Soy. What does this word mean to you? Do you think of a healthy vegetarian food choice? Is it the answer to your lactose intolerance? Or have you heard of the suggested estrogenic effects and fear the potential of manboobs?
Don’t fear just yet, but these are some of the things that people associate with consumption of soy foods. This is partly due to the inclination of popular health media being ever eager to latch onto the latest food or nutrient with supposed health benefits or detriments. Soy, and several of its components, have been in the spotlight in the last several years for both of these reasons. So, I decided to explore the science that lay behind the controversy with soy.
It all started in Asia…
The low observed prevalence of obesity and metabolic diseases in Asian countries prompted exploration into the Asian diet for the key to the seemingly miraculous health. Soy features prominently in Asian culinary fare, and drew interest as the possible secret ingredient.
You are probably familiar with soy products in their various forms from tofu, soymilk, soy sauce, soybean oil, tempeh (fermented soybean cake), to edamame (cooked immature green soybeans). Soy is also found in many protein bars, soy yogurt and ice cream (dairy alternatives), and veggie burgers and hotdogs (meat alternatives). Soy protein is even fortified in many cereals and baked goods. Soy seems to be everywhere and chances are that you consume it in one form or another.
The question is, “ Is this a good thing?”
Soybeans are a type of legume that is oil-rich, exhibiting a healthy fatty acid profile, is a source of complete protein (meaning that it contains all essential amino acids—rare for a plant protein), and contains many of the B-vitamins, and several minerals including: iron, magnesium, calcium, and potassium.
Beyond this nutrient profile, soybeans contain bioactive phytochemicals (naturally occurring compounds in plants that may have physiological properties). One important group of phytochemicals that has garnered much attention in the media, as well as in the scientific community is the isoflavones. These compounds can have estrogen-like effects by binding to estrogen receptors and have been tenuously linked to adverse outcomes—like the feminization of males who ingest high quantities. Studies on animals have found that dietary soy exposure had caused a decrease in androgen hormones (like testosterone) and decreased prostate weight. However, these results are not generalizable to humans as the dose employed in this rodent study was extremely high—well beyond what would be found in dietary content. Messina in a review in The Journal of Nutrition states that,
“…the totality of evidence, especially the clinical data, indicate feminization concerns are unwarranted.”
So, are there any potential health risks?
Well, there are conflicting results on the effect of soy on breast cancer. A review in Molecular and Cellular Endocrinology explains that there is data of both negative and positive effects on breast cancer cell growth. And there is further research that found an interaction between soy and tamoxifen (a popular breast cancer drug). This suggests that perhaps caution should be applied to those who have, or are at high risk of breast cancer.
Another population with which caution would be advised is infants. There is not much data on long-term effects of either soy-based infant formulas, soy exposure via lactation, or in utero exposure. One study in Hormones and Behavior found adverse effects on reproductive development and spatial learning for male offspring of mothers exposed to isolated soy isoflavones throughout pregnancy and lactation. However, this study was conducted in rodents with an isolated isoflavone which is again not a good representation of human exposure. Despite this, infancy is a vulnerable time in which exposure to any potential endocrine-disrupting substances should be limited when possible.
What about everyone else…are there benefits?
Soy has been shown to have beneficial cardiovascular effects. The cholesterol-lowering effects have been well-enough established for the FDA to issue soy foods the health claim of being protective against coronary heart disease. There are also documented benefits of reducing high blood pressure (Messina).
And those feared estrogenic effects may actually be responsible for the observed benefit in reducing the frequency and severity of hot flashes in menopausal women as well as increasing bone density in post-menopausal women.
There have also been suggestions that soy and its constituents can help prevent and/or improve obesity. However, at present there is a lot of conflicting data—so no definite conclusions should be drawn just yet.
An interesting point is that the clear health benefits from numerous studies in Asian populations have not been reproduced with the same dramatic results here. There are two proposed explanations for this observation:
First, the way in which soy is consumed in the US tends to be in the form of much more processed foods, often containing components of soy (like isolated isoflavones, or soy protein) as opposed to whole food preparations as traditionally seen in Asian cuisine (Reinwald et al.). By adulterating the soybean, we may be depriving ourselves of some of the health benefits as the isoflavones may act in concert with other phytochemicals and nutrients in soy.
Second, the health benefits seen in Asian populations may be due to lifetime exposures to soy, and not the short-term exposures evaluated in most intervention studies.
Certainly amongst the abundance of literature on soy research, the aforementioned health outcomes are only a few that may be associated with consumption of soy. From what I could find, there are far more health benefits than consequences in the literature. And there is still a need for more research on the topic to clarify many finer points.
In the mean while, to paraphrase a nutritional epidemiology professor of mine:
To determine if a food is “good” or “bad,” you need to ask the question, “Compared to what?”
If eating soy for you means that it replaces red meat in your diet, for example, that would certainly be beneficial because of the reduction in saturated fat. If, however, your version of eating soy means downing soy protein bars that are also sugar-laden for your snacks…well, you’d probably be better off with an apple and a handful of nuts.
In the end, there seems to be a place on most people’s plates for soy, if they so choose. There is a lot of fear-driven misinformation in the popular media that you should take with a grain of salt. Of course, if you have specific concerns about your individual health, please consult with your physician. And if you do opt for soy, consider consuming it in more traditional, whole-food forms. I will also reiterate the conventional dietary wisdom of exercising moderation and variety in your diet—after all it’s the spice of life.
Ball, E.R., Caniglia, M.K., Wilcox, J.L., Overton, K.A., Burr, M.J., Wolfe, B.D., Sanders, B.J., Wisniewski, A.B., Wrenn, C.C. (2010). Effect of genistein in the maternal diet on reproductive development and spatial learning in male rats. Hormones and Behavior, 57: 313-322. doi:10.1016/j.yhbeh.2009.12.013.
Cederroth, C.R., Nef, S. (2009). Soy, phytoestrogens and metabolism: A review. Molecular and Cellular Endocrinology, 304: 30-42. doi:10.1016/j.mce.2009.02.027.
Messina, M. (2010). Insights gained from 20 years of soy research. The Journal of Nutrition, 140:2289S-2295S. doi:10.3945/jn.110.124107.
Reinwald, S., Akabas, S.R., Weaver, C.M. (2010). Whole versus the piecemeal approach to evaluating soy. The Journal of Nutrition, 140: 2335S-2343S. doi:10.3945/jn.110.124925.
Taku, K., Melby, M., Kronenberg, F., Kurzer, M.S., Messina, M. (2012). Extracted or synthesized soybean isoflavones reduce menopausal hot flash frequency and severity: systematic review and meta-analysis of randomized controlled trials. Menopause: The Journal of the North American Menopause Society, 19(7): 776-790. doi: 10.1097/gme.0b013e3182410159.
Taku, K. Melby, M., Nishi, N., Omori, T., Kurzer, M.S. (2011). Soy isoflavones for osteoporosis: An evidence-based approach. Maturitas, 70: 333-338. doi:10.1016/j.maturitas.2011.09.001.