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Here are some amazing and completely free resources online that you can use to teach yourself data science.Besides this page, I would highly recommend following the Quora Data Science topic if you haven't already to get updates on new questions and answers!Step 1. Fulfill your prerequisitesBefore you begin, you need Multivariable Calculus, Linear Algebra, and Python. If your math background is up to multivariable calculus and linear algebra, you'll have enough background to understand almost all of the probability / statistics / machine learning for the job.Multivariate Calculus: What are the best resources for mastering multivariable calculus?Numerical Linear Algebra / Computational Linear Algebra / Matrix Algebra: Linear Algebra, Introduction to Linear Models and Matrix Algebra. Avoid linear algebra classes that are too theoretical, you need a linear algebra class that works with real matrices.Multivariate calculus is useful for some parts of machine learning and a lot of probability. Linear / Matrix algebra is absolutely necessary for a lot of concepts in machine learning.You also need some programming background to begin, preferably in Python. Most other things on this guide can be learned on the job (like random forests, pandas, A/B testing), but you can't get away without knowing how to program!Python is the most important language for a data scientist to learn. To learn to code, more about Python, and why Python is so important, check outHow do I learn to code?How do I learn Python?Why is Python a language of choice for data scientists?Is Python the most important programming language to learn for aspiring data scientists and data miners?R is the second most important language for a data scientist to learn. I’m saying this as someone with a statistics background and who went through undergrad mainly only using R. While R is powerful for dedicated statistical tasks, Python is more versatile as it will connect you more to production-level work.If you're currently in school, take statistics and computer science classes. Check out What classes should I take if I want to become a data scientist?Step 2. Plug Yourself Into the CommunityCheck out Meetup to find some that interest you! Attend an interesting talk, learn about data science live, and meet data scientists and other aspirational data scientists. Start reading data science blogs and following influential data scientists:What are the best, insightful blogs about data, including how businesses are using data?What is your source of machine learning and data science news? Why?What are some best data science accounts to follow on Twitter, Facebook, G+, and LinkedIn?What are the best Twitter accounts about data?Step 3. Setup and Learn to use your toolsPythonInstall Python, iPython, and related libraries (guide)How do I learn Python?RInstall R and RStudio (It's good to know both Python and R)Learn R with swirlSublime TextInstall Sublime TextWhat's the best way to learn to use Sublime Text?SQLHow do I learn SQL? What are some good online resources, like websites, blogs, or videos? (You can practice it using the sqlite package in Python)Step 4. Learn Probability and StatisticsBe sure to go through a course that involves heavy application in R or Python. Knowing probability and statistics will only really be helpful if you can implement what you learn.Python Application: Think Stats (free pdf) (Python focus)R Applications: An Introduction to Statistical Learning (free pdf)(MOOC) (R focus)Print out a copy of Probability CheatsheetStep 5. Complete Harvard's Data Science CourseAs of Fall 2015, the course is currently in its third year and strives to be as applicable and helpful as possible for students who are interested in becoming data scientists. An example of how is this happening is the introduction of Spark and SQL starting this year.I'd recommend doing the labs and lectures from 2015 and the homeworks from 2013 (2015 homeworks are not available to the public, and the 2014 homeworks are written under a different instructor than the original instructors).This course is developed in part by a fellow Quora user, Professor Joe Blitzstein. Here are all of the materials!Intro to the classWhat is it like to design a data science class? In particular, what was it like to design Harvard's new data science class, taught by professors Joe Blitzstein and Hanspeter Pfister?What is it like to take CS 109/Statistics 121 (Data Science) at Harvard?Course MaterialsClass main page: CS109 Data ScienceLectures, Slides, and Labs: Class MaterialAssignmentsIntro to Python, Numpy, Matplotlib (Homework 0) (Solutions)Poll Aggregation, Web Scraping, Plotting, Model Evaluation, and Forecasting (Homework 1) (Solutions)Data Prediction, Manipulation, and Evaluation (Homework 2) (Solutions)Predictive Modeling, Model Calibration, Sentiment Analysis (Homework 3) (Solutions)Recommendation Engines, Using Mapreduce (Homework 4) (Solutions)Network Visualization and Analysis (Homework 5) (Solutions)Labs(these are the 2013 labs. For the 2015 labs, check out Class Material)Lab 2: Web ScrapingLab 3: EDA, Pandas, MatplotlibLab 4: Scikit-Learn, Regression, PCALab 5: Bias, Variance, Cross-ValidationLab 6: Bayes, Linear Regression, and Metropolis SamplingLab 7: Gibbs SamplingLab 8: MapReduceLab 9: NetworksLab 10: Support Vector MachinesStep 6. Do all of Kaggle's Getting Started and Playground CompetitionsI would NOT recommend doing any of the prize-money competitions. They usually have datasets that are too large, complicated, or annoying, and are not good for learning. The competitions are available at Competitions | KaggleStart by learning scikit-learn, playing around, reading through tutorials and forums on the competitions that you’re doing. Next, play around some more and check out the tutorials for Titanic: Machine Learning from Disaster for a binary classification task (with categorical variables, missing values, etc.)Afterwards, try some multi-class classification with Forest Cover Type Prediction. Now, try a regression task House Prices: Advanced Regression Techniques. Try out some natural language processing with Quora Question Pairs | Kaggle. Finally, try out any of the other knowledge-based competitions that interest you!Step 7. Learn Some Data Science ElectivesData science is an incredibly large and interdisciplinary field, and different jobs will require different skillsets. Here are some of the more common ones:Product Metrics will teach you about what companies track, what metrics they find important, and how companies measure their success: The 27 Metrics in Pinterest’s Internal Growth DashboardMachine Learning How do I learn machine learning? This is an extremely rich area with massive amounts of potential, and likely the “sexiest” area of data science today. Andrew Ng's Machine Learning course on Coursera is one of the most popular MOOCs, and a great way to start! Andrew Ng's Machine Learning MOOCA/B Testing is incredibly important to help inform product decisions for consumer applications. Learn more about A/B testing here: How do I learn about A/B testing?Visualization - I would recommend picking up ggplot2 in R to make simple yet beautiful graphics and just browsing DataIsBeautiful • /r/dataisbeautiful and FlowingData for ideas and inspiration.User Behavior - This set of blogs posts looks useful and interesting - This Explains Everything " User BehaviorFeature Engineering - Check out What are some best practices in Feature Engineering? and this great example: http://nbviewer.ipython.org/github/aguschin/kaggle/blob/master/forestCoverType_featuresEngineering.ipynbBig Data Technologies - These are tools and frameworks developed specifically to deal with massive amounts of data. How do I learn big data technologies?Optimization will help you with understanding statistics and machine learning: Convex Optimization - Boyd and VandenbergheNatural Language Processing - This is the practice of turning text data into numerical data whilst still preserving the "meaning". Learning this will let you analyze new, exciting forms of data. How do I learn Natural Language Processing (NLP)?Time Series Analysis - How do I learn about time series analysis?Step 8. Do a Capstone Product / Side ProjectUse your new data science and software engineering skills to build something that will make other people say wow! This can be a website, new way of looking at a dataset, cool visualization, or anything!What are some good toy problems (can be done over a weekend by a single coder) in data science? I'm studying machine learning and statistics, and looking for something socially relevant using publicly available datasets/APIs.How can I start building a recommendation engine? Where can I find an interesting data set? What tools/technologies/algorithms are best to build the engine with? How do I check the effectiveness of recommendations?What are some ideas for a quick weekend Python project? I am looking to gain some experience.What is a good measure of the influence of a Twitter user?Where can I find large datasets open to the public?What are some good algorithms for a prioritized inbox?What are some good data science projects?Create public github repositories, make a blog, and post your work, side projects, Kaggle solutions, insights, and thoughts! This helps you gain visibility, build a portfolio for your resume, and connect with other people working on the same tasks.Step 9. Get a Data Science Internship or JobHow do I prepare for a data scientist interview?How should I prepare for statistics questions for a data science interviewWhat kind of A/B testing questions should I expect in a data scientist interview and how should I prepare for such questions?What companies have data science internships for undergraduates?What are some tips to choose whether I want to apply for a Data Science or Software Engineering internship?When is the best time to apply for data science summer internships?Check out The Official Quora Data Science FAQ for more discussion on internships, jobs, and data science interview processes! The data science FAQ also links to more specific versions of this question, like How do I become a data scientist without a PhD? or the counterpart, How do I become a data scientist as a PhD student?Step 10. Share your Wisdom Back with the Data Science CommunityIf you’ve made it this far, congratulations on becoming a data scientist! I’d encourage you to share your knowledge and what you’ve learned back with the data science community. Data Science as a nascent field depends on knowledge-sharing!Think like a Data ScientistIn addition to the concrete steps I listed above to develop the skill set of a data scientist, I include seven challenges below so you can learn to think like a data scientist and develop the right attitude to become one.