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The internet is full with stories about how soy contains phytoestrogenes, how the protein is actually not readily used by our bodies etc.

I especially remember quite a long rant in Tim Ferriss's book "The 4 hour body", which I consider a great source for asking questions on this site as, although he claims to be scientific in his approach, relies too much on anecdotes for my taste.

Well, here my question: Are "normal" levels of soy intake harmful to the body? For definiteness, let us define normal as: Using soy milk in your breakfast cereal and having some stir fried tofu once in a while in your dinner or soy bean sprouts in your salad. EDIT: Add a bit of soy sauce once in a while to the mix

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Someone selling a book who claims to be scientific when he isn't is very suspicious. Most people don't seem to have any clue as to what the "scientific method" is and are often impressed by some famous person's anecdote as being a credible source of information (e.g., "that famous actor says her experience is genuine, and since she's also rich she must be right"). There are a lot of books nowadays that are targeting people who are interested in learning more about a healthy diet, and I notice that a lot of them rely on a lot of anecdotes -- don't let anyone shake your sense of doubt there. –  Randolf Richardson Jun 1 '11 at 5:59
    
as with everything, the dose is the poison. I'd consider the intake you consider "normal" already to be excessive however, but then I hardly ever consume soy except sometimes a soy based sauce. –  jwenting Jun 1 '11 at 6:13
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also keep in mind that any diet that is heavy in a single product (be it soy, potatoes, pasta, rice, salad, apples, or whatever) is unbalanced which means you're likely deficient in something. That's far worse than anything in that diet the body can't digest and leaves the body through natural processes. –  jwenting Jun 1 '11 at 6:15
    
Do you mean "harmful" as in "not as healthy/nutritios compared to consuming the identical amount of non-soy-based protein (e.g. beans/peas) that is more easily used"; or "worse than not eating anything at all instead of the soy-based portion of a diet"? E.g "active harm" vs "not as healthful as could be". –  DVK Jun 1 '11 at 6:20
    
Are we excluding things like soy sauce (normal serving sizes) from this one? –  user2466 Jun 1 '11 at 7:11
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6 Answers 6

up vote 17 down vote accepted

Tofu and other soy are listed/recommended (along with many other foods) in the context of Canada's Food Guide (which is a summary of the Canadian government's recommendations for healthy eating).


This page explains why Health Canada (i.e. the relevant department of the government of Canada) does or doesn't warn various categories of people against using using various quantities of soy isoflavone products: it includes this statement for vegetarians.

Soy - One of the nine most common food allergens says that you can be allergic to soy: in which case I suppose it is harmful to you, and you'd better avoid it.


http://www.veganhealth.org/articles/soynorris starts with,

There are at least 30 - 40 scientific papers on soy published each month. If you do a search on PubMed, you will find almost 7,000 papers with “soy” in the title and over 19,000 with soy in the abstract. So, it would be quite easy to build a false case against soy by citing a handful of these thousands of studies.

It then goes on to give its opinion on each of the various ways in which soy is said to be potentially bad for you:

  • Thyroid Problems (people who have a reliable source of iodine can safely eat soy)
  • Breast Cancer (it appears safe for women with and without breast cancer to eat moderate amounts of soy)
  • Dementia (mixed results, slight differences)
  • Feminizing Characteristics (not really, IMO)
  • Kidney Stones (limit soy intake if you are prone to kidney stones because soy is high in oxalates)

My opinion after doing this research is that there is no conclusive (strong, widely accepted) evidence of harm; and that that implies that the 'normal' or 'occasional' dosages mentioned in the OP are, typically, relatively harmless.

It may be worth noting too that there's little or nothing that's absolutely harmless - if you want to worry about soy, it's also possible to worry about the various alternatives (e.g. milk, meat, etc.).

