Another Look at the Evidence on Soy


In her article, Not Soy Fast, Kristin Wartman argues that “…the research is mounting that soy foods are not only questionable in terms of their benefits, but in fact, may be hazardous to your health.”

Wartman describes the Cornucopia Institute’s recent report in which they describe finding residues of hexane in some soy food ingredients. The Cornucopia Institute gave few details about how much hexane they actually found and there is no evidence that the amounts typically found in soy foods are harmful to consumers. However, in the interest of worker and environmental safety, as well as trying to limit any potential harm from hexane residues, I cannot fault anyone for avoiding soy products produced with hexane. As Wartman points out, there are companies that make soy meats without using hexane, such as Tofurky and Field Roast.

But this is where Wartman and I part ways. While there are legitimate concerns regarding soy, Wartman cherry-picked the studies and ignored the vast majority of research. For the topics in this article, I will cover the full range of research findings, both pro and con.

A little background: Soy contains isoflavones which have the ability to bind to estrogen receptors and can affect thyroid hormone (especially if someone has iodine deficiency). There are about 25 mg of isoflavones in one serving of soy.

Breast Cancer

Wartman implicates soy as a cause for breast cancer, mentioning only one study. Unfortunately, she didn’t cite the study correctly, so it is not clear to which she was actually referring. In any case, here is a run down of the research.

Case-control studies on soy and breast cancer have been generally encouraging to those with soy in their diets, with about half associating soy with a lower risk for breast cancer and the other half showing no effects.

Prospective studies, which are generally a higher level of evidence than case-control, have also been very positive. Of the six studies done on populations with higher soy intakes (about one to two servings per day is the typical upper intake amount), the Singapore Chinese Health Study (21), the Shanghai Women’s Study (22, 23), and the Japan Public Health Center study (26) all found that higher intakes of soy were associated with a reduced risk. The Japan Collaborative Cohort Study (17) and the Japan Life Span Study (30) found no association. The European Prospective Investigation into Cancer-Oxford (24), which contained a large number of vegetarians, also found no association. Regarding the lack of association in the European study, some have speculated that in order to receive benefits from soy, exposure must occur during adolescence when breasts are developing, while Western vegetarians often adopt the diet as adults.

As for women with breast cancer, including those with tumors that grow in response to contact with estrogen (known as estrogen receptor positive), the authors of the recently published Women’s Healthy Eating and Living Study write:

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.

Dementia

Wartman cites the 2000 Honolulu-Asia Aging Study, saying that it linked soy with an increased risk of Alzheimer’s disease and brain shrinkage. Actually, this study does not mention Alzheimer’s Disease, although it did measure cognitive function. Let me sum up the evidence on soy and mental cognition.

There have been twelve short-term (lasting one week to a year) clinical trials looking at the impact of soy on cognition, and all have shown soy to be helpful (44, 45, 48, 49, 47 50, 54, 55, 56) or neutral (51, 52, 57).

Epidemiological studies (unlike clinical trials), examine patterns of soy consumption and cognition in specific populations. One such study found tempeh (a fermented soy food) to be associated with improved cognition (9). Three reports from epidemiological studies have associated tofu with reduced cognition in some groups (2, 9, 53), but increased cognition in another group (42), and neutral in others (42, 53). The harmful findings for tofu in the epidemiological studies are likely due to confounding caused by the fact that people of lower economic status have traditionally eaten more tofu in Asian cultures as well as the fact that some tofu has been prepared using formaldehyde (at least in Indonesia from where some of these reports have come). The research as a whole provides little cause for concern.

Infant Formulas

Wartman suggests that soy-based infant formulas are “Perhaps the most alarming…” While I can understand the concern given that some infants are eating nothing but soy, the most important study to date, tracking adults who were fed soy formula as infants, provides assurance that there is no reason to be concerned about thyroid or reproductive function (95). Furthermore, the American Academy of Pediatrics and the National Toxicology Program considers soy formula safe.

The Beginnings Study is an ongoing study examining the effects of formula on child development (86). 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.

Some research shows that is best to choose a soy formula with DHA, and it is important to note that soy-formula is not intended for pre-term infants.

