Unpacking Soy: A Discussion on the History, Health, and Safety of Soy Consumption
Written by Christy Walowit MS, RDN
Imagine walking down the aisles of a grocery store, trying to decide what to buy for your next meal. You come across shelves filled with soy products – soy milk, tofu, soy sauce, and more. But as you reach out to grab one of these items, a thought crosses your mind: Is soy safe to consume? While soy has been a staple in many Asian cuisines for centuries, its introduction to the Western world has been met with mixed reactions. From fears about hormonal imbalances to digestive health, the confusion surrounding soy has only grown stronger over time. Let's delve into the history of soy in the United States, and address the research around soy consumption and safety.
History of Soy in the United States:
The soybean originated in China in the 11th century BCE, but did not become a common food source in the US until the 1900’s. The history of soy in the United States dates back to the early 19th century when Asian immigrants brought soybeans to American shores. Much of soy’s rise in popularity can be attributed to Yamei Kin, a Chinese-born doctor residing in the US. During World War I, the US government faced a meat scarcity and began seeking new protein sources for American soldiers. By the USDA’s request, Kin developed a laboratory to study how the soybean could be used in the United States. Throughout the past century, soy has become increasingly popular in the American diet, and the US became the biggest soy exporter in the 1950s.
Yamei Kin
A Complete Protein:
Soy is a well-known staple in plant-based and traditional Asian diets. Soy is high in polyunsaturated fats, fiber, vitamins, minerals, and is low in unsaturated fats. Because it is a complete protein (meaning–like many meat products–it contains all the essential amino acids), many vegetarians and vegans rely on soy as a non-animal protein source. Despite soy’s widespread popularity, many consumers are uncertain about the health benefits and potential risks of soy consumption.
Does Soy Increase Cancer Risk?
Soy contains phytoestrogens, which are plant compounds that have a similar structure to estrogen in the body. Some worry that consuming soy products could lead to hormonal imbalances or even increase the risk of certain cancers. It is important to clarify that soy phytoestrogens are not the same as estrogen, and soy does not contain estrogen. These concerns may also have originated from research on rodent studies showing components of soy stimulated breast tumor growth (read more here). Fortunately, humans metabolize soy differently from rodents and no research shows higher consumption of soy leads to increased cancer risk.
Zero research shows higher consumption of soy leads to increased cancer risk.
Can Soy Reduce Breast Cancer Risk?
The Shanghai Women’s Health Study is a highly detailed study of soy and breast cancer risk developed between 1997-2000 and follows approximately 75,000 Chinese women1. The study has found a 59% lower risk of premenopausal breast cancer in women who ate higher amounts of soy, and a 43% lower risk when soy was consumed in adolescence (2). Overall, many studies have shown higher soy intake is beneficial in reducing breast cancer risk, and may also protect from endometrial and colorectal cancer (3).
Can Soy Reduce Prostate Cancer Risk?
Soy foods are highly consumed in Asian countries, where the incidence rate of prostate cancer is the lowest (2). A 2017 meta-analysis found that soy intake was significantly associated with a reduced risk of prostate cancer (4). Other studies have found that some components of soy can help protect against the development of prostate cancer (5,6).
Does Soy Consumption Impact Testosterone?
While some may have heard claims that phytoestrogens in soy have feminizing effects or disrupt testosterone in the body, evidence shows that soy does not cause elevated estrogen levels or have a significant effect on hormone levels of people assigned male at birth (7). As we navigate the complexity of beliefs surrounding soy consumption, it is important to recognize negative stereotypes of people with Asian heritage in Western media. For example, a study of Asian American men found that unflattering physical attributes and physical ability distortions were perceived to be Asian male stereotypes (7). These harmful stereotypes may inform negative attitudes and beliefs towards Asian foods such as soy.
Thyroid Function:
Studies addressing the impact of soy consumption and hormone levels vary depending on the specific population and hormonal condition being studied. Overally, studies of healthy individuals have yet to find significant associations between soy isoflavones and thyroid function (10,11). Additionally, individuals with hypothyroidism may require slight adjustments in their medication if consuming soy, but do not need to avoid eating soy products (11).