(1) Satiate your curiosity through dataAs a data scientist you write your own questions and answers. Data scientists are naturally curious about the data that they're looking at, and are creative with ways to approach and solve whatever problem needs to be solved.Much of data science is not the analysis itself, but discovering an interesting question and figuring out how to answer it.Here are two great examples:Hilary: the most poisoned baby name in US historyA Look at Fire Response DataChallenge: Think of a problem or topic you're interested in and answer it with data!(2) Read news with a skeptical eyeMuch of the contribution of a data scientist (and why it's really hard to replace a data scientist with a machine), is that a data scientist will tell you what's important and what's spurious. This persistent skepticism is healthy in all sciences, and is especially necessarily in a fast-paced environment where it's too easy to let a spurious result be misinterpreted.You can adopt this mindset yourself by reading news with a critical eye. Many news articles have inherently flawed main premises. Try these two articles. Sample answers are available in the comments.Easier: You Love Your iPhone. Literally.Harder: Who predicted Russia’s military intervention?Challenge: Do this every day when you encounter a news article. Comment on the article and point out the flaws.(3) See data as a tool to improve consumer productsVisit a consumer internet product (probably that you know doesn't do extensive A/B testing already), and then think about their main funnel. Do they have a checkout funnel? Do they have a signup funnel? Do they have a virility mechanism? Do they have an engagement funnel?Go through the funnel multiple times and hypothesize about different ways it could do better to increase a core metric (conversion rate, shares, signups, etc.). Design an experiment to verify if your suggested change can actually change the core metric.Challenge: Share it with the feedback email for the consumer internet site!(4) Think like a BayesianTo think like a Bayesian, avoid the Base rate fallacy. This means to form new beliefs you must incorporate both newly observed information AND prior information formed through intuition and experience.Checking your dashboard, user engagement numbers are significantly down today. Which of the following is most likely?1. Users are suddenly less engaged2. Feature of site broke3. Logging feature brokeEven though explanation #1 completely explains the drop, #2 and #3 should be more likely because they have a much higher prior probability.You're in senior management at Tesla, and five of Tesla's Model S's have caught fire in the last five months. Which is more likely?1. Manufacturing quality has decreased and Teslas should now be deemed unsafe.2. Safety has not changed and fires in Tesla Model S's are still much rarer than their counterparts in gasoline cars.While #1 is an easy explanation (and great for media coverage), your prior should be strong on #2 because of your regular quality testing. However, you should still be seeking information that can update your beliefs on #1 versus #2 (and still find ways to improve safety). Question for thought: what information should you seek?Challenge: Identify the last time you committed the Base Rate Fallacy. Avoid committing the fallacy from now on.(5) Know the limitations of your tools“Knowledge is knowing that a tomato is a fruit, wisdom is not putting it in a fruit salad.” - Miles KingtonKnowledge is knowing how to perform a ordinary linear regression, wisdom is realizing how rare it applies cleanly in practice.Knowledge is knowing five different variations of K-means clustering, wisdom is realizing how rarely actual data can be cleanly clustered, and how poorly K-means clustering can work with too many features.Knowledge is knowing a vast range of sophisticated techniques, but wisdom is being able to choose the one that will provide the most amount of impact for the company in a reasonable amount of time.You may develop a vast range of tools while you go through your Coursera or EdX courses, but your toolbox is not useful until you know which tools to use.Challenge: Apply several tools to a real dataset and discover the tradeoffs and limitations of each tools. Which tools worked best, and can you figure out why?(6) Teach a complicated conceptHow does Richard Feynman distinguish which concepts he understands and which concepts he doesn't?Feynman was a truly great teacher. He prided himself on being able to devise ways to explain even the most profound ideas to beginning students. Once, I said to him, "Dick, explain to me, so that I can understand it, why spin one-half particles obey Fermi-Dirac statistics." Sizing up his audience perfectly, Feynman said, "I'll prepare a freshman lecture on it." But he came back a few days later to say, "I couldn't do it. I couldn't reduce it to the freshman level. That means we don't really understand it." - David L. Goodstein, Feynman's Lost Lecture: The Motion of Planets Around the SunWhat distinguished Richard Feynman was his ability to distill complex concepts into comprehendible ideas. Similarly, what distinguishes top data scientists is their ability to cogently share their ideas and explain their analyses.Check out https://www.quora.com/Edwin-Chen-1/answers for examples of cogently-explained technical concepts.Challenge: Teach a technical concept to a friend or on a public forum, like Quora or YouTube.(7) Convince others about what's importantPerhaps even more important than a data scientist's ability to explain their analysis is their ability to communicate the value and potential impact of the actionable insights.Certain tasks of data science will be commoditized as data science tools become better and better. New tools will make obsolete certain tasks such as writing dashboards, unnecessary data wrangling, and even specific kinds of predictive modeling.However, the need for a data scientist to extract out and communicate what's important will never be made obsolete. With increasing amounts of data and potential insights, companies will always need data scientists (or people in data science-like roles), to triage all that can be done and prioritize tasks based on impact.The data scientist's role in the company is the serve as the ambassador between the data and the company. The success of a data scientist is measured by how well he/she can tell a story and make an impact. Every other skill is amplified by this ability.Challenge: Tell a story with statistics. Communicate the important findings in a dataset. Make a convincing presentation that your audience cares about.Good luck and best wishes on your journey to becoming a data scientist! For more resources check out Quora’s official Quora Data Science FAQ

I must warn that I am one of those that Dr. Nassim Nicholas Taleb considers to be pseudo scientific quacks that peddle erroneous ideas to suckers. In other words: I am a cognitive psychologist. His statement implies that whatever I say must be considered a priori as being wrong. Read on only if you disagree with Dr. Taleb.Dr. Taleb's article is wrong in so many fundamental ways that it is very hard to even begin to address it. I'll try to tackle most of them, in no particular order.IQ is NOT a measure of "unintelligence", extreme or otherwise. It is a comparative measure of the ability to solve abstract linguistic and logical-mathematical problems. The results show how far each individual is from the average, both in terms of being above or below it.Usefulness, reliability, convenience and praxis have made IQ synonymous with "intelligence" in the scientific community (or pseudo scientific, according to Dr. Taleb). Though there are a myriad of other capacities involving the use of the mind, such as creativity, sociability, leadership, and self-regulation, which some might consider to comprise "intelligence", they are usually referred to as "abilities" or "competences". Its really just a matter of which labels one chooses to use.The observation that IQ explains “only" 13% to 50% of the variance in some tasks merely reflects the fact that performance requires more than just intelligence. Yes, such things include Conscientiousness (which is related to impulse control or the ability to defer satisfaction, i.e., "patience"), but also values, personality, and the way in which all these things relate to sociocultural settings and even physical environment. Actually, one should suspect a "quack" when someone in human or social sciences claims that a single variable alone explains most of the variance of anything. Indeed, the most advanced multivariate statistics in use today were created by psychologists and social scientists in order to