Feminizing Characteristics

Regarding the concern that soy could cause feminizing characteristics in men, there have been two case studies. In one, a man eating twelve servings per day of soymilk developed enlarged, sensitive breast tissue (123). In another, a man with type 1 diabetes was eating 14 servings per day of mostly processed soy foods for one year and developed erectile dysfunction (10), which normalized after ceasing the soy. While I would not recommend eating this much soy, one study used even much higher amounts of isoflavones and found no problems for most men (124).

As for sperm quantity and quality, while one epidemiological study raised concerns, albeit minor, about soy and sperm quantity (14), two clinical studies have shown no effects of soy (15, 126).

Thyroid

I do want to address one more issue that has recently arisen with the publishing of a clinical trial this year in which 16 mg/day of isoflavones in people with mild hypothyroidism appeared to cause an increased rate of advancing to overt hypothyroidism (78). Nine other clinical trials showed no effect of soy on the thyroid compared to placebo in people with presumably healthy thyroids (13, 60, 63, 64, 66, 68, 69, 71, 75) while the remaining five studies found small changes, all without physiological significance (61, 65, 70, 73, 74). People without hypothyroidism should have no problems with soy as long as they get enough iodine, but until we know more, people with mild hypothyroidism might want to avoid soy just to be safe.

Miscellaneous

In Soy: What’s the Harm?, I address other issues Wartman raised, such as mineral absorption (a non-issue) and traditional Asian intakes.

Conclusion

In addition to reducing the risk for breast cancer as mentioned above, soy also provides benefits for preventing prostate cancer, lowering LDL cholesterol, and improving menopausal symptoms. When you add up all the research on soy, there is no reason to think that two servings per day are harmful to most people, and good reason to think soy will provide some health benefits.

Originally published on JackNorrisRD.com

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  1. Rachel
    Thursday, April 14th, 2011
    Looking at health studies is one thing, but when consuming soy, one must also consider the ecological implications. Soy is one of the world's top Genetically Modified crops. Where's your analysis on study's of GM soy affecting the ecosystems and humans?

    Soy in itself may not be damaging to human health overall, but GM foods may be. Please delve into that research as well before creating an article suggesting the safety of soy.

    It is poor form to suggest that eating soy is so acceptable without considering the biggest picture: the effects on the planet.
  2. Thursday, April 14th, 2011
    THank you, Civil Eats and Jack Norris, for publishing this article and giving out solid, reliable information about soy so that we can make informed food choices.
  3. Thursday, April 14th, 2011
    I would like to thank CivilEats.com for publishing this well researched reply.
  4. Thursday, April 14th, 2011
    Thank you for this! Well-researched, level-headed. The hype over soy (and everything else that might be "toxic") freaks me out and it's good to see some evidence that some of the obsession is really just that: hype.
  5. Celeste
    Thursday, April 14th, 2011
    Of course the president and co-founder of Vegan Outreach is going to argue that soy is not harmful.
  6. Amy
    Thursday, April 14th, 2011
    Rachel,
    If you're concerned about the impact of soy on the environment you must also realize that lots of soy is being fed to factory farmed animals, so we should stop eating those as well.
  7. Thursday, April 14th, 2011
    Good point Rachel. The likelihood of soy being GMO is too high for me to even bother with it. Plus it has no taste. There are far better legumes out there. Who needs soy?!
  8. Thursday, April 14th, 2011
    I would encourage anyone who believes the information in this article to do a search for "soy" in the FDA's Poisonous Plant Database. That might make you think twice.
  9. Natalie
    Friday, April 15th, 2011
    That was a fantastic read! Thank you for posting this. :)
  10. Dan
    Friday, April 15th, 2011
    Rachel, if you are concerned about the ecological impact of soy production then I hope you are vegan. After all, over 95% of the soybeans grown are used to fatten up animals for slaughter. When people switch from meat to tofu they actually cause less soy to be grown.
  11. Friday, April 15th, 2011
    Dan, that's quite misleading! There are OTHER ways to eat meat that aren't fed soy besides becoming vegan. It's quite easy to opt out of the industrialized food production (which feeds animals corn & soy), and opt into local, small-scale, pasture-based animal husbandry. It's what I do.

    And while Kristin Wartman may not have cited enough references to make Jack happy, that's not for lack of studies!

    As of 2006, there were 288 references to the toxicity of soy in the FDA's poisonous plants database, 77 studies showing adverse reactions to soy, and 174 studies showing the adverse reactions to isoflavones.