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I'm a little curious to know why there's anti-soy propaganda, but that's probably off-topic everywhere. –  ChrisW Jun 7 '11 at 14:10
    
If 30-40 papers are written about soy every week, one should conclude that if measurable effects were repeatedly found in those studies those results would have already been picked up by the medical world. –  johanvdw Jun 7 '11 at 15:55
    
+1 for noting where soy-phobia gets pseudo scientific validity from. –  djechlin May 11 '13 at 18:09
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This doesn't relate to the phytoestrogen side of things, but to soy products. In particular the so-called 'dark' soy sauces contain a high concentration of free glutamates. Some people are exquisitely sensitive to glutamates (e.g. MSG).

From wikipedia:

Australia and New Zealand

Food Standards Australia New Zealand (FSANZ) cites "overwhelming evidence from a large number of scientific studies" to explicitly deny any link between MSG and "serious adverse reactions" or "long-lasting effects", declaring MSG "safe for the general population". It does, however, describe that in less than 1% of the population, sensitive individuals may experience "transient" side effects such as "headache, numbness/tingling, flushing, muscle tightness, and generalised weakness" to a large amount of MSG taken in a single meal.

[Emphasis mine.]

United States

Monosodium glutamate is one of several forms of glutamic acid found in foods, in large part because glutamic acid, being an amino acid, is pervasive in nature. Glutamic acid and its salts can also be present in a wide variety of other additives, including [...] soy extracts...

[Emphasis mine.]

The toxicity of MSG is an area of some debate. The noted scientist John Olney (of Olney's lesions fame) was an outspoken critic of the addition of MSG to foods. I would say that the vast majority of people are not too concerned about this, or are simply unaware.

There is some evidence that MSG may be linked to obesity.

Association of Monosodium Glutamate Intake With Overweight in Chinese Adults: The INTERMAP Study

This study examines the association between MSG intake and overweight in humans. We conducted a cross-sectional study involving 752 healthy Chinese (48.7% women), aged 40–59 years, randomly sampled from three rural villages in north and south China. [...] With adjustment for potential confounders including physical activity and total energy intake, MSG intake was positively related to BMI. Prevalence of overweight was significantly higher in MSG users than nonusers. [...] This research provides data that MSG intake may be associated with increased risk of overweight independent of physical activity and total energy intake in humans.

On the other side of the coin, a study funded by a Japanese MSG manufacturer (Ajinomoto) found:

MSG intake suppresses weight gain, fat deposition, and plasma leptin levels in male Sprague–Dawley rats

Monosodium l-glutamate (MSG), an umami taste substance, may be a key molecule coupled to a food intake signaling pathway, possibly mediated through a specific l-glutamate (GLU) sensing mechanism in the gastrointestinal tract. Here we investigated the effect of the spontaneous ingestion of a 1% MSG solution and water on food intake and body weight in male Sprague–Dawley rats fed diets of varying caloric density, fat and carbohydrate contents. Fat mass and lean mass in the abdomen, blood pressure, and several blood metabolic markers were also measured. Rats given free access to MSG and water showed a high preference (93–97%) for the MSG solution, regardless of the diet they consumed. Rats ingesting MSG had a significantly smaller weight gain, reduced abdominal fat mass, and lower plasma leptin levels, compared to rats ingesting water alone. Naso-anal length, lean mass, food and energy intakes, blood pressure, blood glucose, and plasma levels of insulin, triglyceride, total cholesterol, albumin, and GLU were not influenced by the ingestion of the MSG solution. These same effects were observed in a study of adult rats. Together, these results suggest that MSG ingestion reduces weight gain, body fat mass, and plasma leptin levels. Moreover, these changes are likely to be mediated by increased energy expenditure, not reduced energy intake or delayed development. Conceivably, these effects of MSG might be mediated via gut GLU receptors functionally linked to afferent branches of the vagus nerve in the gut, or the afferent sensory nerves in the oral cavity.

Summary

For some people, low doses of soy products may cause health problems such as obesity, headache, flushing, sweating, facial pressure or tightness, numbness, tingling or burning in face, neck and other areas, rapid, fluttering heartbeats (heart palpitations), chest pain, nausea, weakness.1


1Mayo Clinic: My favorite Chinese restaurant has a sign that says "No MSG." What is MSG? Is it bad for you?