deal effectively with such complex problems in their field.The criticism stated by Dr. Taleb involving "fat tails", "via negativa not via positiva" and being a "concave" measure is a series of non-issues and expresses profound ignorance on his behalf. A whole other lengthy thread might be initiated on this alone. The fact that many of the so-called "real-world" performance indexes do not usually show a Gaussian distribution (indeed, most often one finds a Pareto or similar distribution) does NOT imply that the association between a normally distributed IQ (or any Gaussian variable) either "doesn't exist" or "is uninformational". At worst, it just means that such associations are better assessed through nonparametric techniques. The same reasoning goes for nonlinear associations, which can be analyzed through nonlinear methods. It is ludicrous to suggest that a nonlinear association between IQ and the SAT is in any way indicative of the uselessness, inadequacy or fallacy of the first. One must also observe that non-Gaussian distributions can frequently be "Gaussianized" through simple mathematical transformations such as taking a natural logarithm or, with a bit more complexity, a Box-Cox transformation, among other methods. Nonlinear association can also be linearized through usually simple transformations. For example, the graph cited from Frey and Detterman (2004) can be easily turned into a strong linear association if one uses Ln IQ instead of "raw" IQ scores. And since when do nonlinear associations imply in pseudoscience?Of course the correlation between IQ and performance gets smaller as one takes higher and higher ranges of IQ. It is a simple, straightforward, diminishing returns or saturation-effect. Indeed, if one takes a high enough range, the correlation would HAVE to be zero. If someone with a certain level of IQ can solve, say, "17+34=?" at a certain speed, a person with a higher IQ will tend to solve it faster, but, as one takes people with higher and higher IQs, the improvements in time would become more and more negligible, for everyone would be giving nearly instantaneous correct responses (I would expect that, in such a scenario, the physiology of eyesight and visual perception, as well as psycho-motor phenomena, would eventually be more relevant for the differences in response time than differences in IQ). Why is this to be considered any sort of argument against IQ's is beyond me. Its is the contrary that would make me scratch my head."It takes a certain type of person to waste intelligent concentration on classroom/academic problems. These are lifeless bureaucrats who can muster sterile motivation." To this I simply refer to the saying from Kurt Lewin: "There is Nothing More Practical Than A Good Theory". Abstract thinking, especially the aloof and detached type, is ESSENTIAL for STEM competences, and one is hard pressed to find skills that are more practical and real world-oriented (or that yield greater employability in the job market, for that matter).The Curse of Dimensionality, including its application to intelligence testing, has been brilliantly addressed by Louis Guttman since 1954 through Multidimensional Scaling, Smallest Space Analysis and Facet Theory. Another interesting, if more limited, approach is the t-distributed Stochastic Neighbor Embedding (t-SNE), by van der Maaten and Hinton (2008). In essence, one uses data to estimate the associations between dimensions and then uses the results to produce a spatial representation of them in a smaller space, measuring and minimizing projection error (Alienation or Stress)."Convexity", as used apparently by Dr. Taleb and his followers alone, is an ill-defined and confusing concept that has most certainly never been measured and tested against IQ scores to substantiate the claim that "IQ doesn't detect convexity".Dr. Taleb argues that the pattern-recognition element underlying IQ tests is not indicative of "true" intelligence, for "Not seeing patterns except when they are significant is a virtue in real life". Is he saying that one can and should discard a pattern due to its lack of significance BEFORE such a pattern is even perceived? How is that even logically possible?Karl Raimund Popper was trained in Psychology and had close ties to the field. What he rejected was the notion that Psychology plays a central role in grounding or explaining some other, non-psychological, type of fact or law (Psychologism) and not Psychology itself.In short, it seems to me that Doctor Nassim Nicholas Taleb has no idea what he is talking about not only regarding IQ tests, but also pertaining to Statistics, Probability Theory and the Scientific Method.P.S. (Jan 7th, 2019)Here are some more rebuttals to Dr. Taleb's arguments and claims:The fact that racists and alike used IQ to pseudo-justify their stances is no more of an argument against IQ than pointing out that Hitler was a vegetarian is an argument against vegetarianism. It is simply an ad hominem fallacy and an appeal to emotion.Dr. Charles Murray has an A.B. in History from Harvard and a Ph.D. in Political Science from the MIT, having extensive research and experience in social topics such as urban education, welfare services, daycare, adolescent pregnancy, services for the elderly, and criminal justice. In 1994 he wrote, with Harvard professor Richard J. Herrnstein, the famous (or infamous, depending on who you ask) The Bell Curve, which presents evidence that intelligence is a better predictor of many factors including financial income, job performance, unwed pregnancy, and crime than one's parents' socio-economic status or education level. He also warned against a trend where the "cognitive elite" are becoming separated from the general population, which he sees as something dangerous. It is exceedingly inappropriate to refer to him as a "mountebank".The similarity between IQ test items and "real-world" tasks is not limited to "some" cases, but to many, including most of the better-payed and most valued activities (e.g., education, clerical work, analyst jobs, STEM occupations, etc.). This is a strength, not a weakness (the opposite might be a weakness).There are numerous studies showing positive associations between IQ and various measure of socioeconomic success, including not only wealth, but also income, longevity, procreation, job performance, job advancement (promotions), college-level employment, attaining advanced degrees, having no criminal record, not requiring welfare, and so forth.The existence of "noise" in the associations between IQ and socioeconomic outcomes is simply the reflection of the fact that, in human and social phenomena, the relationship between variables A and B is nearly always mediated or affected by their interactions with C, D, E, and more, so that if one only considers A and B, the impacts of the others will appear as "noise". The greater the number of other variables affecting the relationship, the larger the "noise" will be. Also, there will always be some "noise" in any measurement due to human errors, the observer effect, Gödel's Incompleteness Theorems, and the fundamental randomness of the Universe. This does not mean that it is useless to measure. Indeed, as George Edward Pelham Box famously said: "Remember that all models are wrong; the practical question is how wrong do they have to be to not be useful."The saying from Box also goes for the deviations of the tails of the IQ distribution from a perfect Gaussian distribution. The extreme values for which there are larger deviations are very rare, thus, having little or no effect upon the vast majority of the uses the score. If one is interested in detailing what happens in such extreme cases, it is just a matter of using the math that is appropriate for them (and, even so, which cases are to be considered as falling in this realm would still be determined by traditional IQ scoring methods - such as all IQs above 125, 133, 140 or 145, for instance).Of course the extreme values of many distributions, including the Gaussian, are always going to be estimated more poorly than the more central values. This happens because such estimates are based on Bernoulli's Law of Large Numbers, and, by definition, the tails of the distribution are less frequent in random samples. There are MANY mathematical ways of dealing with this.There ARE numerous standardized measures of "well-being" and even "sleep" that are widely used for various purposes, ranging from clinical interventions to the guiding of public policies. No one, except Dr. Taleb and, maybe, his acolytes, thinks that they are absurd in essence.The Flynn effect DOES warn us "that IQ is somewhat environment dependent". This is not new. Even the staunchest defenders of a biological basis for IQ still acknowledge that at least some 20% of the variance comes from environmental factors of various types (nutrition, vaccination, breastfeeding, education, use of digital technologies, engagement in social activities, etc.).

