Soy: What We Know

Annette Marin, MD

Few topics in the health food world are more controversial than that of soy. While some think of soy as a health food and want to add it to every meal or recipe, others believe it is the cause of all kinds of problems from infertility to thyroid problems, etc.

Anyone with a background in science knows that there are studies to support just about anything and you can’t put much weight on small studies. A study is only as good as its design and size among other variables. Small studies are good for giving us ideas for future studies but not infrequently are their conclusions proven to be, in fact, the opposite of what originally was concluded. This is no different with soy and you will find numerous small studies on opposite sides of the fray. So how do we approach this topic to find the truth of the matter? What we need to do is focus on what we do know, that is, the outcomes of the large better designed studies. Yet we must also keep in mind that even these are not always above fault. So what do we know about soy?

One thing we know about soy is that Asian people groups have eaten soy for thousands of years. That may seem like enough to settle the debate but we must consider how they have eaten it. Currently in the US we have mostly genetically modified soy added to multiple processed foods from soy protein used as protein supplements to soybean oil in chips and snacks, to highly processed soy veggie meats. None of this is eaten by traditional Asian cultures. So how do they eat soy? They regularly eat minimally processed soy in small quantities, about half fermented such as miso, tempe, natto, and half non-fermented such as tofu, edamame, soy beans. The most conservative thing we can say, then, is that if we eat soy as they eat it we should probably be ok, especially considering they have some of the lowest rates of heart disease, osteoporosis, breast and prostate cancer. Also we should note that some small studies that have shown negative effects of soy have included the most highly refined soy, such as soy protein isolate, which may be very different from minimally processed soy.

Another thing we know is that the longest lived cultures regularly eat legumes. This is seen in Dan Buettner’s Blue Zones study. Soy is a bean. In the China Study it is also seen that 80% of the beans eaten in China are the soy bean and bean intake there is inversely related to heart disease, cholesterol, etc.

We also know is that human studies show a significant decrease in recurrence of breast cancer and improved breast cancer survival with the addition of soy in the diet. The only three human studies we have regarding soy and breast cancer have favorable findings which show that as soy isoflavones increase in the diet, death and breast cancer recurrence decrease. These are not small studies. The first is the LACE study where 2000 breast cancer survivors were followed and another was a large study published in JAMA where 5000 breast cancer survivors were studied.

An additional favorable hormonal effect of soy is demonstrated in its link to delay the onset of menarche in girls. This is a protective benefit since later menarche is linked to decreased rates of a variety of female cancers.

Keep in mind is that it may be possible to get too much soy. Soy is a complete protein like meat and has been found to raise IGF levels similar to meat, at high levels of intake. For soy, the level of intake required to raise IGF levels was 5-17 servings per day. The current studies have found no elevation of IGF with 2-3 servings per day and we have no studies about intakes greater than 3 servings per day.

Just a few answers to concerns may be in order. Soy can affect thyroid function negatively but this effect has only been seen in cases of iodine deficiency. Claims have also been made of it decreasing sperm count which is not true; instead it has increased ejaculate which decreases concentration of sperm, a very different thing and something of which we do not know the meaning. A concern some have of all legumes, including soy, is that they have anti-nutrients which are found to block absorption of minerals and other nutrients. This really should not be a concern since it is only seen in very small studies and especially considering bean eating is associated with improved health in many cultures. Also, phytates have been found to be inversely correlated with cancer, which actually seems to be a good thing.

Overall, soy appears to be healthy. It is a complete protein rich in fiber with positive effects on hormones that has stood the test of time when eaten in moderation in minimally processed forms. It is not necessary to eat soy as a meat replacement since there is adequate protein in a balanced plant based diet. I do not recommend using it regularly as a protein supplement since the average person does not need extra protein and other plant based protein sources are available for those in special circumstances requiring extra protein. Remember that more of a good thing is not always better. A well rounded plant based diet is best. Considering the current evidence, eating soy in moderation as it has been eaten in traditional cultures at up to 2-3 servings per day of minimally processed, non-genetically modified forms seems like a balanced approach.

Sources Soy-Food, Wonder Drug, or Poison? The McDougall Newsletter. April 2005. Vol. 4, No. 4.

How Much Soy Is Too Much?

Guha N, Kwan ML, Quesenberry CP Jr, Weltzien EK, Castillo AL, Caan BJ. Soy isoflavones and risk of cancer recurrence in a cohort of breast cancer survivors: the Life After Cancer Epidemiology study. Breast Cancer Res Treat. 2009 Nov;118(2):395-405.

Shu XO, Zheng Y, Cai H, Gu K, Chen Z, Zheng W, Lu W. Soy food intake and breast cancer survival. JAMA. 2009 Dec 9;302(22):2437-43.

Caan BJ, Natarajan L, Parker B, Gold EB, Thomson C, Newman V, Rock CL, Pu M, Al-Delaimy W, Pierce JP. Soy food consumption and breast cancer prognosis. Cancer Epidemiol Biomarkers Prev. 2011 May;20(5):854-8.

Cheng G, Remer T, Prinz-Lannohl R, Blaszkewicz M, Degen GH, Buyken AE. Relation of isoflavones and fiber intake in childhood to the timing of puberty. Am J Clin Nutr. 2010 Sep;92(3):556-64ge

Soy food intake after diagnosis of breast cancer and survival: an in-depth analysis of combined evidence from cohort studies of US and Chinese women. Am J Clin Nutr. 2012 Jul;96(1):123-32. Nechuta, et al.

Chavarro JE, Toth TL, Sadio SM, Hauser R. Soy food and isoflavone intake in relation to semen quality parameters among men from an infertility clinic. Hum Reprod. 2008 Nov;23(11):2584-90. Epub 2008 Jul 23.