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Although this is a well researched answer, it focuses entirely on MSG, which belongs to a different question. –  Lagerbaer Jun 1 '11 at 15:36
    
OK, I can see that point of view. But free glutamates/MSG are found in high quantities in soy sauce. Even a normal dose will have enough to set of the classic set off symptoms in susceptible individuals. So to the question: "Is soy bad for you", I say: "Yes, with qualifications, for some people." I have no idea if it'll turn men into ladies, etc. however. –  user2466 Jun 1 '11 at 19:34
    
-1 As a vegetarian I eat a lot of soy (e.g. as tofu, as fake/meatless hotdogs, and as soy milk); but I eat no soy sauce (nor MSG) at all. MSG != soya products. –  ChrisW Jun 2 '11 at 0:10
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@ChrisW: I understand this. I'm not talking about tofu either. I did say "In particular the so-called 'dark' soy sauces contain a high concentration of free glutamates." This is why I asked in the comments: "Are we excluding things like soy sauce (normal serving sizes) from this one?" –  user2466 Jun 2 '11 at 0:17
    
It's worth noting that if you avoid soy sauce for this reason, you should also avoid cheese and tomatoes.. –  Brendan Long Nov 21 '12 at 19:24
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  • Soybean fed rats had significantly heavier pancreases and lighter livers.
  • Calves receiving 30% of their protein from cooked soybean flour experienced diarrhea, lost 75 g body weight daily, and several pancreatic ducts became obstructed.
  • 10% of rats fed raw soya flour for a year develop pancreatic cancer, and all rats become more susceptible to pancreatic cancer-causing agents generally.
  • Of rats fed iodine deficient diets, gluten-protein fed rats had thyroid stimulating hormone levels of 5 ng/ml vs soybean-protein fed's 126 ng/ml.
  • Soy protein “significantly decreased” dihydrotestosterone (DHT) and DHT/testosterone in healthy young men.
  • In healthy adults fed 30 g soybeans a day for 3 months, thyroid stimulating hormone rose significantly, and 50% of subjects experienced goiter (swelling of the thyroid) or symptoms such as malaise, constipation, sleepiness.
  • In humans, 60 g daily soy protein for 1 month significantly delayed menstruation, caused a significant 47% increase in estradiol (growth hormone for reproductive organs) during one stage of the menstrual cycle, a significant 67% decrease in luteinizing hormone (triggers ovulation and testosterone production) during midcycle, and a significant 47% decrease in follicle stimulating hormone during midcycle.
  • 5% raw soybean meal protein caused nearly maximal growth retardation in chicks, and supplementing a protein-degrading enzyme fixed this

I can only post two hyperlinks due to my account status, but the sources can be found here, http://flare8.net/health/doku.php/antinutrients_in_food#other_organs

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The site's introduction reads: "'Chronic diseases' such as cancer, Alzheimer's, diabetes, heart attacks, acne, asthma are rampent among developed societies. They are likely wholly preventable by dietary means, and in conjunction with modern medical aid, are likely reversible. –  Lagerbaer Jun 3 '11 at 4:51
    
Everyday health issues commonly associated with aging such as arthritis, decreased energy, allergies, snoring, heartburn, brittle bones, and mood disorders, are not a result of aging, but rather the accumulation of preventable disease, and also are most all reversible. –  Lagerbaer Jun 3 '11 at 4:51
    
Common notions of most health-inducing behavior are either wrong, insignificant, or actually inducing of harm. Whole grains cause disease and saturated fat is the safest nutrient a human could consume. Vegetables, fiber, antioxidants, and chronic cardio do not significantly improve health. Extensive scientific evidence supports the above claims. This wiki currently has over 1,200 references to unique, peer reviewed scientific literature. –  Lagerbaer Jun 3 '11 at 4:51
    
I have some trouble trusting such a page, and the referenced studies might be outliers, so that meta studies show different trends. –  Lagerbaer Jun 3 '11 at 4:51
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Please add a disclaimer when you're linking to your own website. As long as the link to your website is on-topic and you summarize the content here like you did, it is okay to link to your own website. But we do require users to declare their affiliation with a site they link to. –  Fabian Jun 7 '11 at 5:50
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There is many studies done on the health effects of soy. I have not been able to find studies claiming soy is bad for you. Here is a sample of abstracts from recent review studies on the subject. Many more can be found on pubmed if you like.