Average isoflavone intake ranged from 22 to 54 mg per day among food frequency surveys, and two studies from Japan suggest that approximately 5% of adults consume as much as 100 mg of isoflavones per day. Three studies show that the upper range (rather than the average) of soy protein intake is 16 to 18 g/day.An interesting article is by Jack Norris RD.Whats the Harm?Regarding soy and breast cancer, also see The Bottom Line on Soy and Breast Cancer Risk by Marji McCullough, ScD, RD of the American Cancer Society (2012).SummaryThere is a great deal of controversy surrounding soy foods, mostly due to their isoflavones which can bind to estrogen receptors and affect thyroid hormone.There is significant evidence that eating moderate amounts (one to two servings per day) of traditional soy foods, whether fermented or not, can reduce the risk of prostate cancer and can lower LDL cholesterol.People have been concerned that moderate amounts of soy could increase the risk of breast cancer or be harmful to women with breast cancer, especially if their cancer is estrogen receptor positive. However, the research to date has been quite reassuring, showing mostly benefits for breast cancer prevention.Everyone who eats soy should make sure they are getting enough iodine. People with hypothyroidism might need their synthetic thyroid hormone dosage adjusted if they start eating more soy due to the possibility that soy might interfere with it. There is some concern that eating soy could push some people with subclinical hypothyroidism into overt hypothyroidism, so limiting soy for such people might be a good idea.Soy infant formula has been shown to be safe except possibly for infants with congenital hypothyroidism, whose thyroid function should be monitored. Soy formula is not intended for pre-term infants.While one observational study found that tempeh was linked to better cognition in older people, tofu has been associated with worse cognition. This is most likely due to confounding variables, including tofu being processed with formaldehyde in Indonesia. Many clinical studies have found that soy increases cognition. Unless you are in Indonesia, you do not need to worry about tofu harming cognition.Some soy meats or foods containing isolated soy protein are processed with hexane and there may be small amounts of hexane residues in the final product. It is not known if this is harmful, but it might be a good idea to use soy foods from companies who do not use hexane in their processing methods (linked to below).The phytates in soy can lower the absorption of calcium, zinc, iron, and magnesium. However, you do absorb these minerals from soy foods and eating moderate amounts of soy should not cause deficiencies.At moderate amounts, soy does not cause feminine characteristics in men. At high amounts, as in twelve servings a day or more, a small percentage of men who are particularly sensitive to soy might develop tender, enlarged breast tissue.Introduction & Background: Why the Fuss?By far, soy is the most controversial of plant foods, with gluten taking a distant second place.Much of the controversy is due to some fairly unique components of soy, isoflavones. Isoflavones are also called phytoestrogens or "plant estrogens", because they can attach to estrogen receptors in cells. The estrogenicity of soy has raised questions of potential benefits, such as for bone health of post-menopausal women, as well as concerns such as for women with estrogen receptor positive breast cancer.In addition to the isoflavones, soy also contains phytates that can bind minerals and lower their absorption from foods.Between 1990 and 2010, there were over 10,000 peer-reviewed journal articles on soy (122). A large percentage of these were conducted in animals which can make the results irrelevant to humans because species differ in how they metabolize soy isoflavones and because the amount of isoflavones given to the animals is often much greater than any human would eat. This much research makes soy one of the most researched foods and also increases the chances of finding results that are outliers - studies that by random chance, or the inability to control variables, could show soy to be harmful (or helpful) when it actually is not. The large number of studies allow people who want to make a case against soy to simply highlight a handful when the bulk of research provides a different view. Of course, someone could make a case in favor of soy in the same way. It is, therefore, important to perform a comprehensive review of the research on any given topic, which I do below for the most important controversies surrounding the potential harm of soy foods.The IsoflavonesBecause some studies look at individual isoflavones, I will give a little background on them. Table 1 shows the breakdown of the typical isoflavone components of soy foods.Table 1. Typical Isoflavone Content of Soy122IsoflavoneIf Fermented% of Totalgenistingenistein~40%daidzindaidzein~40%glycitinglycitein~5-10%In addition to the three main soy isoflavones, there is a fourth isoflavone-related compound, equol, that is produced from daidzin by bacteria that about 25% of Westerners and 50% of Asians and vegetarians have in their digestive tracts (130). Equol has somewhat more estrogenic activity than daidzein and genistein, which has made comparing equol producers the subject of some specific research referenced below.The isoflavones in soy foods are absorbed almost exactly the same as from supplements (131). After ingesting isoflavones, blood concentrations of genistein and daidzein peak after 5.5 and 7.4 hours respectively; they have a half-life of about 8 hours, meaning 50% will be gone after 8 hours, 75% after 16 hours, and 88% after a day (132). However, if you eat them daily, you will have some in your blood all the time.There are other isoflavones, the most studied of which are the isoflavones in red clover. These are not the same isoflavones as found in soy.Soy ServingsIn the research discussed here, soy is typically described in grams of protein or milligrams of isoflavones. Less frequently, soy is described in grams of total soy foods. To make things more complicated, sometimes the participants in research are given only soy protein concentrate (about 65% protein), the isolated soy protein (about 90% protein), and sometimes only isolated isoflavones.A rough guide is that one serving of soy equals 1 cup of soymilk, or 1/2 cup of tofu, tempeh, soybeans, or soy meats. This is the rough equivalent of about 8 to 10 grams of soy protein and 25 mg of isoflavones. The more processed soy meats tend to have more protein (but fewer isoflavones per gram of protein).Soy FoodsSome people who write about soy suggest that fermented soy foods are the most healthy, and that isolated soy proteins are the most unhealthy. Fermented soy foods are tempeh, miso, and natto. As will be discussed below, for the most part, the evidence does not suggest that fermented soy foods are healthier than traditional non-fermented soy foods (such as tofu and soymilk). And much of the research on soy showing benefits has been performed on isolated soy protein.In the United States, whole soybeans are usually eaten in the form of edamame, tempeh, or soy nuts. Many, but not all, soy meats, as well as texturized soy protein, are made from soy protein concentrate or isolated soy protein. Turtle Island, the makers of Tofurky, use pressed tofu to create their soy meats.Benefits of SoyI do not attempt to comprehensively review the benefits of soy in this article. Because they should also be considered, I will briefly cover them.Lowering CholesterolA 2006 review of six meta-analyses on soy and cholesterol levels concluded, "In summary, the systematic reviews suggest that the effect of [a diet containing isolated soy protein with] around 90 mg/day of isoflavones is to reduce LDL cholesterol modestly (by around 5%), without clear effects on triglycerides or HDL cholesterol." (125)Prostate CancerA 2009 meta-analysis on soy and prostate cancer found that soy, in the highest versus lowest intake categories, was associated with a statistically significant, 26% reduction in prostate cancer risk (0.74, 0.63-0.89) (1). When separately analyzed, studies on non-fermented soy foods yielded a reduced risk of 30% (0.70, 0.56-0.88), and fermented soy foods were not associated with a decreased (or increased) risk of prostate cancer. It should be noted that the highest intake categories in these studies tended to be low, with most being less than one serving per day.Menopausal SymptomsA 2009 paper from the 8th International Symposium on the Role of Soy in Health Promotion and Chronic Disease Prevention and Treatment reported that soy isoflavone supplements containing at least 15 mg of genistein per day have been consistently effective at reducing hot flashes (126). If that much genistein is not provided, then there is little benefit, and is the reason why previous reviews, such as a 2006 meta-analysis (5), provided mixed results.Asian IntakesJapan and China have lower rates of heart disease and many cancers than do Westerners. People have suggested this could be due to the soy in their diet. In response, soy opponents have often argued that the traditional Asian intake of soy is much lower than is commonly thought. Typical Asian intakes of soy vary among countries and areas. In Japan and Shanghai, China, average intakes are about 1.5 servings per day, but many people consume an average of two or more servings per day. About half the soy eaten in Asia is not fermented.Show DetailsBreast CancerSummary: The vast majority of the evidence is that soy is either neutral or protective against breast cancer, including for women previously diagnosed with estrogen receptor positive breast cancer (tumors stimulated by estrogen contact). This evidence is mostly limited to amounts of two servings per day or less.ReviewsIn their 2010 review, Hilakivi-Clarke et al. sum up the evidence on soy and breast cancer (40):Results reviewed here suggest that women consuming moderate amounts of soy throughout their life have lower breast cancer risk than women who do not consume soy; however, this protective effect may originate from soy intake early in life. We also review the literature regarding potential risks genistein poses for breast cancer survivors. Findings obtained in 2 recent human studies show that a moderate consumption of diet containing this isoflavone does not increase the risk of breast cancer recurrence in Western women, and Asian breast cancer survivors exhibit better prognosis if they continue