More details on every topic below, as well as many others, can be found in the companion article to this piece, Soy: What’s the Harm?, just published on VeganHealth.org. The reference numbers in the article below link to the references of Soy: What’s the Harm?
I approached each subject regarding soy without a foregone conclusion, trying to get to the truth rather than putting a positive spin on the results. While I think there is a limit on how much soy is safe to eat each day, that limit has not yet been determined. There is a great deal of evidence, however, that unless you have thyroid issues or are allergic to soy, two servings per day is perfectly safe.
I want to give special thanks to Jean Bettanny for her careful editing of Soy: What’s the Harm?
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 alternative 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.
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 (121) 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.
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.
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.
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).
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.
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.
This article was written by Jack Norris, a Registered Dietitian and President and co-founder of Vegan Outreach. Vegan Outreach produces Why Vegan and other booklets and their Adopt a College program hands booklets to over 750,000 students every semester. In 2005, Jack was elected to the Animal Rights Hall of Fame.
Along with Ginny Messina, MS RD, Jack has written the book Vegan For Life: Everything You Need to Know to Be Healthy and Fit on a Plant-Based Diet.