A brief historical overview of the past two decades of soy and isoflavone research.
Messina M - 2010

During the past 20 years, a remarkable amount of research into the health effects of soy consumption has been conducted, which in large part can be attributed to the presence of isoflavones in the soybean. Isoflavones first came to the attention of the scientific community in the 1940s because of fertility problems observed in sheep grazing on a type of isoflavone-rich clover. In the 1950s, as a result of their estrogenic effects in rodents, isoflavones were studied as possible growth promoters for use by the animal feed industry, although shortly thereafter, it was shown that isoflavones could also function as antiestrogens. Despite this early work, it was not until the 1990s, largely because of research sponsored by the U.S. National Cancer Institute, that the role of soyfoods in disease prevention began to receive widespread attention. Subsequently, isoflavones and soyfoods were being studied for their ability to alleviate hot flashes and inhibit bone loss in postmenopausal women. In 1995, soy protein attracted worldwide attention for its ability to lower cholesterol. At this same time, isoflavones began to be widely discussed as potential alternatives to conventional hormone therapy. In 2002, it was hypothesized that individuals possessing the intestinal bacteria capable of converting the soybean isoflavone daidzein into the isoflavan equol were more likely to benefit from soy intake. More recently, in vitro and animal research has raised questions about the safety of isoflavone exposure for certain subsets of the population, although the human data are largely inconsistent with these concerns.

The health consequences of early soy consumption.
Badger TM, Ronis MJ, Hakkak R, Rowlands JC, Korourian S. - 2002

Soy formula is made with soy protein isolate containing isoflavones (SPI+) and supports normal growth and development in term infants. Recent data suggest that there are no long-term adverse effects of early exposure to soy formula through young adulthood. It is as yet unknown whether soy formula consumption by infants will result in health problems or benefits upon aging, but multigenerational animal studies with diets made with SPI+ have not revealed any problems. Soy isoflavones can function as estrogen agonists, antagonists or selective estrogen receptor modulators, depending on the conditions, and much research has focused on health effects of purified isoflavones. Results from several studies suggest that the effects of diets made with SPI+ differ significantly from those of diets to which purified soy isoflavones are added. Furthermore, it seems that soy protein processed to contain lower levels of isoflavones also provides significant health benefits. Further research is needed to confirm the results of the few studies that have been conducted and new studies are needed to investigate the more subtle effects that could occur during development or that could surface later in life.

Is soy consumption good or bad for the breast?
Hilakivi-Clarke L, Andrade JE, Helferich W. - 2010

Genistein in soy activates estrogen receptor (ER)-α and ERβ and acts as an estradiol in multiple target tissues. Because estrogens increase breast cancer risk and genistein promotes the growth of ER-positive human breast cancer cells, it has remained unclear whether this isoflavone or soy is safe. 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. The mechanisms explaining the breast cancer risk-reducing effect of early soy intake or the protective effect in Asian breast cancer survivors remain to be established. We propose that the reduction in risk involves epigenetic changes that result in alterations in the expression of genes that regulate mammary epithelial cell fate, i.e. cell proliferation and differentiation. Lifetime soy consumption at a moderate level may prevent breast cancer recurrence through mechanisms that change the biology of tumors; e.g. women who consumed soy during childhood develop breast cancers that express significantly reduced Human epidermal growth factor receptor 2 levels. More research is needed to understand why soy intake during early life may both reduce breast cancer risk and risk of recurrence.

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Soy is not a healthy food. in China it was used in crop rotation to add nitrogen to the soil and prevent pathogens with being in the rotation of crops. Fermented Soy is fine (miso) but not all this soy. Sad that many babies have to suffer from this soy formulas as people think it is healthy. It also is known to have heavy metals in soy formula.