consuming a soy diet.Retrospective StudiesIn retrospective studies, past diets (provided by memory) of subjects with breast cancer are compared to those without breast cancer. Retrospective studies are less expensive and faster than following people prospectively (forward in time) but are considered less reliable due to inaccuracies in recalling past diet. They generally provide ideas of what connections to investigate prospectively. Nonetheless, the findings from the retrospective studies have shown soy to be either neutral or beneficial in protecting against breast cancer.Click for DetailsObservational Studies of Women Initially without CancerIn typical observational studies, you start with people who are apparently healthy and follow them prospectively, without any clinical intervention. The observational studies on soy and breast cancer can be divided into two groups - those done on populations with very low intakes and those done on populations with higher intakes.The studies done on populations with very low intakes do not provide much evidence. In a couple of cases they actually found an association that reached statistical significance, but if you look enough for associations, you are bound to find some eventually due to random chance.Observational Studies with Very Low Soy IntakesAs stated above, studies with very low soy intakes do not provide much evidence. But for the record, since 2001 there have been six observational studies on such populations. One of those studies, EPIC-Norfolk (2004), did find an association between soy intake and breast cancer, but the intake of soy was so small as to make the finding likely due to random chance.Show DetailsObservational Studies with Higher Soy IntakesOf the studies done on populations with higher soy intakes (about one to two servings per day as a typical upper intake amount), the Singapore Chinese Health Study (21), the Shanghai Women's Study (22, 23), and the Japan Public Health Center (26) study all found that higher intakes of soy were associated with a reduced risk of cancer. The Japan Collaborative Cohort Study (17) and the Japan Life Span Study (30) found no association. European Prospective Investigation into Cancer-Oxford (24) which contained a large number of vegetarians, also found no association, but this could be because exposure to soy might be more protective when breast tissue is developing during adolescence, while Western vegetarians often come to the diet as adults.Show DetailsBreast Cancer Survival and RecurrenceWomen’s Healthy Eating and Living Study, USA (2011)The Women’s Healthy Eating and Living (WHEL) study is a randomized controlled trial of a high fruit-vegetable-fiber and low fat dietary intervention in early stage breast cancer survivors (121). It had a median follow-up of 7.3 years from the time of enrollment. Soy intake was measured post-diagnosis (median 2 years, range: 2 months to 4 years) using a food frequency questionnaire that included specific items for "Meat Substitutes (such as Tofu, Veggie Burgers)," and "Soy Milk", as well as an opportunity to include other soy foods and supplements.Isoflavone intake (the marker for soy) was unrelated to the risk of recurrence regardless of hormone receptor status or Tamoxifen use. No significant increased or decreased risk was associated with any specific level of intake. Risk of death tended to be lower as isoflavone intake increased (p for trend=0.02). Women at the highest levels of isoflavone intake (>16.3 mg/day isoflavones; equivalent to at least 1⁄2 cup soymilk or 2 oz tofu) had a non-significant 54% reduction in risk of death compared to the lowest one-fifth of soy intake.The authors state:Our study is the third epidemiological study to report no adverse effects of soy foods on breast cancer prognosis. These studies, taken together, which vary in ethnic composition (two from the US and one from China) and by level and type of soy consumption, provide the necessary epidemiological evidence that clinicians no longer need to advise against soy consumption for women diagnosed with breast cancer.Of the other studies, one from the Cancer Hospital of Harbin Medical University, China (2010) (31), the Shanghai Breast Cancer Survival Study (2009) (11), and the Long Island Breast Cancer Study (2007) (32) found soy to be associated with a lower risk of breast cancer or death in some subgroups. Neither those studies nor the Shanghai Breast Cancer Study (2005) (33) or the Life After Cancer Epidemiology Study from Northern California and Utah (2009) (34) found any statistically significant increase in breast cancer, including among women with estrogen positive breast cancer (11, 33, 34).Show DetailsBreast Tissue and Nipple AspirateNon-lactating nipple aspirate might be a risk for breast cancer, although the data is mixed, and it is likely that what is more important is whether certain types of cells are found in the aspirate (129).There have been three studies measuring the effects of soy on nipple aspirate. One was conducted in women diagnosed with breast cancer and it found increased aspirate after two weeks in the high soy diet, although there was no increase in breast cell proliferation (35). In two longer term studies of six months on women without a diagnosis of breast cancer, one study found increased nipple aspirate in women while on a higher soy diet of 45 mg of isoflavones per day (there was no placebo group) (37). The other found no increase in nipple aspirate on the high soy diet (2 servings per day) when compared to the low soy diet (< 3 servings per week) (129). At this time, it appears that there is little concern regarding soy and nipple aspirate in amounts up to two servings per day.In two other studies of breast tissue, no difference was found between the high and low soy diet groups (38, 39)Show DetailsThyroidSummary: Almost everyone who has a reliable source of iodine can safely eat soy without it causing thyroid problems. Although most studies that have measured thyroid function and soy intake have found no problems, a 2011 study (78) of people with subclinical hypothyroidism found an increased rate of progression to overt hypothyroidism. For such people, it might be wise to limit soy. People with overt hypothyroidism who substantially change their soy intake might need to talk to their doctor about adjusting their synthetic thyroid medication.IntroductionThe thyroid gland produces hormones, triiodothyronine (T3) and thyroxine (T4), that regulate metabolism. Thyroid stimulating hormone (TSH), made by the anterior pituitary gland, increases in response to the body's need for more T3 or T4.Isoflavones in soy can inhibit thyroid peroxidase (TPO) (77), an enzyme involved in the synthesis of the thyroid hormones. This mostly happens when someone is deficient in iodine. If left unchecked, this can lead to hypothyroidism and even an enlarged thyroid gland, known as a goiter.ReviewsIn their 2006 review article, Messina and Redmond write (77):The preponderance of evidence from clinical trials involving healthy adult men and women indicates that neither soy protein nor isoflavones adversely affect thyroid function. As noted the adverse effects reported by one Japanese study [Ishizuki Thyroid Clinic, (67)] are biologically implausible and contrast with the results of 13 other trials. Thus, despite their ability to [inhibit thyroid peroxidase] in vitro and in vivo in rodents, isoflavones do not appear to cause thyroid hormone abnormalities in euthyroid individuals [people with genetically normal thyroid function]. Nevertheless, research aimed at determining whether further depression of thyroid function in individuals with pre-existing subclinical hypothyroidism occurs in response to soy intake is warranted.Messina and Redmond also point out that soy might interfere with the absorption of synthetic thyroid hormone.In their 2002 review article, Doerge and Sheehan write (76):Iodine deficiency is an emerging concern in elderly Americans. Consumption of iodized salt, the primary source of dietary iodine, may decrease with the desire or need to reduce the possible hypertensive effects of high salt intake. The data presented here suggest that elderly women need to be aware of, and monitored for, possible thyroid problems resulting from consumption of soy products. Those post-menopausal women who consume large amounts of soy products may be at higher risk.Primary StudiesHull Royal Infirmary, United Kingdom (2011)This study was done on people who were found to have subclinical hypothyroidism (defined as a TSH value between 5 and 15 mU/liter; normal range is 0.5–4.7 mU/liter) (78).It was a randomized, double-blinded, crossover study of 60 patients (8 men, 52 women), ages 44–70. The two study periods lasted eight weeks each and consisted of 30 g of soy protein that provided either 2 mg of isoflavones or 16 mg of isoflavones with an eight week washout period in between. An isoflavone intake of 2 mg per day is typical of Western populations, while 16 mg is typical of vegetarian intakes. The subjects had normal iodine levels ( >100 µg/liter).Six patients, all women, developed overt hypothyroidism after the 16 mg isoflavone phase, while none did after the 2 mg phase. The overt hypothyroidism appeared permanent, with no reversal on isoflavone withdrawal, although to determine this fully they should have discontinued thyroid hormone for a period, which they did not do. The underlying cause is unclear, although acceleration of an underlying autoimmune process is a possibility.The authors state:In a prospective study looking into the spontaneous course of hypothyroidism in females, the rate of progression of subclinical hypothyroidism to overt hypothyroidism was 5.6%/yr. Extrapolating the data from this 8-wk study to give a rate of progression per year, it