by Jack Norris, RD
Soy contains isoflavones, which have the ability to bind to estrogen receptors. There are about 25 mg of isoflavones in one serving of soy.
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 studies, 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 (1), the Shanghai Women’s Study (2, 3), and the Japan Public Health Center study (4) all found that higher intakes of soy were associated with a reduced risk. The Japan Collaborative Cohort Study (5) and the Japan Life Span Study (6) found no association.
The European Prospective Investigation into Cancer-Oxford (7), 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 add soy to their diets 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 (8) 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.
A 2019 dose-response meta-analysis of prospective cohort studies found that a 10 mg/day increase in soy isoflavone intake was associated with a 9% decreased risk of breast cancer mortality, and a 5 g/day increase in soy protein intake was associated with a 12% reduction in breast cancer death (48).
Also see Soy and Cancer Risk: Our Expert’s Advice from the American Cancer Society.
The 2000 Honolulu-Asia Aging Study linked soy with lower cognitive function and brain shrinkage. Here’s a more thorough review of the research on soy and mental cognition.
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 (21). Three reports from epidemiological studies have associated tofu with reduced cognition in some groups (21, 22, 23), but increased cognition in another group (24), and neutral in others (23, 24). 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 Indonesian tofu has been prepared using formaldehyde.
The research as a whole provides little cause for concern.
The concern about infant formula is understandable giving that infants will be eating a large portion of their diet as soy.
The American Academy of Pediatrics and the National Toxicology Program considers soy formula safe.
The most important study to date, tracking adults who were fed soy formula as infants, provides assurance that there’s no reason to be concerned about thyroid or reproductive function (25).
The Beginnings Study is an ongoing study examining the effects of formula on child development (26). It’s 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 it’s best to choose a soy formula with DHA, and it’s important to note that soy formula isn’t 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 (27). 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 (28), 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 (29).
Isoflavones can affect thyroid hormone levels—especially if someone has iodine deficiency. A 2011 clinical trial found that 16 mg/day of isoflavones in people with mild hypothyroidism appeared to cause an increased rate of advancing to overt hypothyroidism (33).
Nine other clinical trials showed no effect of soy on the thyroid compared to placebo in people with presumably healthy thyroids (34, 35, 36, 37, 38, 39, 40, 41, 42), while the remaining five studies found small changes, all without physiological significance (43, 44, 45, 46, 47).
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. Please see Soy Part 2—Research for more details.
When you add up all the research on soy, there’s no reason to think that two servings per day are harmful to most people, and good reason to think soy provides some health benefits.
Last updated April 2011
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29. Fischer L, Mahoney C, Jeffcoat AR, Koch MA, Thomas BE, Valentine JL, et al. Clinical characteristics and pharmacokinetics of purified soy isoflavones: multiple-dose administration to men with prostate neoplasia. Nutr Cancer 2004;48(2):160–70.
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