Soy causes digestive problems in many. Bad stuff.

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Welcome to Skeptics! We expect users to back up any significant claim with a reliable source. Please edit your answer to include the references on which you based your answer. –  Fabian Jun 4 '11 at 7:42
    
In France they use maize/corn only for cattle, not for human consumption: and, in fact, what they're planting is a different variety of corn, which doesn't taste sweet in the way that North Americans are used to. In the past they used to practice crop rotation in Europe too (e.g. wheat, turnips, barley and clover) but that doesn't imply that wheat, turnips, are barley are inedible. I do agree with you that soy-milk isn't the same as infant formula, but I don't think that answers the question either. –  ChrisW Jun 4 '11 at 12:22
    
Please don't just link to a mostly unrelated website, you can of course link to specific articles relevant to the topic, but your link was very close to spam, so I removed it. –  Fabian Jun 4 '11 at 12:44
    
Soy is bad for you articles.mercola.com/sites/articles/archive/2010/09/18/… –  Tom Stickel Jun 6 '11 at 22:44
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If it seems like soy foods appeared out of nowhere to be regarded as the “miracle health food” of the 21st Century, it’s because they did.

From 1992 to 2006, soy food sales increased from $300 million to nearly $4 billion, practically overnight, according to the Soyfoods Association of North America. This growth came about due to a massive shift in attitudes about soy. And this shift was no accident—it was the result of a massive investment in advertising by the soy industry that’s been wildly successful.

Soy is indeed big business, very big business.

From 2000 to 2007, U.S. food manufacturers introduced more than 2,700 new soy-based foods, and new soy products continue to appear on your grocer’s shelves. Fermented soy products are the only ones I recommend consuming.

These are the primary fermented soy products you’ll find:

Tempeh a fermented soybean cake with a firm texture and nutty, mushroom-like flavor. Miso, a fermented soybean paste with a salty, buttery texture (commonly used in miso soup). Natto, fermented soybeans with a sticky texture and strong, cheese-like flavor. Soy sauce, which is traditionally made by fermenting soybeans, salt and enzymes; be wary because many varieties on the market today are made artificially using a chemical process. Please note that tofu is NOT on this list. Tofu is not fermented, so is not among the soy foods I recommend.

Vitamin K2: One of the Major Benefits of Fermented Soy

One of the main benefits of fermented soy, especially natto, is that it is the best food source of vitamin K2. Vitamin K2 is essential to preventing osteoporosis, cardiovascular disease, and diseases of the brain such as dementia, and protecting you from various cancers including prostate, lung, liver cancer and leukemia.

Vitamin K acts synergistically with vitamin D to keep you healthy.

http://articles.mercola.com/sites/articles/archive/2010/09/18/soy-can-damage-your-health.aspx

http://articles.mercola.com/sites/articles/archive/2010/09/18/soy-can-damage-your-health.aspx

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The fact that it's big business doesn't imply that it's bad. –  ChrisW Jun 7 '11 at 4:25
    
"1992 to 2006" is hardly "overnight"; perhaps there is a better way to word that. :P –  MrHen Jun 7 '11 at 4:34
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The website from Joseph Mercola is not a reliable resource. He's certainly biased as he earns his money from the product sold on his website. He advocates a lot of questionable claims, e.g. HIV does not cause AIDS and vaccines and most prescription drugs are harmful. –  Fabian Jun 7 '11 at 5:56
    
@Fabian - So who are you going to believe, then? A multi-billion dollar, financially inter-dependent system that includes big Pharma, big Agribiz, and big Government, not to mention mainstream medical professionals from every country in the world? –  ChrisW Jun 7 '11 at 12:00
    
Dr. Mercola is a Medical Doctor who prescribed drugs daily for years and saw the destruction and decided to go with preventative medicine. Mercola.com is highly reputable and he sites scientific studies and journals all the time. Pharmaceutical company earn money (Billions) by paying the FDA, Lobbyist, Television channels to promote their drug, so are they biased as they earn money.... hmmm –  Tom Stickel Jun 7 '11 at 17:55
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