was expected that in our study population, 3.36 cases per year would progress to overt hypothyroidism; this translates into a standardized rate ratio of 3.6 (95% confidence interval = 1.9, 6.2), i.e. supplementation with 16 mg phytoestrogen caused a 3-fold increase in progression from subclinical to overt hypothyroidism in this study....There are currently no data to guide what effect longer periods of exposure would have on thyroid function, with the possibilities that more overt hypothyroidism may result, stabilization may occur, or thyroid function could improve with thyroid adaptation to the phytoestrogen load.The authors conclude that female vegetarian patients with subclinical hypothyroidism may need careful monitoring.The good news was that the high isoflavone phase produced a significant reduction of blood pressure, c-reactive protein, and insulin resistance. It should also be noted that this is just one study which is rarely enough to be considered conclusive.Secondary StudiesThe studies below, while not primarily looking at thyroid function, did measure it while studying other effects of soy.Studies showing no effects on thyroid hormone were:54 mg/day genistein in post-menopausal women for 3 years (13)76 mg/day isoflavones in post-menopausal women for 2 years (75)10 mg/day of equol in post-menopausal women for 1 year (63)90 mg/day isoflavones on post-menopausal women for 6 months (60)65 mg/day isoflavones in men and women for 3 months (69)48 mg/day isoflavones on children for 2 months (66)62 mg isoflavones on men for 2 months (68)54 mg/day isoflavones on post-menopausal women for 7 weeks (71)A study on soy's effects on protein synthesis by the liver showed no effect on thyroid binding globulin (TBG) after 118 mg/day isoflavones for 3 months in post-menopausal women (72).The following studies showed minor changes in thyroid hormone but were not believed to be of physiological significance:90 mg/day isoflavones in post-menopausal women for 6 months (73)132 mg/day isoflavones in post-menopausal women for 3 months (70)128 mg/day isoflavones in pre- and post-menopausal women for 3 months (61)40 mg/day isoflavones in pre-menopausal women for 1 month (74)140 g/day soybeans in female students for 1 week (65)One study of 50 g/day soy protein in men for 4 weeks found that T4 changed from baseline, but did not differ from the placebo group (64).Finally, one study found that 30 g/day of pickled soybeans didn't change thyroid hormone levels but caused a goiter in half the subjects.Show DetailsInfant FormulaSummary: The American Academy of Pediatrics and the National Toxicology Program considers soy formula safe for term infants. There has been one cohort study looking at the general health of adults who were fed soy formula as infants. It found no reason to be concerned about thyroid or reproductive function (95). The Beginnings Study is an ongoing study examining the effects of formula on development. It is in its early stages with findings from children only a year old, but to date no negative effects of soy have been found on growth, sex organs, or neurological development compared to children on cow's milk formula. It is best to choose a soy formula with DHA. Soy-formula is not intended for pre-term infants.ReviewsExcerpts from Setchell et al's 1997 review paper on soy infant formulas sums up the controversy (79):When bodyweight and milk intake are taken into account, an infant exclusively fed soy-based formulas would be exposed to a fairly constant dose [of isoflavones] throughout early life. This dose represents a 6–11 fold higher level of intake of isoflavones than the bodyweight-adjusted intake (0.7 mg/kg per day) found to cause significant modifications to the hormonal regulation of the menstrual cycle of western women.... These values are also much higher than plasma isoflavone concentrations of Japanese adults, which range from 40 to 240 ng/mL.Seven years later in 2004, Merritt and Jenks wrote (82):Largely as a result of research in animal models, concerns have been voiced regarding isoflavones in soy infant formulas in relation to nutritional adequacy, sexual development, neurobehavioral development, immune function, and thyroid disease. We discuss the available clinical evidence regarding each of these issues. Available evidence from adult human and infant populations indicates that dietary isoflavones in soy infant formulas do not adversely affect human growth, development, or reproduction.American Academy of Pediatrics Committee on Nutrition (2008)While the American Academy of Pediatrics (AAP) recommends not using soy formula unless cow's milk formula is contraindicated or the parents are vegan, they say that there is no reason to think that it harms development in term infants (84):In summary, although studied by numerous investigators in various species, there is no conclusive evidence from animal, adult human, or infant populations that dietary soy isoflavones may adversely affect human development, reproduction, or endocrine function... [A]n increased incidence of feminization in male infants or an increased incidence of hypospadias in high soy-consuming populations have not been observed.Regarding infants with thyroid problems:Consumption of soy products by infants with congenital hypothyroidism complicates their management, as evidenced by a prolonged increase in thyroid-stimulating hormone when compared with infants not fed soy formula; the authors of 2 studies suggested closer monitoring and a possible need for an increased dose of levothyroxine.The AAP does not recommend soy infant formulas for pre-term infants due to studies showing poor bone growth in pre-term infants on soy-formula compared to cow's milk formula designed for pre-term infants. There is also a concern that aluminum levels in soy formula could be a problem for infants with undeveloped kidneys. They say, "Term infants with normal renal function do not seem to be at substantial risk of developing aluminum toxicity from soy protein-based formulas."National Toxicology Program (2009)The National Toxicology Program's Center for the Evaluation of Risks to Human Reproduction convened an expert panel on December 16-18, 2009 to evaluate soy infant formula (85). They concluded that there is minimal concern for adverse developmental effects in infants fed soy formula.ResearchOther studies include:Three case studies of infants with goiter in 1960, before soy formula was fortified with iodine (80).A 1990 retrospective case-control study showing children diagnosed with autoimmune thyroid disease were more likely than controls to have received soy formula as infants (81).A 2001 study of young men and women who had eaten soy infant formula found no reason to be concerned about thyroid or reproductive function, although the women had slightly longer duration and greater discomfort of menstrual bleeding (95).A 2008 cross-sectional study showing that female toddlers who received soy had more breast bud tissue left (93).A 2008 cross-sectional study found female girls on soy formula had a higher maturation index which the authors thought should be examined in future studies (94).A 2009 cross-sectional study found no difference in sex hormone levels in children fed soy formula (83).A 2010 study found that women who had received soy formula as infants were more likely to have non-cancerous uterine fibroids (92).Show DetailsCognitionSummary: There have been 12 short-term clinical trials looking at the impact of soy on cognition, and all have shown soy to be helpful or neutral. A longitudinal study found tempeh to be associated with improved cognition. Three reports from longitudinal studies have associated tofu with reduced cognition in some groups, but increased cognition in another group, and neutral in others. This is likely due to confounding. Based on the research to date, there should be little concern about eating soy, including tofu, with regards to cognitive decline.Long-term StudiesHonolulu-Asia Aging Study (2000)In a study of elderly Asian men living in Hawaii, food intake was collected once at baseline (1965–1967) and one more time during 1971 – 1974. Cognitive function was then measured in 1991–1993, and brain size was measured in those who had died (2).Midlife tofu consumption by itself (controlling for no other factors) explained 2.3% of the variation in mental cognition scores. A linear regression model that included only age, education and history of a prior stroke explained 27.8% of the variation. After controlling for these three most important factors, midlife tofu consumption remained statistically significant even though it then explained only 0.8% of the variance in test scores. Despite this, the researchers state that, "In this study population, 20% to 25% of the burden of cognitive impairment appears attributable to midlife tofu consumption — an effect size of enormous public health importance..."Upon autopsy of those who died, smaller brain size was associated with increased tofu consumption.In an accompanying editorial, Grodstein et al, write (41):Clearly, these results are interesting, although must be considered preliminary; relatively few subjects consumed tofu at high levels, and the confidence intervals around the estimates of effect presented are wide, indicating the limited precision of these results... Importantly, we do not know if tofu itself was the cause of these numerous indications of accelerated brain aging or if tofu is merely a marker for some other unfavorable exposure. For example, in this population, men with higher tofu intake (and more traditional diets) likely came from poorer immigrant families and perhaps experienced more childhood privation, which may be related to their brain development and subsequent cognitive function.In 2010, Hogervorst et