in foods or supplements, it is safe for healthy individuals to consume soy products such as soymilk, meat alternatives, tofu, edamame, and tempeh. When added to the diet, soy is a nutrient-dense source of high-quality protein that can provide health benefits, particularly when used as a substitute for red or processed meat.[3]
References
[1] Lee GA, Crawford GW, Liu L, Sasaki Y, Chen X. Archaeological soybean (Glycine max) in East Asia: does size matter? PloS One. 2011;6(11):e26720. doi:https://doi.org/10.1371/journal.pone.0026720
[2]Harvard School of Public health. Straight Talk about Soy. The Nutrition Source. Published August 6, 2018. https://www.hsph.harvard.edu/nutritionsource/soy/
[3] Messina M, Duncan A, Messina V, Lynch H, Kiel J, Erdman JW. The health effects of soy: A reference guide for health professionals. Frontiers in Nutrition. 2022;9:970364. doi:https://doi.org/10.3389/fnut.2022.970364
[4] Center for Food Safety and Applied Nutrition. Food Allergies. FDA. Published November 3, 2020. https://www.fda.gov/food/food-labeling-nutrition/food-allergies
[5] Jung Hyun Kang, Dong Z, Seung Ho Shin. Benefits of Soybean in the Era of Precision Medicine: A Review of Clinical Evidence. Journal of microbiology and biotechnology. 2023;33(12):1552-1562. doi:https://doi.org/10.4014/jmb.2308.08016
[6] Allred CD, Allred KF, Ju YH, Virant SM, Helferich WG. Soy diets containing varying amounts of genistein stimulate growth of estrogen-dependent (MCF-7) tumors in a dose-dependent manner. Cancer Res. (2001) 61:5045–50.
[7] Qiu S, Jiang C. Soy and isoflavones consumption and breast cancer survival and recurrence: a systematic review and meta-analysis. European Journal of Nutrition. 2018;58(8):3079-3090. doi:https://doi.org/10.1007/s00394-018-1853-4
[8] van Die MD, Bone KM, Williams SG, Pirotta MV. Soy and soy isoflavones in prostate cancer: a systematic review and meta-analysis of randomized controlled trials. BJU International. 2014;113(5b):E119-E130. doi:https://doi.org/10.1111/bju.12435
[9] Messina M. Soy and Health Update: Evaluation of the Clinical and Epidemiologic Literature. Nutrients. 2016;8(12):754. doi:https://doi.org/10.3390/nu8120754
[10] Al-Shaar L, Satija A, Wang DD, et al. Red Meat Intake and Risk of Coronary Heart Disease among US men: Prospective Cohort Study. BMJ. 2020;371:m4141. doi:https://doi.org/10.1136/bmj.m4141
[11] Zuo X, Zhao R, Wu M, Wan Q, Li T. Soy Consumption and the Risk of Type 2 Diabetes and Cardiovascular Diseases: A Systematic Review and Meta-Analysis. Nutrients. 2023;15(6):1358. doi:https://doi.org/10.3390/nu15061358
[12] Li L, Lv Y, Xu L, Zheng Q. Quantitative efficacy of soy isoflavones on menopausal hot flashes. British Journal of Clinical Pharmacology. 2015;79(4):593-604. doi:https://doi.org/10.1111/bcp.12533
[13] Chen LR, Ko NY, Chen KH. Isoflavone Supplements for Menopausal Women: A Systematic Review. Nutrients. 2019;11(11):2649. doi:https://doi.org/10.3390/nu11112649
[14] Taku K, Melby MK, Kronenberg F, Kurzer MS, Messina M. Extracted or synthesized soybean isoflavones reduce menopausal hot flash frequency and severity. Menopause: The Journal of The North American Menopause Society. 2012;19(7):776-790. doi:https://doi.org/10.1097/gme.0b013e3182410159
[15] Akhlaghi M, Ghasemi Nasab M, Riasatian M, Sadeghi F. Soy isoflavones prevent bone resorption and loss, a systematic review and meta-analysis of randomized controlled trials. Critical Reviews in Food Science and Nutrition. Published online July 10, 2019:1-15. doi:https://doi.org/10.1080/10408398.2019.1635078
[16] Mohsen Akhavan Zanjani, Rahmani S, Sanaz Mehranfar, et al. Soy Foods and the Risk of Fracture: A Systematic Review of Prospective Cohort Studies. Complementary medicine research. 2021;29(2):172-181. doi:https://doi.org/10.1159/000519036
[17] CDC. Subjective Cognitive Decline — A Public Health Issue. www.cdc.gov. Published March 25, 2019. https://www.cdc.gov/aging/data/subjective-cognitive-decline-brief.html#:~:text=Subjective%20Cognitive%20Decline%20(SCD)%20is
[18] Cui C, Birru RL, Snitz BE, et al. Effects of soy isoflavones on cognitive function: a systematic review and meta-analysis of randomized controlled trials. Nutrition Reviews. 2019;78(2):134-144. doi:https://doi.org/10.1093/nutrit/nuz050
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