Digestive Health:
The presence of phytates and lectins in soy has raised concerns about nutrient absorption and digestive health. In high amounts, compounds can interfere with the body's ability to absorb minerals like iron and zinc. This effect can be minimized by eating soy as part of a balanced and varied diet. Additionally, lectins may cause negative side effects such as bloating, gas, and diarrhea when ingested, however, people rarely ingest enough to experience adverse symptoms. Lectins have many other health benefits, including slowing the digestion and absorption of carbohydrates and acting as antioxidants in the body (9).
Final Thoughts:
One final factor to consider is the type of soy consumed. For example, soy foods like tofu and edamame may have different nutrient composition than processed soy products like soy protein isolates and textured vegetable protein. Ultimately, while individual responses to soy may vary, soy is considered very safe to consume and is associated with many health benefits.
Ultimately, while individual responses to soy may vary, soy is considered very safe to consume and is associated with many health benefits.
While some studies (particularly animal studies) have caused confusion around the health and safety of soy consumption, research in humans shows soy has beneficial or neutral effects, especially when consumed as part of a balanced and varied eating pattern. Research suggests that consuming soy may help in lowering cholesterol levels, reducing the risk of heart disease, and lowering the risk of certain cancers. It may be helpful to consult with a Registered Dietitian when considering your own dietary needs and preferences when incorporating soy products into your meals.
References:
Zheng W, Chow W-H, Yang G, et al. The Shanghai Women’s Health Study: rationale, study design, and baseline characteristics. Am J Epidemiol. 2005;162(11):1123-1131. doi:10.1093/aje/kwi322
Straight Talk About Soy. The Nutrition Source. Published August 6, 2018. Accessed September 6, 2020. https://www.hsph.harvard.edu/nutritionsource/soy/
Rowe IJ, Baber RJ. The effects of phytoestrogens on postmenopausal health. Climacteric. 2021;24(1):57-63. doi:10.1080/13697137.2020.1863356
Applegate CC, Rowles JL, Ranard KM, Jeon S, Erdman JW. Soy Consumption and the Risk of Prostate Cancer: An Updated Systematic Review and Meta-Analysis. Nutrients. 2018;10(1):40. doi:10.3390/nu10010040
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 Int. 2014;113(5b):E119-130. doi:10.1111/bju.12435
Zhang HY, Cui J, Zhang Y, Wang ZL, Chong T, Wang ZM. Isoflavones and Prostate Cancer: A Review of Some Critical Issues. Chin Med J (Engl). 2016;129(3):341-347. doi:10.4103/0366-6999.174488
Reed KE, Camargo J, Hamilton-Reeves J, Kurzer M, Messina M. Neither soy nor isoflavone intake affects male reproductive hormones: An expanded and updated meta-analysis of clinical studies. Reprod Toxicol. 2021;100:60-67. doi:10.1016/j.reprotox.2020.12.019
Wong YJ, Owen J, Tran KK, Collins DL, Higgins CE.. Asian American male college students' perceptions of people's stereotypes about Asian American men. Psychology of Men & Masculinity. 2012; 13(1), 75–88. https://doi.org/10.1037/a0022800
Lectins. The Nutrition Source. Published January 24, 2019. Accessed September 27, 2020. https://www.hsph.harvard.edu/nutritionsource/anti-nutrients/lectins/
Dillingham BL, McVeigh BL, Lampe JW, Duncan AM. Soy protein isolates of varied isoflavone content do not influence serum thyroid hormones in healthy young men. Thyroid. 2007;17(2):131-137. doi:10.1089/thy.2006.0206
Messina M, Redmond G. Effects of soy protein and soybean isoflavones on thyroid function in healthy adults and hypothyroid patients: a review of the relevant literature. Thyroid. 2006;16(3):249-258. doi:10.1089/thy.2006.16.249