al, reference this study (42):People of low socioeconomic status were hypothesized to eat more soy (as an animal protein substitute because it is cheaper). However, low socioeconomic status in itself is associated with a faster cognitive decline and an earlier onset of dementia and could thus have mediated the association between high soy intake and low memory function.Kame Project, King County, Washington (2000)The Kame Project is a prospective study of Japanese Americans aged 65 years or older living in King County, WA (53). Cognitive ability was measured at the beginning and after two years. Tofu consumption was categorized as low (<1/wk), moderate (1–2/wk), and high (3+/wk). At the beginning of the study, among women taking estrogen replacement therapy, high tofu consumption was associated with lower cognitive scores. No other statistically significant differences were found at baseline, and all analyses were adjusted for age, education, and language spoken. Upon testing at two years, tofu consumption was not associated with the rate of cognitive change either among men, women, or women on estrogen replacement therapy.Tofu and Tempeh Study, Indonesia (2008 & 2010)A 2008 study from Indonesia found that among people aged 52 to 98, tempeh intake was associated with slightly better memory scores (9). The authors suggested that tempeh might be good for memory because the bacteria used in the tempeh starter, Rhizopus oligosporus, produce folate which is thought to protect memory.However, increased tofu consumption was linked to slightly worse memory scores (-0.18, p = .05). The authors state that, "According to the Departments of Public Health at the Universities of Jakarta and Yogyakarta, formaldehyde is often added to tofu (but not tempeh) to preserve its freshness. Formaldehyde can induce oxidative damage to the frontal cortex and hippocampal tissue...." As of January 2011, the Indonesian government was still trying to end the practice of adding formaldehyde to tofu (43).In 2010, this research group published a follow-up paper (42). It was a cross-sectional study of 151 men and women (most from the previous report) over the age of 56. Both immediate and delayed recall were tested. Median and mean tempeh and tofu consumption was seven times a week, ranging from never to three times a day. Before adjusting for age, sex, and education, tempeh and tofu were associated with better immediate recall; after adjusting, the associations were no longer significant. In the group younger than 73 years, higher tofu consumption was significantly associated with better immediate recall even after the adjustments.There are a lot of theories as to why this follow-up study had opposite findings of their first study (one being that the older people who were eating more tofu had died), but it is an indication that tofu, itself, does not cause cognitive decline.California Seventh-day Adventists (1993)Seventh-day Adventists (SDA), ages 65 and over who had not eaten meat in the previous 30 years were about one-third as likely to develop dementia as their regular meat-eating counterparts (58). While soy intake was not compared to dementia rates, another study (59) of California Seventh-day Adventists showed SDA vegetarians to eat an average of 3.5 servings of meat substitutes (which usually contain soy) per week compared to only 1.4 servings by their meat-eating counterparts.Clinical TrialsThere has been a plethora of short-term (lasting one week to a year) clinical trials on soy and cognitive function. Four studies found that soy had benefits compared to the placebo (44, 45, 48, 49). One study found no decline in cognitive function in the soy group whereas the placebo declined (47). Four studies found a benefit to soy but there was no placebo group (50, 54, 55, 56). Three studies found no benefit of soy (51, 52, 57). No study found the placebo group to do better than the soy group on any measurement of cognitive function.Show DetailsConclusionThe short-term clinical trials have been very favorable towards soy and cognitive function. The one long-term study that assessed a non-tofu soy food, tempeh, found a protective effect in the first analysis, and no effect after adjustments in a follow-up.The main concern appears to be tofu which has recently been prepared with formaldehyde in Indonesia and was a food traditionally eaten by poorer people with less education. The research can be summarized as follows:The Honolulu-Asia Aging Study found tofu consumption during the 1960s and 1970s was associated with cognitive decline in the 1990s (2).An Indonesian study of older people found that tofu was associated with slightly lower memory scores, possibly due to formaldehyde in processing (9). A follow-up report found that tofu was linked with better memory scores among the younger subjects (42).The Kame Project, a study of older Japanese Americans found that at baseline, and only among women taking estrogen replacement therapy, tofu consumption was associated with lower cognitive scores. Testing again two years later, this association was no longer there (53).It seems safe to assume that tofu does not have an appreciable effect on cognitive function and that these associations are due to confounding variables. A study on non-Asians and tofu consumption (where it cannot be linked to lower socioeconomic status or formaldehyde) could shed more light on this subject.HexaneSummary: There is little evidence to determine whether the hexane residues in some processed soy meats are safe over the long term, though hexane processing can be harmful to workers and the environment. It seems prudent to buy most of your processed soy from companies that do not use hexane extraction.Hexane is sometimes used by the soy industry to separate the oil from the protein in soybeans. The protein is then used for soy meats and other products that contain soy protein extract. This process inevitably leaves some hexane residues in the products.The Cornucopia Institute is a public interest group that promotes organic agriculture. In November 2010, they released a report Toxic Chemicals: Banned In Organics But Common in "Natural" Food Production. The report makes a persuasive argument that hexane use is dangerous to workers and the environment (the evaluation of which is beyond the scope of this article), and for those reasons alone it seems better to use soy meats processed without hexane. Luckily, there are soy meats available that do not use hexane such as Tofurky, Field Roast, Wildwood, Amy's Kitchen, and some (though not all) Boca products (link). Foods certified as "USDA Organic" are produced without using hexane extraction.There is not much evidence that the amount of hexane found in most veggie burgers is harmful. The U.S. Food and Drug Administration has not set an upper limit on how much hexane residue can be in foods, whereas the European Union prohibits hexane residues greater than "30 mg/kg [30 ppm] in the defatted soya products as sold to the final consumer" (118). The Cornucopia Institute found some soy ingredients to have hexane levels as high as 21 ppm, but did not provide details on the average hexane levels they found in any final food products.More info on hexane in soyfoods:Hexane in Soy Food – Berkeley Wellness | May 01, 2012Do Veggie Burgers Contain Hexane? – Shereen Jegtvig, MS | Updated Feb 07, 2014Mineral AbsorptionSummary: Eating a few servings of soy per day is unlikely to result in calcium, zinc, iron, or magnesium deficiency.Table 2. Phytate Content of Selected Foods97FoodRange or Averageg/100 gWheat0.52 - 1.05Corn0.62 - 1.17Barley0.85 - 1.18Oats0.90 - 1.42Peas0.72 - 1.23Soybean1.20 - 1.75Soy isolate0.82Lupine bean0.76 - 1.63Vital gluten1.90Soybeans contain phytic acid, also known as phytate, which can inhibit the absorption of calcium, zinc, iron, and possibly magnesium. Many whole plant foods contain phytate, but soy has more than most. Hidvegi and Lasztity estimated the phytate content per 100 grams of food (97) in Table 2.While phytic acid has a bad reputation, it has some benefits. In their 2002 review, Minerals and Phytic Acid Interactions: Is It a Real Problem for Human Nutrition?, Lopez et al point out that phytates can prevent lipid peroxidation, iron oxidation of colorectal tissue, and calcium-based kidney stones (98).CalciumCalcium AbsorptionThree studies on adults (99,100, 101) have shown calcium to be absorbed from soy at rates comparable to that of cow's milk. One study on Chinese boys found calcium from soy milk to be absorbed at 43% while calcium from cow's milk was absorbed at 64% (109, see Table 3).Show DetailsTable 3. Mineral Absorption Rates in Chinese Boys109FoodCalcium(%)Zinc(%)Iron(%)Soy milk43.511.36.7Cow's milk64.231.215.5Dephytinized soy milk50.920.120.6Soy and BonesAlthough a 2010 (3) and 2008 (4) meta-analysis each found that soy isoflavone supplements of 82 and 90 mg/day, respectively, increases bone mineral density in menopausal women, a more recent, clinical trial found little benefit from 80 or 120 mg of soy isoflavone supplements for three years (6).There is a plethora of evidence that soy does not harmbones; including a cross-sectional study from the Chinese University of Hong Kong (2003) which found that women who ate the most soy (10 g/day of soy protein or more) had greater bone mineral density than those in the lower intake groups (104).Show more clinical trials on soy and bone mineral density or bone markers.ZincStudies have shown that the zinc in soy is absorbed at a rate of about 10 to 20%.Show DetailsIronIron from plant foods is generally not absorbed as well as iron from meat. But a type of iron in soy, ferritin iron, is absorbed at about 30% among people with low iron stores, and this is a high rate of absorption. Ferritin iron makes up a large percentage