First, let me start by saying I wasn’t sure what to think of soy before I started updating my soy knowledge last week. I learned in school that it is high in fiber, it is the only vegetable that is a “complete protein” meaning it has all 9 essential amino acids, and if eaten regularly it will help lower cholesterol. I guess you could say that for most of my life, I have been “pro-soy”, and I would recommend it to most of my patients. Even after my week of research on soy, I am not an expert. I don’t expect to know everything there is to know about nutrition. As new research is published, my recommendations may change slightly. In this article I am reporting a comprehensive view of what we know now in 2015 about soy.
My curiosity about soy started a few years ago when a friend had a boy baby who couldn’t eat cow’s milk. She was concerned about feeding him soy milk because of the “estrogen like” components of soy products, and therefore, was feeding him goat’s milk. I thought to myself that she was being paranoid, but she’s the mom, so whatever she thinks is best probably is. I didn’t really think about soy much over the last 2-3 years until recently moving back to the United States. In the last 6 weeks since we have been back, I have heard soy come up several times. The two questions that keep coming up are: Is it ok for young boys and men to eat soy? And, is it ok to eat soy if you are at risk for or have had breast cancer? I decided that since school (I graduated with my Masters degree in Nutrition in 2006) was almost 10 years ago, it was time to update my knowledge on soy. Here’s what I found…
Ok, before I tell you what I found, let me tell you how I found it, and why I think that is important. The internet is one of our greatest resources for nutrition knowledge. It has a lot of information that is true, but it also has a lot of information that is speculation or just someone’s opinion. How do we know what to believe? One of the most valuable tools I learned in all my years of higher education was how to interpret research studies and put them into practice to make recommendations for my patients. It would be impossible to read all of the research out there now about soy. Well, maybe not impossible, but it may become your life’s work. My approach was to search for all meta-analysis research studies related to this topic to see what their conclusions were. For those of you slightly deficient in the science category, a meta-analysis is a type of research that strives to combine multiple research studies in hopes of finding a pattern. A pattern is what we are looking for! When multiple studies show the same result, we can be confident in making a recommendation. If you are pressed for time and don’t want to read all of my explanations, this article is a little long, skip to the bottom, where I outline my recommendations in bold. To eat soy, or not to eat soy, that is the question…
I found 4 meta-analysis studies, and one original research study I think are worth mentioning. The first one1 was an analysis of 8 studies in total that looked at the consumption of soy in men and it’s relation to prostate cancer. The conclusion was that the consumption of soy was associated with lower incidence of prostate cancer. There wasn’t a lot of discussion in the end of this analysis because the conclusion was clear. To play devil’s advocate with this one, I would say, “well, did you look at what else these men were eating, and correct for those things?” It is very possible that men who eat soy also do many other healthy behaviors like eat lots of fruits and veggies, how do you know it was the soy that reduced their risk? The answer would have been something like this, “that is why we don’t say the consumption of soy caused lower incidence of prostate cancer”. There is a big difference in saying something is associated with versus something is the cause of.
The second analysis2 looks at the effects of soy protein or isoflavones (a component found in soy thought to have estrogen like effects) on hormones in men. 15 placebo controlled studies and 32 other reports were analyzed. In words you all will understand, there would have been no mental difference in the men as they all thought they were taking soy. Second, there were a lot of men included in this analysis, it was not a small study. Third they looked at 4 markers of male hormones in the blood, not just testosterone. The conclusion was that consuming soy or isoflavones did not have an effect on any of the male hormones. So is it ok to give soy to boys? We don’t know that answer from this study as it only looked at men. Are there any studies on boys specifically? I found 1, it is mentioned later, but only looks at boys and girls given soy infant formula as babies.
The third analysis3 was my favorite because there was a lot in their conclusions discussing limitations in studying soy and its effects. This analysis looked at soy intake in relation to breast cancer. They reviewed 18 studies and concluded that soy intake was moderately associated with reduced breast cancer risk. The discussion goes on to say how many of these studies didn’t look at pre versus post-menopausal women. Additionally, they commented that soy additives and processed soy foods (soy based protein drinks or bars) may react differently in your body and may not be as advantageous for your health as a true soy food like tofu or edamame. There have been little to no studies on this topic. There may also me some significance in the timing of when soy foods are consumed related to puberty. There are many studies that show that exposure to soy early in life for girls lowers their breast cancer risk later in life. Lastly, an excerpt from this analysis states, “If the association between greater soy intake and lower breast cancer risk is real, what could explain the apparent lack of association between intake levels and size of the risk reduction in Western and Asian studies? Breast cancer risk for the average woman is much higher in Western women (133 per 100 000) (77) than in Asian women (39 per 100 000) (78), undoubtedly reflecting a higher burden of risk factors such as late age at first full-term pregnancy, early menarche, obesity/lack of physical activity, alcohol consumption, and adverse nutritional factors.”