of the iron found in soybeans; up to 90% (127). Some soy foods have a decent amount of iron, including extra firm tofu (3.35 mg per 1/2 cup), edamame (1.75 mg per 1/2 cup), and soy nuts (1.7 mg per 1/4 cup). There is no reason to worry that moderate amounts of soy might cause iron deficiency.Also note that adding vitamin C to a meal (in a dose of about 100 mg) has been shown to significantly increase absorption of the iron in plant foods in numerous studies. I found no studies on vitamin C's effect on iron absorption from soy foods in adults, but two showed that it significantly increased iron absorption from soy formula in infants (114, 115).Show DetailsMagnesiumI found no studies looking at the effect of soy on magnesium absorption. Phytates have been shown to bind magnesium, so it would be no surprise if the magnesium in soy is absorbed at a lower rate than from other foods. Leafy green vegetables, whole grains, nuts, and seeds are the best sources of magnesium and people eating such foods while adding a few servings of soy per day should not be in danger of magnesium deficiency.FermentationIn their 2007 review, Hotz and Gibson say that fermentation can reduce the amount of zinc and iron-binding phytates in soybeans and should, therefore, increase the amount of zinc and iron absorbed from fermented soy foods such as tempeh and miso (111).The effects of fermenting soy foods on rates of zinc absorption have not been tested in humans, but one study from the University of the Witwatersrand, South Africa (1990) found that the iron in tempeh was absorbed better than from other soy foods, at a rate of 10 – 15%; the iron from tofu was absorbed at a range of about 3 to 8% (113). Again, absorption rates of plant iron vary greatly according to someone's current iron status and the amount of iron in the food, which can account for a large range of absorption amounts.Feminizing CharacteristicsSummary: It requires twelve servings of soy (and probably much more for most men) to have any sort of noticeable feminizing effects. While one epidemiological study raised concerns about soy and sperm quantity (14), two clinical studies have shown no effects of soy on sperm quality or quantity (15, 126).Case ReportsIn 2011, there was a case report of a 19-year old vegan male who was eating a great deal of soy foods - enough to provide 360 mg of isoflavones per day, the equivalent of about 14 servings (10). He also had type 1 diabetes. After eating this way for a year, he developed low free testosterone levels and erectile dysfunction. After ceasing the soy products (and the vegan diet), his symptoms normalized within a year. There was no reason why he needed to stop being vegan rather than just cutting way down on, or eliminating for a period, soy foods.A 2008 case report described a 60 year old man who developed gynecomastia (the enlargement of the mammary glands in a male) after drinking 3 quarts of soymilk (the equivalent of 12 servings) a day (123). His breasts returned to normal after discontinuing the soymilk.Gynecomastia - Male BreastsIn contrast to the case report above, in a 2004 study, 20 men with prostate cancer were given either 450 mg (amount found in about 18 servings of soy) or 900 mg (amount found in about 36 servings of soy) of isoflavones for 84 days (124). Two of the men, who had no gynecomastia at baseline, receiving the 900 mg developed mild cases of gynecomastia. One man was taking a drug that likely increased the gynecomastia. Two men had hot flashes probably related to the isoflavones. Given the very large amounts of isoflavones they were taking, this study should allay fears of feminization in men rather than cause any sort of alarm.SpermA 2008 epidemiological study found that men attending a fertility clinic had lower sperm concentrations if they ate more soy, but the amount of soy was very small (≥ .3 servings/day) and part of the difference could be explained by a higher ejaculate volume (14). Two more recent clinical trials using 62 mg/day of isoflavones for two months and 480 mg/day of isoflavones for three months found no difference in sperm count, concentration, or motility.Show DetailsReviewMessina sums up the rest of the literature on soy and feminizing characteristics in men in his 2010 review (122):In contrast to the results of some rodent studies, findings from a recently published metaanalysis and subsequently published studies show that neither isoflavone supplements nor isoflavone-rich soy affect total or free testosterone (T) levels. Similarly, there is essentially no evidence from the nine identified clinical studies that isoflavone exposure affects circulating estrogen levels in men. Clinical evidence also indicates that isoflavones have no effect on sperm or semen parameters, although only three intervention studies were identified and none were longer than 3 months in duration. Finally, findings from animal studies suggesting that isoflavones increase the risk of erectile dysfunction are not applicable to men, because of differences in isoflavone metabolism between rodents and humans and the excessively high amount of isoflavones to which the animals were exposed. The intervention data indicate that isoflavones do not exert feminizing effects on men at intake levels equal to and even considerably higher than are typical for Asian males.EndometriumA 2009 meta-analysis of five retrospective and two prospective studies found that participants with a higher soy intake (roughly one serving per day) had a reduced risk for endometrial cancer and ovarian cancer, when compared with lower soy intakes. Two other studies of three years in duration found that soy isoflavones in the amounts of 70, 80, and 120 mg/day did not have a negative effect on the endometrium. However, one study using 90 mg of isoflavones for five years (equivalent to 3.5 servings of soy per day) did result in cell growth (though non-cancerous) of the endometrium among some subjects. Women at risk for endometrial cancer might use caution in eating more than one serving per day of soy.Show DetailsOvarian FunctionA 2010 review by Wendy N. Jefferson of the Laboratory of Reproductive and Developmental Toxicology, National Institute of Environmental Health Sciences, summarized the literature on soy and ovarian function (117):For the most part, the studies conducted to date suggest that a diet containing lower levels of soy, e.g. 1–2 servings of soy/d, as part of a well-balanced diet should not pose harmful effects on the function of the ovary as it relates to ovulation. These levels are similar to that found in a traditional Asian diet (10–25 mg/day isoflavones) and even up to 50 mg/day isoflavones has little impact on serum circulating levels of hormones involved in reproduction. Although the levels of phytoestrogens typically found in soy foods pose minimal risk in the adult female, the female reproductive system is dependent on hormones for proper function and phytoestrogens at very high levels can interfere with this process.Additionally, a 2009 meta-analysis of five retrospective and two prospective studies found that participants with a higher soy intake (roughly one serving per day) had a reduced risk for endometrial cancer and ovarian cancer, when compared with lower soy intakes (119).FertilityA 2014 cross-sectional study from Adventist Health Study-2 found that a higher intake of isoflavones was associated with an increased rate of never becoming pregnant and of being childless (136). Among the highest soy intake category of > 50 mg/day (average of 79 mg/day or about three servings), compared to those in the lowest intake category (average of 3.7 mg/day), there was a 15% higher rate of being childless (1.15, 1.02-1.29). This finding was adjusted for age, educational level, and marital status.The authors did not distinguish between involuntary and voluntary childlessness.This issue warrants further attention, but given that it is a cross-sectional study and the findings were rather weak, it seems unlikely that isoflavones cause childlessness.GlutamateIsolated soy protein is rumored by some to have unusually large amounts of MSG (it doesn't), and MSG, in turn, is rumored to be an excitotoxin (for practical purposes, it's not) that will destroy brain cells. I cover this topic in some blog posts at Jack Norris RD (link).HypospadiasSummary: Two studies have examined soy intake and risk of hypospadias. Neither found a statically significant risk with more soy intake. It appears that factors other than soy are the most likely the cause of hypospadias, although more studies are warranted.Hypospadias is a male birth defect in which the opening of the urethra (the tube through which urine passes) is not located at the tip of the penis as it should be. The milder forms of hypospadias, which are by far the largest number of cases, are relatively easy to repair which usually results in a penis with normal or near-normal function and appearance, and no future problems (135).A 2004 case-control study from the Netherlands (133) found no link between maternal soy intake and an increased risk of hypospadias. Women who had a soy protein intake of 20 g or more had the exact same risk as those eating no soy (1.0, 0.5-2.2).A 2000 UK prospective study, of 7,928 boys born to mothers taking part in the Avon Longitudinal Study of Pregnancy and Childhood, found 51 cases of hypospadias (134). There was a trend of more soy meats and soy milk being associated with an increased risk for hypospadias, but the findings were not statistically significant, nor were the results adjusted for any confounding variables.Two of three studies have found an increased risk of hypospadias if the mothers were vegetarian; more information can be found in the Vegan Health Home Page article, Hypospadias and Vegetarian Diets.