The last analysis4 I looked at reviewed the benefits of soy intake compared to LDL and triglyceride levels. This analysis confirmed that soy intake is beneficial to reduce LDL cholesterol levels and triglyceride levels, and will raise the beneficial HDL levels in the blood. The discussion of this study was interesting… “However, concerns in the Western world about the effects on boys and men from eating soy foods have been raised repeatedly and addressed in numerous laboratory and population studies. Researchers at the University of Pennsylvania surveyed men and women who, as babies, were given soy-based formulas to see if any of them had sexual, fertility or hormonal problems; they were then compared to a matched group raised on cow’s milk formula. The only discrepancies noted were some minor menstrual complaints among the women (their periods lasted one-third of a day longer and they reported slightly more menstrual pain). Results were published in the August 15, 2001, issue of the Journal of the American Medical Association.”5
Here is my last thought before I present my recommendations. Has anyone ever heard of nutrigenomics? It is a term used to describe the study of the way nutrition interacts with our genes. There is more and more research in this field and I think it is one we will see a lot more of in the years to come. Nutrigenomics studies are to blame for some of the nutrition discrepancies out there right now. For example, sodium really isn’t that bad for most of us, but if we have a particular gene, it may still be. Or, coffee, good or bad? Your genes may explain this one too. There are a lot more of these questions and discrepancies out there, and the thought is that someday, dietitians will be able to look at a map of your genes and give you specific recommendations for your diet based on that map! We are a little while from that right now for two reasons: 1. Expense of gene mapping, and 2. Limited research in nutrigenomics. I hope I will see this come to fruition in my lifetime.
- I see no compelling evidence to limit eating REAL soy foods like tofu, soy milk, soy nuts, edamame, or tempeh for most individuals including women at risk or recovering from breast cancer.
- Concerning pre-pubescent boys, the research is extremely limited, I would recommend limiting soy intake in this group, if you made me say a number, I guess I would say less than 2 servings per week, although, admittedly I have no evidence for this recommendation since the research is so limited.
- I don’t think soy is AMAZINGLY beneficial, although it does have benefits for blood cholesterol levels.
- I would NOT recommend soy based supplements like protein powders, bars, or formulas. I agree with the theory that the more processed a food is, the more potential for harm. Eat REAL foods people!
My last thought is not specific to soy. Why do we have to LOVE or HATE a particular food? Can’t we just enjoy it sometimes without going crazy about it? Can’t we just limit it a little if necessary? I have said this in one of my previous posts,
“Foods are not inherently bad or good, but we make them either depending on the amount we eat, or the frequency we eat them.”
A good thought in which to end.
- Meta-analysis of soy food and risk of prostate cancer in men. Yan L, Spitznagel EL. International Journal of Cancer. 2005 Nov 20;117(4):667-9.
- Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis. Hamilton-Reeves JM1, Vazquez G, Duval SJ, Phipps WR, Kurzer MS, Messina MJ. Fertility and Sterility. 2010 Aug;94(3):997-1007.
- Meta-Analysis of Soy Intake and Breast Cancer Risk. Bruce J. Trock, Leena Hilakivi-Clarke, Robert Clarke. Journal of the National Cancer Institute. 2006 Apr 5;98(7):459-71.
- Meta-analysis of the effects of soy protein containing isoflavones on the lipid profile. Siyan Zhan and Suzanne C Ho. American Journal of Clinical Nutrition. Aug 2015, Vol 102.
- Exposure to soy-based formula in infancy and endocrinological and reproductive outcomes in young adulthood. Strom BL, Schinnar R, Ziegler EE, Barnhart KT, Sammel MD, Macones GA, Stallings VA, Drulis JM, Nelson SE, Hanson SA. Journal of the American Medical Association. Aug 2001; 286(7):807-14.